CRREL REPORT 96-12

Ice Action on Riprap

Small-Scale Tests

Devinder S. Sodhi, Sharon L. Borland and Jesse M. Stanley September 1996
Abstract: We conducted 35 small-scale experiments

to assess the damage on riprap-covered banks by ice

shoving. A review of literature on this subject revealed

very little experience or guidance available for the design of riprap in the cold regions, where presence of

moving ice can cause substantial damage to a

riprapped bank. During the experimental program, we

changed the slope of the model riprap bank, the size

and the mix of rocks, and the thickness of model ice

sheets. Results of these tests are presented in terms of

measured hori-zontal and vertical forces, outcome of

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Cover: Model riprap before (left) and after a test in which ice rode up the bank.

interaction as pileup or ride-up events, and damage to

the model riprap bank. From the observations made

during the tests, the damage to the riprap appears to

take place during pileup events, because the incoming

ice sheet is forced to go between the riprap and the

piled-up ice, bringing with it rocks from the bottom to

the surface of an ice pile. To sustain no damage to the

riprapped protective layer, maximum rock size (D100)

should be twice the ice thickness for shallow slopes

(1V:3H) and about three times the ice thickness for steeper slopes (1V:1.5H).
CRREL Report 96-12

Ice Action on Riprap

Small-Scale Tests

Devinder S. Sodhi, Sharon L. Borland and Jesse M. Stanley September 1996

Prepared for

OFFICE OF THE CHIEF OF ENGINEERS

Approved for public release; distribution is unlimited.

US Army Corps


of Engineers

Cold Regions Research &


Engineering Laboratory
PREFACE

This report was prepared Dr. Devinder S. Sodhi, Research Hydraulic Engineer, Ice

Engineering Research Division, Research and Engineering Directorate, Sharon L. Borland,

Civil Engineer, Plans and Programs Office, and Jesse M. Stanley, Jr., Mechanical Engineering Technician, also of the Ice Engineering Research Division, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire. Funding was

provided by U.S. Army Corps of Engineers Civil Works Project CWIS 32548, Ice Effects on

Riprap.

The authors thank Kevin Carey and James Wuebben for technically reviewing the manuscript of this report.

The authors are grateful to J. L. Wuebben, J.-C. Tatinclaux, J. Wilkinson, C. Schelewa,

M. Hopkins, R. Demars, R. Haehnel, P. Stark, and K. Chafe for their help in various ways

in completing this project.

ii
iii

CONTENTS

Preface .................................................................................................................................. ii

Introduction ......................................................................................................................... 1

Review of slope protection in the cold regions .............................................................. 1

Experiments ......................................................................................................................... 6

Setup ................................................................................................................................ 6

Sizes and gradation of stones ....................................................................................... 8

Placement of model riprap ........................................................................................... 9

Ice growth procedure .................................................................................................... 9

Measurement of model ice properties ........................................................................ 10

Carriage speed ................................................................................................................ 10

Data acquisition ............................................................................................................. 10

Procedure ........................................................................................................................ 11

Results .................................................................................................................................. 12

Observations on ice ride-up and pileup ..................................................................... 12

Measured horizontal and vertical forces .................................................................... 15

Observed failure of riprap ............................................................................................ 16

Failure and nonfailure of riprap vs. ratio of ice thickness to rock size .................. 19

Discussion ............................................................................................................................ 20

Summary and conclusions ................................................................................................ 20

Literature cited .................................................................................................................... 20

Appendix A: Design of riprap protection against waves ............................................. 23

Appendix B: Review of slope protection systems used in the Arctic .......................... 25

Appendix C: Plots of horizontal and vertical forces measured during the tests....... 27

Appendix D: Photographs and contour plots of the model riprap bank before

and after each test .......................................................................................................... 37

Abstract ................................................................................................................................ 65

ILLUSTRATIONS

Figure

1. Ice issues relating to Arctic slope protection and beach designs ....................... 2

2. Typical beach designs for Arctic structures ........................................................... 2

3. Typical winter ice action on sloping beaches ........................................................ 3

4. Typical ice action on caisson/berm structures ..................................................... 3

5. Forces acting during ice interaction with a sloping surface ............................... 5

6. Aluminum box used for model riprap bank ......................................................... 6

7. Box and the supporting platform prior to their installation under the

carriage .................................................................................................................. 6

8. Model supporting platform ..................................................................................... 7

9. Model support system .............................................................................................. 7

10. Partially installed layers of sand, filter fabric, and stones used to simulate

a riprap-protected bank ....................................................................................... 8

11. Refrigerated test basin in the Ice Engineering Facility ........................................ 9

12. Placement of grid over the model riprap bank..................................................... 10
iv

Figure

13. Measurement of the distance of the riprap surface from the strings at each

grid point ............................................................................................................... 11

14. Leveling rocks of model riprap in final preparation prior to each test ............. 12

15. Cutting of slots in the model ice sheet prior to each test .................................... 12

16. Ride-up event ............................................................................................................ 13

17. Model riprap before and after a test in which ice rode up the bank ................. 13

18. Ice sheet after it is broken and pushed up the riprap bank ................................ 14

19. Stones brought up to the surface of an ice pile ..................................................... 14

20. Sequence of ride-up and pileup events for test 20 ............................................... 14

21. Typical plot of the horizontal component of the force acting on the riprap ..... 15

22. Typical plot of the vertical component of the force acting on the riprap .......... 15

23. Plot of maximum force recorded during each test vs. bρwgL2 ...................................... 16

24. Ice pileup after test 4 ................................................................................................. 16

25. Model riprap before and after test 4 ....................................................................... 17

26. Model riprap before and after test 5 ....................................................................... 17

27. Contour plots of the model riprap before and after test 5 .................................. 18

28. Rocks found on top of the ice pile during melting of ice after test 20 ............... 18

29. Sand bed of model bank deformed by ice action after test 8 .............................. 18

30. Damage to filter fabric by ice action after test 30 ................................................. 19

31. Plot of cumulative probability of damage vs. the ratio of ice thickness to

D10 rock diameter ................................................................................................. 19

TABLE

Table

1. Values of various parameters used during the small-scale tests and some of

the results obtained from the tests ..................................................................... 11
INTRODUCTION

According to the Coastal Engineering Research

Center (1984), riprap is defined as a protective layer of quarry stones randomly placed to prevent

erosion, scour or sloughing of an embankment of

bluff. As listed in the publication by Construction

Industry Research and Information Association

(1991), the functions of a rock breakwater include

attenuation of wave transmission and overtopping,

dissipation of wave energy, and filtration. These

two manuals contain extensive information on the

design, construction and maintenance of the rock

structures, so that the structures can perform their

intended tasks with acceptable risk of failure and

maximum possible benefit-to-cost ratios. In Appendix A, we have presented the formulas to compute

the size of stones to be used for riprap protection

against waves (Coastal Engineering Research Center 1984) and in a flood control channel (U.S. Army

Corps of Engineers 1991). However, the guidance

for the design of shore protection systems in the

cold regions, where ice is present, is limited. This

can largely be attributed to the small amount of

coastal construction in the cold regions and particularly in the Arctic. Except for a few artificial

islands built along the southern coast of Beaufort

Sea for exploration of oil and gas resources in the

1970s and the 1980s, most of the construction of

dams and coastal facilities in the cold regions has

taken place outside the Arctic areas.

As in the case of open water, the processes taking place between an advancing ice sheet and individual rocks in a riprap structure are not simple

to model theoretically. Small-scale physical model

study is one of the means to assess the suitability

of riprap as a slope protection system against ice

action. In the following, we present a brief review

of slope protection system employed for installations in the cold regions, state the objectives of this

study, describe the procedure followed in conducting small-scale tests to assess the damage of riprap

by direct ice action, and present the results obtained from this small-scale study.

REVIEW OF SLOPE PROTECTION

IN THE COLD REGIONS

A few (≈ 10–20) man-made islands were built

since the 1970s for exploration of oil and gas along

the southern coast of the Beaufort Sea. Innovative

means of slope protection for the Arctic coastal and

offshore facilities were developed to minimize

their construction costs. Other constraints associated with the construction in that area are the short

construction season and high costs of labor, equipment and material.

The presence of ice is the major difference between the design of slope protection in the cold

and warm regions (Croasdale et al. 1988). During

the winter months, the presence of ice limits the

wave climate and erosion, but the ice itself may

damage the slope protection system. When the ice

edge retreats during the summer months, zones

of armor already damaged by ice may possibly

further be damaged because of storm-generated

waves in the open water. Three issues that may

affect the design of slope protection in the Arctic:

1) an ice sheet may ride up the sloping surface and

damage the surface facilities, 2) grounded rubble

ice around an island or adjacent to a beach may

form a protective barrier between the moving ice

and a structure, and 3) the interaction between a

discrete, thick ice feature and an underwater berm

of beach or island slope may absorb the energy of

the moving ice feature (Croasdale et al. 1988).

These are illustrated in Figure 1.

As shown in Figure 2, three types of slope protection designs have been used for the Arctic:

1) sacrificial beaches, 2) sandbag-retained islands,

and 3) caisson-retained islands (Croasdale et al.

Ice Action on Riprap

Small-Scale Tests

DEVINDER S. SODHI, SHARON L. BORLAND AND JESSE M. STANLEY
Figure 1. Ice issues relating to Arctic slope protection and beach designs

(after Croasdale et al. 1988).

Figure 2. Typical beach designs for Arctic structures (after Croasdale et

al. 1988).

2

1988). These designs evolved with the construction

of islands in increasing depth of water from the

early 1970s to the late 1980s.

Figure 3 shows a typical sequence of ice action

on sloping beaches in the Arctic. At the start of a

winter season, thin ice moves against an island and

forms a grounded rubble ice zone. Part of this rubble ice close to the water surface freezes and forms

a stable, protective rubble zone around an island.

Any further ice action takes place at the outside

edge of the consolidated rubble ice. In deeper

water, caisson-retained islands (Fig. 4) have suc-
Figure 3. Typical winter ice action on sloping beaches (after Croasdale

and Marcellus 1978).

Figure 4. Typical ice action on caisson/berm structures (after Croasdale

et al. 1988).

3
cessfully been used along the Canadian and the

Alaskan offshore regions. The rubble ice may not

remain stable in water depths greater than 10 m

(33 ft), and an underwater berm may be needed

for the caisson-retained islands to stabilize the protective rubble ice zone.

Some of the types of slope protection armor that

have been used are sandbags (Gadd 1988), concrete mats (Leidersdorf 1988), riprap and armor

stones (McDonald 1988), and large precast concrete armor units (Collins 1988). In Appendix B,

we discuss the experiences with each of these types

of slope protection armor.

Matheson (1988) surveyed the performance of

riprap slope protection of Canadian and other

dams. He found that the dominant mechanism of

riprap deterioration was freeze-thaw action on

riprap consisting of bedded sedimentary rocks,

resulting in delamination and breakdown. While

the sedimentary limestone and sandstone were

prone to breakdown, such degradation was found

to be negligible in the case of igneous and metamorphic rocks, e.g., granite. He described the following ice-related mechanisms that cause riprap

damage: 1) “plucking” of riprap by rising and falling water levels, and 2) ice shoving. He found that

the plucking action can affect riprap with rock size

up to approximately the ice thickness, and that the

riprap damage due to winter drawdown was significant for steeper slopes (1 vertical:1.75 hortizontal) and minimal for shallow slopes (1V:3H).

Doyle (1988) described the damage to public

and private property, including the destruction of

a bridge and several riprapped banks, from a sudden river ice breakup in January 1984 on the Nicola

River and its two main tributaries in British Columbia, Canada. He attributed all damage either

to the severe ice run within the channel or to flow

forced out over the floodplain by ice jams. The ice

run, caused by runoff generated by warm weather moving into the watershed after extremely cold

weather, damaged seven riprapped bends in the

Coldwater and Nicola Rivers. The riprap protection on 1V:2H slope banks consisted of a single

layer of well-graded angular rocks having a maximum (D100) and median (D50) diameter of 750–

900 mm and 500–600 mm, respectively. His field

evidence suggests that the running ice took out

individual rocks from riprap-protected banks,

causing total destruction of the protective works.

The damage sustained by trees in the path of running ice on the floodplains also would tend to corroborate the conclusion that running ice was able

to remove moderately heavy rocks. Because such

4

an event is believed to occur rarely, the repair to

ice-damaged riprap was done in a conventional

manner, rather than adopting a different, perhaps

more costly, design.

Wuebben (1995) reviewed the literature on ice

effects on riprap, river hydraulics and scouring of

river beds. He pointed out that nonuniform or unsteady flows during ice jams can cause serious

scour and degrade riprap performance. Ice jams

can cause stages comparable to rare open water

events, and it is necessary to consider protection

of upper limits of banks.

Ice can damage a slope protection system by

direct impact and shoving action. Even when the

damage caused by direct ice action on an offshore

artificial island may not be extensive, it can be

exacerbated by the wave action during storms in

the summer months (Gadd 1988). In a similar way,

river ice can damage slope protection armor at a

few places, and this damage can be extended over

a large area by high water levels and currents following the breakup of ice.

When an ice sheet moves against a sloping bank

protected with a riprap armor, the ice may ride up

or pile up. Sodhi et al. (1983) conducted small-scale

experimental studies to understand the factors that

lead ice to pileup and found that ice pileup occurs

more often on a rough sloping surface than on a

smooth surface. At present, the set of conditions

or parameters that can predict a ride-up or a pileup of an ice sheet is not exactly known. Extensive

ride-up of an ice sheet is not preferred because it

may damage onshore facilities. A rough sloping

surface may promote an ice pileup because the

forces required for continued ride-up of an ice

sheet become large. Croasdale et al. (1988) discussed the strategies to be employed in the design of a sloping surface to promote an ice pileup.

McDonald (1988) discussed the ice forces on a

coastal structure caused by a moving ice floe or

by thermal expansion of a restrained ice sheet. The

global force exerted by an ice sheet against a large

structure is not uniformly distributed across the

width of the structure. The force (or pressure) that

may develop on an individual armor unit may be

very high at one place and almost negligible at

another place. This is attributed to nonsimultaneous local ice failure, resulting in variation of ice

pressure with respect to time and location. The

normal component of this force may cause a pressure buildup to the ice crushing pressure, which

is less than that sufficient to damage most stones.

The shear component of ice force may act on an

individual stone protruding into the ice mass, and
the pressure on an individual rock can be as high

as that to crush ice. The shear component can introduce a rotation and dislodging of an individual

stone if opposing moment cannot be developed

from neighboring stones. To prevent such damage

from ice, the individual rocks that make up the armor stone or riprap layers should be placed on the

slope in such a way that the surface of the protective layer is relatively smooth. This is contradictory to the design practice of riprap to resist wave

action, where a rough surface is intended to dissipate the wave energy.

Croasdale (1980) presented a two-dimensional

formulation of the forces that result from an interaction between a floating ice sheet and a sloping

structure, as shown in Figure 5. The horizontal force

against a sloping surface results in a vertical force

at the point of contact, which deflects the ice sheet

up. When the maximum stress in the ice sheet exceeds the flexural strength, the ice sheet fails in

bending. The broken pieces of ice are pushed up

the slope and cause additional resistance to the

advancing ice sheet. Nevel (1983) has given a similar, but more rigorous, formulation of ice failure by

bending.

In general, the ice sheet deflected up by the sloping surface fails in bending into large ice blocks,

which may further break into smaller blocks as they

move upward and over irregular surface of the

stones. Because of the nonsmooth surface of a riprap protection system, a detailed analysis of interactions between the ice blocks and the rocks is very

complex. The damage in a riprap armor takes place

by removal and transport of stones by ice, and keeping track of progressive damage is impossible in a

simple theoretical analysis (e.g., Croasdale 1980,

Kovacs and Sodhi 1980).

Using a two-dimensional, discrete-particle analysis, Hopkins (1995) simulated onshore pileup and

ride-up of an ice sheet moving against an inclined

surface. He compared the results of his simulation with measured values of forces and energy

during a small-scale experiment. As part of this

study, he also attempted a two-dimensional, discrete-particle simulation of ice action on a sloping

surface protected by two layers of stones. The simulation showed movement of a few rocks until

they were embedded in an ice pile. Though this

simulation looks to be promising, the two-dimensional aspect of this analysis is a limiting factor in

establishing a relationship between the riprap

damage and the extent of ice action. Small-scale

experimentation is one of the means with which

we can study the ice effects on riprap armor in

terms of stone size, ice thickness, slope angle, and

ice flexural strength and elastic modulus.

The purpose of this study is to assess the damage to riprap protection by the shoving action of

an ice sheet. The focus of this study is for seasonally occurring ice on rivers and lakes in the cold

regions, such as the northern tier of the United

States. In those areas, it is possible for riprap to

have varying degrees of fixity to the bank during

a given ice impact event. The degree of attachment

can range between two extremes: 1) when a riprap

is strongly attached to the bank by being frozen in

place, and 2) when a riprap is not frozen to the

bank. Generally, a riprap frozen to the underlying

bank will be less prone to damage by ice action.

Because ascertaining and simulating varying degree of fixity of a riprap to the bank is difficult, we

chose to conduct the physical model test program

using the riprap in an unfrozen condition.

The objectives of this small-scale experimental

study are 1) to conduct small-scale tests of ice shoving against a sloped surface protected with a

5

Force from

Ice Blocks

Force Pushing

the Ice Sheet Reaction

Force

Z

Ice Blocks

Floating Ice Sheet

α

Figure 5. Sketch of forces acting during ice interaction with a sloping

surface, where α is slope angle from the horizontal andz is the height

of pushed-up ice blocks.
model riprap, 2) to measure the forces generated

during the ice–riprap interaction during the tests,

and 3) to determine the conditions when ice shoving damages a riprap armor. In this report, we describe the experimental apparatus and procedure

used to simulate a riprap-protected sloping bank

being impacted by model ice, present the data obtained during the 35 tests, and discuss the results

and their implications on design criteria for riprap

protection in ice-affected areas. The focus of this

study is for situations in rivers and lakes in the

cold regions. Generally, a riprap frozen to the underlying material will be less prone to ice damage. We decided to keep the model riprap armor

in an unfrozen condition so that the results of this

study will be conservative for the riprap armor

frozen to the material underneath.

EXPERIMENTS

We constructed a physical model of an inclined

riprap-covered embankment at a scale of oneeighth the actual size, based on typical ice flexural strength. To simulate ice shoving on a riprap

bank, we mounted the model under the carriage

spanning the refrigerated test basin of CRREL’s

Ice Engineering Facility, and pushed it against ice

sheets grown in the test basin. We could vary the

slope of the model riprap surface to three particular values and change the size and the mix of rocks

for different tests. We installed load cells at appropriate places in the supporting structure of the

model to measure the horizontal and the vertical

forces generated as a result of interaction between

the ice and the riprap. Thirty-five tests were conducted to model various combinations of the variables.

6

Figure 7. Box and the supporting platform prior to their

installation under the carriage.

a. Side view.

b. Front view.

Figure 6. Aluminum box used for model riprap bank.

Setup

Figure 6 is a photograph of the aluminum box

in which we built each model riprap bank. It was

1.32 m (52 in.) wide and 2.44 m (96 in.) long. We

attached sides to the box to confine the sand bed,

filter, model riprap and ice in the model area during a test. We left the top side of the model bank

open so that the ice reaching the top end during a

test could fall into the basin. We made the bottom

of this box with plywood reinforced with angle

iron, and we attached 25-mm- (1-in.-) × 25-mm-

(1-in.-) aluminum angles to the plywood surface,

extending from side to side.

We installed the box containing the model riprap

in a supporting platform (Fig. 7), which enabled

us to measure the horizontal and the vertical forces on the bank during the tests. Figure 8 shows a

schematic drawing of a two-tiered frame, which

supported the model riprap bank. We installed the

whole assembly under the main carriage beams so
that the model was partially submerged in the basin (Fig. 8). Figure 9 shows the engineering drawings of the frame. We suspended the upper frame

(1) from steel box sections (2), which were bolted

to the carriage, with the help of four bar linkages

(3) so that it could freely swing back and forth and

allow transmission of horizontal forces on the model bank through two load cells (4) installed on the

carriage. We supported a lower frame (5) under the

upper frame (1) through three vertical load cells(6).

Figure 9 also shows the location all load cells. We

installed the model bank (7) on the lower frame (5),

as shown in Figure 8. We supported one end of the

model riprap frame (7) at a pivot point (8) on the

lower frame (5) and the other end on two steel bars

7

Carriage I-Beams

Direction of Travel

Ice Sheet

2

3

4

5

6 6

7

8 9

2

3 1

Figure 8. Model supporting platform. Items numbered in this

sketch are described in the text.

2 ft -0 in. 6 ft -8 in.

1 ft -3 in.

Wall Box

(6 × 4 × 3/16 in.)

Wall Box

(7 × 5 × 5/16 in.)

5 ft -3 in.

5000 lb Load Cell

(3 locations)

7500 lb Load Cell

(2 locations)

10 ft -0 in.

Figure 9. Model support system, in which the locations

of five load cells are shown.

(9). By adjusting the length of the two steel bars,

we could change the slope of the model riprap surface. We could set three slope angles, 18°, 27°, and

34° from the horizontal, corresponding to vertical

to horizontal distance ratios of 1V:3H, 1V:2H,

1V:1.5H , respectively.

All load cells were calibrated to maximum capacity using standard calibration load cells prior

to the test program. After installing the test fixture under the carriage, we calibrated the load

measuring system, by applying a known load

through another calibrated load cell, and compared the known load to the sum of horizontal

and vertical forces measured by the load cells in

the model.
The model bank riprap installation simulated

a typical field installation as much as possible. We

placed a 38-mm- (1.5-in.-) deep layer of sand on

the bank (held in place by the transverse aluminum angle strips), covered the sand with a filter

fabric, and placed manufactured stones to model

the riprap material (Fig. 10). The practice in many

field applications is to apply the riprap over a filter material, which prevents fine materials of an

underlayer from being washed through the voids

of a coarse upper layer. Where geotechnical conditions permit, a filter cloth (or geotextile) is often

specified for the filter to lower construction costs.

This type of filter material offers less frictional resistance to the riprap compared to a filter made of

gravel. In previous laboratory studies on the angle of repose of riprap, the riprap was found to be

less stable when placed on filter cloth than when

placed on a simulated gravel filter. We used the

filter cloth for the underlying filter material in our

model, because the use of a filter fabric in our tests

will better simulate the conditions for the growing number of bank protection projects using geotextile filters, and it will be conservative for those

riprap installations using a gravel filter.

Sizes and gradation of stones

We used scaled-down stones in approximate

gradations as suggested by the U.S. Army Corps

of Engineers (1991) to model the riprap. In general, we followed the guidance given by the Corps

on the stone characteristics and installation techniques for the purpose of making model ripraps.

The guidelines and the convention used in specifying stone size are summarized here for reference.

Stone shape should be blocky, not elongated, to

enable the stones to “nest” together more effectively

to increase their resistance to movement. We used

rocks within a certain range of shape factor, which

is defined roughly as the ratio of the longest dimension to the shortest dimension of the rock, to

limit the number of rocks that were too elongated

to be effective as riprap. The guidance provide by

U.S. Army Corps of Engineers (1991) is that no more

than 30% of the material should have shape factors

greater than 2.5, and no more than 15% should exceed 3.0.

The stone size is defined as the equivalent spherical diameter D of a stone of a given weight W, and

it can be calculated using the standard equation for

computing the volume of a sphere:

D = {6W/(πγs)}1/3

where γs is the specific weight of a stone. Conventionally, stone size is expressed as Dn , where D

denotes the equivalent spherical diameter of the

stone and the subscript n denotes the percentage

of the total weight of the graded material that contains stones of smaller weight. Similarly, the stones

can be graded according to the weight expressed

as Wn, where W denotes the weight for which a

percentage n is lighter on the cumulative distribution curve. In the U.S. Army Corps of Engineers

(1991), standardized gradations having a narrow

range in sizes (1.4 < D85/D15 < 2.2) are given in tabular form. The manual recommends that the gradation limits should not be too restrictive to make the

production cost high. In general, the guidance simply suggests that gap-graded or widely graded

stone distributions may not be suitable as effective

riprap. According to the manual, most graded

riprap have ratios D85/D15 less than 3; for our tests,

we followed this guidance for the model riprap to

have the ratio D85/D15 less than 3.

We obtained manufactured stone from a local

quarry (West Lebanon, N.H.) for the model riprap.

The rock from this source is a metamorphic and

has a density of 2600 kg/m3 (160 lb/ft3). The shape

of an individual rock was typically blocky to slightly elongated with angular edges. We manually sifted the rocks to reject rocks with shape factor greater than 2.5. The large-size stones were sieved into

the desired size fractions because of the unavailability of standard sieves and the difficulty in handling.

Although stone size is often represented in terms

of the D50 or D30 size for open-water riprap installations, we have found that, for the case of ice shov8

Figure 10. Partially installed layers of sand, filter fabric, and stones used to simulate a riprap-protected bank.

In this view, the model and supporting platform are attached to the main carriage in the test basin.
ing, D100 is a better indicator of performance. We

used four different sizes of stone in this study: a

relatively large stone size of D100 = 127 mm (5 in.),

a medium stone size of D100 = 76 mm (3 in.), and a

small stone size of D100 = 38 mm (1.5 in.). The thickness of the riprap blanket was approximately 1.5

to 2 times the D100 stone. We determined the bulk

rock porosity by weighing a known volume of

rocks, and found it to be approximately 0.30 for

all size distributions of rock used.

We scaled down the distribution of stone sizes

for D100 = 127 mm (5 in.) from a recommended

distribution of riprap in the U.S. Army Corps of

Engineers (1991). Approximately, the distribution

of stone by weight was given in the following: 127-

mm- (5-in.-) diam. stone 15%, 102-mm- (4-in.-)

diam. stone 45%, 76-mm- (3-in.-) diam. stone 24%,

and 38 mm- (1.5-in.-) diam. stone and smaller 16%.

This distribution is equivalent to D85 = 102 mm (4

in.), and D15 = 38 mm (1.5 in.). The ratio D85/D15

= 2.7.

The distribution of stone sizes for D100 = 76 mm

(3 in.) was more uniformly graded with basically

two stone sizes in the mix. Approximately 50% by

weight of the total stone distribution was 76-mm-

(3-in.-) diam. stone and the diameter of the other

50% of the stone was 38 mm (1.5 in.) and less. The

resulting D85 = 76 mm (3 in.), and D15 = 38 mm

(1.5 in.) gave a D85/D15 ratio equal to 2.

The distribution of stone size for D100 = 38 mm

(1.5 in.) was a well-graded standard mix direct

from the plant designed to be used for grading

septic systems. The shape of this rock was slightly

blocky with angular edges. We estimated the ratio D85/D15 for this mix was about 2.

Placement of model riprap

Because we did not want any rock breakage

(which was observed to occur as a result of rough

handling during previous small-scale tests), we

distributed the larger stones evenly over the model

bank by hand as widely and evenly as possible.

Although placing the riprap by hand was a laborious and time-consuming process, we made every attempt to place the stone in a consistent fashion for each test run. We did not dump the stones

from a great height, nor were they keyed in by

hand. Because the stone placement took considerable time, we did not replace all the rock after every test, although that would have been ideal. Instead of removing and then replacing all the stone

to ensure that the riprap was not being “wedgedin” to the bank by repeated ice shoving events,

we carefully rearranged the rocks on the bank after every test. When relatively large quantities of

rocks were shoved by the ice over the top of the

bank into the basin during a test, we replaced the

lost rocks with a mix of rocks that had the same

gradation as that of the original mix.

Ice growth procedure

The test basin at CRREL is 36.6 m (120 ft) long,

9.1 m (30 ft) wide, and 2.4 m (8 ft) deep (Fig. 11). It

holds a solution of urea in water, 1% by weight, at

a temperature of approximately 0°C (32°F). To

grow an ice sheet, the temperature in the room is

lowered to –10°C (14° F). The water is kept mixed

with a bubbler system to prevent ice growth during this pull-down time. When the ambient temperature in the room reaches –10°C (14° F), we

spray fresh water into the air above the water surface to seed an ice sheet. During the seeding process, ice crystals form in the air and fall onto the

surface of the water. This seeding technique results in growth of small-size columnar ice crystals. The room is then cooled further to a temperature of –20°C (–4°F) to proceed with the ice

growth.

Total growth time of our model ice sheets depends on the required ice thickness. Once the

thickness is achieved, the desired strength was

obtained by “tempering” the ice sheet at a higher

room temperature of 0°C (32° F). The ice impurities (trapped between crystal platelets) weaken the

ice when the ice warms slowly. The flexural

strength of the ice is measured during the tempering process until the desired strength is reached.

The desired flexural strength of a model ice sheet

is about 87.5 kPa (12.5 psi), which is one-eighth of

the full-scale value of 700 kPa (100 psi). While test9

Figure 11 Refrigerated test basin in the Ice Engineering Facility.
ing with thin ice sheets, we did not temper the ice,

because the maximum flexural strength of thin ice

sheets is usually less than 87.5 kPa (12.5 psi).

Measurement of model ice properties

The ice sheet was characterized by measuring

the characteristic length and the flexural strength.

These properties were measured in situ using standard testing procedures, as described below.

The characteristic length was measured by placing a known dead weight P on a floating ice sheet

and measuring the resulting elastic deflection δ (Sodhi et al. 1982). The characteristic length L was then

calculated as

L = [P/(8ρwgδ)]1/2

where ρwg is the specific weight of water.

We measured the flexural strength by cutting

cantilever beams and breaking them in the upward

or downward direction. The dimensions of the

beam were proportional to the ice thickness. The

length of beams was about five to six times the ice

thickness, and the width was about two times the

ice thickness. We measured the maximum force F

(applied at their tips) required to break the beams,

and also the dimensions of broken beams. The flexural strength σf was then calculated as

σf = 6Fλ/(wh2)

where λ = the beam length

w = the width

h = the ice thickness.

Carriage speed

On rivers, water velocities during spring breakup can exceed 5 m/s (16.40 ft/s) in extreme events

(Wuebben 1995). The ice action would most likely

be a combination of ice shoving and shearing along

a river bank at high velocity (≈ 1 m/s or 3.28 ft/s).

In a reservoir situation where there could be purely normal ice shoving action on a bank, the ice

would be driven by wind or thermal expansion at

speeds of less than 1 m/s (3.28 ft/s).

We used the main carriage to push the model

through an ice sheet at a constant speed of 40 cm/

s (1.31 ft/s) for all tests but the first two. We conducted the first two tests at speeds of 80 cm/s (2.62

ft/s) and 20 cm/s (0.66 ft/s) for shakedown purposes. We found little effect of speed on the measured forces if the ice failure took place in bending.

A speed of 40 cm/s (1.34 ft/s) in model tests scales

to full-scale speed of 1.13 m/s (3.71 ft/s), which is

10

Figure 12. Placement of grid over the model riprap bank.

a reasonable speed of ice shoving during a typical

ice breakup.

Data acquisition

We measured or noted the following variables

during each test: ice thickness, flexural strength

of ice, carriage speed, horizontal force, vertical

force, topography of riprap before and after each

test, and an assessment of damage. In some tests,

we measured the topography of the ice pile and

its maximum height. In Table 1, we have given the

values of parameters or variables that were either

set or measured during each test. We measured

and recorded the horizontal and the vertical forces along with the carriage speed using a data-acquisition system, which we programmed to scan

at 150 samples/second per channel. We used antialiasing, 45-Hz low-pass filters to remove highfrequency noise at the analog input stage. We programmed the data acquisition system to acquire

the data when the carriage was in motion, i.e., start

and stop triggers were set on the carriage movement. We also saved pre- and post-trigger data for

a period of 10 seconds.

To profile the rock surface, we placed a frame

containing a 102- × 102-mm (4-in. ×4-in.) grid made

of strings (Fig. 12) on top of the fixed aluminum

sides of the model bank, which provided a fixed

reference for each set of measurements. With a

ruler, we measured the perpendicular distances

from the rock surface to the strings at each grid

point (Fig. 13). The width of the grid frame was

the same as that of the box (1.32 m or 52 in.) and

its length was 1.52 m (60 in.). With this grid, we

measured the riprap surface profiles at 168 grid

points, which enabled us to get a reasonably good

profile. We profiled the rocks before and after each
ice-free test. This technique provided a repeatable

means of quantifying the amount of rock moved

during a test by ice shoving.

Procedure

Before each test, we carefully leveled the rocks

on the model riprap bank and filled in gaps in the

riprap layer where necessary, as shown in Figure

14. The angle of the model bank was adjusted by

changing the effective length of the support rods

at the front (submerged) end of the model. Prior

to freezing an ice sheet, we adjusted the water level

in the test basin so that the model was submerged

about halfway. We measured the water level relative to the model bank for each test and profiled

the rocks using a grid technique explained above.

We took photographs of the rocks directly above

the model from a fixed vantage point before, dur11

Table 1. Values of various parameters used during the small-scale tests and some of the results obtained

from the tests.

Stone Maximum

Carriage size Ramp Ice Flexural strength Characteristic horizontal

speed (D100) slope thickness Down Up length Level of damage force

Test Date (cm/s) (mm) (deg) (mm) (kPa) (kPa) (m) 0 = none; 3 = H (N)

1 29 Nov 93 80 38 18 50 118 70 0.90 0 8007

2 3 Dec 93 20 38 18 45 93 44 0.64 3 5338

3 8 Dec 93 40 38 18 40 106 45 0.58 1 1779

4 10 Dec 93 40 38 18 43 104 45 0.66 3 6895

5 15 Dec 93 40 38 18 35 90 37 0.40 3 4226

6 7 Jan 94 40 38 18 34 90 0.42 2 3114

7 12 Jan 94 40 38 18 24 102 40 0.37 2 2224

8 21 Jan 94 40 38 18 35 57 0.38 3 2447

9 26 Jan 94 20 38 18 35 75 31 0.40 2 2669

10 28 Jan 94 40 38 27 40 103 50 0.54 2 4671

11 2 Feb 94 40 38 34 31 80 31 0.36 1 2669

12 4 Feb 94 40 38 34 36 98 37 0.47 1 4893

13 9 Feb 94 40 38 27 37 107 34 0.50 1 3781

14 11 Feb 94 40 38 27 41 90 51 0.54 2 6228

15 16 Feb 94 40 38 34 38 108 59 0.54 3 5560

16 18 Feb 94 40 127 18 37 95 31 0.47 0 4448

17 23 Feb 94 40 127 18 33 79 32 0.38 0 2224

18 25 Feb 94 40 127 18 54 116 79 0.84 0 3559

19 1 Mar 94 40 127 34 53 100 52 0.84 1 11121

20 8 Mar 94 40 127 27 53 108 56 0.79 0 8674

21 11 Mar 94 40 127 27 55 85 42 0.75 1 11009

22 15 Mar 94 40 127 34 56 118 68 0.80 0 8585

23 18 Mar 94 40 127 34 41 128 72 0.58 0 7340

24 22 Mar 94 40 127 34 34 108 41 0.42 0 3225

25 25 Mar 94 40 127 18 43 90 45 0.69 0 6450

26 5 Apr 94 40 76 18 42 124 56 0.58 0 6005

27 7 Apr 94 40 76 18 40 94 0.53 0 6005

28 12 Apr 94 40 76 18 65 117 53 0.84 3 7562

29 15 Apr 94 40 76 27 55 101 0.90 1 13345

30 19 Apr 94 40 76 34 55 95 0.97 3 17793

31 22 Apr 94 40 76 18 42 122 49 0.56 2 3559

32 26 Apr 94 40 76 27 46 102 0.63 0 6895

33 29 Apr 94 40 76 34 46 114 56 0.63 1 4448

34 4 May 94 40 76 18 25 122 49 0.31 0 6228

35 6 May 94 40 76 27 33 93 31 0.40 0 3114

Figure 13. Measurement of the distance

of the riprap surface from the strings at

each grid point.
12

Figure 15. Cutting of slots in the model ice sheet prior

to each test.

Figure 14. Leveling rocks of model riprap in final preparation prior to each test.

ing and after each test.

After measuring and documenting the ice properties, we cut slots in the ice sheet along the length

of the tank using chain saws mounted on a slowmoving carriage (Fig. 15). These slots were located such that the ice would not hit the side walls of

the model riprap bank. This left a long strip of ice

sheet no wider than the model bank, approximately 1.22 m (48 in.) wide, and as long as the test basin, approximately 30 m (100 ft) long. On each side,

we cut two more slots 15 cm (6 in.) apart, and removed the ice between these slots.

Immediately prior to each test run, we recorded the initial values measured by the load cells.

When we moved the carriage, the model riprap

moved against the model ice sheet. We continued

the test as long as there was sufficient ice to shove

onto the model bank. After completion of each test,

we recorded final force values.

We made video recordings of the ice interaction with the model bank during each test. We recorded the first eight tests from the front, and the

remaining tests from the side. Some of the video

recordings show the initial ice ride-up, followed

by a gradual transition to ice pileup.

After the ice pile was removed from the model,

we photographed the rocks again from the same

position above the model to serve as a record of

rock displacement. We profiled the rocks again using the grid technique explained above.

RESULTS

As mentioned earlier, Table 1 lists the values of

parameters or variables that were either set or

measured during each test. It also lists the maximum horizontal force measured during each test

as well as the severity of damage to the model

riprap bank. Some of the observations made during the tests are given below.

Observations on ice ride-up and pileup

We observed ice ride-up only during two tests:

3 and 18. The slope angle α of the bank in both

these tests was 18.4° (1V:3H). During an ice rideup event, we observed that the ice sheet was being pushed up the riprap slope and continually

slid up and over the top edge of the model and

back into the tank. During the ice ride-up events,

the ice sheet dislodged some rocks upon initial

impact, but generally did not displace enough

rocks to expose the bank. Figure 16a shows an ice

sheet riding up the model riprap bank for test 3.

The action of the ice sheet sliding on top of the

riprap shaved the underside of the ice sheet and

filled in the interstitial spaces between the rocks,

causing the riprap surface to become smooth. Figure 16b shows the riprap surface filled with ice

shavings after we removed the ice sheet. Figure

17a and 17b show the riprap bank before and after test 3, respectively. As can be seen, there was

very little displacement of the riprap.

During the initial stages of a test with a pileup,

an ice sheet would contact the riprap, break into

pieces, and ride up on top of the riprap, as shown

in Figure 18. Although the ride-up continued for

a long time in a few tests, the ice would usually

pile up above the waterline and offer resistance to

the incoming ice pieces. As the ice pile grew larger, there were instances when we observed an ice

sheet pushing under the ice pile, gouging the model riprap, and bringing up small stones (Fig. 19).

Figure 20 shows a sequence of photographs of an

ice pileup during its formation. These photographs

were taken during test 20, in which the bank angle was 27° (1V:2H) and the ice thickness was 53
13

Figure 16. Ride-up event.

a. Ice riding up during a test run. b. Riprap with shaved ice packed in the interstitial spaces

between the rocks after the ice sheet was removed at the

completion of a test.

Figure 17. Model riprap before and after a test in which ice rode up the bank.

a. Before. b. After.

mm (2.1 in.). As shown in Figure 20a, the ice sheet

broke into roughly rectangular-shaped pieces approximately 25 cm (10 in.) square as soon as the

ice sheet contacted the bank above the waterline.

Figure 20b shows the broken-up ice sheet going

over the top edge of the bank and starting to pile

up on one side of the riprap bank. Figure 20c shows

the ice sheet riding up and over the top edge of

the bank on one side of the model, and beginning

to pile up on the other side. As more of the ice

sheet continued to shove onto the bank, the ice

pieces stacked up on the previously deposited ice

pieces (Fig. 20d). At this stage, no ice was being

pushed off at the top edge of the bank because all

the ice pieces piled up high, creating a steep incline for the incoming ice sheet. Figure 20e and f

show that the ice sheet piled up and crushed into

the ice pile. In these last two views, it is possible

to see rocks being brought to the surface of the ice

pile by the ice sheet. We observed the rocks being

transported from the bed to the surface of the ice

pile during most of the tests in which the ice piled

up on the bank.

At times, the ice sheet would buckle one or more
Figure 18. Ice sheet after it is broken and pushed

up the riprap bank.

Figure 19. Stones brought up to the surface of an

ice pile.

Figure 20. Sequence of ride-up and pileup events for test 20, in which the ice thickness was 53 mm (2.1 in.), and the

bank slope was 27° (1V:2H).

a.

b.

c.

d.

e.

f.

14
times during a test. As discussed later, the forces

associated with buckling represent the maximum

force that a model ice sheet could impart on the

model riprap. Buckling events represent largescale failure of the model ice sheet, and this did

not cause large-scale failure of riprap in our tests.

However, the model ice sheet caused small-scale

damage by removing individual stones one at a

time.

Measured horizontal and vertical forces

The forces on the model during open water test

runs (without any ice) were much smaller than

those measured during tests with model ice. Figure 21 shows typical time-history plots of the horizontal force, which were obtained by summing

the load measured by two load cells (Fig. 8). This

plot shows a slight increase in the force, caused

by the edge of the ice sheet first coming in contact

with the model, at about 7 seconds after the start.

For the next 10 seconds, the ice rode up and over

the model bank, as indicated by the relatively constant horizontal force. This force was the result of

the friction force between the ice sheet and the

riprap. For the next 30 seconds, the horizontal force

gradually increased until it reached a peak value

at about the 50-second mark. Figure 21 has several cycles of increasing force followed by sudden

reductions in force during the remaining duration

of the test. Each reduction in force was due to buckling failure of the ice sheet during the interaction.

In this study, buckling failure limits the force that

could be imparted to the model riprap by the model ice sheet.

To obtain the total vertical load acting on the

model riprap, we summed the output of three load

cells placed between the two frames (Fig. 8). Figure 22 shows a typical time-history plot of vertical force, which generally increased with time because the force was largely a measure of the weight

of accumulated ice on the model. During a few

tests, one of the forward two load cells for measurement of the vertical force stopped functioning.

Because of symmetry in the longitudinal direction,

the measured force from the other forward load

cell was multiplied by two and added to the measured value from the third load cell to get the total

vertical load. After the defective load cell was replaced with a new load cell, the outputs of three

load cells were added to obtain the total vertical

load.

Plot of maximum horizontal force

In Appendix C, we have given the plots of horizontal and vertical forces from all ice tests. In Table 1, we have also tabulated the maximum horizontal force in each test. These maximum force

values were in the range of 1.8 and 17.8 kN (405–

4000 lb).

During the model tests, we observed crushing,

bending and buckling failures of the ice sheet during its interaction with a model riprap bank. Crushing of ice occurred in localized areas where the ice

came in contact with the stones during ice ride-up.

An intact sheet predominantly broke up into pieces by bending failure that took place in longitudinal as well as transverse directions. These ice pieces either rode over or piled on top of the riprap

surface. Ride-up occurred for low values of the

bank slope, whereas a pileup formed for high values of the bank slope. At times, the ice sheet plowed

through the piled up material, gouged the riprap

at the bottom, and brought out stones with it. This

took place as long as the ice sheet was capable of

exerting sufficient force, which was limited by the

buckling failure of ice. Therefore, the maximum

15

Figure 21. Typical plot of the horizontal component of the force acting on the riprap.

Figure 22. Typical plot of the vertical component of the force acting on the riprap.
force that we could expect to measure during a

test is the force required to buckle an ice sheet.

The buckling force of a semi-infinite beam of floating ice is between bρwgL2 and 2bρwgL2, corresponding to the frictionless and hinged boundary

conditions (Hetenyi 1946), respectively, where b is

the width of the beam, ρwg is the specific weight

of water, and L is the characteristic length of floating ice sheet. Figure 23 shows the plot of maximum measured force vs. bρwgL2 for all tests. Most

of the data fall between the lines having a slope of

1 and 2, indicating that the boundary condition

on the edge of the ice sheet was something between that of frictionless and hinged.

Though buckling failure limits the horizontal

force in model tests, the forces in full-scale situations will be limited by the amount of environmental forces (wind and water stresses or thermal

expansion) acting on an ice sheet, by the momentum of moving ice floes, or by the forces required

to fail an ice sheet in bending, crushing, or buckling. During initial stages of ice shoving, the bending failure of ice will limit the force that an ice sheet

can exert on the riprap, because this failure mode

requires the least force per unit width of the ice

sheet. If the ice sheet forms an ice pileup, the ice

sheet may be possibly confined to fail in crushing

or buckling. It is also possible for the zone of failure to progress offshore, creating a rubble field.

Observed failure of riprap

 An ice pileup consisted of the ice sheet breaking into small pieces and piling one on top another.

An ice pileup most often caused failure of the

riprap when the advancing ice sheet was forced

to go between the riprap and the piled up ice. In

general, we defined the riprap to have failed if

enough rock material was displaced to expose underlying areas of the riprap-protected bank. For

example, the ice piled up significantly in test 4, as

shown in Figure 24. During that test, the ice

gouged away enough riprap to expose large areas

of the bank, as shown in Figure 25. In test 5, an ice

pileup formed below the water line, as can be seen

by the damage to riprap shown in Figure 26. Figures 27a and b show the contour plots of the rock

surface, which were generated from the rock profile data taken before and after test 5. Such contour plots help to visualize the total displacement

of the riprap during a test. The contour plots

16

Pile Up Event

Ride Up Event

Slope 2:1

Slope 1:1

0 2 4 6 8 10 12

Estimated Buckling Force (kN)

18

16

14

12

10

8

6

4

2

Maximum Horizontal Force (kN)

Figure 23. Plot of maximum force recorded during each test vs. bρwgL2,

where b is the width of ice sheet, ρwg is the specific weight of water, and L is

the characteristic length of floating ice sheet.

Figure 24. Ice pileup after test 4.
shown in Figure 27 show a typical pattern of ice

action on riprap, in that the rock was gouged out

just below or at the waterline and was deposited

in a hump above the waterline.

During a test in which ice piled up, we observed

17

Figure 25. Model riprap before and after test 4. The difference between these photographs

indicates the extent of rock movement.

Figure 26. Model riprap before and after test 5. The difference between these photographs

indicates the extent of rock movement.

a. Before test. b. After test.

a. Before. b. After.

that some of the rock material was removed from

the bed and deposited on top of the ice pile. In some

tests, the rocks were tossed from the bed and over

the top edge of the bank. Figure 19 shows the rocks

in the ice pile towards the end of the test 20. After
18

Figure 27. Contour plots of the model riprap before and after test 5. These contour plots show

both extent and amount of rock movement during test 5. The numbers along contour lines denote

measured distance from the string in the grid frame.

a. Before. b. After.

Figure 28. Rocks found on top of the ice pile during melting of ice after test 20.

this particular test, we allowed the ice to melt on

top of the damaged riprap and observed that the

model bank was barren of rock under the last bit of

ice, as shown in Figure 28. Later, when the ice melted

away completely, the rocks on top of the ice were deposited onto the barren patch, though not quite covering it. We observed this “self healing” behavior in

many tests after an ice pileup. In the open-water bank

protection by riprap, this self healing of riprap also

takes place, which makes riprap a popular choice for

projects that require low maintenance.

After an ice run on the smallest stone size (test

8), we observed that the ice sheet had deformed the

sand bed as well as the riprap, as shown in Figure

29. After test 30, we observed a far greater amount

of ice damage than usually encountered; the ice tore

the filter fabric (Fig. 30). These observations indiFigure 29. Sand bed of model bank deformed by ice

action after test 8.
cate that the small-stone riprap was not only inadequate to resist these ice actions, but also made

the underlying bank material highly vulnerable

to damage by ice.

In Table 1, we have given numerical values to

the level of riprap failure, as visually apparent to

us in terms of exposure of filter fabric underneath

the stones. A failure level designated by 0 was

assigned to tests when we did not observed significant displacement of rocks. A failure level designated by 1 was assigned to slight damage during

tests in which we observed small areas, on the

order of 20 cm2 (3 in.2), of filter fabric. A failure

levels designated by 2 was assigned to damages

in those tests where we observed moderate area

of riprap damage (20–65 cm2 or 3–10 in.2). Lastly,

a failure level of 3 was assigned to damages in

those tests in which we observed large areas of riprap gouged from the bank, resulting in exposed bank

areas of 65 cm2 (10 in.2) or more. In Appendix D, we

have also presented the photographs of the riprap

taken before and after each test.

Failure and nonfailure of riprap vs.

ratio of ice thickness to rock size

The damage to the riprap takes place as a result

of interaction between an ice sheet and an individual rock. However, we do not know the exact

nature and magnitude of the ice pressure on an

individual rock. It is for this reason that we have

done model tests, as in the case of waves and currents, to determine the ice damage to the riprap.

We conducted model tests with three bank

slopes, and the damage the riprap was noted in

each test. For each bank slope angle, we calculated to the ratio of ice thickness to the maximum rock

diameter and ranked this ratio from smallest value

to the largest value. When the outcome of a test was

associated in terms of riprap failure or nonfailure,

we found that there was no damage to the riprap

when the ice thickness was small compared to the

maximum rock size. On the other hand, we found

that the ice sheet always damaged the riprap when

it was large compared to maximum rock diameter.

In Figure 31, we have plotted these results for each

bank slope in terms of cumulative probability of riprap failure vs. ratio of ice thickness to D100 rock diameter. There was a range of ice-thickness/rock-size

ratios in which the damage to the riprap was random.

The plots in Figure 31 show that the values of

ice-thickness/rock-diameter (h/D100) ratio at which

transition from nonfailure to failure takes place decrease with the increase in the bank slope angle. By

combining the data from all tests for the three bank

slope angles, we have plotted the cumulative probability of riprap failure with respect to the ratio of

ice thickness to nominal rock diameter. These plots

indicate that there is no damage to the riprap when

h/D100 is less than 0.35 and that the riprap is always

damaged when h/D100 is greater than 1. From the

results shown in Figure 31, the riprapped banks

with shallow slope (1V:3H) are less prone to damage by ice shoving than those with steeper slopes

(1V:1.5H). For shallow slope (1V:3H) banks, the rock

size should be about twice the ice thickness to sustain no damage by ice shoving. For steeper slopes

(1V:1.5H), the rock size (D100) should be about three

times the ice thickness to prevent damage by ice

shoving.

19

Figure 31. Plot of cumulative probability of damage vs. the ratio of

ice thickness to D100 rock diameter.

Figure 30. Damage to filter fabric by ice

action after test 30.

Slope 1V:3H

Slope 1V:2H

Slope 2V:3H

Combined Data

0 0.5 1.0 1.5 2.0 2.5

Ice Thickness/Rock Size (h/D100)

1.0

0.8

0.6

0.4

0.2

Cumulative Probability of Damage
DISCUSSION

The functions of a riprap armor are to protect a

sloping embankment against waves or currents in

a channel at a minimum-cost balance between initial construction cost and future maintenance. In

cold regions, the design procedures against wave

action or erosive forces of currents remain the same

as those in warm regions. In addition, Matheson

(1988) discussed that one should consider the

breakdown of rocks due to freeze-thaw action,

plucking of rocks by rising and falling ice sheets

due to water level changes, and shoving action by

moving ice sheets.

The observations made in relation to dams and

rivers (Matheson 1988, Doyle 1988, Wuebben 1995)

have been discussed earlier. To avoid plucking of

rocks by rising ice sheets, Matheson (1988) suggested that riprap should have a D50 in excess of

the maximum winter ice thickness. Though based

on a limited number of tests, the results of this

study show that the maximum rock size (D100) in

a riprap should be two to three times the ice thickness to avoid any damage to riprap by ice shoving. Costs of construction and maintenance for a

given site should also be considered. It may be

cost-effective to protect those areas of an embankment that are more prone to damage by ice shoving, according to the results of this study. Frequency of damage by ice shoving is another factor that

needs to be considered in the design of a riprap

protection of a bank, because it may be cost-effective to repair riprap damage that occurs rarely

(Doyle 1988).

SUMMARY AND CONCLUSIONS

A review of literature on ice effects on riprap

revealed practically no guidance available for design of riprap in the cold regions, where the presence of moving ice can cause considerable damage to a riprapped bank. To generate data on riprap

damage by ice shoving, we conducted a series of

small-scale tests in which we pushed a model

riprap-covered embankment against 1.22-m-wide

model ice sheets grown in the test basin of our laboratory. During the 35 tests, we changed the slope

of the model riprap bank, the size and the mix of

rocks, and the thickness of model ice sheets. Except during the first two tests, we moved the carriage at a constant speed of 40 cm/s. The interaction between the model riprap bank and the ice

mostly resulted in ice pileup, except during two

tests when ice rode up the bank. We measured the

horizontal and the vertical components of the

interaction force during the tests. We observed that

the ice sheet buckled many times during a test, and

that the buckling failure limited the maximum force

an advancing ice sheet could exert during the smallscale tests.

There was very little or no damage to the riprap

during ice ride-up events, which occurred on shallow-slope (1V:3H) banks. Most of the riprap damage occurred when the ice piled up on the riprap,

and the incoming ice sheet was forced to go between the riprap and the piled-up ice. We observed

that some of the rock material was removed from

the bed and brought up to the surface of the ice

pile. The most severe damage occurred at or below the water level. We considered the riprap to

have failed if the rock material was displaced by

ice to expose a certain amount of area on the bank.

From the results of these tests, we conclude that

the maximum size (D100) of rocks should be twice

the ice thickness for shallow slopes (1V:3H) to sustain no damage by ice shoving, and that the maximum rock size (D100) should be about three times

the ice thickness for steeper slopes (1V:1.5H).

LITERATURE CITED

Construction Industry Research and Information

Association (1991) Manual on the Use of Rock in Coastal and Shoreline Engineering. Construction Industry

Research and Information Association, CIRIA Special Publication 83, CUR Report 154. Rotterdam:

A.A. Balkema Publishers.

Collins, J.I. (1988) Large precast concrete armor

units in the Arctic. In Arctic Coastal Processes and

Slope Protection Design (A.T. Chen and C.B. Leidersdorf, Ed.). Technical Council on Cold Regions Engineering Monograph, American Society of Civil

Engineers, New York, p. 208–215.

Coastal Engineering Research Center (1984) Shore

Protection Manual. U.S. Army Corps of Engineers

Waterways Experiment Station, Vicksburg, Mississippi, Vol. I and II.

Croasdale, K.R. (1980) Ice forces on fixed, rigid

structures. In Working Group on Ice Forces on Structures (T. Carstens, Ed.). USA Cold Regions Research

and Engineering Laboratory, Special Report 80-26,

p. 34–106.

Croasdale, K.R., R.W. Marcellus (1978) Ice and

wave action on artificial islands in the Beaufort Sea.

Canadian Journal of Civil Engineering, 5(1): 98–113.

Croasdale, K.R., N. Allyn and W. Roggensack (1988)

Arctic slope protection: Considerations for ice. In

Arctic Coastal Processes and Slope Protection Design

(A.T. Chen and C.B. Leidersdorf, Ed.). Technical

20
Council on Cold Regions Engineering Monograph,

American Society of Civil Engineers, New York, p.

216–243.

Doyle, P.F. (1988) Damage resulting from a sudden

river ice breakup. Canadian Journal of Civil Engineering, 15(4): 609–615.

Gadd, P.E. (1988) Sand bag slope protection: Design, construction and performance. In Arctic Coastal

Processes and Slope Protection Design (A.T. Chen and

C.B. Leidersdorf, Ed.). Technical Council on Cold

Regions Engineering Monograph, American Society of Civil Engineers, New York, p. 145–165.

Hopkins, M.A. (1995) The ice pile-up problem: A

comparison between experiments and simulations.

In Ice Mechanics—1995 (J.P. Dempsey and Y.D.S.

Rajapakse, Ed.). American Society of Mechanical

Engineers, New York, AMD, vol. 207, p. 211–218.

Hetenyi, M. (1946) Beams on Elastic Foundations. Ann

Arbor, Michigan: University of Michigan Press.

Kovacs, A. and D.S. Sodhi (1980) Shore pile-up and

ride-up. Field observations, models, theoretical

analyses. Cold Regions Science and Technology, 2: 209-

288.

Leidersdorf, C.B. (1988) Concrete mat slope protection for Arctic applications. In Arctic Coastal Processes and Slope Protection Design (A.T. Chen and C.B.

Leidersdorf, Ed.). Technical Council on Cold Regions Engineering Monograph, American Society

of Civil Engineers, New York, p. 166–189.

Matheson, D.S. (1988) Performance of riprap in

northern climates. Contract report to the Canadian

Electrical Association, CEA No. 625 G 571, Acres

International Limited, Winnipeg, Manitoba, Canada.

McDonald, G.N. (1988) Riprap and armor stone. In

21

Arctic Coastal Processes and Slope Protection Design

A.T. Chen and C.B. Leidersdorf, Ed.). Technical

Council on Cold Regions Engineering Monograph,

American Society of Civil Engineers, New York, p.

190-207.

Nevel, D.E. (1983) Pressure ridge forces. In Proceedings, 7th International Conference on Port and Ocean

Engineering Under Arctic Conditions (POAC ‘83), 5–9

April, Technical Research Centre of Finland, Helsinki,

Finland. Espoo, Finland: Valtion Teknillinen Tutkimuskeskus, vol. 1, p. 212–220.

Sodhi, D.S., K. Kato, F.D. Haynes and K. Hirayama

(1982) Determining the characteristic length of model ice sheets. Cold Regions Science and Technology, 6(6):

99–104.

Sodhi, D.S., K. Hirayama, F.D. Haynes and K. Kato

(1983) Experiments on ice ride-up and pile-up.

Annals of Glaciology, 4: 266–270.

U.S. Army Corps of Engineers (1991) Hydraulic design of flood control channels. Engineer Manual EM

1110-2-1601, Washington, D.C.

Van der Meer, J.W. (1988) Rock slopes and gravel

beaches under wave attack. Doctoral thesis, Delft

University of Technology; also Delft Hydraulics

Communication 396.

Wuebben, J.L. (1995) Ice effects on riprap. In River,

Coastal and Shoreline Protection: Erosion Control Using Riprap and Armourstone (C.R. Thorne, S.R. Abt,

F.B.J. Barends, S.T. Maynord and K.W. Pilarczyk,

Ed.). New York: Wiley, p. 513–530.
23

Wind-generated waves induce one of the most

powerful forces on coastal structures. A given structure may be subjected to nonbreaking, breaking,

and broken waves during different stages of a tidal

cycle. The significant wave height is obtained from

historical data to represent the wave condition at a

site. The riprap stability formula (Coastal Engineering Research Center 1984), based on small-scale

and large-scale model testing, is given by:

 

W gH

K S 50

3

3 1 = −

ρ

θ

r

D r ( ) cot (A1)

where W50 = median weight of individual armor

unit in primary cover layer,

ρr = density of armor unit

g = acceleration due to gravity

H = design wave height at the structure

(≈ H1/10 average of the highest 10%

of all waves)

Sr = ρr/ρw ratio of the armor density to

the water density

θ = angle of structure slope measured

from the horizontal

KD = stability coefficient that varies primarily with the shape of the armor

units, roughness of the armor unit

surface, sharpness of edges, and the

degree of interlocking obtained in

placement

= 3.5 for breaking waves

= 4.0 for nonbreaking waves.

Because of its simplicity, the above formula is

easy to work with. The values of KD have been derived for a wide range of armor units and configurations. However, the above formula has many

limitations. This formula is based on small-scale

model tests, and does not account for the level of

damage, storm duration and wave period.

As reported in CIRIA Special Publication 83

(1991), Van der Meer (1988) proposed a number of

formulas for deep and shallow water conditions

and for plunging and surging waves. These formulas, which incorporate the effect of wave period,

the storm duration, the permeability of the structure, and clearly defined damage level, are given

in CIRIA Special Publication 83 (1991).

Design of riprap protection of

flood control channels

The guidance for the riprap slope protection of

flood control channels (U.S. Army Corps of Engineers 1991) applies to open channels having a slope

of less than 2%. The ability of the riprap material

to resist the erosive forces of channel flow depends

on the following parameters: shape, size, weight

and durability of stone; gradation and layer thickness of riprap; and alignment, cross section, gradient and velocity distribution in a channel. The basic equation for the representative stone size in

straight or curved channels is given by

 

D SCC C d v

K gd 30

1 2

1

2 5

= −





 



















 f svT w

s w

γ

γ γ

/ .

(A2)

where D30 = riprap size of which 30% is finer by

weight

Sf = safety factor (≈ 1.1)

Cs = stability coefficient for incipient

failure

Cv = coefficient related to vertical

velocity distribution

CT = thickness coefficient

d = local depth of flow

γw = specific weight of water

γs = specific weight of stone

v = local average velocity

K1 = side slope correction factor =

1 2 2 − (sin / sin ) θ φ

θ = angle of side slope

φ = angle of repose

g = acceleration due to gravity.

Recommended values of various coefficients

and factors are given in EM 1110-2-1601 (U.S. Army

Corps of Engineers 1991). Special attention needs

to be given for the design of toe protection and filter material between the riprap and the underlying material. Construction quality needs to be

maintained for the size, the shape and the gradation of stones.

APPENDIX A: DESIGN OF RIPRAP PROTECTION AGAINST WAVES
25

Sand bag slope protection

Since 1972, sand bag slope protection has been

used effectively in the offshore islands built along

the Canadian and the Alaskan coasts of the southern Beaufort Sea. Gadd (1988) described the design

and the development of slope protection by the use

of sand bags. He discussed the placement of sand

bag armor on two artificial islands: Resolution Island in 2.4-m water depth, and Seal Island in 12-m

water depth. Both these islands had a design life

of three years. Because of the limited design life

and also unavailability of large stone at the sites,

sand bag slope protection was considered to be the

only viable option.

Resolution Island required about 100,000 m3 (3.5

× 106 ft3) of gravel that was delivered to the site by

barges from a gravel pit onshore. The 1.5-m3 (53-

ft3) bags were filled from a barge-based bagging

plant and were placed end-to-end fashion (no overlapping) with a 60-ton crane at a maximum rate of

30 bags per hour. After completion of construction

in 1980, annual inspection of slopes above and below water were conducted. Damaged bags were

replaced in 1981 and 1984. Because of shallow

water and resulting mild environment, damage

due to wave action alone was not observed. The

principal cause of bag loss was due to ice abrasion

and puncture, followed by fill loss due to wave

action on torn bags.

Seal Island was constructed in 1982 about 17 km

(10.5 miles) northwest of Prudhoe Bay. The environmental forces of moving ice and summer wave

action were larger at this sites than those at any

previous site in Alaska. To construct the island core,

about 535,000 m3 (18.9 × 106 ft3) of gravel was transported to the site from an onshore quarry in winter over an artificially thickened floating ice road.

About 17,100 gravel bags, each 3 m3 (106 ft3) capacity, were used to protect the slope. Based on the

results of model testing, the bags were arranged in

a 50% overlap configuration above –6 m (–20 ft)

level to enhance their stability. Bags were produced

on-site at an average rate of 400 bags per day. Floating ice damaged the sand bags located between

elevations +1.5 m (+5 ft) and –6 m (–20 ft). The ice

would tear the bag fabric, allowing the gravel fill

to escape. During two late summer storms, many

bags were removed by the 2-m-high wave action,

yielding armor gaps near the waterline. Due to the

50% overlap bag placement configuration, there

were abundant bags that could slide down slope

to cover the voids and provide slope protection

for the life (ca. 5–10 years) of the island.

Sand bag slope armor has proven to be effective protection for short-lived offshore structures

in the Arctic. However, it can be damaged by ice

during both summer and winter, and also by ultraviolet radiation affecting the strength of bag fabric. Periodic repairs should be conducted to prevent progressive deterioration and to prolong the

functional life of a sand-bag slope armor system.

Concrete mat slope protection

As mentioned above, the sand bag armor is

susceptible to damage from ice impacts and loss

of fabric strength from exposure to ultraviolet radiation. Moreover, the filling and placement of

sand bags are time-consuming and labor-intensive

operations. Because suitable quarry sites are scarce

along the Beaufort Sea coast, articulated concrete

mats have been selected as an effective and economical slope protection in the Arctic since 1980.

Leidersdorf (1988) has given a summary of Arctic

experience with concrete mat armor installed at

six sites along the Alaskan Beaufort Sea coast. Potential advantages of concrete mat armor are 1)

low weight per unit area because of interblock linkages, 2) resistance to damage from ice impacts and

low temperatures, 3) ability to accommodate

changes in the subgrade without major fill loss,

and 4) modular placement and removal. Leidersdorf (1988) discussed hydraulic, ice, and material

design considerations; fabrication and installation

procedures; and additional research needed to

understand their failure modes, hydraulic stability, and long-term durability. The capital costs of

installing a concrete mat armor are higher than

those for sand bag armor, but the maintenance

costs are comparatively lower. Hybrid slope protection systems, consisting of sand bag armor and

concrete mat armor, have been used to cut down

the construction cost. On Northstar Island, the

concrete mat extended from +1.5 m (+5 ft) to –6 m

(–20 ft) levels, and sand bag armor was used above

and below the concrete mat armor (Leidersdorf

1988).

APPENDIX B: REVIEW OF SLOPE PROTECTION SYSTEMS USED IN THE ARCTIC
Riprap and armor stone

McDonald (1988) discussed the use of quarry

stone for slope protection in the northern environment. He cites two examples where stone was used

for slope protection, both in Nome, Alaska. The

first example is the 1067-m- (3500-ft-) long seawall,

constructed in 1950. It has been overtopped by

wind-driven, first-year ice many times without

sustaining any damage. The seawall contains

125,000 tons of granite stone, ranging in size up to

6 tons. The second example is that of jetties built

for the Nome Harbor in 1920. These jetties were

protected with stone, which suffered damage from

storms and ice. These were replaced with steel

sheet pile and concrete jetties in 1939.

Experience with the action of multiyear ice on

riprap stone does not exist in the Arctic(McDonald

1988). This may be attributed to lack of quarries

and transportation systems along the Beaufort Sea

coast. McDonald (1988) discussed the damage

caused by the buoyant uplift of submerged ice frozen to stones. He mentioned that loss of stone due

to buoyancy of ice is not common and is usually

restricted to small-size stones.

Collins (1988) reviewed the behavior and performance of large concrete units for arctic slope

protection, and found that there is little knowledge

of their expected performance in thick ice. He commented that the experience of slope protection in

the Great Lakes, Gulf of Labrador, and Gulf of Finland cannot be extrapolated to high Arctic conditions.

26
APPENDIX C: PLOTS OF HORIZONTAL AND VERTICAL FORCES

MEASURED DURING THE TESTS

27
28
29
30
31
32
33
34
35
36
APPENDIX D: PHOTOGRAPHS AND CONTOUR PLOTS OF THE

MODEL RIPRAP BANK BEFORE AND AFTER EACH TEST

This appendix contains photographs and contour plots of the model riprap bank before and

after each test. The numbers along contour lines denote the distance measured from the strings

in the grid frame.

37

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September 1996

Ice Action on Riprap:

Small-Scale Tests CWIS 32548

Devinder S. Sodhi, Sharon L. Borland and Jesse M. Stanley

U.S. Army Cold Regions Research and Engineering Laboratory

72 Lyme Road CRREL Report 96-12

Hanover, New Hampshire 03755-1290

Office of the Chief of Engineers

Washington, DC 20314-1000

Approved for public release; distribution is unlimited.

Available from NTIS, Springfield, Virginia 22161.

70

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL

We conducted 35 small-scale experiments to assess the damage on riprap-covered banks by ice shoving. A review of literature on this subject revealed very little experience or guidance available for the design of riprap

in the cold regions, where presence of moving ice can cause substantial damage to a riprapped bank. During

the experimental program, we changed the slope of the model riprap bank, the size and the mix of rocks, and

the thickness of model ice sheets. Results of these tests are presented in terms of measured horizontal and vertical forces, outcome of interaction as pileup or ride-up events, and damage to the model riprap bank. From the

observations made during the tests, the damage to the riprap appears to take place during pileup events, because the incoming ice sheet is forced to go between the riprap and the piled-up ice, bringing with it rocks from

the bottom to the surface of an ice pile. To sustain no damage to the riprapped protective layer, maximum rock

size (D100) should be twice the ice thickness for shallow slopes (1V:3H) and about three times the ice thickness

for steeper slopes (1V:1.5H).

Bank protection Lake ice Sea ice

Cold regions Riprap Shore protection

Ice shoving River ice Small-scale tests