LANDSLIDES OF THE CINCINNATI, OHIO, AREA

Anchoring of Thin Colluvium by Roots

of Sugar Maple and White Ash

on Hillslopes in Cincinnati

U.S. GEOLOGICAL SURVEY BULLETIN 2059-E
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Anchoring of Thin Colluvium by 

Roots of Sugar Maple and White Ash 

On Hillslopes in Cincinnati

By MARY M. RIESTENBERG

LANDSLIDES OF THE CINCINNATI, OHIO, AREA

U.S. GEOLOGICAL SURVEY BULLETIN 2059-E

Studies of root morphology, distribution, 

and pull-out resistance show how 

tree roots help stabilize thin colluvium

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Robert M. Hirsch, Acting Director

For Sale by U.S. Geological Survey, Map Distribution

Box 25286, MS 306, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Riestenberg, Mary M.

Anchoring of thin colluvium by roots of sugar maple and white ash on hillslopes in 

Cincinnati / by Mary M. Riestenberg.

p. cm. (U.S. Geological Survey bulletin; 2059) (Landslides of the Cincinnati, Ohio, 

area;E)

Includes bibliographical references.

Supt. of Docs, no.: I:19:3:2059E

1. Soil-binding plants Ohio Cincinnati Region. 2. Soil stabilization Ohio 

Cincinnati Region. 3. Slopes (Soil mechanics) Ohio Cincinnati Region. 4. Roots 

(Botany) Ohio Cincinnati Region. 5. Sugar maple Ohio Cincinnati Region 

Roots. 6. White ash Ohio Cincinnati Region Roots. 7. Colluvium Ohio 

Cincinnati Region. I. Title. II. Series. III. Series: Landslides of the Cincinnati, 

Ohio, area; ch. E. 

QE75.B9 no. 2059-E 

[S627.P55] 

557.3 s dc

[551.3'07] 93-32563

CIP
CONTENTS

Abstract.......................................................................................................................... El

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

Acknowledgments ............................................................................................... 2

Roots act as soil anchors ............................................................................................... 2

Measurement of pull-out resistance of tree roots .......................................................... 2

Spatial distribution of roots in colluvium ...................................................................... 9

Morphology and distribution of the roots of sugar maple ................................... 9

Morphology and distribution of the roots of white ash ....................................... 9

Comparative analyses of roots of sugar maple and white ash ............................. 11

Stability analysis of a hillslope underlain by colluvium anchored by sugar maple

and white ash roots .................................................................................................... 11

Critical spacing of ash trees on a hillslope .................................................................... 19

Critical thickness of rooted colluvium .......................................................................... 21

Discussion .................................................................................................................... 22

References cited ............................................................................................................ 24

FIGURES

1. Map showing location of study sites ........................................................................................................................... E3

2. Photographs of roots penetrating the shear surface of a shallow, planar landslide..................................................... 4

3. Sketch and photograph of a root deformed along five shear surfaces......................................................................... 5

4 Sketches and photographs of two deformed roots exposed at the excavated shear plane........................................... 6

5. Photograph of the Vermeer spade................................................................................................................................ 7

6. Scale drawings of the field setup used to test pull-out resistance................................................................................ 8

7 Sketches and force-displacement graphs illustrating the three general categories of roots ........................................ 10

8. Graphs showing the tensile breaking force measured during extraction..................................................................... 12

9. Graphs showing predicted tensile breaking force, predicted force to break all termini at once, and measured

	tensile breaking force versus diameter.................................................................................................................. 13

10 Plan and cross section of the excavated roots of sugar maple 1 at the Delhi site........................................................ 14

11. Graph showing density of roots as a function of soil depth........................................................................................ 15

12. Plan and cross section of the excavated roots of sugar maple 2 at the Spring Grove site........................................... 16

13. Plan and cross section of the excavated white ash at the Spring Grove site ............................................................... 17

14. Sketches showing lateral views of sugar maple and white ash roots removed from the colluvium by a

	Vermeer spade....................................................................................................................................................... 18

15. Stereonets of cross-sectional area of roots cutting the surface of a hemisphere at a radius of 30 cm from the base

	of trunk................................................................................................................................................................. 19

16. Graphs showing the cross-sectional area of roots of the hand-excavated trees as a function of depth....................... 19

17. Rose diagrams of occurrence and azimuths of lateral roots........................................................................................ 20

18. Cross section of trench wall showing density of roots in colluvium underlying a mature forest in Cincinnati.......... 21

19. Graph showing stability analyses of a soil block inclined at 30° with and without roots........................................... 23

20. Graph showing the effect of spacing of ash trees on stability of a hillslope............................................................... 23

21. Graphs showing factor of safety versus displacement for colluvium anchored by sugar maple and ash roots........... 24

TABLE

1. Stability analysis of colluvium anchored by white ash roots...................................................................................... E22

III

Anchoring of Thin Colluvium by Roots of Sugar Maple and 

White Ash on Hillslopes in Cincinnati

By Mary M. Riestenberg1

ABSTRACT

Tree roots effectively anchor colluvium as much as 1 m 

thick on steep hillslopes bordering the Ohio River and its 

tributaries in the Cincinnati, Ohio, area. This ability is demonstrated through detailed analyses of shallow landslides on 

forested hillslopes and through studies of the morphology, 

distribution, and pull-out resistance of roots of two species of 

trees that dominate the forests. The roots penetrate the clay 

colluvium, cross shear zones, and anchor the colluvium to 

the underlying weathered limestone and shale bedrock.

The extent to which a tree species can anchor colluvium and stabilize a hillslope depends on the morphology 

of its root system. A small (8-16 cm diameter) sugar maple 

characteristically has a well-developed lateral root system, 

but no dominant taproot. It can stabilize surficial soil, but 

its shallow root system cannot stabilize colluvium thicker 

than 0.5 m. A white ash of similar diameter has welldeveloped shallow roots and a stout, deeply penetrating taproot. It can stabilize soil as much as 1 m thick.

Stability analyses indicate that small white ash trees 

may be widely spaced and still stabilize a hillslope. For 

instance, white ash as much as 7 m apart will anchor a 43 cm 

thickness of colluvium on a 30° hillslope.

INTRODUCTION

Previous laboratory studies have shown that roots 

increase the shear strength of soil, but have failed to demonstrate how roots affect the strength of soil in place on hillslopes. This study investigates the interaction of roots and 

landslide shear surfaces and quantifies the resulting 

enhancement of soil shear strength.

Tree roots increase both the tensile and shear strength 

of soil. The addition of tree roots to soil increases its tensile

'Department of Geology, University of Cincinnati, Cincinnati, Ohio. 

Present address: Department of Chemistry, Geology Division, College of 

Mount St. Joseph, Cincinnati, OH 45233.

strength from three to five times (Kaul, 1965; Endo and 

Tsuruta, 1969a); the shear strength of the soil increases with 

increasing mass of roots per unit volume of soil (Endo and 

Tsuruta, 1969a; O'Loughlin, 1972; Ziemer, 1981). Endo and 

Tsuruta (1969b), Gray (1974, 1978), Waldron (1977), and 

Wu (1984) contend that roots increase shear strength by 

increasing the cohesion of the soil. The increase in cohesion 

of rooted soil ranges from one to 20 kilopascals (O'Loughlin 

and Ziemer, 1982), and the cohesion increases with increasing cross-sectional area of roots per area of the shear surface 

(Waldron, 1977; Wu and others, 1979; Waldron and others, 

1983; Wu, 1984).

Measurements of shear strength demonstrate that both 

peak and residual strengths of the soil are increased by the 

presence of roots (Manbeian, 1973) and that rooted soil can 

be deformed by shear to a greater extent than fallow soil 

before it fails (Kaul, 1965). Rooted soil not only has a higher 

peak shear strength than fallow soil, but it also accommodates greater displacement before reaching its peak shear 

strength. Waldron and others (1983) demonstrated that the 

shear strength peaks at 18 mm displacement in soils with 

alfalfa and pine roots.

A few studies have been made of the anchoring 

capacity of roots. Fraser (1962) studied windthrow and 

the associated forces required to pull out spruce and fir 

trees, and determined that the force necessary to pull out a 

tree increases with loading rate. The pull-out resistance 

increases with root size and with the maturity of the root 

system, and decreases if the roots are diseased (Ortman 

and others, 1968) or have begun to decay (Kitamura and 

Namba, 1968).

In a previous study of the contribution of roots to hillslope stability, Riestenberg and Sovonick-Dunford (1983) 

concluded that roots contributed 85 percent of the shearing 

resistance at the base of a shallow landslide, and the soil 

contributed the remaining 15 percent. However, I have 

reexamined many of the assumptions used in our previous 

study and found them inconsistent with my more recent 

field observations and the laboratory measurements of 

others. In particular, Riestenberg and Sovonick-Dunford 

(1983) assumed that all roots break simultaneously and that

El
E2 LANDSLIDES OF THE CINCINNATI, OHIO, AREA

the full tensile strength of each root was mobilized parallel 

to the shear surface. However, my observations indicate 

that many of the roots were pulled out of the materials 

underlying the shear surface and failed at less than their 

full tensile strength. My field experiments indicate that root 

pull-out resistance is mobilized gradually and that roots fail 

at different amounts of displacement, depending on their 

morphologies. Laboratory experiments (Fleming and Johnson, 1994) also indicate that shear strength of the colluvium is much higher than previously estimated (Fleming 

and others, 1981; Riestenberg and Sovonick-Dunford, 

1983).

This paper presents evidence that roots, like artificial 

ground anchors (Hanna, 1982), are pulled from the colluvial 

soil by landsliding. These observations are then supported by 

pull-out tests on roots exposed in excavations and by tensile 

tests on excised roots. Next the paper describes the morphology and distribution of the roots of sugar maple and white 

ash, species which co-dominate the hillslopes. Lastly, data 

on the pull-out resistance of roots, root distribution and morphology, soil strength, and thickness of colluvium are combined in stability analyses to determine the effects of tree 

spacing and colluvium thickness on the stability of wooded 

hillslopes.

ACKNOWLEDGMENTS

I am grateful to many people from the University of 

Cincinnati for assistance with this research. I thank David 

Nash, Susan Sovonick-Dunford, and Arvid Johnson for 

invaluable advice and encouragement throughout the 

research. Thanks to David Nash, Kenneth Cruikshank, and 

Roger Stuebing for writing inventive computer programs 

and to Daniel Joyce for designing and building the field 

equipment. I am indebted to family and friends Kitty and Ed 

Riley, Dottie Kennedy, Allan Taylor, Patrick Fox, Connie 

Bertka, Virginia Pfaff, George Losonsky, Bill Haneberg, 

Mary Gamer, Amy Lamborg, Jim Beaujon, Scott Brockman, 

and Ron, Beth, Ted and David Riestenberg for their generous help with the fieldwork. The Sisters of Charity and the 

administrators of Spring Grove Cemetery kindly allowed me 

to use their properties as field sites. The research was supported by Sigma Xi, the Hillside Trust of Cincinnati, and the 

Department of Geology at the University of Cincinnati.

ROOTS ACT AS SOIL ANCHORS

There is overwhelming evidence that roots act as soil 

anchors, transmitting tensile forces across a discontinuity, 

the shear surface, to the underlying bedrock. A detailed field 

study at the Delhi landslide (fig. 1) indicates that roots 

become stretched along surfaces during soil failure. Observations indicate that they are pulled from the soil or fail in

tension, as would flexible cables or ground anchors (Hanna, 

1982), rather than fail in shear.

Roots penetrating the basal shear surface of the Delhi 

Pike landslide (fig. 1; see Fleming and Johnson, 1994, for a 

complete description) have been moved and realigned by a 

few centimeters of landslide movement. Figure 2A shows 

about 70 roots penetrating the basal shear surface of the 

landslide. The roots are clustered in groups where they penetrate the shear surface through fissures. Above and below 

the shear surface the roots are nearly perpendicular to the 

shear surface. At the shear surface, centimeters-long segments of the roots are aligned parallel with striations that 

indicate the direction of sliding (fig. 2B). Similar realignment of a root that penetrates five shear surfaces is shown 

in figure 3. Figure 4 also shows closeup views of roots that 

have been deformed and reoriented parallel with the striations on the shear surface.

Deformation and reorientation of the roots parallel with 

the striations in the failure surface indicate that the roots are 

pulled out of the soil during sliding. Above and below the 

shear surface, the roots appear to have maintained their original, relatively random orientations as they became 

deformed near the shear surface. Roots grow in length only 

at their tips (Kramer and Kozlowski, 1979), so the deformed 

segments must have been pulled into the plane of the shear 

surface from their original positions above or below it. 

Moreover, the thin layer of remolded colluvium that surrounds the root shown in figure 3 also suggests that the root 

was pulled lengthwise through its own hole in the colluvium.

Additional evidence that roots are pulled from the soil 

during sliding was observed at the Rapid Run landslide 

(fig. 1; see Riestenberg and Sovonick-Dunford, 1983, for a 

complete description). The slope failed as a series of blocks 

or slide elements, each supporting a segment of the mapledominated forest. Each slide element was bounded by 

scarps draped with roots. Many of the roots had broken in 

tension, but many others were pulled out of the colluvium 

intact. Furthermore, lengths,of the roots varied, indicating 

that the roots probably had failed after varying amounts of 

soil displacement.

MEASUREMENT OF PULL-OUT 

RESISTANCE OF TREE ROOTS

I verified the notion that roots act as anchors by measuring the pull-out resistance of roots exposed in excavations 

and comparing it to the laboratory-measured tensile strength 

of roots. After briefly describing the methods I used for 

these measurements I will summarize the results.

The pull-out resistance of the roots of sugar maple 

(Acer saccharum M.) and white ash (Fraxinus americana 

L.), two dominant species on Cincinnati's hillslopes, was 

measured in excavations dug by a Vermeer spade at the 

Spring Grove site (fig. 1). The site is forested, slopes gently,
ANCHORING OF THIN COLLUVIUM BY ROOTS ON HILLSLOPES IN CINCINNATI E3

84°40'

SPRING GROVE CEMETERY SI

RAPID RUN 

Rapid Run LANDSLIDE

Figure 1. Location of the two study sites and river bluffs. Contours in inset show elevation in feet. The bluffs along the Ohio River and 

its tributaries, indicated by stippling in the illustration, are forested and contain many shallow, planar landslides. The failures occur at the 

interface of the colluvium and die weathered bedrock. Tree roots penetrate the failure surfaces and are involved in the shallow landslides. 

The ridge near the Ohio River is the site of both the Delhi and Rapid Run landslides. The Spring Grove site is on a gently sloping, upland 

part of Cincinnati. All these sites are underlain by the same type of colluvium soil and support forests dominated by sugar maple and 

white ash.

and is underlain by the silty clay colluvium described as 

Eden soil by Lerch and others (1982), the same soil that 

underlies the ridge site. The Vermeer spade is a truckmounted, four-bladed hydraulic spade, which can remove a 

small tree, complete with the soil surrounding its central root 

system. It is used by nurseries for removal and transplanting 

of saplings (fig. 5). This type of excavation was useful for 

this study because the Vermeer spade severs roots at a specified distance from the tree and cuts them cleanly, with little 

or no tearing. The spade cuts a conical soil face about a tree, 

exposing the three-dimensional root distribution. The cut is 

1.1 m in diameter at the ground surface and comes to a point 

approximately 0.85 m below the ground surface.

The colluvium at the Spring Grove test sites was kept as 

moist as possible by daily watering throughout the test 

period (May through July) in an effort to reproduce the soil 

moisture conditions of winter and spring, when shallow 

landslides in Cincinnati are usually active.

Trees with diameters ranging from 8 to 16 cm were 

tagged and excavated. The colluvium outside the conical 

excavations was mapped, and the roots exposed at the surface labeled with a tag listing species, tree number, and 

individual root number. Measurements of the diameters of 

the roots projecting from the excavation face, their depth, 

and their orientations were also recorded. Bark was 

removed from the exposed ends of the roots to prepare
E4 LANDSLIDES OF THE CINCINNATI, OHIO, AREA

V ; -:* N*"" 

»..-*>» .. rf ..'- "Jb***1?"^** *:' * " * :^^ ? * -> 1*

-*?' -S"** '' JSP' "^ ' Tf"' - r'- < £-, ,2

"^ ^'*.,* : ;/--** " -?>c.- "*-^PJ - -.. »» » «r -*!«.* , * _*

Figure 2. Roots penetrating the shear surface of a shallow, planar landslide. A, The surface, measuring 2.53 m2 in area, is exposed 

at 1 m below the ground surface. Seventy roots project through the shear surface. B, Roots are reoriented at the shear surface in the 

direction of landslide movement (Tape measure calibrated in centimeters.)
ANCHORING OF THIN COLLUVIUM BY ROOTS ON HILLSLOPES IN CINCINNATI E5

Plunge/Azimuth 

of root

® 26S/5

(2) 87S/155

(3) 35N/38

(4) 60S/353

(5) 11S/170

(6) 1N/10

(7) 67S/9

Strike-dip of 

shear planes

A93-15S 

B 62-47S 

C 42-46S 

D 58-35S 

E 70-35S

Orientation of 

slickenlines

38 V"-

67

Figure 3. A root deformed along five shear surfaces in a zone of anastomosing surfaces near the landslide toe. The 28-cm long, deformed 

root follows a tortuous path through a film of remolded colluvial soil. The colluvium outside the film exhibits little evidence of deformation. 

The deformation of the root is controlled by the positions of the shear surfaces and the direction of displacement. The root is realigned on 

each shear surface in the direction of shear as indicated by striations on the shear surface.

them for gripping with bias-woven, steel cable pullers as 

part of the pull-out test described below.

A boat winch of 5.3 kilonewtons capacity was modified 

to pull out the roots (fig. 6A). The hand-operated winch was 

anchored, and its load cable attached to the root by a cable 

puller. In some tests a pulley was used to increase the 

mechanical advantage. The cable was threaded through a 

pulley supported by a beam to eliminate friction and to 

accommodate alignments of the cable (fig. 6B).

For every root pulled, the following parameters were 

measured or observed:

1. Diameter of root at soil face.

2. Diameters of all truncated ends.

3. Length of root from cut end at excavation face to 

largest truncated terminus.

4. Order of root pulled, with the first-order roots being 

smallest (that is, having no smaller roots branching off them) 

and second-order roots being those from which first-order 

roots branch, and so on.

5. Depth of root in the colluvium at face of excavation.

6. Azimuth and plunge of root.
E6 LANDSLIDES OF THE CINCINNATI, OHIO, AREA

Shear 

plane

Weathered bedrock

Weathered bedrock

Figure 4. Two deformed roots exposed at the excavated shear surface. The roots are pulled out of their preslide position and realigned 

at the shear surface during landsliding. A, The root is realigned in the direction of shear at the shear surface. The root is deformed more 

than 90° from its orientation below the surface. B, The root is realigned from its orientation below the shear surface in the direction of shear 

at the failure surface. It is abruptly deformed again above the surface at an angle approaching 90° from its orientation at the surface.
ANCHORING OF THIN COLLUVIUM BY ROOTS ON HILLSLOPES IN CINCINNATI E7

Figure 5. The Vermeer spade used to expose the roots of some of 

the trees. The hydraulically driven spade removes a cleanly cut 

cone of roots from the colluvium, allowing subsequent study of 

root characteristics. It also exposes truncated roots on the conical 

trench face for measurement of pull-out resistance.

7. Species of root. Sugar maple and ash sometimes 

protruded from the same soil face but were easy to distinguish from each other; the ash has a distinctive thick, palegray-brown bark, and is ring porous, whereas the sugar 

maple has a deep brown bark, underlain by a red phloem 

layer, and is diffuse porous.

Graphs relating load, displacement, and time were constructed for each root removed from the colluvium.

The pull-out resistance of the maple and ash roots was 

compared to their tensile strength, which was measured in 

the laboratory using a testing machine with a capacity of 22 

kilonewtons. Fresh, green roots were collected, cut into 

lengths appropriate for the testing machine (20 to 30 cm), 

and kept moist. The bark was shaved from the ends of the 

root to reduce slippage of the roots during testing. Electrician's cable pullers were used to hold the root. The bias 

weave of the cable puller allowed for tight holding with little 

damage to the root. The root was placed between two rods: 

one was stationary and the other displaced at a rate determined by the operator. The time for an individual test varied 

from 15 seconds to about 1 minute. Displacement was 

increased and the load was monitored until the root broke in 

tension.

The average tensile strengths of sugar maple root and of 

white ash root are 2.85x104 kilopascals (correlation coefficient r2 = 0.90) and 2.22X104 kilopascals (r2 = 0.83), respectively; this difference in tensile strength is not statistically 

significant. The results of the laboratory testing of root 

strength in association with the field measurements of root 

pull-out resistance illustrate the following:

1. Although the graphs of pull-out resistance versus 

displacement for the tested roots vary widely, they can be 

divided into three general categories (fig. 7). In all three categories, force initially rises abruptly with little or no displacement; but this initial peak is followed by a gradual 

reduction of force in category I roots, by continued high 

force leading up to a final abrupt fall-off in category II roots, 

and by an interim reduction of force and then a second abrupt 

peak in category III roots.

The three basic force-displacement categories are associated with different root morphologies. Category I roots are 

generally long (0.8-1.5 m), straight segments with few to 

moderate numbers of branches. They have one or two large, 

truncated termini aligned in the same direction in which the 

root was pulled. Category II roots are short (0.1-0.7 m), 

highly branched segments that have large numbers of thin 

truncated termini. Most category III roots are forked into 

two major branches, which diverge at an angle of 45° or 

more in most cases. A sketch of each root is shown with a 

graph of force versus displacement in figure 7.

2. The measured tensile breaking force (mtbf) of a root 

being pulled from soil increases with root diameter for both 

sugar maple and white ash (fig. 8). The tensile breaking 

force, or pull-out resistance is the final peak in force which 

causes failure; all forces applied subsequently are smaller in 

magnitude. It may or may not be the greatest force which 

was applied to the root in the pull-out test. The "m" in the 

abbreviation "mtbf denotes that the breaking force was 

measured in field tests, rather than predicted ("p") from laboratory tests of root strength.

3. The laboratory-measured tensile force required to 

break a root of a specified diameter (ptbf) is greater than the 

field-measured pull-out resistance for a root of a similar 

diameter (mtbf). The difference between pull-out resistance 

(mtbf) and predicted tensile breaking force (ptbf) is shown 

by the best-fit lines in figure 9. The pull-out test shows that 

root failure occurs at various distances away from the 

applied force, as demonstrated by the different lengths of 

broken roots of similar diameters. Thus, the root fails as an 

anchor; it displaces under tension and then breaks within the 

soil at a distance from the applied axial force.

4. The load rate does not affect the measured force 

needed to break a root. This independence was demonstrated 

in a stepwise multiple regression test of variables possibly 

affecting root strength (Riestenberg, 1987). This relationship 

differs from that described by Fraser (1962) in his study of 

the pull-out resistance of the roots of softwoods, in which he 

showed that the measured force increased with loading rate.

5. The branches of a root break sequentially as the root 

displaces within the soil. However, the sequence of failure of 

the branches within the soil cannot be determined from the 

field data. The force to break all truncated termini at once 

(srs) was estimated by measuring the diameters of all the 

broken ends on roots extracted by the pull-out tests, calculating the force needed to make each of these breaks based on
E8 LANDSLIDES OF THE CINCINNATI, OHIO, AREA

SIDE END

10 CENTIMETERS

, , , I

Cable 

puller,

Barrier Barrier

Spring 

scale

Tape 

recorder

Chart recoder

B

Figure 6. Field setup used to test pull-out resistance. A, Side and end views of the modified winch used to pull roots from Vermeer 

excavations. As the load on the cable increases, the calibrated springs between the two boards attached to the winch compress. The compression is sensed by a potentiometer and recorded on a chart recorder. Winch is calibrated at the start of each field session. B, The entire 

testing array. Both load and displacement are measured for each root and recorded on a chart recorder powered by a portable generator.

laboratory tests of root strength, and then summing all the 

forces thus calculated. In all cases the resulting total force 

was greater than that predicted by laboratory tests to break a 

root of the diameter exposed at the excavation face (ptbf) 

(fig. 9), which in turn is greater than the field-measured pullout resistance (mtbf). This difference in breaking forces

develops because the sum of the cross-sectional areas of the 

termini of broken branches is greater than the cross-sectional 

area of the root trunk exposed at the soil face, and the breaking strength of the roots within the species is essentially a 

function of cross-sectional area for the range of root diameters tested.
ANCHORING OF THIN COLLUVIUM BY ROOTS ON HILLSLOPES IN CINCINNATI E9

6. There is no preferred plane in the colluvial soil 

within which roots tend to break, despite differences in soil 

texture and gradients in soil moisture.

SPATIAL DISTRIBUTION OF ROOTS 

IN COLLUVIUM

In order to estimate the areal spacing, sizes, and types 

of root anchors that would most likely project through a 

shear surface at some discrete depth within the soil, I studied 

the root systems of sugar maple and white ash, tree species 

that dominate the Cincinnati hillslopes. This study was limited to trees ranging from 6 to 16 cm in diameter, because 

trees of this size dominate the Rapid Run landslide and 

because trees of this size may be removed by a Vermeer 

spade. Roots were excavated and described at the Delhi and 

Spring Grove sites. Various excavation and sampling methods were combined for the study. Three trees were handexcavated, one at the Delhi site and two at the Spring Grove 

site. The root morphologies were compared with those of 11 

trees at Spring Grove that were excavated by a Vermeer 

spade.

MORPHOLOGY AND DISTRIBUTION OF THE 

ROOTS OF SUGAR MAPLE

Figure 10 shows the root system of the understory-sized 

sugar maple chosen for excavation at the Delhi site. The 

maple was growing 2-3 m from canopy trees within an area 

in which the soil had been thinned to 0.5 m by landsliding. 

The tree, designated sugar maple 1, had a height of 6 m, 

diameter at breast height (dbh) of 6 cm, and a crown width 

of 4 m. The roots of the maple were excavated with a trowel 

and a hand rake. The diameter, azimuth, and plunge of the 

main roots and branch roots were measured between 

branches at 10 cm from branching points.

The taproot of the excavated maple vertically penetrates the upper 20 cm of soil, then branches, narrows, and 

bends to align itself subparallel with the ground surface 

(fig. 10). The large lateral roots originate in swelled areas 

on the tap root and radiate from the base of the tree. The 

swelled areas project laterally about 4-5 cm or more from 

the tap, then terminate where the branching occurs (fig. 10). 

Individual lateral roots taper gradually and branch at random; the branches grow at an angle of about 45° from the 

root and are one-third to one-half the diameter of the root. 

The roots taper, at first markedly, then more gently, until 

they reach about crown width (in this case, about 2 m from 

the trunk), where they terminate in highly branched fans of 

tiny rootlets. These terminal rootlets are nonwoody and 

fragile. In addition, many sinker roots, about 1 cm or less in 

diameter, originate from the large lateral roots and from 

branches off the laterals.

Figure 11 demonstrates the high concentration of the 

roots of the excavated maple near the ground surface and the 

abrupt decline in concentration with depth. The high concentration of roots in the top 0.6 m of soil is consistent with 

root distributions described in hardwood forests studied by 

Scully (1942), Kochenderfer (1973), and Stout (1956). A 

maple excavated at the Spring Grove site (sugar maple 2, fig. 

12) has a similar shallow rooting habit, even though it grew 

in deeper colluvium than sugar maple 1 and was not 

obstructed by bedrock. This understory-sized tree grew in a 

heavily wooded area of the Spring Grove woodlot dominated 

by sugar maple and white ash. At the excavation site, the 

land slopes at 8° and the soil averages 1.5 m in thickness. 

The soil type at each site is the same, and both trees grow 

within a mature hardwood forest. The only known differences between the sites are the ground slope and aspect and 

the depth to bedrock.

The root system of the sugar maple at Spring Grove 

(fig. 12) has the same general form as that of sugar maple 1 

at the Delhi site (fig. 10). The taproot abruptly reorients from 

vertical to parallel with the ground surface. Swelled projections from the taproot branch near the trunk into lateral roots 

which radiate from the center of the tree. The laterals branch 

at high angles to the source root. The abundant branching 

and rapid tapering of the roots gives them a spindly appearance. Upon tapering to about 0.5 cm in diameter, the roots 

project as more gradually tapering tubes for as much as 2 m. 

These narrow tubes terminate in fans at about crown width 

(3-4 m from the center of the tree).

Root asymmetry has been correlated with surface gradient (Parizek and Woodruff, 1957), but for both sugar 

maples 1 and 2, the radial distribution of roots seems unaffected by slope, at least within 50 cm of the trunk (figs. 10 

and 12).

Roots of several sugar maples ranging from 8 to 15 cm 

dbh were observed at excavations made by a truck-mounted 

spade to determine whether the distributions described for 

the two hand-excavated maples are representative of small 

maples growing in colluvium in Cincinnati (Riestenberg, 

1987). The spade-excavated sugar maples show essentially 

the same rooting habit near the base of the tree that was seen 

in the hand-excavated sugar maples; the numbers and crosssectional area of roots are highest near the ground surface, 

and diminish in number with depth. The roots displayed the 

same spindly form as those of the excavated trees, with the 

laterals tapering highly as they branched, and the taproot 

reorienting from vertical to become parallel with the slope 

near the base of the tree.

MORPHOLOGY AND DISTRIBUTION OF THE 

ROOTS OF WHITE ASH

A white ash, 8.8 cm in diameter, was selected for hand 

excavation at the Spring Grove site. It grew 5 m from the
E10 LANDSLIDES OF THE CINCINNATI, OHIO, AREA

Category I

Category II

- 0.5 -i

0 4 8 12

DISPLACEMENT, IN

CENTIMETERS 0=1.56 cm

4 8 12 

DISPLACEMENT, IN 

CENTIMETERS

D=1.04cm

0.6

0.4

O 

2 

- 0.2

LU 

O 

DC

OLL.

0.0, 5 10 15

DISPLACEMENT, IN

CENTIMETERS

20

D=0.52 cm

sugar maple that was excavated from the woodlot. Ail roots 

of the excavated white ash larger than 0.075 cm are woody. 

The branches extend to a depth of 1 m below the ground surface and then reorient horizontally. The taproots are intermittently branched; the branches are at high angles to the 

taproot and are generally less than 1 cm in diameter.

The taproot has the form of a cone, rather than the cylindrical shape of the laterals. Its branches also taper strongly

n n -T i i i i I i i i I I I I I I I I I I I I I I I I I I I

U ' U0 10 20 30 

DISPLACEMENT, IN 

CENTIMETERS

0.8u i i i i i i i i i i i i i i i i i i i i i i i i i i i

D=1.12cm

0.0 5 10

DISPLACEMENT, IN

CENTIMETERS

D=0.85 cm

Figure 7 (above and facing column and facing page). Three general categories of roots and their force-displacement relationships. 

As any root of these three categories is pulled out, force initially 

rises abruptly with little or no displacement, but this initial peak is 

followed by gradual reduction of force in category I roots, by continued high force leading up to a final abrupt fall-off in category II 

roots, and by an interim reduction of force and then a second abrupt 

peak in category III roots. Category I roots are generally long and 

poorly branched, category II roots are short and highly branched, 

and most category in roots fork in two major branches. In these figures, SM stands for sugar maple, D is the diameter of the illustrated 

root segment at its proximal end, and mtbf, the measured tensile 

breaking force, is the final peak in force that causes failure; all 

forces applied subsequently are smaller.

as they project nearly vertically into the colluvium. The lateral roots radiate from the taproot and disperse about the 

trunk. In some places individual laterals originate directly 

from the taproot whereas, in other spots, groups of two or 

more fuse at their heads to form swellings on the side of the 

taproot. As the laterals radiate from the tree, they generally 

project in the same direction in which they started (fig. 13). 

The roots are circular in cross section and taper gradually, so 

they appear almost cylindrical in shape, in contrast to the 

highly tapering and branched roots of the maple. This difference in taper gives the two species their characteristic 

shapes: the sugar maple appears to be spindly, and the ash 

appears to be massive.
ANCHORING OF THIN COLLUVIUM BY ROOTS ON HILLSLOPES IN CINCINNATI Ell

Category III

0 5 10 15 20

DISPLACEMENT, IN

CENTIMETERS

D=1.57 cm

0 5 10 15

DISPLACEMENT, IN

CENTIMETERS

D=0.50cm

Some lateral roots of ash terminate as fans of tiny rootlets; others terminate abruptly (fig. 13). The termini of the 

blunt lateral roots are sealed with a waxy covering and 

reflect root mortality or cessation of growth (Kramer and 

Kozlowski, 1979). (Blunt termination is observed infrequently in sugar maple).

Unlike the shallow roots of the sugar maple, the lateral 

roots of the ash may plunge steeply. One lateral of the ash 

projects at an angle 55° from horizontal, and continues to a 

depth of 1 m below the ground surface. It has 34 branches 

along its 1.8-m length beyond the point at which it reorients 

horizontally. It has a higher rate of taper than the horizontal 

laterals.

The spade-excavated roots of five white ash trees at 

the Spring Grove site are similar in form to the handexcavated ash roots. Roots exposed by both hand excavation and truck-mounted spade are distributed in two distinct 

directions within the soil: a set of laterals extends nearly 

horizontally, and a very large, branched taproot projects 

vertically. The laterals are large, have few branches, and 

taper gradually. The dominant vertical taproot is typical of 

ash trees (Riestenberg, 1987).

COMPARATIVE ANALYSES OF ROOTS OF 

SUGAR MAPLE AND WHITE ASH

The observations of the roots of sugar maple and white 

ash demonstrate that rooting habits differ greatly between 

the two species and that, in general, the overall root configurations for both species are predictable. Figure 14 shows the 

roots of sugar maple and white ash trees that were excavated 

by a Vermeer spade. The trees have the same diameter trunk, 

but the roots are quite different in form. The root balls shown 

in figure 14 represent 2 of 11 trees that were removed from 

the Spring Grove site.

The differing root distributions of sugar maple and 

white ash are illustrated by stereonets showing contours of 

root area as a function of orientation at a distance of 30 cm 

from the bases of the three fully excavated trees (fig. 15). 

The sugar maple roots are concentrated at low angles from 

the base of the tree, whereas the white ash roots are concentrated in two orientations: one nearly horizontal and the other 

vertical.

Root area and depth are compared for the two species in 

figure 16. The cross-sectional area of roots penetrating various depths for all three excavated trees is highest near the 

ground surface, and decreases rapidly with depth. The root 

areas of the sugar maples approach zero at a depth of 60 cm, 

but the tap root of the ash projects to a depth of 110 cm.

Rose diagrams of orientations of the lateral roots of the 

fully excavated trees and those sampled by the Vermeer 

spade excavations show similarities between the two species 

(fig. 17). The measurements are made at a distance of 30 cm 

from the base of the tree trunk. The laterals tend to be spaced 

nonpreferentially about the trunk, despite differences in 

slope. No preferred orientation of roots was observed on the 

more steeply sloping Delhi site.

Roots of trees larger than the fully excavated trees 

show distributional trends similar to those described above. 

Observations of trench faces cut near trees of varying sizes 

showed that roots are most numerous in shallow soil to a 

depth of 6-8 cm and then decrease sharply in abundance 

with depth. Kramer and Kozlowski (1979) point out that 

rooting depth shows no correlation with the above-ground 

size of the plant. Few roots larger than 1 cm in diameter 

extend through soil 2 m below the ground surface (fig. 18).

STABILITY ANALYSIS OF A

HILLSLOPE UNDERLAIN BY

COLLUVIUM ANCHORED BY SUGAR

MAPLE AND WHITE ASH ROOTS

The data collected on pull-out resistance and distribution and morphology of roots can be incorporated into a 

stability analysis to determine how effective the roots of 

sugar maple and white ash are in stabilizing hillslopes. The
E12 LANDSLIDES OF THE CINCINNATI, OHIO, AREA

_ Sugar Maple, fi=Q.

c/3 5

z 

o

4

LLJ 

O

DC 

O

u. 2

0.2 0.6 1.0 1.4 1.8 2.2 

DIAMETER AT SOIL FACE, IN CENTIMETERS

2.8 3.0

5 -

01 3 O

DC

O

White Ash, ^=0.49

 t

2 3 

DIAMETER AT SOIL FACE, IN CENTIMETERS

Figure 8. The tensile breaking force (mtbf) measured for two roots during extraction. The factor that 

best predicts the pull-out resistance of a root is its diameter. A secondary factor affecting pull-out resistance is the length of the root. Correlation coefficient for each root type shown as r2 .
ANCHORING OF THIN COLLUVIUM BY ROOTS ON HILLSLOPES IN CINCINNATI E13

35

30

25

20

uJ 15 

O DC 

O

10

SUGAR MAPLE

EXPLANATION

 mtbf

o srs

1 2 3 

ROOT DIAMETER, IN CENTIMETERS

25

Figure 9. Root diameter versus predicted 

tensile breaking force (ptbf), predicted force to 

break all termini at once (srs), and measured 

tensile breaking force (mtbf). (See figures 7 

and 8.) The force predicted to break all termini 

within the colluvium at once (srs) is greater 

than the force predicted to break a root of the 

diameter exposed at the trench face (ptbf), 

which in turn is greater than the measured tensile breaking force (mtbf) applied to the root. 

These observations invalidate the assumptions 

used in the model for the Rapid Run landslide 

(Riestenberg and Sovonick-Dunford, 1983) 

that is, that all roots break at once and that the 

tensile breaking force is determined by the 

diameter of the root that projects through the 

shear surface in the soil.

20

15

10

-iiip -irWHITE ASH

EXPLANATION

 mtbf

2 3 

ROOT DIAMETER, IN CENTIMETERS
E14 LANDSLIDES OF THE CINCINNATI, OHIO, AREA

PLAN VIEW

>t from 

here

20 30 40 CENTIMETERS

J_______I______I

OBLIQUE VIEW

Figure 10. Plan and cross-sectional views of the excavated roots of sugar maple 1 from the Delhi site. Root diameters, in 

millimeters, are shown in the plan view. The roots are numerous, are highly branched, and are concentrated within the 

uppermost soil horizons. The roots taper markedly with distance from the trunk, a characteristic associated with frequency 

of branching.
ANCHORING OF THIN COLLUVIUM BY ROOTS ON HILLSLOPES IN CINCINNATI E15

o

W 4 a. *

O

ROOT DIAMETERS 

1-2 mm 

2.1-5 mm 

5.1-10 mm 

10.1-19 mm 

>19 mm

20 30 40 50 60 

DEPTH, IN CENTIMETERS

70

Figure 11. The density of roots as a function of soil depth. The 

cross-sectional area of roots per area of soil exposed at a trench 

face decreases markedly with depth (Riestenberg and SovonickDunford, 1983).

stability analysis uses data from the Delhi and Spring 

Grove sites along with measurements of the residual 

strength of the colluvium made by Fleming and Johnson 

(1994). For an anchored soil block lying at equilibrium on 

an inclined surface, such as the bedrock-soil interface, factors that resist failure of the block are the strength of the 

soil, Ss, and the strength of the anchors; factors that drive 

failure are the unit weight of the soil mass, y, and the slope 

angle, P Soil strength is defined by the Terzaghi-Coulomb 

equation as:

Ss = c + tf tan (1)

where

c = cohesion for effective stress

o' = effective normal stress, and

<J> = soil internal friction angle for effective stress 

For a block of soil of volume v resting on an inclined plane 

(hillslope), assuming the water table is at the ground surface 

and flow is parallel to the slope, the force resisting basal 

shearing is:

ASS - v(yryw) cos P tan <|> + cA (2)

where

v = AT, volume of soil block

A = area of base of block

P = slope angle

T = thickness of soil

Yj = unit weight of the saturated soil, and

jw = unit weight of water

If an anchor projects through the block into the underlying 

material at an angle, a, from normal, the block resists shear 

by:

ASa = F cos a tan <|> + F sin a (3)

where

F = tensile force on anchor (assumed positive), and 

ASa = resistance of anchored block to shear. 

The forces that resist failure, Fr equal ASa plus ASS, the 

block's resistance to shear. The forces driving failure of the 

anchored block are:

(4)

Thus, the factor of safety against sliding, FS, is equal to:

(, 1 - Y-" cosB o + cosa F } tan<b ^ + F since + (A 

 

V 

I Y, Y,v ) Y,v U Jy,

sinp

(5)
E16 LANDSLIDES OF THE CINCINNATI, OHIO, AREA

PLAN VIEW

10 20 30 40 50 CENTIMETERS

OBLIQUE VIEW

Ground surface

10

^ 20

- 30

40

50 u

Figure 12. Plan and cross-sectional views of the excavated roots of sugar maple 2 at the Spring Grove site. Root 

diameters, in centimeters, are shown in the plan view. The general distribution and morphology of the roots are similar 

to those of the maple excavated at the Delhi site. This tree, however, grows on gently sloping terrain underlain by deep 

colluvium, whereas the one at the Delhi site grew on a steep hillslope underlain by a thin mantle of colluvium.
ANCHORING OF THIN COLLUVIUM BY ROOTS ON HILLSLOPES IN CINCINNATI

PLAN VIEW

E17

LATERAL VIEW

90 L

Figure 13. Plan and cross-sectional views of the excavated roots of a white ash at the Spring Grove site. Root diameters are shown in 

centimeters. The roots of the ash tree have two contrasting orientations: one set of roots is nearly parallel with the ground surface, and 

another set is at high angles to the ground surface. The tree is similar in girth to the excavated sugar maple, which grew next to it, but the 

roots of the ash extend deeper into the colluvium.
E18 LANDSLIDES OF THE CINCINNATI, OHIO, AREA

ASH

MAPLE

Ground surface

20

30

40

50-

60-

70

40 CENTIMETERS

Figure 14. Lateral view of sugar maple and white ash roots removed from the colluvium by the Vermeer spade. The orientations and 

characteristics of the roots near the trunk are similar to those described for the hand-excavated roots. The ash, in contrast to the highly 

branched maple, has a root system that is dominated by a large tap root. (Illustration by Ted Riestenberg).

As the anchored soil displaces, tensile forces, F, are 

mobilized in the roots that anchor the mass to a stable substrate below the colluvium. The tensile forces that develop 

in the roots are a function of the root type and size, and the 

magnitude of displacement of the anchored soil block. For 

example, a category I root (fig. 7) will develop peak resistance to failure at about 2 cm of displacement, at which point 

the root will contribute its greatest force resisting failure.

The computer program DEPTH (written by Roger 

Stuebing, University of Cincinnati) uses measurements from 

hand-excavated trees of the azimuth, plunge, and starting 

and ending diameters of each unbranched segment of root to 

determine the numbers and diameters of roots that penetrate 

planes parallel to the ground surface at various depths 

beneath a tree (Riestenberg, 1987). The computer program 

lists the sizes and orientations of the roots projecting through 

soil planes at depths selected for the stability analyses.

The force-displacement relationships of 19 roots, each 

belonging to one of the three root categories (fig. 7), are used 

in developing stability analyses of wooded hillslopes. The 

roots chosen are of differing sizes and were growing at various orientations within the soil before they were pulled out.

A root is selected from the 19 type roots to represent 

each root segment enumerated by the program for the 

stability analysis of soil of a thickness corresponding to the

chosen depth or soil plane. The root is selected on the basis 

of its size, orientation, and type. For instance, for a small 

root projecting through a shallow plane, a type root is 

selected from the set of category II roots, because those 

roots are highly branched as shallow roots observed in the 

excavations. A steeply plunging root is represented by a 

type root from either category I or category III, depending 

on the depth of the plane. A root that projects through a 

deep plane is poorly branched, like roots of category I, and 

a root projecting through a more shallow plane is branched 

like a category III root.

Figure 19 and table 1 illustrate the results of a stability 

analysis for a block of colluvium inclined at 30°. The block 

width and length are each 1 m; its thickness is 0.43 m. The 

soil properties used in the analysis are the residual strength 

values measured for remolded colluvium (Fleming and 

Johnson, 1994). Constants used in this analysis and those 

that follow are listed in table 1.

The stability analysis uses several roots, each of which 

has a different relationship between force and displacement. 

The root anchors are assumed to be aligned perpendicular 

to the shear surface, so a, the root angle, is zero. This simplifying assumption is based partly on observations of the 

orientations of small roots that cross shear surfaces, and 

partly on the limitations set by the field testing of pull-out
ANCHORING OF THIN COLLUVIUM BY ROOTS ON HILLSLOPES IN CINCINNATI

SUGAR MAPLE 1 SUGAR MAPLE 2 ASH

E19

Figure 15. Stereonets of cross-sectional area of roots cutting the surface of a lower hemisphere at a radius of 30 cm from the base of the 

trunk. The contour interval is 2 cm2. The two species show contrasting root distribution. The sugar maple roots are most concentrated at 

low angles from the horizontal, and the white ash roots are concentrated at low and high angles from the horizontal. The sugar maple is 

shallow rooted; the white ash is deeply rooted due to its dominant, vertically oriented tap root.

100

110

10 20 30 40 50 

ROOT AREA, IN SQUARE CENTIMETERS

60

Figure 16. The cross-sectional area of roots of the handexcavated trees as a function of depth. The roots of both species 

are concentrated in number and size in shallow colluvium, and 

decrease in numbers and cross-sectional area with depth. The 

maple roots extend to 0.6 m and the white ash roots to 1.1 m 

below the ground surface. The plot is drawn using orientations 

and dimensions of the roots of the fully excavated trees.

resistance. The size of the Vermeer excavations used in 

field testing limited the pulling direction to nearly perpendicular to the excavation face, collinear with the long axis 

of the exposed roots.

The number and sizes of roots that penetrate the shear 

surface at 0.43 m depth were calculated for the trees that 

were hand excavated at the Spring Grove site (fig. 19) (Riestenberg, 1987). The depth of 0.43 m chosen for the analysis 

approaches the critical thickness of the block without 

roots that is, the thickness at which the factor of safety 

(resisting force/driving force) equals unity. This thickness is 

0.42 m. Figure 19 shows that for colluvium anchored by 

either maple or ash, the factor of safety exceeds 1 at the first 

hint of movement. The subsequent increase with further 

displacement is much greater for colluvium anchored by ash 

than for that anchored by maple. This difference in anchoring is due to the difference in root mass at depth between the 

two species.

CRITICAL SPACING OF ASH TREES 

ON A HILLSLOPE

The density of roots within soil affects the soil's resistance to shear, and, because roots are most concentrated 

near the base of the tree from which they originate, it follows that close spacing of trees will strengthen soil. A stability analysis of an inclined colluvial soil block supporting 

an ash tree shows that the factor of safety decreases with 

increasing spacing of trees (fig. 20). The analysis uses the 

dimensions of the excavated ash from Spring Grove and a 

soil block 0.43 m thick. The area of the block and, hence, 

the shear-surface area vary. The initial area of the block's 

shear surface is 1 m2, which is equivalent to 1-m spacing
E20 LANDSLIDES OF THE CINCINNATI, OHIO, AREA

SUGAR MAPLES ASH TREES

Spring Grove site

11.9cm dbh

49 observations

(3 in the longest

petals)

vDelhi Pike site

6.0 cm dbh / 

13 observations / 

(1 in each petal)

W

Spring Grove site 

10.7 cm dbh 

28 observations 

(2 in the longest 

petals)

Spring Grove site 

7.1 cm dbh 

24 observations 

E (2 in the longest 

petals)

Spring Grove site

8.3 cm dbh

19 observations

(1 in each petal)

Spring Grove site 

8.0 cm dbh 

13 observations 

(2 in the longest 

petal)

Spring Grove site

14cm dbh

30 observations

(3 in the longest

petals)

W r

Spring Grove site 

13cm dbh 

33 observations 

E (2 in the longest 

petals)

Figure 17. Azimuths of lateral roots of sugar maple and white ash trees from the Delhi and Spring Grove sites. In both species, root distribution appears random and unaffected by slope; none of the specimens showed any strong directional trend. U marks upslope side of 

each diagram; dbh is diameter at breast height.
ANCHORING OF THIN COLLUVIUM BY ROOTS ON HILLSLOPES IN CINCINNATI E21

White Ash

8 cm dbh

1 m from trench

-1 mEXPLANATION

Root Diameters: 

+ 2.1-5 mm 

X 5.1-10 mm

 10.1-17 mm 

A 17.1-25 mm

 >25 mm

Juniper

8 cm dbh

1 m from

trench

Figure 18. Density of roots in colluvium underlying a mature forest, exposed in a trench at the Delhi site. Contours show the 

distribution of more than 400 very small roots (<2 mm in diameter) that project through the trench face; contour values show 

the number of <2-mm roots in each lOxlO-cm square. Larger roots (>2 mm) are shown individually by symbols representing 

various ranges of root diameters. The roots are most concentrated near the colluvium surface, and drastically decline in number 

and cross-sectional area with increasing colluvium depth. The roots project most deeply into the soil directly below individual 

trees. Two trees near the trench are a white ash (on the left) and a young juniper. The contour pattern under the ash tree reflects 

its deeply penetrating tap root. The juniper roots are concentrated near the ground surface.

between trees, if the tree is centered on the soil block. At 

1-m spacing, the peak factor of safety is 2.1. With 

increased block surface area, the factor of safety decreases, 

until it approaches unity at 7-m spacing between ash trees, 

or 0.14 trees per meter. This is the critical spacing for ash 

trees of the same girth as the fully excavated tree in soil 

with the physical properties of the colluvium.

A spacing of 7 m between small ash trees corresponds 

to about 200 trees per hectare. This density compares with 

that of an actual mixed forest growing on a steep hillside; the 

hillslope at the Rapid Run site had a density of 259 trees per 

hectare east of the landslide and 176 trees per hectare west of

the landslide. The forests were not monocultures of ash trees, 

of course, and other factors may have varied, but the comparison shows that the critical density calculated for ash trees is 

close to that observed on the stable portions of hillsides adjacent to failed areas.

CRITICAL THICKNESS OF ROOTED 

COLLUVIUM

Not only the spacing of trees within a soil mass affects 

soil stability; the number of roots and their anchoring effect
E22 LANDSLIDES OF THE CINCINNATI, OHIO, AREA

Table 1. Stability analysis of colluvium anchored by white ash roots

[The ash tree excavated at the Spring Grove site had six roots of various diameters (as shown below) that would have projected through a plane 0.43 m below the surface. Table

of safety that the excavated ash tree would have provided. Two of the test roots are listed twice because they each are used to represent two of the excavated roots]

Root diameter (cm): 

Excavated ash root .... 

Field test root ............

Root number .....................

Displacement (cm)

0 ................................

1 ................................

2 ................................

3 ................................

4 ................................

5 ................................

10 ................................

20 ................................

30 ................................

0.9 

1.3 

11.22

1.2 

1.3 

11.22

1.8 

1.6 

7.22

2.4 

2.3 

7.20

2.6 

2.6 

11.13

2.8 

2.6 

11.13

Measured pull-out resistance (kN)

0 

.15 

.40 

.40 

.40

.40 

.18 

.03 

0

0 

.15 

.40 

.40 

.40

.40 

.18 

.03 

0

0 

.44 

1.56 

3.29 

1.01

1.58

.05 

0 

0

0 

0 

0 

0

.33

.66 

2.85

.74 

0

0 

3.4 

4.0 

4.0 

4.0

4.0 

3.6 

3.6 

2.5

0 

3.4 

4.0 

4.0 

4.0

4.0 

3.6 

3.6

2.5

Total 

resisting 

force1 

(kN)

4.27 

7.37 

8.54 

9.25 

8.44

8.81 

8.58 

7.56 

6.33

Driving 

force2 

(kN)

4.3 

4.3 

4.3 

4.3 

4.3

4.3 

4.3 

4.3 

4.3

Factor 

of 

safety3

0.99 

1.71 

1.98 

2.15 

1.96

2.05 

1.99 

1.76 

1.47

1 Equals [v(YfYw) cos P+£roots cos a] tan <t>+£roots sin a+c(A), which, using the parameters listed below, reduces to approximately

4 * ' 27+0.41 v>^*2 roots, 

2Equals vfy) sin P = 4.3 kN (using parameters listed below). 

3Equals resisting force/driving force.

The hypothetical block of colluvium anchored by the ash tree has the following characterisitcs:

Area (A) = 1 m2 Unit weight of colluvium (yf) = 20 kN/m3

Volume (v) =0.43 m3 Unit weight of water (yw) = 9.8 kN/m3

Slope (P) = 30° Water table is at ground surface

Angle of internal friction (<f>) = 22.4° Root angle (a) = 0°

Cohesion (c) = 2.7 kN/m3 ^roots = ^um °^ f°rces contributed by all roots

decline with depth, and, beyond some critical depth, they 

have no anchoring effect at all. An analysis of the factor of 

safety of rooted soil as a function of depth demonstrates the 

effect of rooting depth on slope stability. Figure 21A shows 

the effect of the root system of the excavated sugar maple 2 

(Spring Grove) on colluvium at 0.5 m depth. The root configuration and density used in the analysis were derived from 

number, type, and orientation of roots at a given depth as 

indicated by the computer program DEPTH for the excavated sugar maple.

Under a normal stress equivalent to 0.5-m-thick colluvium overburden, the factor of safety for the colluvium alone 

would be less than one (0.90). With maple roots penetrating 

the colluvial soil, the peak factor of safety at 0.5 m depth is 

1.01. The peak is reached at 2-cm displacement. No large 

roots project through the soil at greater depth.

Figure 21B illustrates the effect of ash roots on the 

factor of safety, based on data from the hand-excavated 

tree at Spring Grove. The ash roots penetrate deeper into 

the colluvium than do sugar maple roots, and so they 

increase soil strength at depths as great as 1 m. The initial

rise in strength to its peak value is similar to that of the 

colluvium with sugar maple roots; it rises steeply to a peak 

at about 3 cm of displacement, and then diminishes with 

further displacement. Another peak in strength follows at 

5-cm displacement, smaller in magnitude than the first 

peak. The patterns of force versus displacement differ with 

depth. The curve illustrating the anchoring by ash roots at 

0.43-m depth (fig. 19) has the general shape of a category 

II root's force-displacement curve (fig. 7). The similarity 

arises because category II roots dominate at shallow depth. 

With increasing depth, the ash roots behave more like category I and III roots. Hence, these root categories are used 

in the stability analysis, and their characteristic forces sum 

into the relationships illustrated in figure 21 (Riestenberg, 

1987).

DISCUSSION

Tree roots stabilize thin colluvium on Cincinnati's hillslopes by acting as soil anchors, which transfer shear stress 

at the failure surface to a stable underlying substratum. The
ANCHORING OF THIN COLLUVIUM BY ROOTS ON HILLSLOPES IN CINCINNATI E23

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 

DISPLACEMENT, IN CENTIMETERS

Figure 19. Stability analyses of a colluvium block inclined at 

30° with and without roots. The numbers, sizes, and types of roots 

that project through a shear plane with an area of 1 m2 at a depth 

of 0.43 m are assumed to be the same as those of one handexcavated sugar maple or white ash trees. The factor of safety is 

the ratio of the resisting force to the driving force. The colluvium 

penetrated by ash roots develops a peak factor of safety of 2.30 (at 

15 cm displacement); the colluvium penetrated by sugar maple 

roots reaches a peak factor of safety of 1.25. The factor of safety 

of fallow colluvium at 0.43 m depth is 0.99.

roots project through the soil, across the shear surface, to the 

weathered, fissured bedrock below. Roots are realigned at 

the shear surface by the movement of the overlying, failing 

soil. This realignment pulls the root terminus out from its 

preslide growth position. The root resists failure by a combination of its bond with the soil, its size, and its strength. 

The strength of soil can be greatly increased by the addition 

of root anchors. A critical spacing between root anchors 

may be determined for soil of different thicknesses. Small 

white ash trees of the same girth as those studied may be 

spaced 7 m from each other and anchor a hillside mantled 

with 0.43 m of saturated colluvial soil sloping at 30°.

The most effective root configuration has one or more 

deep taproots. The analysis of the anchoring effects of ash 

and maple shows clearly that ash trees anchor soil far better 

than maples. The sugar maple has as much biomass below 

ground as the ash but is shallow rooted and, therefore, does 

not penetrate to a depth at which failure is likely to occur. 

Neither of the sugar maples that were hand excavated for this 

study had roots that extend as much as 0.8 m into the soil, but 

the roots of the hand-excavated white ash penetrate to 1.1 m 

depth and would provide a factor of safety greater than 1 for 

a 0.8-m-thick block of soil on a 30° slope.

Maple roots do have some favorable characteristics. 

The sugar maple has many more category II roots than does 

the ash, and these roots are highly branched and resist soil 

shear with a continually high force as displacement

4 8 12 16 20 24 

DISPLACEMENT, IN CENTIMETERS

28

4 8 12 16 20 24 

DISPLACEMENT, IN CENTIMETERS

Figure 20. The effect of spacing of ash trees on stability of a hillslope. Stability analyses of a 0.43-m-thick colluvium layer supporting ash trees indicate that the factor of safety is sensitive to tree 

spacing. Given a slope of 30°, the critical spacing of ash trees of 

the size excavated at Spring Grove is 7 m, equivalent to 0.02 trees 

per square meter or 200 trees per hectare. The 1-m curve shown at 

top here differs from the top curve on figure 19 because different 

sets of type roots were used in generating the plots.

progresses (fig. 7). By contrast, category I and category III 

roots have varying resistances with displacement. Because 

maple roots are shallow and highly branched, they probably 

reduce surficial erosion better than the sparsely branched 

roots of white ash; this function may not be crucial in an 

established forest, though, for leaf litter and understory trees 

could serve the purpose as well. Despite the fact that the category II roots of the sugar maple seem to be better anchors, 

this advantage is meaningless if they don't penetrate to 

depths at which landslide shear surfaces are likely to form. 

The ash roots are effective anchors at greater depths than 

maple roots simply because they are present at greater 

depths.

Observations of tree populations and distribution on the 

failed and stable portions of local hillslopes support the
E24 LANDSLIDES OF THE CINCINNATI, OHIO, AREA

A, Maple, 0.5 m depth, FSCO |=0.90

4 8 12 16 20 24 28 

DISPLACEMENT, IN CENTIMETERS

Figure 21. Factor of safety versus displacement for a 1-m2 block 

of colluvium anchored at various depths by roots from a sugar maple (A) or an ash tree (B). FScol is the factor of safety of fallow colluvium with no roots. At 0.5 m depth, maple roots increase the 

factor of safety from 0.90 at no displacement to 1.01 at 2 cm displacement. Hence, 0.5 m would be the critical thickness for the 

block anchored by maple roots, as the factor of safety would fall below 1 for any thicker block. Ash roots, however, could increase this 

factor to 1.94 at 3 cm displacement The stabilizing effect of these 

roots decreases with depth as the roots become smaller and less 

abundant. Changes in root type with depth account for the differences in patterns of force between part B, above, and figure 19.

conclusions given here about critical spacing and the relative 

effectiveness of maples and ashes. The Rapid Run landslide 

(Riestenberg and Sovonick-Dunford, 1983) averaged 0.5 m 

in thickness and supported a monoculture of sugar maples. 

These maples had girths similar to those of the two that were 

excavated by hand. Maples of this girth are nominally able 

to anchor soil as much as 0.5 m thick, but only if they are 

closely enough spaced. The density of trees on the landslide 

mass was about one-half that of trees in the stable forested 

area adjacent to the landslide. The spacing of the trees on the

stable portion of the hillslope averaged 7 m, the same as the 

critical spacing calculated for the ash tree on soil 0.43 m in 

thickness. The maple trees on the landslide were so widely 

spaced that they exceeded even the critical spacing calculated for the more deeply rooted white ash. Hence, the landslide was probably inevitable.

A limitation of this study is that it analyzes the root systems of trees of relatively small girth. Larger trees were not 

studied for reasons stated earlier in the text, but one may 

make certain assumptions about the root systems of larger 

trees from observations made at trench faces and exposures 

along streams. Roots are most concentrated within the 

uppermost 0.5 m of soil, taper in numbers to about 0.8 m, 

then fall off abruptly in numbers and become sparsely distributed and smaller in girth at greater depths.

With knowledge of the distribution of roots for a species and the pull-out resistance from the soil, one can calculate the resistance offered by a root system to failure. But 

trees also can stabilize the soil by reducing the moisture 

content of the soil through transpiration during the growing 

season. The timing of Cincinnati's landslides may reflect the 

stabilizing effect that transpiration has on hillslopes. They 

occur in late winter and in early spring before leaf-out. By 

summertime, when the field tests of the roots' pull-out resistance were done, the soil is normally drier and more stable 

than in the spring. To restore springlike moisture conditions, 

the field site was continuously watered throughout the testing period.

The effectiveness of roots in stabilizing a hillslope 

depends upon the displacement of soil that can be tolerated 

at a given site. The roots' resistance to failure peaks at about 

3-4 cm of soil displacement. This displacement may be 

allowable in some areas, such as on hillslopes where the soil 

is thin and engineering structures are anchored in bedrock, 

but not in other areas where damage to structures will occur 

at small displacements.

A tree species selected for stabilization of a hillslope 

must have strong roots which penetrate deeply into the soil; 

the species must have the potential to thrive in the local climate and soil, and to tolerate competition with local biota. A 

species with the above characteristics as well as a high rate 

of transpiration would be ideal for stabilization of shallow 

soil. Trees may be combined with engineering materials in 

some cases to produce an inexpensive, attractive means of 

hillslope protection (Gray and Leiser, 1982).

REFERENCES CITED

Endo, Taizo, and Tsuruta, Takeo, 1969a, On the effect of the tree's 

roots upon the shear strength of soil, in 1968 Annual Report of 

the Hokkaido Branch, Forest Experiment Station: Sapporo, 

Japan, p. 167-182. [In Japanese with English summary. 

English translation, by Joseph M. Arata and Robert R. Ziemer, 

obtained from U.S. Forest Service, Pacific Southwest Forest 

and Range Experiment Station, Arcata, CA 95521.]
ANCHORING OF THIN COLLUVIUM BY ROOTS ON HILLSLOPES IN CINCINNATI E25

 1969b, A report in regard to the reinforcement action of 

the vegetational roots upon the tensile strength of the natural 

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Fleming, R.W., and Johnson, A.M., 1994, Landslides in colluvium, 

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Ortman, E.E., Peters, D.C., and Fitzgerald, P.J., 1968, Vertical-pull 

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Manuscript approved for publication July 28,1993. 

Published in the Central Region, Denver, Colorado. 

Edited by Peter L. Martin and Vera Sable. 

Graphics by Wayne Hawkins. 

Photocomposition by Marie Melone.

Cover design by Arthur L. Isom, based on an original drawing by 

K. A. Richards, University of Cincinnati.

U.S. GOVraXMENT PRINTING OFFICE: 1994-573-038/86014



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