UNITED STATES 

DEPARTMENT OF THE INTERIOR

J. A. KRUG. Secretary

GEOLOGICAL SURVEY

W. E. WEATHER, Director

CIRCULAR 13

ROCK-BURSTS IN THE GRANITE QUARRIES 

AT BARRE, VERMONT

PK. C*TY DP

U. B. GEOLGGICAL SURVEY! 

PUBLIC lf,JUJr;!L-Z! OFFICE 

SAN FRANCISCO, CALIFORNIA

By 

Walter S. White

WASHINGTON 

1946

ROCK-BURSTS IN THE GRANITE QUARRIES AT BARRE, VERMONT

By Walter S. White

Introduction ...............................

Geology....................................

Rock-bursts................................

Relation of cracks to geologic features of 

the granite ..........................

Relation of cracks to quarrying..........

Cause of rock expansion................

Amount and force of expansion..........

Results of rock expansion..............

Factors controlling rupture of distorted 

blocks..............................

CONTENTS 

Page Page

1 Rock-bursts Continued

1 Factors controlling rupture of distorted

3 blocks Continued

Size, shape, and support of blocks.... 7

3 Concentration of pressure and

3 movement........................ 7

3 Recommendations............................. 11

3 Avoidance of extensive free faces.......... 11

5 Distribution of pressure in quarries ....... 11

Sloping sheet fractures ................... 15

7 Quantitative measurement of expansion...... 15

ILLUSTRATIONS

Page

Plate 1. Diagrammatic views of Barre granite quarries, showing cracks due to rock-bursts.............. 6

Figure 1. Index map of Barre, Vermont, granite area................................................ . 2

2. Cross section of a granite hill, showing sheeting nearly parallel to slope of hill................. 4

3. Cross section of a newly opened quarry in unsheeted granite, showing how granite expands ....... 4

4. Plan (A) and section (B) of part of a quarry .................................................. 8

5. Even distribution of pressure and distortion in bowl-shaped quarry in unsheeted granite.......... 9

6. Distortion in rectangular quarry in sheeted granite ......................................... . 10

7. Distortion in rectangular quarry in unsheeted granite ........................................ 12

8. Cracks next to keyway in steeply sloping sheet (bed) ......................................... 13

9. Diagram comparing present method of cutting keyway with recommended procedure............. 14

10. Plan and section of ideal bowl-shaped quarry ............................................... 14

INTRODUCTION

During the last few years the quarrymen of 

Barre, Vermont, have been troubled by rock-bursts 

in the granite. Partly freed blocks develop fractures 

at oblique angles to the natural partings of the rock, 

and much stone is thereby lost. Preliminary study of 

the problem indicates that the fractures are caused by 

expansion of the rock as it is freed. Probably little 

can be done to reduce the pressure 1 of this expansion, 

but certain modifications of quarrying practice should 

reduce the number of destructive fractures.

The writer spent eight days in a preliminary field 

study of the problem in November and December, 1944. 

In the limited time available no quantitative study of the rock 

expansion and bursting was made nor was an exhaustive 

study of the literature of rock-bursts attempted. Preparation of this report was delayed by more pressing wartime 

duties. All conclusions and recommendations are tentative, 

and point to the need for quantitative measurement of the

factors involved.

i

The word "pressure", as used in this article and by the 

quarrymen concerned, refers to the force generated by elastic expansion of the granite, a force that causes drill holes to close, 

displaces large blocks of granite, and causes rock-bursts. 1

The writer is grateful to the management and 

quarrymen of the Barre quarries visited for their 

interested cooperation and many courtesies extended. 

Preliminary observations and notes on the rockbursting by M. P. Billings and A. E. J. Engel of the 

U. S. Geological Survey have proved most helpful. Previous geologic studies in the Barre area by R. H. Jahns 

.and the writer provided a background knowledge of the 

regional geology. The writer is much indebted to 

L. W. Currier and R. H. Jahns of the Geological Survey 

and Prof. Francis Birch of Harvard University for 

critical review of the manuscript.

GEOLOGY

A preliminary map of the Barre granite area 

(fig. 1) made by the writer in July 1940, shows the 

distribution and shape of the granite bodies. They are 

narrow tabular bodies, more or less parallel with the 

enclosing rjcks ("slates") on a regional scale, 

but sharply crosscutting in detail. The bodies 

strike north-northeast to northeast and probably 

dip (slope) steeply northwest. The rocks have 

undergone very little deformation since the granite 

was intruded.
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The granite has been described in detail by several writers. 2 It is a medium- to fine-grained rock, 

ranging in composition from granodiorite to granite. 

Most of the granite in the quarries is massive and 

lacks a conspicuous linear or platy arrangement of 

minerals; schlieren (streaks of black mica) are very 

rare. Balk3 found that where foliation and linear 

elements are present, they lie in planes more or less 

parallel with the boundaries of the granite bodies. The 

principal joints also strike parallel to the boundaries, 

but dip steeply southeast. Persistent joints or groups 

of joints, called "headers" by the quarrymen, generally 

have this orientation.

The rift, or direction along which the granite 

splits most easily, trends northeast and dips steeply 

southeast to vertical. It is nearly parallel to the 

principal joints, or "headers", and may be closely 

related to them in origin. The hardway, or direction 

of most difficult splitting, is also vertical and trends 

across the granite bodies at right angles to the rift. 

The lift, which is intermediate between the rift and 

the hardway in ease of splitting, is essentially horizontal.

In most of the quarries the granite is fractured 

into sheets, or "beds", that lie more or less .parallel 

to the surface of the ground. The sheeting, or "bedding", 

is horizontal near the tops of hills; on the sides, it 

slopes in the same direction as the surface of the ground, 

but generally less steeply (fig. 2). Most sheets exposed at the natural surface of the ground are a foot or 

less in thickness; sheets 200 feet below the surface 

may be as much as 30 feet thick. There is practically 

no sheeting in the northeast part of the Rock of Ages 

quarry. Sheeting is generally attributed to upward 

and sideways expansion of the granite as the overlying 

rocks are gradually eroded away. 4 'This belief is confirmed by the fact that the pressure due to rock expansion (see below) is small or absent in highly sheeted 

granite near the surface on hills. The sheets have probably already relieved the pressure by expanding sideways 

in such a way as to spread the hill a fe"w inches or feet. 

Slight arching and lateral movement of wedge-shaped 

sheets in the direction of taper may also afford local 

relief of pressure.

ROCK-BURSTS 

Relation of cracks to geologic features of the granite

Plate 1 contains five oriented sketches, drawn 

in isometric projection, showing cracks that formed 

during or following quarrying operations. The cracks 

slope in all directions and may be gentle or steep. It 

is clear, therefore, that they have no consistent relationship tCKthe natural parting directions, sheeting, 

planar structures, and boundaries of the granite, all 

of which have a more or less constant orientation 

throughout the district.

2Balk, Robert, A contribution to the structural relationa of 

the granitic intrusions of Bethel, Barre, and Woodbury, Vermont: 

Vermont State Geologist, Ann. Rept.for 1925-26, p. 50-72, 1927. 

Dale, T. N., U. S. Geol. Survey, Bull. 738, p. 121-143, 1923- 

Finley, G. I., The granite area of Barre: Vermont State 

Geologist, Ann. Kept, for 1901-02, p. 46-60, 1902.

3Balk, R., op. cit., p. 60 and map, p. 52.

4For discussion and bibliography, see Jahns, R. H., Sheet 

structure in granites; Jour. Geology, v. 51, p. 71-81, 1943.

Relation of cracks to quarrying

The cracks have a very consistent relationship 

to the shape of the part of the quarry in which they lie; 

without exception they slope towards the deepest adjacent opening or high free face. They are certainly 

the result of the tendency of the granite to expand into 

the quarry opening. The cracks are a product of 

"exfoliation", a name that geologists have given to a 

process common in nature. Blocks of massive rock, 

like granite, in the course of time almost invariably 

tend to fracture in such a way that corners and sharp 

angles are broken off.

Cause of rock expansion

When the granite was formed by the cooling of 

molten liquid millions of years ago, there were several 

thousands of feet of rock above the present surface of 

the land. Since that time the land surface has been 

worn down to its present level, and the compression 

caused by the weight of this overlying rock has been 

removed. Just as certain deep-sea fish expand or 

even burst when brought to the surface, the granite, 

which was originally deep and greatly compressed, 

now tends to expand in all directions because of the 

removal of the overlying rocks.

All the pressure phenomena observed at Barre 

can be attributed to expansion after compression. The 

quarrymen understandably refer to the weight of rock 

in hills behind quarries or in high walls as the source 

of the pressure, but pressure generated in this manner 

is probably negligible.

Amount and force of expansion

A distinction must be made between the amount 

of expansion and the force of expansion ("pressure"). 

The amount is small and the force relatively large.

The amount of expansion depends upon the size of the 

mass of granite that is expanding and on the amount a unit 

mass expands upon being freed. Dale 8 found at Stone Mountain, Georgia, andWestford, Massachusetts, that blocks 

expanded about 0.1 percent, that is, a block that was 

1,000 inches long before quarrying would be 1,001 inches 

long after being freed. At Barre, masses of rock have locally been displaced 2 or 3 inches; this indicates that the 

mass of rock that is expanding is well over 100 feet long in 

the direction of expansion. At many such places the granite 

in the quarry walls must be expanding into the quarry.

The force of expansion is tremendous, as any quarryman can testify. This force amounts to many thousands of 

pounds per square inch before quarrying begins, and is often 

sufficient to crush the ".cores". 6 B ut as quarrying proceeds, 

and the rock expands into the new opening, the pressure disappears rapidly. As described in the preceding paragraph, 

the expansion in a quarry required to reduce the pressure to 

zero may amount to 2 or 3 inches, but at most places may not 

be readily detected by visual examination.

Apparently the expansion is not everywhere uniform. 

Channels parallel to the rift are more commonly 

closed up than hardway channels at the E. L. Smith

B Dale, T. N., op. cit., p. 30.

"A "core" is the rock left between two adjacent drill holes 

in a channel.
5heetin(j (bedding)

Figure 2. Cross section of a granite hill, showing sheeting nearly parallel to slope of hill, 

probably forms by upward' and outward expansion of granite.

Sheeting (bedding)

Figure 3. -Cross section of a newly opened quarry in unsheeted granite, showing how granite expands. Dashed 

lines represent original position of rocks that now form walls. Amount of expansion greatly exaggerated. 

Double arrow is axis of maximum stretching.
and Wetmore & Morse quarries. Vertical expansion 

of the granite is probably negligible, because near the 

surface of the earth the rocks have already expanded 

upwards. This universal upward expansion has undoubtedly contributed to the formation of the shee'ts or 

"beds" in the granite. The following discussion of the 

localization of rock-bursts is limited largely to consideration of the effects of sideways expansion; furthermore, the differences in the amount of expansion 

in different horizontal directions are neglected for lack 

of quantitative data.

Results of rock expansion

Expansion of the granite results in movement of 

rock toward available open spaces and, _ indirectly, to 

the distortion and fracture of rock masses that resist 

this movement. This distortion may take place in the 

walls or floor of a quarry, or in partly detached blocks 

within the quarry. The rock at the top of a typical 

quarry may be several inches nearer the center of the 

quarry than it was before the opening was made. As 

indicated above, the amount that the walls must move 

inward to dissipate the force of expansion is relatively 

small. But until dissipated by this small movement, 

the force is tremendous, and more than adequate to 

distort or break large blocks of granite that resist it.

Rock movement is aided by the bedding or sheeting typically present in the Barre quarries. If no sheets 

were present, inward movement of the top of quarry 

walls would be resisted by the strength of the granite 

beneath. The sheeting, however, permits greater freedom of movement, and individual sheets can slide a 

little over underlying'sheets that have not yet been freed. 

To some extent, therefore, quarry walls are like the 

sides of a deck of cards rather than like the sides of a 

block of wood. The analogy is imperfect, however, 

because sliding on the sheeting surfaces is strongly 

resisted by friction and by the many interlocking projections and hollows that characterize these surfaces. 

The typical nature of the rocks lies somewhere between 

the examples of a deck of cards and a block of wood. 

To simplify the descriptions and illustrations that follow, 

sheeting is for the most part neglected; in deep quarries 

there is moi*e movement, and hence more rock-bursting 

in sheeted than unsheeted granite, but the types of bursts 

are the same. In a quantitative'study, sheeting would 

require far fuller consideration.

Another factor that undoubtedly influences the 

expansion, and therefore the results of expansion, is 

the jointing (headers), so common in the Barre quarries. 

Where joints are common in the granites of other regions, sheeting is generally rare or absent, and most 

geologists accept this fact as evidence that the force of 

expansion in the rock has been dissipated by slight 

movements on the joint planes. 7 The absence of sheeting in the north part of the Rock of Ages quarry, close 

to a large header, is very suggestive in this connection. 

There are, however, rock-bursts in this part of the 

quarry^which is ample evidence that the force of expansion there is still large. Furthermore, joints or 

headers are fully .as abundant in the well-sheeted rock 

of the E. L. Smith quarry as in the unsheeted granite 

of the Rock of Ages quarry. The prominent joints 

throughout the district dip (slope) steeply, and probably

Johns, op. cit., p. 75. Gives bibliography.

permit very little sideways expansion of the granite. 

For the present the writer proposes to omit consideration of the role played by joints in reducing the pressure 

of expansion within specific blocks of ground, but such 

a role must certainly be investigated in a more detailed 

study.

Rock-bursts occur when the sideways movement 

of rock toward an opening operates to distort a block 

of granite beyond its breaking point. Blocks may be 

distorted in at least three ways, and the resulting 

fractures are of three types. The commonest type is 

illustrated diagrammatically in figure 3. The walls, 

pressing inward, move point A sideways to a new position at B. The axis of maximum stretching is shown 

by the double arrow, and cracks form perpendicular 

to this line. It should be noted that these cracks slope 

toward the nearest free face. Most of the cracks indicated in the diagrams of plate 1 are of this type. If 

the rock at the bottom of the vertical face were as free 

to move inward as the rock at the top, because of slip 

on sheeting surfa'ces, no such fracture would form. 

The fact that most of the rock-bursts at Barre take 

place along this type of fracture, rather than along the 

type described in the following paragraph, is clear 

evidence that slip on sheeting surfaces is of minor 

importance.

The second type oi fracture is nearly vertical 

(pi. IE, behind derrick 5A) and is found only where a 

sheet has been freed to slide over the underlying sheeting fracture. The fracture passes completely through 

a sheet and does not affect the sheet below. Its origin 

may be visualized by regarding figure 3 as a map rather 

than a cross section. This type of fracture is rare, 

but the common fractures in which corners are broken 

off may be combinations of types 1 and 2.

The third type of fracture is flatter, and appears 

to have formed where very little sideways yielding wa& 

possible, and the rock could only expand upward. Such 

fractures are not common and generally release only a 

thin slab.

All the fractures represent the results of expansion into the nearest open space. Viewed very simply, 

they represent new sheeting or bedding fractures that 

originate when man changes the topography of the surface of the earth. And like the natural sheeting fractures that faithfully follow the slopes of hills and valleys, 

but with more subdued contour (fig. 2), these new fractures slope toward the new man-made valleys, the quarries (fig. 3). And also like the natural sheeting fractures,, 

the new fractures tend to form in such a way that corners 

are broken off.

Brief mention should be made of the fact that 

some blocks, apparently intact when freed, subsequently 

develop cracks. The writer was not present when such 

a crack developed, but a possible explanation was suggested by observations on cracks in blocks that had not 

yet been freed. A large number of cracks that were 

observed in one face do not persist- to the corners that 

bound adjoining faces. The crack, which may be very 

obvious and large enough to accommodate a knife blade 

in the middle of a face, disappears when followed towards 

an adjacent face. This fact suggests that there may be 

cracks within a block that do not extend to the faces, and 

subsequent-splitting or handling of the freed block may 

cause these cracks to open up to the outer surfaces.
C. E.LSMITH, DUFFEE QUARRY D. WETMORE a MORSE QUARRY

Cracks and broken corner 

due to rock-bursts during 

quarrying

ij , Imaginary vertical surface where 

\-> ± \ high walls have been cutaway in 

________diagram fa show quarry bottom___

Plate 1. Diagrammatic views of Barre granite quarries, showing cracks due to rock-bursts.
Factors controlling rupture of distorted blocks

As suggested above, partly freed granite blocks 

will crack when the amount of distortion or extension 

at any point exceeds some allowable quantity. Therefore any factor that limits the amount of distortion is 

an important control of rupture. The two factors that 

appear most significant are (1) the size, shape,-and 

support of a given block, and (2) the extent to which 

the strains and movement in the rock are permitted to 

be concentrated at certain points within a quarry. 

Sheeting has an important bearing on the second factor. 

Proper control of these factors should substantially 

reduce the number of rock-bursts at Barre.

Size, shape, and support of blocks. A simple 

example will demonstrate that the shape and size of a 

block control the amount of distortion. The end of a 

10-foot diving board is depressed less than half as 

much by its own weight as the end of a 20-foot board 

would be. In the same way, a point at the top of a 

40-foot vertical wall in unsheeted granite, under constant pressure from within the wall, can move more, 

in proportion to its height, than a point at the top of a 

12-foot vertical wall under the same unit pressure. 

Distortion is therefore greater, and with it the chance 

of fracture.

The manner in which lateral support may offer 

resistance to bending and reduce distortion is illustrated in figure 4, a plan and section of a part of a 

hypothetical quarry, closely resembling the situation . 

in plate IB. When the block to the right of line DE 

(fig. 4A) was removed, expansion of the granite caused 

point B to move to B1 . This distortion may or may 

not have caused a crack, but if a crack formed, it 

would cut the surface of the block along the heavy 

black line. If no fracture formed and a channel were 

cut from F to D, point D would expand to D'. The 

distance D-D1 is considerably larger than B-B1 . The 

bending of boards offers a comparable example; a 

heavy diving board, free at one end, bends down farther than the same board would bend if it were supported at both ends. The fracture that would form 

after the channel is cut at DE would follow the dotted 

line EF. The removal of lateral support by the cutting 

of channel DF permitted the distortion that broke the 

block.

It is clear, therefore, that, other things being 

equal, blocks with extensive free faces are more apt 

to break than small blocks, and blocks are most apt 

to break when lateral support is removed. The size 

of blocks that can be freed on two or more sides without breaking-is not everywhere the same, and depends 

on lateral support, local distribution of pressure within 

the sheets and perhaps, to a, less extent, on variation 

in the tensile strength of the granite. But observations 

in the Barre quarries visited suggest that where 2 or 3 

free sides of a block are1 over 20 feet high, there is 

great danger of breaking, particularly where one fixed 

side is against a main quarry wall; blocks in any location with 2 or 3 free sides 40 feet high are almost 

certain to burst. Blocks with only one free vertical 

face, like that shown in figure 4A may burst where 

the supporting walls at the sides are more than 75 feet 

apart.

Concentration of pressure and movement. With 

modern quarrying practice, pressure is concentrated

at certain points within the quarry, and the walls are 

fairly free to move inward. The reasons for this concentration can be made clear by two examples. If two 

sealed, hollow tin cans, one oblong in shape and the 

other spherical, are carried to great depth in water, 

the oblong can will collapse at much shallower depththan the spherical one; hence the spherical design of 

the bathysphere. Similarly, an arch has much greater 

load capacity than a span bridge of equal weight. In 

both the sphere and the arch, there is relatively uniform 

distribution of pressure throughout the structure, and 

maximum use is made of the compressional or crushing 

strength of the material to prevent inward movement. 

In certain materials, including granite, the crushing 

strength far exceeds the tensional or bending strength; 

Modern quarries resemble the oblong can or the span 

bridge.

Before the advent of the compressed air drill, 

maximum use was made of joints and other natural 

breaks in the rock to loosen blocks with a minimum 

of drilling. The shape of blocks was of less importance than the ease with which they could be loosened, 

because it was cheaper to shape the stone by splitting 

and throw away a large proportion than to attempt to 

cut out blocks with squared faces. This practice resulted in essentially bowl-shaped or oval quarries 

(fig. 5), because all projecting masses of rock were . 

easily attacked and therefore were generally removed. 

These bowl-shaped quarries favored a relatively uniform distribution of the pressure due to expansion of 

the rock, and reduced the amount of distorting inward 

movement of the walls to a minimum. In such quarries the force that can be concentrated at any one point 

is probably rarely adequate to break much stone, because the rock at any one point is prevented from moving 

inward by the rock beside it, as in an arch (fig. 5A). 

As the diameter of the quarry decreases downward, 

the amount of inward movement is progressively less. 

In figure 5B, the dashed line represents the original 

position of the rock that now forms the walls, and their 

present position is shown by a solid line. The distance 

between these lines is, of course, tremendously exaggerated. The gradual downward decrease in the distance between the dashed and solid lines, however, 

suggests the uniform downward decrease in the amount 

of inward movement, and indicates the manner in which 

distortion is evenly distributed through the whole quarry 

wall. A bowl shape can be considered a "natural" shape 

in contrast with a rectangular shape, because, in nature, 

exfoliation tends to eliminate corners.

In modern quarrying with compressed-air drills, 

most of the cost of the operation is in channeling, for 

the removal of large squared blocks permits a big saving by substantially cutting down the amount of waste. 

It generally results, however, in rectangular openings. 

The amount of distortion and inward movement of the 

walls in a rectangular quarry are very uneven, and 

permit the concentration at certain points of forces 

that are capable of breaking the stone. The ideal distribution of these forces is somewhat different in 

sheeted and unsheeted granite.

In well-sheeted granite, inward movement of the 

walls is almost as great at the bottom of the quarry as 

at the top, and is restricted only by the friction between 

sheets or beds and by the support of the other walls in 

corners (figures 6B and 6C). It will be noted in figure 6A 

that angles in the four corners of the quarry that were
/,// / / // / 

am quQTry wall

'

'Figure 4. -Plan (A) and section (B) of part of a quarry. Section is drawn along line AC of (A). Dashed line DE 

represents original position of rock at edge of bench before rock to the right of DE was removed. Dashed line 

B'D1 represents edge of bench after cutting of channel DF. Amount of distortion greatly exaggerated.
PLAN

AT LEVEL OF 

GROUND SURFACE

. ' . * T 

(A) PLAN . '

(B) SECTION

Figure 5. -Even distribution of pressure and distortion in bowl-shaped quarry. Dashed lines represent original 

position of rock that now forms walls of quarry. Amount of distortion greatly exaggerated.
SECTION

to"

SECTION

Figure 6. -Distortion in rectangular quarry in sheeted granite. Section in (B) is drawn along line WX of (A) and 

section in (C) along line YZ. Dashed lines represent original position of rock that now forms walls of quarry. 

Amount of distortion greatly exaggerated.

10
formerly right angles are now acute. Blocks of granite 

that remain unquarried in these corners are squeezed 

into a diamond shape, and their fracture is almost inevitable after the rock in the middle of the quarry has 

been removed. In one such corner in the Wells-Lamson 

quarry, the rock is under such great distorting pressure 

that practically every channel drilled in the bottom is 

accompanied by bursting. Along the sides, the rock is 

pushed inward with even greater force, but will not 

fracture if it is free to slide over the sheeting surface 

beneath it. Any block that resists the inward movement 

of the sides, however, will probably fracture.

In unsheeted granite, there is very little distortion 

of the shape of the floor of the quarry, particularly in 

corners, because the floor and the walls are part of the 

same unit of rock, and brace each other (fig. 7A). But 

along the sides, particularly at the center, the walls are 

bent inward above the floor level, and the original right 

angle between the wall and the floor, is made acute (fig. 7C); 

the upper surface of any block remaining unquarried in 

this angle is pushed toward the opening farther than the 

attached lower surface, and the block win, fracture. In 

plate 1A, a view of unsheeted granite, it is clear that 

most of the fractures are located along the side of the 

quarry at some distance from the corner.

As stated above, the friction between sheets or 

beds makes the behavior of most of the granite at Barre 

lie somewhere between the ideal behavior of sheeted and 

unsheeted granite. Therefore fractures occur along the 

sides as well as in the corners of quarries in sheeted 

granite.

Where the sheets or beds are not horizontal, the 

maximum pressure is exerted in a direction parallel to 

the sheets rather than horizontally. In the bottom of the 

northeast Wetmore & Morse quarry, movement of one 

sheet over another has formed openings along an irregular 

sheeting surface. In this place (upper right-hand part 

of pi. IE), where the sheeting dips (slopes) 25° to 30° SE, 

expansion within a single sheet has formed cracks which 

have different orientations on opposite sides of a removed 

block. Both types of cracks can be found in this quarry, 

and they are diagrammatically illustrated in figure 8. 

If the sheeting were flat, the cracks would presumably 

slope at about the same angle but in opposite directions. 

This special occurrence is mentioned for its possible 

usefulness.

RECOMMENDATIONS

The following recommendations for local modifications of quarrying practice are based on a purely 

qualitative consideration of the factors involved in 

rock-bursts. Specific figures are avoided, in .general, 

because no actual measurements of rock expansion were 

made, and because in the time available it was not possible to isolate and evaluate the relative importance of 

all the contributing factors at the points of observation. 

The recommendations are made with full knowledge that 

it would be uneconomic to follow them faithfully because 

(1) the shapes of present openings are largely fixed by 

past operation; (2) it is impractical to quarry a lot of unmarketable stone for the purpose of avoiding a few cracks 

in the part of the quarry that produces marketable stone; 

(3) it may be cheaper, in general, to sacrifice a little 

stone to rock-bursts than to adopt a more costly improvement of method and save the stone; and (4) adoption of

some pf the recommendations would require abandoning 

parts of some quarries, such as the south end of the 

Wells-Lamson-quarry, with consequent loss of investment in derrick location and completed channels. 

The recommendations are made in the hope that where 

several methods of cutting out a given mass of stone 

are similar in cost, or where other considerations permit a choice in location of a new operation, the recommendations will be helpful in showing which choice is 

best from the point of view of avoiding rock-bursts.

Avoidance of extensive free faces

Except near the surface in hillside quarries, 

wide and high vertical or bedding surfaces, particularly 

near main quarry walls, generally lead to rock-bursts. 

Where two sides of a block are free to expand into an 

opening, and these sides are as much as 40 feet high 

and long, bursting appears to be almost inevitable. 

Deep keyways 8 are almost everywhere accompanied by 

cracking of the blocks at either side. A method of 

quarrying by which such bursts can generally be avoided 

is as follows:

In figure 9, a cross section of a hypothetical 

keyway, the solid line represents a keyway about 36 feet 

deep and the dotted lines the blocks at either side. The 

heavy lines represent cracks that formed during the cutting of the lowest block (block 6). The high vertical 

walls have permitted sufficient distortion of the blocks 

on either side to cause bursting. Viewed more simply, 

it is quite clear that no rock would have been present 

to burst next to the keyway if the granite had been 

quarried as a series of blocks 12 feet thick and 24 feet 

wide in the order of the numbers in figure 9.

Distribution of pressure in quarries

Modern quarries permit a concentration of pressure in corners, and this unequal distribution can be 

corrected only by development of bowl-shaped quarries 

(fig. 10). Such an extreme shape is clearly impractical 

for many reasons, notably the shape of present openings, 

the distribution of good stone, and maximum utilization 

of hoist capacity, -It requires that a bowl shape, once 

attained, must be preserved by an equal rate of deepening 

of all parts of the quarry as operations proceed. Although a bowl shape can only be considered as ideal, 

the principle involved should be followed wherever there 

is a choice of two or more locations for a new operation 

within a quarry. Overdeepening of large holes and deep 

narrow cuts with vertical walls like the northeast 

end of the Rock of Ages quarry (pi. 1A), should 

be avoided whenever possible. Where it is necessary 

to make such holes or "cuts, an attempt should be 

made to keep the bottom in a series of steps about 

12 feet high and 25 feet wide with the lowest ,step 

or level in the middle of the cut. It is doubtful, 

however, that even this procedure would have prevented all the cracks shown in plate 1A; the cut is 

too narrow and the large, high rift face too long 

and straight. A broad flat bottom will cause extreme wedging of tolls in channels, and new cuts 

into the floor of the quarry will generally permit 

rock-bursts.

8 A keyway is the first cut from a lower bench into higher 

beach, and frees the second side of the two blocks it separates. 

See opening behind Derrick 5A in plate IE.
FLOOR PLAN

x«.x -x x vs i z'x.

Figure 7. Distortion in rectangular quarry in unsheeted granite. Section in (B/ is drawn along line W'X' of (A), 

and section in (C) along line Y'Z1 . Dashed lines represent original position of rock that now forms walls of 

quarry. Amount of distortion greatly exaggerated.

12
Figure 8. Section showing cracks next to keyway in steeply sloping sheet (bed). Large arrows show direction in

which maximum pressure is exerted.

13
SECTION

Keyway

............ I

Figure 9. Diagram comparing present method of cutting keyway with recommended procedure. Solid line shows 

* a keyway as cut at present. Numbers in blocks bounded by dotted lines give order of cutting blocks in 

recommended procedure.

Figure 10. Plan and section of ideal bowl-shaped quarry. Bowl shape is preserved by quarrying in benches and 

avoiding long straight faces. Quarry is 80 to 100 feet deep.

14
In addition to reducing rock-bursts, the ideal 

bowl-shaped quarry or its modifications would present 

fewer difficulties of closing of channels by lateral pressure.

Sloping sheet fractures

In figure 8, the cracks to the right of the keyway 

have destroyed some stone, but those to the left are 

more or less parallel to the "lift", and little if any 

stone is lost. It is immediately apparent that if the 

keyway had been cut next to the main quarry wall on 

the right, most of the cracks that formed during quarrying would be useful rather than destructive. Use of 

this principle can be made where the sheeting slopessteeply at the Whetmore & Morse and E. L. Smith 

quarries.

Quantitative measurement of expansion

The maximum size of free faces and the optimum 

slope and curvature of quarry walls to reduce rock-bursts

to a minimum are functions of (1) the amount a unit 

of granite expands upon being freed (this may vary 

with depth and direction), (2) the size of the mass of 

granite that contributes to displacement at a given 

point, (3) the forces that resist expansion, particularly the rigidity of large masses, and the friction 

between beds and the interlocking of beds, (4) the 

ratio of vertical to lateral (bedding plane) expansion, 

and (5) time. All these factors except the first are 

interdependent. Vertical expansion is resisted by 

bending strength, but the bending caused solely by vertical expansion is probably far less than that necessary 

to rupture the rocks except possibly at the base of high 

vertical quarry walls.

The relative importance and quantitative values 

of these various factors are not known, and can be 

found only by careful and detailed measurement of the 

movements that take place as a block is quarried. 

Such measurements should be made over an area of 

several hundred feet under different quarrying circumstances, and for as long a time as is necessary 

to make significant observations.

15 IKT.-DW. an.. »».. D.C.««TM