slope stability determination methods may be divided into three broad categories

8/12/2019 Slope Stability Determination Methods May Be Divided Into Three Broad Categories

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Slope stability determination methods may be divided into three broad categories: i) Methods

suitable for slopes in soil like materials, where the strength of the material can be determined

from testing small specimens of the material in a laboratory. Classical soil mechanics slope stability

analysis and material strength testing methods are applicable. ii) Methods suitable for slopes in

hard jointed rock where the stability of the slopes is controlled by the discontinuities in the rock

material. Slope stability is evaluated using traditional techniques such as joint surveys to

determine the orientation, continuity spacing and strength properties of the jointing and some

laboratory strength testing on representative joints and gouge materials. Because of its high

strength, little failure occurs through intact rock material. The potential for failure is dependent on

the presence, orientation and strength along joints. iii) Methods suitable for weak rock masses,

where failure can occur both through the rock mass, as a result of a combination of macro and

micro jointing, and through the weak rock material. Determination of the strength of this category

of rock mass is extraordinarily difficult since the size of representative specimens are too large for

laboratory testing and the combinations of micro and macro jointing, rock alteration and hard and

soft zones is too complex for detailed determination and analyses. Strength estimation for such

rock masses is usually based on some form of classification technique.

This paper is concerned largely with the description of a classification for weak rock masses, which

has evolved on projects undertaken by the consulting firm Steffen Robertson & Kirsten (SRK). Back

analysis of failed slopes involving rock mass materials which have been described by the SRK

classification has enabled rock mass strengths to be assigned to the various class intervals.

FACTORS CONTROLLING THE STRENGTH OF WEAK ROCK MASSES

For the purposes of this paper a weak rock mass is defined as any rock mass in which the effective

shear strength parameters are less than:

effective cohesion, c' - 25 psi (0.2 Mpa) effective friction angle, 0'30

Rock masses, with such low strengths, may occur as a result of a number of independent factors as

illustrated in Figure 2.

i) Weak rock material (Figure 2.(i)

Where the sole reason for a weak rock mass strength is the low strength of the rock material it is

more properly classified as having a soil like strength. An appropriate classification for rock and

soil, based on simple tests, from which an estimate can be made of the uniaxial compressive

strength is given in Table 1 (Jennings and Robertson, 1969). Extremely weak "rocks", or more

properly materials with a soil strength but a rock appearance, can be tested in the laboratory to

determine the shear strength. Such material may be referred to as either soils or rocks but use of

the strength qualifies SI to SS, as indicated on Table 1, serves to indicate that their strength falls

into the soil classification.


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ii) Intense Jointing (Figure 2(ii)) Where the jointing is sufficiently intense and has a sufficient range

of orientations, a stepped failure surface can develop through the mass, at any orientation, with a

resultant low shear strength. Thus while the jointing may be non-isotropic, the shear strength may

be considerably less so. The spacing between joints may be sufficiently large that it is impractical

to select a laboratory scale specimen which is representative. The difficulty of cutting and

preparing a specimen without disturbing the joints and effecting the shear strength of the

specimen is an additional impediment to such testing.

iii) Rotational Freedom (Figure 2(iii)) This form of failure may be illustrated by the failure surface

that would form through a gravel pile. The individual rock fragments, which would interlock if they

were prevented from rotating, have rotational freedom resulting from the loose packing of the

gravel particles. With a high rationality freedom (such as exhibited by even sized marbles in a pile)

very low shear strengths result. Rotational freedom increases if the intact rock fragments are

equidimensional, rounded and within a matrix of either voids or a very low shear strength (soft)

material. An example of a rock mass with very high rotational freedom would be spherically

weathered dolerite boulders in a matrix of soft (in-situ weather dolerite) clay. Again it is difficult tocut, prepare and test laboratory sized specimens for strength testing. Most often, weak rock

masses occur as a result of a combination of each of (i) to (iii) above. Typically a rock material may

have weathered to produce a soft rock (R2) or very shift rock (RI) rock material between closely

spaced fractures along which more extensive weathering has occurred to produce zones of S3

to 55 strength material. The failure surface is a complex combination of failure through soft intact

rock, along weak joints and through very soft (soil like) weathered zones which squeeze and

deform to allow harder rock blocks to rotate. Figure 3 is a typical example.

CLASSIFICATION OF WEAK ROCK MASSES

Bieniawski, 1974, introduced a geomechanics classification of rock masses for application to

tunnelling. This classification system, shown in Table 2, has been widely adopted as the CSIR rock

mass classification system. In this classification system the rock mass is assigned rating points for

five factors and the resulting total is termed the Rock Mass Rating or RMR. Hoek and Brown, ]980,

correlated the RMR values with rock mass strength as determined from back analysis of stress

condition surrounding underground failed and unfailed openings. Strength estimation using the

Hoek and Brown correlation is referred to as the MS system. Robertson, Olsen and Pierce, 1987,

applied the RMR and MS rating and strength estimation systems to the rock mass in the pit walls

of the Island Copper mine in British Columbia. They performed back analyses of pit slope failures

and found that for weak rock masses, this method provides a very poor and inaccurate strengthestimate. They proposed a modified rating system, termed the Island Copper Rock Mass Rating

(]LC-RMR} in which the rating method was modified for RMR values less than 40. By correlating

the ILC-RMR with rock strengths determined from back analyses of failed slopes they developed a

method of rock mass strength estimation for the very weak rock mass zones at Island Copper

mine. The method of RMR rating values assignment used in the ILC-RMR system was entirely

different from that used in Table 2. There is consistent for both weak and strong rock masses. To


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this end the author proposed the SRK Geomechanics Classification of Rock Masses as described in

Table 3. Since this rating system was developed specifically for rock mass strength estimation for

rock slope stability analyses, the resulting rating is referred to as

No. Description

SI VERY SOFT SOIL - easily moulded with fingers, shows distinct heel marks.

S2 SOFT SOIL - moulds with strong pressure from fingers, shows faint heel marks.

S3 FIRM SOIL - very difficult to mould with fingers, indented with finger nail, difficult to cut with

hand spade.

S4 STIFF SOIL - cannot be moulded with fingers, cannot be cut with hand spade, requires hand

picking for excavation. S5 VERY STIFF SOIL - very tough, difficult to move with hand pick,

pneumatic spade required for excavation

RI VERY WEAK ROCK - crumbles under sharp blows with geological pick point, can be cut with

pocket knife.

R2 MODERATELY WEAK ROCK - shallow cuts or scraping with pocket knife with difficulty, pick point

indents deeply with firm blow.

MODERATELY STRONG ROCK - knife cannot be used to scrape or peel surface, shallow indentation

under firm blow from pick point. STRONG ROCK - hand-held sample breaks with one firm blow

from hammer end of geological pick. VERY STRONG ROCK - requires many blows from geological

pick to break intact sample.

the Slope Rock Mass Rating or SRMR. The SRK-RMR system has been checked at the Island Copper

mine and has been found to yield rating values similar to those obtained using the ILC-RMR

system. Consequently the strength/rating correlation found by Robertson, Olsen and Pierce, 1987,

is directly applicable to the SRK-RMR system, i.e., to the SRMR values. The SRK-RMR system was

also applied for rock mass classification at the Getchell Mine, Nevada, and a strength correlation

performed, similar to that at Island Copper Mine, using a combination of back analyses of failed

slopes and laboratory strength test results. Table 4 summarizes the results of the strength/rating

correlation found for the weak rock masses at these mines.

BASIS FOR SRK GEOMECHANICS CLASSIFICATION OF ROCK MASSES

The difference between the CSIR and SRK classification systems can be seen in a comparison of

Tables 2 and 3. (i) Rating for Groundwater The amount of water present in the rock mass does not

influence the rock mass strength. It is a destabilizing force, and should be accounted for as such in

any stability analysis. The groundwater parameter is therefore dropped in SRK-RMR system. To

maintain the validity of the CSIRRMR correlation for stronger rock masses, the maximum rating

value for the parameter (15) has been added to parameter I (strength of intact rock). This results

in the rating assignment of the highest intact rock


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strength class increasing from 15 to 30. (ii) Rating for Intact Rock Strength Apart from the

increased rating value resulting from the groundwater modification the rating for this parameter is

unchanged for rock of hardness RI (refer Table 1) or greater. Additional classes and ratings have

been added for materials in the soil strength range S5 to Si. This allows the effects of very weak,

soil like, materials to be included in the rock mass rating.

(iii) Rating for RQD The conventional RQD is replaced by a new parameter; the Handled ROD

(HRQD). The HRQD is measured in the same way as the RQD after the core has been firmly

handled and "worried" in an attempt to break the core into smaller fragments. During "handling"

the core is firmly twisted and bent but without substantially force or use of any tools or

instruments. RI rock core, without planes of weaknesses, will not break under such handling.

The adoption of HRQD allows account to be taken of weakly cemented joints. It also prevents the

assignment of large rating values for continuous core in soft soil like materials, clays for example.

(iv) Rating for Spacing of Discontinuities As for the previous parameters, the discontinuity spacing

is determined from the "handled" core. For core with sticks of RI or harder material and no weakly

cemented joints there is little difference between the handled

and unhandled values. The rating for handled core reduces where sticks of soils like hardness can

be broken up by hand.

(v) Rating for Condition of Discontinuities This rating is unchanged except that all core with a rock

material hardness of less than RI is assigned to the lowest category with a rating of zero. This

prevents the assignment of high rating values to extremely weak rock material (S5 say} with clean

closed rough joints. Rock material with a hardness of R] has a mximum rating of 10.

METHOD OF APPLICATION OF SRK-RMR FOR ROCK SLOPE STRENGTH ESTIMATION

Core taken from very weak rock masses tend to be very variable as illustrated by the typical core

sample on Figure 3. Portions of a rock core may have SRMR ratings of 50 or higher and be

immediately adjacent to portions having ratings of 20 and lower. A method of logging and analysis

is required to determine "average' rock mass strength but which distinguishes substantial zones of

weaker mass from zones of stronger mass, as, for example, in the cored borehole illustrated in

Figure 4. The practice used by Steffen Robertson and Kirsten is to assign a SRMR to each one foot

interval of the core. This assignment can be done rapidly once representative sections of the core

have been SRK-RMR rated in detail, and the core logger has developed a set of 'reference' ratings.

A profile of rock mass rating values is then prepared by calculating the moving average of theSRMR values for a 20 section of the core. A typical profile is illustrated in Figure 5. Also shown on

this figure is the standard deviation of the SRMR values determined for each 20 foot string of

values used to calculated the moving average.

A failure surface passing through a weak rock mass will tend to be selective and pass preferentially

though material which is weaker than the average encountered in any borehole core. This

selective process is illustrated in Figure 3. Thus it is appropriate to use an SRMR value, less than


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the average value, to be representative of the material along the failure surface. The practice

developed at SRK is to subtract one standard deviation of the rating values from the mean. This

value of the SRMR is also shown on Figure 5. The significance of this reduced value of the SRMR is

that approximately 86% of core encountered in the 20 foot sample length will have rating values

greater than the reduced value. Using the reduced value SRMR profile, the rock mass can be

divided into zones of stronger and weaker rock mass strength, as illustrated in Figure 4. Where the

rating is 40 or greater it is anticipated that slope stability will be determined by the orientation and

strength along discontinuities. Where the rating is less than 30 failure may occur through the rock

mass at any orientation. For such weak rock zones the rock mass strength is estimated from a

rating/strength correlation as is illustrated by the results on Table 4. Until considerably more case

histories of such correlations have been performed it is premature to accept the values in Table 4

as typical of all weak rock masses.

slope stability determination methods may be divided into three broad categories

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