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A Fundamental Approach to Slope Stability Problems in Hong Kong

D. J. Sweeney & P. K. RobertsonFugro (Hong Kong) Limited

There are many types of slope in Hong Kong and a simple classification is shown in figure 1. It is perhaps an indication of the complexity of the slope stability problem in Hong Kong that this paper deals only with slope type 2a - cut slopes in residual soil. However, many of the concepts discussed will be applicable to natural slopes in residual soil and to the design of new cut slopes, both permanent and temporary.

It is not the intention of this paper to introduce new concepts in geotechnical engineering, but to show how considerable progress in slope stability investigations in Hong Kong may be made by application of established geotechnical principles that are both practical and economic and which may easily be used by an experienced geotechnical engineer.

A large number of steep, high cut slopes in residual soil exist in Hong Kong and many failures of such slopes have occurred, almost invariably during heavy rainfall. As a result, the stability of many cut slopes is now in question. Very often, inadequate investigation and analysis of such slopes is made and unacceptably low factors of safety are obtained. (The minimum acceptable factors of safety in a situation where the consequences of failure are severe are presently 1.4 and 1.12 for rainstorm conditions with 10 year and 1,000 year return periods respectively.) Consideration is then given to design of remedial works and some of the more usual solutions are as follows: 1 - Cut back slope to a flatter angle. 2 - Install horizontal drain holes. 3 - Install permanent ground anchors. 4 - Construct retaining structure in front of slope.

Unfortunately, use of these methods is often severely hampered by lot usage and lot boundary restrictions, and even if such methods are feasible, cost is likely to be high. It is therefore often possible to make a strong case for carrying out a detailed and careful site investigation and stability analysis to determine if there really is a significant risk of slope failure. Even if remedial works are found to be necessary, a detailed investigation and stability analysis will almost certainly result in significant economies in remedial works design compared with a design based on conservative assumptions necessitated by inadequate data.

Figure 1. Classification of slopes in Hong Kong

The most important factors affecting slope stability are: 1 - Slope height and steepness. 2 - Any imposed loads. 3 - Geology. 4 - Soil strength in terms of effective stress shear

strength parameters.5 - Soil suction. 6 - Groundwater behaviour. 7 - Acceptable factor of safety. 8 - Surface protection to slope.

Discussion will concentrate on items 3 to 7, though some interesting concepts relating to slope height and steepness will emerge.

GEOLOGY

Almost always, the residual soils in Hong Kong exhibit an increase in strength with depth. This is

CLASSIFICATION OF SLOPES IN HONG KONG

1. NATURAL SLOPES

2. CUT SLOPES

3. FILL SLOPES

4. COMPOSITE SLOPES

CATEGORY 2A

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Figure 2.

Figure 3.

Figure 4.

c' = 5 kPaφ' = 35°

c' = 10 kPaφ' = 40°20 m

BOREHOLE

BEDROCKc' = 5 kPa F = 1.08φ' = 35° }

σ1 - σ32

σ1 + σ32

σ1 - σ32

σ1 + σ32

Q

Q

P

P

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d

c' = d/cosφ'φ = sin-1 tanψ'

range of shear strengths used in stability analysis

stress range for most slope stability problems

usual stress range for triaxial testing

actual shear strength in low stress range

stress range for most slope stability problems

usual stress range for triaxial testing

ψ'

F = 1.4

obvious from a study of standard penetration test results, blowcounts for driving sample tubes, voids ratio measurements, visual examination of soil samples or exposures and so on. A site investigation which does not define the strength variations at a site must be regarded as inadequate and a stability analysis which does not take account of these strength variations must also be inadequate.

It is quite common to see soil strength data in terms of effective stresses plotted on the well known p - q diagram or modified Mohr envelope, with a large amount of scatter of the results about the best fit strength envelope. Much of this scatter is almost certainly caused by irrational selection and grouping of samples for testing. For example, a set of samples may all be described as completely weathered granite, but standard penetration tests carried out immediately below the samples may give N values varying between, say, 5 and 300. If the strengths of all the samples in this set are plotted on the same p - q diagram, it is hardly surprising that there will be a large amount of scatter.

Selection of samples for effective stress shear strength testing, and for grouping on p - q diagrams should be carried out logically based on the strength indicators mentioned. Two of the most reliable indicators are standard penetration test N values (if the tests are carried out in a consistent manner) and visual examination - the ‘thumb test’ is still one of the most reliable in geotechnical engineering. In adopting this procedure the authors have usually found that scatter on p - q diagrams is much reduced and typical results are presented in this paper.

In general it is found that shear strength increases with depth. A very simple example of how this can affect a slope stability analysis is shown in figure 2. Good borehole cover and frequent sampling andeffective stress shear strength testing are required todefine the strength zones shown. Definition of thesezones is obviously essential if the safety of the slopeis to be realistically assessed. Consider, for example,the failure surface shown. If c' = 5 kPa and ø' = 35degrees are used along the entire slip surface, thefactor of safety is less than 1.1. However, for the actualconditions shown, factor of safety is 1.4.

Geology also affects groundwater behaviour, and data will be presented which shows that groundwater behaviour is strongly influenced by bedrock topography. The site investigation must define both bedrock topography and the position of the groundwater table. For groundwater, the minimum requirement is to determine the position of the dry season groundwater table and this is very often found to be close to bedrock surface. In addition, it is usually desirable and often essential to have some data on the response of the groundwater table to heavy rainfall.

It is important to stress that weathering profiles in the Hong Kong residual soils are often erratic and if important decisions regarding slope stability are to be made based on the definition of strength zones, the borehole cover must be adequate and interpretation must be made by an experienced geotechnical engineer.

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BASIC CONCEPTS

It is generally assumed by designers that the shear strength of the Hong Kong residual soils may be expressed in terms of effective stress shear strength parameters and that the soils are partly saturated during the dry season. Also it is generally stated that soil suctions resulting from partial saturation increase the effective inter-granular stresses in the soil and hence contribute to slope stability. It has been suggested that the stability of many soil slopes in Hong Kong depends wholly upon the maintenance of suction. However, for every slope that has failed, there are many more still standing and some have stood for 40 years or more. Many are not well protected against ‘infiltration of rainwater, and during successive heavy rainstorms it is likely that suctions fall to quite low values. This must be especially so in coarse grained decomposed granite soils - yet the slopes still stand.

Perhaps there has been a tendency to underestimate the basic shear strength of the Hong Kong residual soils. Basic shear strength is defined here in terms of effective stress shear strength parameters, c' and ø', which may be measured by testing fully saturated samples. Perhaps also traditional methods of slope stability analysis are over-simplified and err on the conservative side.

It is likely to be some time before practical techniques are developed and commonly used for making reliable in-situ measurements of suctions and their variation with time, and before sufficient data is available for suctions to be used with confidence in geotechnical analysis and design. Furthermore, it may

be that research will eventually show that only quite low suctions are admissible, particularly for the more coarse grained granitic soils.

As we require reliable evaluation of slope stability now, it is logical to develop detailed methods of investigation and analysis which will allow accurate assessment of slope stability using basic shear strength, and which take little or no account of soil suction. If we discount suction, we know that in most cases our analysis will be conservative.

It is common practice in Hong Kong to determine effective stress shear strength parameters in the consolidated drained triaxial compression test using effective confining pressures which are much higher than those existing in-situ along potential failure surfaces. Effective cohesion c' is determined by linearly extrapolating a considerable distance back to the ordinate, as illustrated in figure 3. For medium dense soils, c' may be of the order of 20 to 30 kPa (see, for example, figure 6) and for dense soils much higher ø' is usually found to be in the range 30 to 35 degrees. However, because of the usual scatter of results and the uncertainty involved in extrapolating back to the ordinate, it is usual to reduce c' to only about half of its original value or even to take c' = 0.

The resulting range of shear strengths used in most slope stability analyses is indicated in figure 3. This approach does not accurately define the shear strength of soils in the low stress range appropriate to most slope stability problems and may lead to excessively conservative design. On the other hand, some designers use the full value of c' obtained from linear

Figure 5. Grain size distribution

SITE 1

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extrapolation to the ordinate. This is equally incorrect and may lead to dangerous design.

In order to avoid these problems the authors carry out triaxial tests at quite low effective confining pressures which are representative of those existing along the relatively shallow failure surfaces that are usually critical. In selecting stress levels for testing, account must be taken of the stress path during testing which, in a drained test, travels upwards and away from the origin at 45 degrees. It has been found that the shear strength envelope curves downwards at low stress levels, as indicated in figure 4. This results in increased ø' values; c' is of course reduced but, because long extrapolation back to the ordinate is not required, values of c' measured may be regarded as reliable. The comparison between the actual curved low stress strength envelope and assumed design envelopes is illustrated in figure 4.

SHEAR STRENGTH

The main soil types that form slopes in Hong Kong are derived from the decomposition of granitic and volcanic rock. These residual soils are usually granular with predominant particle sizes lying in the sand/silt range. In general, percentages of clay size particles are small though volcanic soils with high clay contents are encountered.

There are two principal components in the shear strength of Hong Kong’s residual soils. These are inter-granular bonding and frictional strength. At depth the fresh rock is strong with high inter-granular bonding. Towards the ground surface, the rocks become increasingly weathered and inter-granular bonding decreases. Because of the original discontinuous structure of the rock mass, bonding also varies laterally, with decreased bonding close to relic joints. This variation in bonding may contribute to the scatter seen in laboratory strength data.

The inter-granular bonding is thought to be a chemical bonding created during the formation of the rock. This bonding or ‘cohesion’ should not be confused with the cohesion exhibited by some clays. Clays derive their cohesive strength principally from overconsolidation, with Van Der Waal’s forces playing an important role in the bonding of the compressed clay structure.

Frictional strength is derived from the resistance generated at mineral to mineral contacts and from interlocking of the grains. The denser the material the greater the interlocking. However, the effect of interlocking decreases with increasing confining stress as particles become abraded at contact points and particles break. This is particularly true for the angular quartz grains and friable feldspars occurring in Hong Kong. Almost certainly, this effect contributes to the observed curvature of the Mohr envelope where

Figure 6. Figure 7.

Figure 8. Figure 9.

300

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BASE ENVELOPE

φ' = 34°c' = 20 kPa

φ' = 37°c' = 12 kPa

BASE ENVELOPE

BASE ENVELOPE

MULTI-STAGE TEST RESULTSCOMPLETELY WEATHERED GRANITE14 samplesSPT-N 8 to 47

SITE 1

SITE 1

1st STAGE RESULTSSTRESSES ≤ INSITU CONDITIONS

Effective Normal Stress, σ-, kPa

φ' = 37°c' = 12 kPa

φ' = 34°c' = 20 kPa

Effe

ctiv

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ear

Stre

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, kPa SITE 1

MOHR - ENVELOPE

SITE 1

1st STAGE RESULTSIN-SITU STRESS CONDITION

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friction angle, ø', decreases with increasing confining stress.

Because of the important frictional component of shear strength in Hong Kong’s residual soils, critical failure surfaces in slopes are usually quite shallow and stress levels on potential failure surfaces are correspondingly low. Therefore, shear strength parameters c' and ø' must be defined within the appropriate low stress range. There are difficulties associated with testing samples at low stress levels and tests must be carried out with great care.

LABORATORY TEST RESULTS

This section discusses various problems in making laboratory measurements of effective stress shear strength parameters which will represent the field behaviour of residual soils. Triaxial test data from two sites are presented by way of illustration.

Samples from both sites were of decomposed granite, and typical grading curves are given in figures 5 and 14. Most samples were taken using a specially developed open drive sampler with a thin metal liner to produce a 64 mm diameter sample. For comparative purposes, some Mazier (triple tube core barrel) samples were also taken. Mazier samples were either 65 mm or 75 mm in diameter. In most tests, samples were saturated under back pressure prior to consolidation and shearing. However, a few samples were obtained from boreholes made using dry percussion techniques and tested at natural moisture content in order to investigate the effect of partial saturation on test results.

All tests were carried out in the laboratory of Fugro (Hong Kong) Limited, except for tests with internal load cells which were carried out in Fugro’s UK laboratory. All test carried out were multi-stage consolidated drained triaxial compression tests and test procedures are described in Appendix A. This test defines the effective stress shear strength envelope directly, without the requirement of measuring pore water pressure. Much of the residual soil deposits in Hong Kong are medium dense or dense and such soils dilate during drained shearing with failure tending to concentrate along a single shear plane. This is significant in fully drained multi-stage testing as the subsequent stages will tend to shear along the same prefailed surface. A further problem is that for a bonded material, multi-stage testing may progressively destroy the bonding, such that second and third stage tests produce unrepresentatively low shear strengths. By using good testing techniques, as outlined in Appendix A, these problems can be largely eliminated.

An alternative to consolidated drained testing, is consolidated undrained testing with pore water pressure measurement, but it is difficult to achieve accurate results at low stress levels with conventional

null-indicator pore water pressure measuring apparatus. However, electrical pressure transducers are now available and this makes accurate low stress level consolidated undrained triaxial tests with pore water pressure measurement feasible. In this type of test, failure does not tend to concentrate on a single failure plane, and so an improvement in the accuracy of multi-stage testing may be achieved compared with consolidated drained testing.

Figure 6 shows the results of 14 multi-stage tests on completely weathered granite plotted on a p-q diagram. Samples for testing were selected from several boreholes made in a large cut slope on Hong Kong Island. Standard penetration test N values determined immediately below each sample varied from 8 to 47. Scatter of results is quite small particularly at low stress levels. The plot has been

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divided into two ranges, a low stress range of less than 150 kPa and a high stress range of greater than 150 kPa. Best fit straight line envelopes through each range are shown along with the corresponding values of c' and ø'. The results indicate a slightly curved envelope. This curvature is shown more clearly on the unmodified Mohr envelope in figure 7. It is of interest to note that the break in the envelope is located closer to 100 kPa on this plot.

It should be noted that a small but significant cohesion intercept has been defined. As there may be some loss in accuracy in the instruments for measuring effective confining pressure at low stress levels, the reliability of such a result must be investigated. In order to achieve this, the authors have carried out many tests at essentially zero effective confining pressure. In these tests, the cell pressure and back pressure supply are connected to the same pressure supply regulator, so that effective confining pressure is known to be essentially zero. Samples tested under these conditions have been found to have significant compressive strengths, compatible with the cohesion intercepts determined from low stress level testing. Several of the test results presented in this paper were carried out at zero effective confining pressure.

In order to investigate the many variables associated with the testing, various sets of data were isolated and replotted on the best fit strength envelope obtained from all the test results, which is referred to as the base envelope.

Figure 8 shows the results of all first stage tests carried out at confining stresses comparable to in-situ stresses. These results lie close to the base envelope and indicate that the multi-stage technique has been successfully used to define the true effective stress strength envelope. Figure 9 shows the results of all first stage tests carried out at confining stresses lower than and equal to in-situ stresses. Once again. these lie close to the base envelope, indicating that the effect of overconsolidation resulting from testing at stresses lower than those existing in-situ is not significant. This is probably a consequence of the granular nature of the soils tested. For fine grained soils with significant clay content, such as some volcanic soils, overconsolidation may cause a slight upward curvature of the strength envelope at low stress levels. However, caution should be exercised in taking advantage of such additional strength in design as the soil in-situ may not be overconsolidated to the same degree.

In order to investigate the effect of ram friction on strengths measured at low stress levels, a set of tests was carried out using internal load cells and pressure transducers. Results are presented in figure 10 and lie close to the base envelope indicating that ram friction does not significantly influence the results of tests carried out without internal load cells, provided that good testing techniques are used.

Some workers (for example Lumb 1975) have suggested that samples should be tested in the

laboratory at their in-situ degree of saturation so that apparent cohesion due to negative pore pressure, or suction, can be measured. Figure 11 shows the results of two multi-stage triaxial tests carried out on samples obtained from boreholes made using dry percussion techniques and tested partially saturated at their natural moisture contents. These results show no significant departure from the base envelope obtained from testing fully saturated samples. A possible explanation is that any suctions in the samples had an insignificant effect on shear strength. Alternatively, volume changes in the sample during consolidation and shearing may have reduced the suctions to a level where their effect on shear strength was insignificant; however, this would only have occurred if initial suctions were small.

Figure 13.

Finally, figure 12 shows the results of tests carried out on Mazier samples. Results lie close to the base envelope suggesting that sample disturbance caused by Mazier sampling was comparable with that caused by good quality open drive sampling.

A series of tests was also carried out on samples of completely to highly weathered granite obtained from the same site but generally at greater depth. N values measured immediately below these samples ranged from 150 to 300. Test results are presented in figure 13 and the increase in strength for deeper samples is clearly shown. The additional strength is almost certainly due to the higher inter-granular bonding and the denser packing of particles. The average grading of these samples was slightly coarser than for the overlying completely weathered granite.

Figures 15 to 17, show results or a testing programme carried out for a site in the New Territories. A similar pattern of results has been obtained.

GROUNDWATER

The importance of rising groundwater table in slope stability problems is widely appreciated but prediction of rises under the influence of Hong Kong’s severe rainstorms is difficult. In most site investigation

BASE ENVELOPE FOR CWG

φ' = 41°c' = 26 kPa

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MULTI-STAGE TESTS RESULTS COMPLETELY TO HIGHLY WEATHERED GRANITE

SITE 1

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reports, either there is a complete absence of reliable groundwater data, or dry season conditions only are given. In these circumstances it is necessary to guess or attempt to predict the groundwater rise under severe rainstorm conditions.

A common method devised by Beattie and Chau (l976) and frequently used by designers in Hong Kong is to consult charts relating thickness of wetting band to ground permeability for rainstorms of different statistical return periods. (At present slopes in Hong Kong are required to satisfy specific factor of safety requirements for both 10 year and 1,000 year return periods.) The assumption is then made that the wetting band descends until it meets the groundwater table and produces a rise in groundwater table equal to the thickness of the wetting band. The wetting band approach cannot be regarded as satisfactory as it does not take account of either complex rainstorms patterns or geological conditions.

A rainstorm with a return period, of say, 1000 years may include periods of extremely high intensity rainfall of short duration or periods of relatively low intensity rainfall of long duration. In the case of the short duration rainstorm, the rainfall intensity will probably be much greater than the capacity of the ground to accept infiltration and much of the rainwater will run off. The consequent rise in groundwater table may be relatively small. On the other hand, a long duration storm may have an intensity approximately equal to the infiltration capacity of the ground and run-off will be small. This condition will probably lead to the highest groundwater response. Several

other factors also affect surface infiltration such as topography, vegetation and topsoil permeability.

Bedrock topography has a strong influence on rise in groundwater table. Numerous permeability tests have indicated that Hong Kong’s residual soils generally have a permeability one or two orders of magnitude greater than that of the underlying bedrock. (Occasionally, a zone of bedrock with open joints is encountered resulting in higher permeability than the overlying soil, but the effect of this is usually localised and is unlikely to have an important influence on groundwater behaviour.) During the dry season, groundwater is often observed at or below bedrock surface. The storage capacity of the rock mass will be equal to the volume of the rock mass discontinuities and this will usually be quite small.

On the other hand the storage capacity of the overlying soil approaches the volume of air voids which may be as high as 10 or 15 per cent of the total soil volume. During heavy rainfall, water descending to bedrock will rapidly fill the small air space in the rock mass discontinuities, and the groundwater table will rise rapidly to bedrock surface, Above bedrock surface, the rise in groundwater table will be strongly influenced by the relatively higher air voids volume in the soil, and rate of rise in groundwater table will be much smaller. The importance of this phenomenon is illustrated by the following example.

Three piezometers were installed in an extensive cut slope in the Peak district of Hong Kong Island.The piezometers were monitored for almost two years and results are presented in figure 18. During Tropical

Figure 14.

SITE 2

B. S. STANDARD SIEVE (mm) HYDROMETER

% R

ETAIN

ED

PRO

JECT

GR

AIN

SIZE DISTR

IBU

TION

% PA

SSING

GRAIN SIZE IN MILLIMETERS

COBBLECOARSE MEDIUM FINE COARSE MEDIUM FINE COARSE MEDIUM FINE

GRAVEL SAND SILT CLAY

SYMBOL BORING SAMPLE DEPTH DESCRIPTION

Brownish yellow, dense, silty SAND with some finegravel (C. W. G.)

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Storm Ellen in August 1976, the groundwater level in two of the piezometers rose by about 12 m whereas in the third piezometer installed a short distance away, groundwater rose by only about 3 m. A study of the borehole records shows that for the two piezometers exhibiting high response, dry season groundwater level was well below bedrock surface and most of the groundwater rise occurred in bedrock. On the other hand, for the third piezometer, the dry season groundwater level was approximately at bedrock surface and most of the groundwater rise occurred in soil. The pattern of groundwater behaviour described above has been observed at several other sites in Hong Kong.

It is clear that successful prediction of groundwater rise during rainstorms will only become possible when geological and hydrogeological influences are more fully understood and when records of comprehensive groundwater monitoring in a wide variety of situations are available.

FACTOR OF SAFETY

Selection of acceptable factor of safety is one of the most important aspects of a slope stability study. Present requirements in Hong Kong for a high risk slope are a factor of safely of 1.4 during a rainstorm with a return period of 10 years and 1.12 during a rainstorm with a return period of 1,000 years.

Though there is a clear need for guidance in Hong Kong for selection of factor of safety, it is not entirely logical to work to a system which does not take adequate account of the nature of the slope stability investigation - for example, the degree to which the geology and groundwater regime have been defined and understood, or the quality and type of laboratory testing to define effective stress shear strength parameters.

Most slope stability analyses carried out in Hong Kong are over-simplified and there is undoubtedly a difference between calculated factor of safety and actual factor of safety. The more important factors which may influence the difference are discussed below. Factors tending to produce conservative analysis are:

a - Plane strain effects

The pattern to strain in most slope failures is close to plane strain, whereas, mainly for convenience, soil strength is almost always measured in the triaxial apparatus where the pattern of strain is quite different. Research has shown that soil strengths measured in plane strain apparatus are generally significantly greater than strengths measured in triaxial apparatus, especially for dense sandy soils at low stress levels. A summary of a large number of plane strain tests is given by Lee (1970).

b - Sample disturbance

It is difficult to assess the effect of sample disturbance on soil strength measured in the laboratory. However, the strength of Hong Kong residual soils is strongly influenced by structure and, even if sampling is carried out with extreme care, it is reasonable to expect that the sample will suffer some loss in strength. A possible exception is the case of very loose soils with poor structure which may be densified during sampling operations with a consequent increase in strength. There is a great need for comparative studies involving triaxial test specimens from borehole samples and triaxial test specimens trimmed from block samples taken in trial pits or trial shafts.

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c - Soil suction

Though soil suctions and their influence on soil strength have not yet been quantified, it is probable that soil suction does make a significant contribution to slope stability. The increase in soil strength due to suction is likely to be greater in the fine grained volcanic soils than in the coarser grained granitic soils. In many situations, there is no reason to expect a reduction in soil suction during rainfall - for example where there is extensive surface protection against infiltration and the groundwater table is well below the toe of the slope. Even when heavy infiltration does occur on an unprotected slope, it is unlikely that there is a complete loss of suction.

Factors tending to produce unconservative analysis are:

a - Relic joints

Large scale plane or wedge failures are rare in rock cuttings in the geological formations of Hong Kong, and it may be concluded that joint systems are generally favourable with respect to large scale instability. This must also apply to soil slopes derived from in-situ weathering of rock as the pattern of joints will remain the same. Large scale soil slope failures in which relic joints play a significant part are therefore likely to be rare. However, small scale instability in

rock cuts in Hong Kong is very common and relic jointing may significantly increase the danger of small slips in steep soil slopes cut at, say, 70 to 80 degrees.

Figure 18. Rainfall and groundwater records

Figure 19.

20 30 40 50 60 70 80 90

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Figure 4 Variation of Failure Probability with Factor of Safety (after Li and White 1987b)

b - Perched water table

Flow of water vertically downwards through a homogeneous soil will not create positive pore water pressure. However, the presence of relatively impermeable layers will interfere with the vertical flow and may result in a local build up of positive pore water pressure. Attempts to identify such a condition by drilling and permeability testing will almost certainly be unsuccessful. However, if such a condition is present in an existing cut this should be manifest by weep marks or burst chunam where perched water has flowed out of the slope in the past. As with relic joints, this type of perched water table is likely to be most important in relation to small scale instability in soil slopes, since perched water tables will probably not be very extensive.

The uncertainties in analysis described above raise an important question regarding soil suction. As mentioned earlier, many geotechnical engineers have suggested that the stability of a large number of soil slopes in Hong Kong depends wholly upon the maintenance of suction. This is because slope stability analyses using unrealistically low effective stress shear strength parameters yield factors of safety less than unity. When low stress level plane strain test results on high quality samples are available, the number of soil slopes in Hong Kong for which soil suction must be involved to demonstrate stability, may be much smaller than is presently thought.

An obvious question to ask at this stage is why do cut slopes in Hong Kong frequently fail during heavy rainfall? Figure 19 shows the relationship between slope angle, slope height and factor of safety for a cut slope in soil with shear strength c' = 10 kPa and ø' = 40 degrees. The water table is below the toe of the cut and no account has been taken of soil suction. The curves have been produced from the stability charts by Hoek and Bray (1974).

Consider, for example, a 30 m high cut slope at an average angle of 45 degrees. Even for fully saturated conditions (but water table below the toe of the cut) the factor of safety is greater than 1.1. For 40 m height, the factor of safety is only slightly less than 1.1. On the other hand, a slope only 10 m high but cut at 65 degrees has a factor of safety less than unity; for angles greater than 65 degrees, factor of safety decreases rapidly.

This suggests that a particularly dangerous category of slopes is those between, say, 10 m and 15 m high, and cut at angles greater than 60 or 65 degrees. Slopes in this category are most likely to be reliant on soil suction for their stability and are also most likely to be adversely affected by relic joints and perched water table effects. This reinforces the contention that such slopes will usually be the most troublesome and this is borne out by observation of old slip scars around Hong Kong.

For many slopes, the 10-year rainstorm criteria of a factor of safety of 1.4 is more difficult to achieve than the 1,000 year rainstorm case where only 1.12 is required. In some cases a smaller factor of safety for the 10 year storm condition may be acceptable and this should be at the discretion of the geotechnical engineer responsible for the slope. His judgement should take account of the factors discussed above.

RECOMMENDATIONS FOR FURTHERRESEARCH

Research into the stability of residual soil slopes in Hong Kong is at a very early stage. Some recommended lines of further research are as follows: 1 - Effective stress testing at low stress levels to

determine basic soil strength - some data has been presented in this paper but far more testing is required in order to develop a full understanding of the factors influencing effective stress shear strength parameters such as grading, density, fabric, structure, mineralogy, etc.

2 - A study of soil strength under plane strain conditions as triaxial testing may lead to significant underestimates of soil strength.

3 - A study of the effects of sample disturbance on soil strength.

4 - Suction measurements and the influence of suction on soil strength.

5 - Monitoring of groundwater levels in order to develop a better understanding of groundwater behaviour during and after heavy rainfall in a wide variety of situations.

REFERENCES

Slope failures in Hong Kong. Peter Lumb, Quarterly Journal of Engineering Geology, 1975, Vol. 8, pp 31 to 65.

The assessment o f l ands l ide po ten t ia l wi th recommendations for future research. A.A. Beattie and E.P.Y. Chau, the Journal of the Hong Kong Institution of Engineers, Vol. 4, No. 1, 1976,

Comparison of plane strain and triaxial tests on sand. Kenneth L. Lee, Journal of the Soil Mechanics and Foundations Division, Proceedings of the American Society of Civil Engineers, May 1970.

Rock Slope Engineering. E. Hoek and J. Bray, the Institution of Mining and Metallurgy, London 1974.

The Measurement of Soil Properties in the Triaxial Test. Alan W. Bishop and D.J. Henkel, Edward Arnold Ltd, London 1962.

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APPENDIX A

Details of laboratory testing

Standard Wykeham Farrance apparatus was used for all tests and a schematic drawing of the apparatus is shown in figure A1. Testing was generally in accordance with procedures described by Bishop and Henkel (1962). Prior to consolidation and shearing, samples were saturated in stages under back pressure keeping a small positive effective confining pressure at all times. Saturation was considered to have been achieved when B values of 0.95 or greater were obtained.

Following saturation, samples were consolidated under various confining pressures and volume changes were recorded. Rate of shearing was calculated in the conventional way from consolidation records. Samples were then sheared at constant rate of strain and volume changes were recorded. Satisfactory dissipation of excess pore water pressure during shear was checked by monitoring pore water pressure at the top of the sample.

For first and second stages of multi-stage tests, shearing was stopped at or slightly before failure.

With the exception of tests carried out with internal load cells, deviator stress was measured using a proving ring outside the triaxial cell. In order to reduce errors in measurement of deviator stress resulting from ram friction, the proving ring was set to zero when the ram was moving down before making contact with the top of the sample. Accuracy in measurement of low effective confining pressures was increased by using the same pressure gauge to measure both cell pressure and back pressure.

Tests on partially saturated samples were carried

out by placing samples on the triaxial apparatus pedestal without a porous stone and without water in the lines. Samples were consolidated and sheared with lines open to atmospheric pressure.

DISCUSSION

In the discussion it was pointed out that for drained triaxial compression tests carried out at zero effective confining pressure, the results when plotted on a p - q diagram would lie on a line drawn at 45 degrees through the origin. The authors agreed that this was so.

It was also suggested that progressive failure should be taken into account in slope stability analyses in Hong Kong. The authors commented that there may be some progressive failure effect in slopes with factors of safety close to unity, but otherwise the effect was unlikely to be of great importance.

Two speakers stressed the importance of relic joints with respect to slope stability. One said that he had observed a major movement on a relic sheeting joint, but that movement stopped before failure occurred. The authors expressed the belief that their discussion of relic joints in the paper gave a reasonable perspective to the importance of relic joints in slope stability problems in Hong Kong.

Two speakers also remarked on the importance of the micro-structure of Hong Kong soils on their behaviour.

One speaker suggested that it would be a good idea if someone were to instrument a slope and cause it to fail.

Another speaker said that he could not believe that slope stability analyses for Hong Kong slopes could be as simple as had been suggested in the paper.

Figure A1. Schematic layout of triaxial equipment

REGULATOR

0 - 1000 kPa BACK PRESSURE0 - 1000 kPa

REGULATOR

CELL PRESSURE VOLUME CHANGE

STRAIN CONTROLLED

AIR AIR

PORE PRESSURE0 - 1000 kPa

NULLINDICATION

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12

The authors replied that they did not think that the approach presented was simple, and that they had in fact drawn attention to several complex factors that required research.

Further explanation was also requested for the groundwater behaviour shown in the example, in particular the apparent constant groundwater levels which were recorded for a long period after Tropical Storm Ellen and the subsequent fall in early summer of 1977. The authors replied that the constant groundwater levels corresponded approximately with

bedrock surface and a possible explanation was that groundwater in the overlying soil, built up during the previous rainy season, was continually replenishing the groundwater within the bedrock.

Speakers who took part in the discussion were Messrs. Howat, Lau and Malone (PWD), Koo (Scott, Wilson, Kirkpatrick & Partners), Dutton (Binnie & Partners), Endicott (P & T Geotechnics), Nunns and Davies (Ove Arup & Partners) and Low (Low & Partners).

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