Slope stability and stabilization - G&P Geotechnics Sdn Bhd

1 CHAPTER 10. Slope stability and stabilization Tan Yean-Chin & Chow Chee-Meng Gue & Partners Sdn Bhd, Kuala Lumpur, Malaysia Tropical residual soils are derived from in situ weathering and decomposition of rock and have characteristics that are quite different from those of transported soils. Some methods commonly adopted in Malaysia for Slope stabilization work are discussed, highlighting good practices and relevant design guidelines. Some as- pects on measurement of strength parameters and groundwater profile assessment together with a discussion on stability analysis are also presented. CHARACTERISTICS OF TROPICAL a) Upper zone RESIDUAL SOIL Slope This zone consists of highly weathered (or de- composed) and leached soils often reworked by Residual soils are derived from in situ weathering burrowing animals and insects or cultivation, and and decomposition of rock and remain at their origi- intersected by root channels.

2 This zone is also nal location. The stratigraphy may be a continuous possibly subjected to some transport processes. gradation from the fresh, sound unweathered parent b) Intermediate zone rock through weathered soft rock and hard soil to This zone consists of highly weathered to moder- highly weathered material containing secondary de- ately weathered material (50% to 90% rock) but posits of iron, alumina, silica or calcium salts with exhibits some features of the structure of the par- its original rock texture completely destroyed ent rock and may contain some core stones (or (Blight 1997a). Based on the above definition, it is boulders). clear that residual soils would have characteristics c) Lower zone that are quite different from those of transported This zone consists of fresh rock and slightly soils. The same also applies with respect to the be- weathered material.

3 Havior of a residual soil Slope as compared to a Sometimes, behavior of a residual soil Slope is transported soil Slope . Figure illustrates the governed more by its structural features such as rel- typical weathering profile for a residual soils show- ict jointing, bedding or slickensiding which it has ing gradual changes of gradation from sound un- inherited from the parent rock. The influence of such weathered rock (Grade I) to highly weathered structural features cannot be ignored in the analysis (decomposed) material (Grade VI). of Slope stability and stabilization involving residual The weathering process leading to the formation soils. For example, a Slope with pre-existing shear of residual soils is highly complex and weathering plane which dips in the direction of the Slope ( day- sequence such as those proposed by van der Merwe lighting') as shown in Figures and will (1965) cannot be used as a general guide to relate definitely have a lower factor of safety compared to the properties of residual soils to its parent rock.

4 The a Slope without such structural feature or one in weathering process is influenced by climate and which the direction of dip is in the opposite direc- drainage conditions and any changes to the two fac- tion. In addition, Blight (1997a) also highlighted the tors may arrest or reverse certain stages of weather- difficulties of determining the relevant engineering ing. It has also been demonstrated extensively in the properties of residual soils. This is due to the weath- literature that behavior of residual soils in different ering characteristics of residual soils which tend to regions of the world (or even within the same re- form soil which consists of aggregates or crystals gion) shows distinct differences in characteristics. bonded together by a combination of capillary forces However, Blight (1997a), based on the works of and bonding.

5 These aggregates of soil particles can Vargas & Pichler (1957), Ruxton & Berry (1957) be easily broken down and become progressively and Little (1969) summarised that a residual soils finer if the soil is manipulated. will typically consist of three zones (Figure ): 159. Grade VI. residual soils Upper Zone Grade V. completely decomposed Grade III. Intermediate Grade I. moderately Zone Fresh rock decomposed (Lower Zone). Figure Typical weathering profile of residual soils. Pre-existing shear plane Figure Structural features in residual soil Slope . In brief, the main characteristics of residual soils a) The potential failure surface and mode of failure. are: b) The groundwater hydrology, and therefore the a) Very heterogeneous; this makes sampling and critical pore pressure distribution in the Slope . testing for relevant engineering parameters diffi- c) The erosion characteristics of the soil materials.

6 Cult. The Geotechnical Engineering Office, GEO (for- b) Usually high permeability, therefore susceptible merly Geotechnical Control Office, GCO) of Hong to rapid changes in material properties when sub- Kong, has adopted a system for granites in which a jected to changes of external hydraulic condition. profile is logged according to six rock material In general, the formation process of residual soils grades' given by GCO (1988). Table presents is complex and is very difficult to model and the modified grades classification based on the generalize. Therefore, a simplified weathering pro- above reference for ease of classification. For geo- file which differentiates the material into different technical design of slopes, materials of Grade I to III. grades' is used to describe the degree of weathering are usually treated as rock' and materials of Grades and the extent to which the original structure of the IV to VI (upper zone) as soil'.

7 Rock mass is destroyed varying with depth from the It is also important to be aware of the contribu- ground surface. The weathering profile is important tion of capillary forces, bonding and structural fea- for Slope stability analysis in residual soils because tures on the shear strength of residual soils. There- it usually controls: fore, proper determination of relevant engineering 160. ' peak = ' cr + max ( ). where 'peak = peak strength; 'cr = critical state strength; and max = maximum angle of dilation. Table Material grade classification system modified from GCO (1988). Descriptive Grade General characteristics term Residual soils VI Original rock texture completely destroyed Can be crumbled by hand and fin- ger pressure Completely V Rock wholly decomposed but Figure Possible platy' soil materials from a residual soil decomposed rock texture preserved Slope .

8 No rebound from N Schmidt Hammer Can be crumbled by hand and fin- properties for residual soil Slope is essential in order ger to produce design that is safe but not over- Easily indented by point of geo- conservative. logical pick Slakes when immersed in water Completely discoloured compared with fresh rock MEASUREMENT OF STRENGTH Highly de- IV Rock weakened and can be bro- PARAMETERS AND GROUNDWATER composed ken by hand into pieces PROFILE FOR ASSESSMENT OF Slope Positive N Schmidt rebound value stability up to 25. Makes dull sound when struck by hammer Stress-Strain and Shear Strength Characteristics of Geological pick cannot be pushed Residual Soils into surface Does not slake readily in water The special features encountered in residual soils Hand penetrometer strength index that are mainly responsible for the difference in greater than 250kPa stress-strain and strength behavior in comparison Individual grains may be plucked from surface with transported soils are listed in Table Completely discoloured compared One of the significant characteristics of residual with fresh rock.

9 Soils is the existence of bonds between particles. These bonds are a component of strength (can be re- Table Comparison of residual soils and transported soils flected as apparent cohesion, c') and initial stiffness with respect to various special features that affect strength that is independent of effective stress and void ra- (from Brenner et al. 1997). tio/density. The bonding also contributes to appar- Factors affecting Effect on residual Effect on trans- ent' overconsolidated behavior of the soils. strength soils ported soils Stress History Usually not im- Very important, Vaughan (1988) highlighted some of the possible portant. modifies initial causes of the development of bonds as: grain packing, a) Cementation through the deposition of carbon- causes overcon- ates, hydroxides, organic matter, etc. solidation effect.

10 B) Solution and re-precipitation of cementing agents, Grain / Particle Very variable, More uniform;. such as silicates. Strength varying mineral- few weak grains ogy and many because weak par- c) Cold welding at particle contacts subjected to weak grains are ticles become high pressure. possible. eliminated during d) Growth of bonds during chemical alteration of transport minerals. Bonding Important compo- Occurs with geo- In engineering applications, these bonds are pur- nent of strength logically aged de- mostly due to re- posits, produces posely omitted (on the conservative side) because it sidual bonds or cohesion intercept is easily destroyed and not reliable for design. In ad- cementing; causes and yield stress, dition, these bonds also contribute to the peak cohesion intercept can be destroyed strength of the soil.