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Investigation of Slope Failures in Soil Mechanics

Hamed Niroumand1, Khairul Anuar Kassim1, Amin Ghafooripour2,

Ramli Nazir1, Sayyed Yaghoub Zolfeghari Far1

1 Department of geotechnical engineering, Faculty of civil engineering, Universiti Teknologi Malaysia, E-mail: niroumandh@gmail.com

2 Department of Structural Engineering & Vibrations, School of the Built Environment, Heriot – Watt University, Dubai, UAE

ABSTRACT Slope failures are disasters that happen all around the world. Occurrence of slope failures depends on number of factors. To safeguard the safety of the public from slope failure hazards, proper geotechnical input by the engineers with geotechnical experience is very important. The geotechnical input includes four important stages namely, planning, design, construction and maintenance. Whenever failures occur, engineers are responsible to the problems. The paper observed the assessment of slope failures.

KEYWORDS: Failure, Slope, Failure mechanism, Slope stabilization

INTRODUCTION In Malaysia, the construction of residential buildings on hill-site has increased tremendously due

to lack of suitable flat land and other factors like beautiful scenery, fresh air, exclusiveness, etc. However, the collapse of Block 1 of Highland Towers on 11th December 1993; one of the first high-rise developments on hill-site in Kuala Lumpur, has worried many people. Safety of building on hill-site is often a topic of discussions among engineers and public. The discussion intensifies each time after a slope failure being highlighted by media. Slope failures are disasters that happen all around the world. Occurrence of slope failures depends on number of factors. The understanding of these contributing factors is essential in any slope failure investigation. Therefore, the knowledge on types of slope failures, mechanism of slope failures, and causes of slope failures are essential for any slope design and remedial work.

TYPES OF SLOPE FAILURE Slope failure can be classified in many types. For the purpose of this paper, slope failures are

classified as below:

Slide Slide can be defined as movement of soil mass, which is parallel to planes of weakness, and

occasionally parallel to slope. Figure 1 shows slide failure. Slides in soil, will have rotational or translational movement. The behavior of the slide depends mostly on the type of material and whether

Vol. 17 [2012], Bund. R 2704 that material is: (a) homogeneous (isotropic) material (similar properties in all directions), as shown in Figure 2, (2) inhomogeneous (anisotropic) material with planes of weakness, as shown in Figure 3

Figure 1: Slide

Figure 3: Slope failure of inhomogeneous (anisotropic) materials with planes of weakness

Figure 2: Slope failure of homogeneous (isotropic) materials (similar properties in all directions).

slips

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Figure 5: Slip

Figure 4: Flow

Figure 6: Creep

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Slip Slip is a complex movement of materials on a slope, includes rotational slip. Figure 5 shows slip

failure.

Creep Creep is defined as gradual movement of slope materials. Figure 6 shows creep failure.

Torrent Torrent is a sporadic and sudden canalized discharge of water and debris. Figure 7 shows torrent

failure.

Failure Mechanism in Slope During the hot and dry days, the slope face become desiccated and shrunken especially the newly

cut slopes. The extent and depth depend mainly on plasticity of the soil and type of slope protection. During rainstorm, water percolates in to cracks or other expose surface, causing the slope mass to swell and saturated with corresponding reduction in shear strength gradually through seepage, migration of soil particles and gradually increase in void ratio in soil mass. Initially water percolates downward into the slope mainly through the desiccation cracks and in response to the suction pressure of the top stratum of dried soil. As the outer face of slope swells and saturated, the permeability parallel to the slope face increase with continued rainfall, seepage develops parallel to the slope face. Reduction in shear strength due to saturation and swelling coupled with the condition of seepage, failure eventually occurs if the shearing resistance is equal to or less than the shearing force (Neoh, 2001). In soil mechanics, it is essential to understand the forces acted on slopes, which is the driving force and the resisting force, which prevent from slope failing. Factor of Safety (FOS) is the ratio of resisting forces to driving forces. Generally, the FOS equation is:

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Slopes with lower FOS means the potential of failing are higher then slopes with higher FOS. Therefore slopes with higher FOS are safer.

Causes of Slope Failure It is essential to understand the causes of slope failures, which are indicated in an analysis or

which have developed in practice. These causes may be summarised simply as follows:

a) The slope is too high or too steep for materials of which it is composed. b) The materials are too weak to sustain the slope at its present profile. c) The pore water pressures are too high, and thus adversely affect the soil strength. d) The materials contain weak inclusions or discontinuities. e) The slope is affected adversely by some external influence, for example applied loads

from structures or excavation at or near the toe of the slope.

Many studies have been done to find out the causes of slope failure. Slope failures statistics based on “Lecture Notes for highway Slope Management”, IEM/JKR (1997) are given below:

A survey of 322 slope failures revealed that:

89% are cut slopes (54% soil slopes + 11% rock slopes + 24% soil/rock slopes)

8% are fill slopes 2% are retaining walls 1% are natural slopes

Neoh (2001) points out that findings from another survey of 260 case histories about causes of slope failures are:

90 % infiltration 38% seepage 30% perched water table 8% wash-out / erosion 2% rise in main water table 2% others (pipe leakage etc)

Seldom can a slope failure be attributed to a single definite cause. Slope stability is a wide and complex subject involving with many geotechnical principles, some of which are very empirical or statistical in nature and required site verification by experienced engineers during construction.

Factors that have significant impact on slope stability are:

Slope geometry (Height of slope (H), angle of slope (), shape, adjacent and upslope and down slope topography)

Soil unit weight Shear strength of soil Pore water pressure or suction Geological settings or discontinuity

Research papers and proceedings related to slope failures and slope stability, causes of slope failures varies. For the purpose of this study, only relevant causes of slope failures in Malaysia are discuss. These causes include:

Geological Causes

a) Erosion b) Weathered materials

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c) Weak materials d) Contrast in permeability e) Contrast in stiffness

Physical Causes

a) Intense rainfall b) Perched water table c) Rapid draw-down d) Flood e) Extreme infiltration f) Seepage

Human Causes

a) Excavation of slope at it’s toe b) Loading of slope at it’s crest c) Irrigation d) Deforestation e) Artificial vibration (blasting, piling etc) f) Water leakage from utilities

Prolong and high intensity of rainfall especially during the two monsoon periods every year allows rainwater infiltrates with ease into a slope and causes saturation at shallow depths in the field during the service life of a slope. Figure 8 shows the possible hydrological effects of rainfall on permeable slopes. Some of the rainwater runs off the slope and may cause surface erosion if there is inadequate surface protection. If soil has high permeability, majority of the water will infiltrates into the subsoil. This causes the water level in the slope to rise or it may cause perched water table to be formed above some less permeable boundary (e.g. clay seams). Above the water table, the degree of saturation of the soil increases thus reduces the soil suction (i.e. negative pore pressure) (Gue & Tan, 2000). Failures in cut slopes of residual soils might be caused by ‘wetting-up’ process, which decreases the soil suction and hence the decrease in soil strength. Premchitt, (1985) shows evidence suggesting that transient rises in groundwater table are responsible for some rain-induced landslides.

Figure 8: Effects of rainfall on high permeable slope

Rise in water level

Rainfall

Surface Runoff + Erosion Cl l

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Shear Strength of Soil Shear strength of soil is the internal resistance per unit area that the soil can offer to resist failure

and sliding along any plane inside it. Knowledge of shear strength is required in solution of problems concerning the stability of soil masses and slope stability. For cut slope, effective stress (drained or long term condition) is normally more critical than total stress (undrained) condition. Therefore, effective stress strength parameters c’ and ’, determined from testing of representative samples of matrix materials are used in analyses. In Malaysia, normally Isotropic Consolidated Undrained Triaxial Tests (CIU) are carried out on large diameter undisturbed soil samples using Mazier sampler. Sample of about 70 mm without trimming is ideal. Samples should not install with side drains to prevent formation of insistent layers soil samples (Tschebotarioff, 1950). It is important that the soil samples are tested at stresses comparable to those in the field, and should be saturated. It is appropriate to measure strength parameters on saturated soil samples because the residual soils are usually of high permeability (usually 10-4 to 10-6 m/sec) (Gue & Tan, 2002). The shear strength of the soil may be represented graphically on a Mohr diagram as shown on Figure 9. For simplicity of analysis, it is conventional to use a c’-’ soil strength model as expressed in the equation below:

= c’ + n’tan’

where, = shear strength of soil.

n’ = effective normal stress at failure.

’ = effective angle of friction (degree).

c’ = apparent cohesion (kPa).

Slope Stabilization and Protection The purpose of slope protection is to protect the slope against erosion by surface runoff, to reduce

infiltration and also to enhance slope landscape with environmental friendly outlook. Slope stabilisations or remedial works are measures in strengthening the slope stability. Proper slope stabilisations depend on appropriate selection of methods, proper specifications, proper construction procedures and good maintenances. When attempting to stabilise an existing slope failure, a number

c’

3’ 1’

’

Figure 9: Mohr diagram

Failure envelope

Vol. 17 [2012], Bund. R 2710 of possible remedies are open to engineers. There are many types of slope stabilisations applied in Malaysia, such as:

Change of geometry Retaining wall Geotextile Soil nailing Slope drainage Turfing Shotcreting

Change of Geometry – Cut and Fill Change of geometry of a slope is one of the most common methods used in slope stabilisation

because it is often most economical. Generally the procedures are grading a slope angle to a uniform flatter angle, concentrate the filling at the toe of the slope, creating a berm in the section and reduce overall slope height and or reduce the slope angle. The practice in Malaysia for designing cut slopes normally uses berms of 1.5m wide at 5m to 6m vertical slope intervals. Geotechnical Manual for Slopes (GCO, 1991) of Hong Kong recommends that the vertical interval of slopes should not be more than 7.5m. The typical gradient of cut slopes normally range between 1V: 1.75H to 1V: 1.5H for grade V & VI materials. The reason for having berms with 1.5m wide is easy for maintenance. For fill slopes, similar to cut slopes, berms of 1.5m wide at 5m to 6m vertical slope interval are commonly used for fill slopes in Malaysia. Usually the fill slope is at one vertical to two horizontal angle (1V: 2H) depending on the subsoil conditions and the material used for filling (Gue & Tan, 2002). Neoh (2001), mentions the following:

For cut slopes, the common factors influence slope stability or failure are as follows:

a) Presence of perched ground water table. b) Excessive infiltration from upslope increases the unit weight of soil. c) Presence of unsuitable geological discontinuity/settings.

For fill slopes, the common factors influence slope stability or failure are as follows:

a) Low shear strength of filling due to poor compaction. b) Excessive infiltration increases the unit weight of soil. c) Poor foundation soil. d) Creeping toe.

Retaining Wall Retaining wall is a wall built to keep a bank of earth from sliding or water from flooding

(Webster's NewWorld Dictionary,1988). There are many types of retaining walls used in Malaysia. The common types are as follows:

Gravity Wall Gravity wall is made of mass concrete and rubble stone with nominal reinforcement near the wall

surface to limit cracks. The stability of gravity walls depends on to the self-weight of the wall. This type of wall is uneconomic because the large quantity of material is used only for its dead weight. Figure 10 shows an example of gravity wall.

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Gabion Wall Gabion wall is free-draining walls constructed by filling large baskets with broken stone. The

baskets are made from galvanised steel mesh or woven strips. A typical basket is rectangular with dimensions of about 1 m x 1 m. Retention is achieved from a combination of the stones weight, and its interlocking and frictional strength. The wall face is battered at approximately 6 degrees from the vertical. They are constructed with either a stepped face or a stepped back up to a maximum height of about 6 m. Due to its durability it is generally used as temporary retaining wall. Figure 16 shows an example of gabion wall.

Contiguous Bored Pile Wall Contiguous bored pile wall is constructed in a line with a clear spacing between the piles of 75 to

100 mm. Therefore they cannot be used as water retaining structures. Their main use is in clay soils where water inflows are not a problem. However they have also been used to retain dry granular materials or fills. Where water is not a problem the spacing of the piles can be adjusted so long as the gap between piles is such as to prevent soil collapse between them. Generally, contiguous bored pile wall is used in slopes that has high risk of collapse during excavation for construction. It is also one of the most expensive retaining walls. Figure 17 shows an example of contiguous bored pile wall.

Figure 16: Gabion Wall

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Geotextile Geotextile is a geosynthetic fabric,either woven or non-woven, applied to either soil surface or

between materials. The purpose of geotextile is to reduce erosion by storm generated water by providing filtration, separation, or stabilisation properties. Geotextile is used in cut slopes where soil is composed of weak materials. The function of geotextile is to transfer the excessive shear stress from weak soil to tension in geotextile. With geotextile, steeper slope can be constructed to gain more space and thus increase of FOS. Figure 18 shows a type of Geotextile.

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Slope Drainage From the preceding discussion, it will be seen that control of water must be considered. The main

purpose of surface drains is to collect surface runoff from slope as much as possible and convey away from slope as fast and as far as possible so as to reduce infiltration and erosion caused by rainfall.

Turfing Turfing is the process of covering slope surface with turf. Close turfing with cow grass is common

to provide immediate protection. This method is very laboured intensive and usually only applied to gentle slope (1V: 1.5H) for residual soils.

Shotcreting Shotcrete is sprayed concrete or mortar. Shotcrete is expensive and is used when slope steep

(>450) and genarally for highly fractured weathered rocks. Typical shotcrete slope protection should consist of minimum 75 mm to 100 mm thick sprayed cement/sand mixture (1:3) with a layer of wire mesh to reduce shrinkage and thermal cracking. Adequate subsoil drainage should be provided especially where water is observed seeping from the surface or where water seepage may be expected (Neoh, 2001). Figure 21 shows an example of shotcreting.

Maintenance of Slopes Although lack of maintenance of slopes and retaining walls are not the direct causes to failure.

However, failure to maintain particularly after erosion may propagate and trigger slope failures. Therefore regular inspection and maintenance of the slopes are necessary (Gue & Tan, 2002). Awareness alone is not sufficient, engineers and personnel involved in slope maintenance should also know how to properly carry out the work. A set of standards of good practice slope maintenance is needed. A good guideline from GEO of Hong Kong like “Geoguide 5 – Guide to Slope Maintenance” (1995) for engineer and “Layman’s Guide to Slope Maintenance” which is suitable for the layman should be referred. Geoguide-5 (1995) recommends maintenance inspections be sub-divided into three categories:

Figure 21: Shotcreting.

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(A) Routine Maintenance Inspections, which can be carried out adequately by any responsible person with no professional geotechnical knowledge (layman). (B) Engineer Inspections for Maintenance, which should be carried out by a professionally qualified and experienced geotechnical engineer. (C) Regular Monitoring of Special Measures, which should be carried out by a firm with special expertise in the particular type of monitoring service required. Such monitoring is only necessary where the long term stability of the slope or retaining wall relies on specific measures, which are liable to become less effective or deteriorate with time. This measure is seldom carried out in Malaysia.

Malaysia, which has at least two monsoon seasons, Routine Maintenance Inspections (RTI) by layman should be carried out as a minimum twice a year for slopes with negligible or low risk-to-life. For slopes with high risk-to-life, more frequent RTI is required (once a month frequency). In addition, it is good practice to inspect all the drainage channels to clear any blockage by siltation or vegetation growth and repair all cracked drains before the monsoon. Inspection should also be carried out after every heavy rainstorm. Category B Engineer Inspection for Maintenance, should be taken to prevent slope failure when the Routine Maintenance Inspection by layman observed something unusual or abnormal, such as occurrence of cracks, settling ground, bulging or distorting or wall or settlement of the crest platform. Geoguide-5 (1995) recommends as an absolute minimum, an Engineer Inspection for Maintenance should be conducted once every five years or more as requested by those who carry out the Routine Maintenance Inspections. More frequent inspections may be desirable for slopes and retaining walls in the high risk-to-life category.

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