Keywords

1 Introduction

The studied cut slope is situated in front of the Faculty of Social Sciences and Humanity (FSSK), Universiti Kebangsaan Malaysia (UKM), Malaysia. The UKM campus area situated in Bangi, Selangor is encompassing the Kuala Lumpur International Airport, Kuala Lumpur—the main commercial centre, and Putrajaya—the administrative capital. This studied slope area is located about 35 km south of Kuala Lumpur city, 45 km from Kuala Lumpur International Airport (KLIA) and 20 km from Putrajaya. The coordinate of this slope is 02° 55′ 40.94″ N and 101° 46′ 51.48″ E (Fig. 1).

Fig. 1
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Location map of the study area, UKM campus, Malaysia

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This research lies on its specific focus to assess the stability of the slope and to identify the existing and to predict likely mode of instabilities or slope failures and then assess/recommend to the suitable stabilization/protection measures. This research involves identification of landslide hazards, geological structures, identification of discontinuity, measurement of discontinuity orientation and delineation of weathering grade. This slope is sensitive to landslide hazard aspect due to two landslide occurrences in this slope.

In this slope, first landslide occurred in the 1990s (actual date unknown) and second landslide occurred in 09 November 2012. The residual soil, phyllite, graphitic phyllite, quartzite, oxidized layer/band and quartz vein is exposed in cut slope. Most part of the hill is covered by residual soil but some part of the hill is covered by weathered phyllite and quartzite. The residual soil, completely weathered phyllite, highly weathered phyllite and quartzite belongs to weathering grade VI, V and IV respectively. The graphitic phyllite and quartzite are generally medium strong rocks and low durability, while the phyllite is generally weak rock and very low durability. The slope mass is moderately to heavily jointed. The slope face appeared rough and planar. Two fault line and one recumbent fold was exposed in studied slope.

2 Methods

In order to assess the stability of the slope, analyzing a satellite image of the area, followed by slope mapping, soil and rock testings, discontinuity survey, data have been plotted into stereographic plot, kinematic stability analysis of the slope and finally assess the risk mitigation measures. The slope orientation and slope geometry has been measured by Brunton compass during field work. The lithology and weathering grade have determined by walkover survey and some laboratory testings. The discontinuity survey has conducted by scan line method and randomly at three sections (A, B and C) from five sections because different types of discontinuity have been exposed at three sections of studied cut slope in accordance with I.S.R.M. (1978).

The collected discontinuity and slope orientation data compiled and analyzed statistically by plotting onto equal area net and lower hemisphere projections e.g. pole plot, contour plot, and mean great circle plot. Then interpretation and identification of major and minor discontinuity sets were done and calculated their mean orientations. This work has been done by Stereo 32 software. Friction circle (ϕ = friction angle) has been plotted. The friction angle along the major joints in the saturated completely to highly weathered phyllite is about 32° is assumed for this research case (Xu et al. 2012). This value is considered reasonable, since some of the major joints are normally wavy, tight, dry and have rough surface. The area bounded by the friction circle and slope face (the shaded area in the stereoplots) is considered as critical area. If any discontinuity plane or an intersection between two or more discontinuity planes fall within this area, the rock block or slab bounded by that particular discontinuity planes is likely to be unstable. The potential mode of failure can either be one or combination of the modes of failure e.g. Circular failure, Planar failure, Wedge failure and Toppling failure which have been determined in accordance with Hoek and Bray (1981).

The average great circle was plotted on stereogram and a hypothetical profile line were drawn perpendicular to the slope face on stereogram at three sections of studied slope for further analysis. Along X–Y section calculate the true dip and apparent dip of major discontinuities on stereogram. Then draw a cross section based on apparent dip and shown potential slope failure direction.

For discontinuity survey and detail study this studied slope was dissected by following five sections those are shown on the Table 1. Three sections have been selected from five sections of studied slope for the purpose of kinematic stability assessment.

Table 1 The summary of the slope sections of the studied slope in UKM campus
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3 Results and Discussions

3.1 Discontinuity Survey

During field work, three types of discontinuity (joint, foliation and fault) have been measured. All discontinuity parameters e.g. types, orientation, length, aperture, filling, roughness and water have measured. Discontinuity sets and average orientation of the discontinuities are shown on the Table 2.

Table 2 The summary of discontinuity data from different section of the studied slope
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3.2 Slope Stability Assessment

The slope stability assessment adopted in this study is to highlight any potential danger and mode of weathered rock cut slope failure. Therefore the kinematic slope stability assessment is most suitable. The use of stereographic projection allows the three dimensional orientation data to be represented and analyzed in two dimensions. The stereographic analyses consider only angular relationships between lines, planes and both of lines and planes. Based on this stereographic analysis, the potential mode of slope failures can be determined and assisting the engineer to design or planning the suitable stabilization measures.

The result of the kinematic stability analysis indicates that the three sections of this slope are considered unstable as shown on Fig. 2. The summary of kinematic stability analysis from different section of the studied slope is shown on the Table 3. Field observation indicates that a number of loose/unstable wedges and blocks are readily bounded by dilated/opened up joints, suggesting subtle movements which need urgent attention and stabilization or protection works.

Fig. 2
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The stereographic (contour) plot of discontinuity data and kinematic stability analysis of section A, B and C of the studied slope

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Table 3 The summary of kinematic stability analyses from different sections of the studied slope
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The result of cross section showing the likely structural discontinuities orientation and the potential sliding plane and mechanism of major failure in the different sections of studied slope in UKM. X–Y profile line has been drawn on stereogram and cross sections and the interpreted failure modes are shown on Fig. 3.

Fig. 3
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The X–Y profile of stereographic projection of major discontinuities and cross sections showing the likely mechanism of major failure in the section A, B and C of the studied slope

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4 Risk Mitigation Measures

The studied slope is stabilized by steel-wire mesh coupled with active soil nails to stabilize and reinforce the slope. A retaining wall is constructed along the toe of the slope and a drainage control system is installed to intercept and divert surface runoff from flowing over the slope. A hydroseeding method is also applied to the slope for soil improvement to support vegetation growth on poor sub-soil conditions. Details of the recommended stabilization and protection measures for the slope are given below.

4.1 Wire Mesh with Soil Nail

The high tensile strength steel-wire mesh is laid down to cover the entire slope face. The wire mesh panels are then tied back and secured to the slope face with steel rope or cable net, system spike plates specially adapted to the net cover, border steel ropes or cables as well as an adequate nailing adapted to the site specific situation. Border ropes have to be able to transfer the load from the net onto the nails in the border sections. The soil nails can provide deeper stabilization by holding the mesh to the surface throughout. These designs are largely dependent on the ability of the system to transfer forces from the facing material to the anchor points.

This method is preferred compared to shotcrete or other impermeable slope protection structures mainly because it allows the groundwater and surface runoff to percolate in and drain out freely through it, thereby permitting healthy and lasting surface vegetative cover. The flexibility and 3D characteristics of the wire mesh and cable netting system allow the mesh to be installed on undulating slope surface and to wrap around existing big trees without removing or cutting down the trees (Jamaluddin 2010; Fatzer et al. 2006).

The wire mesh is used in combination with the active soil nails, provides a flexible and strong surface protection and stabilization cover by being able to lay taut against the rock surface. Soil nailing is in situ reinforcement technique (internally stabilized system) which consists of long steel rods inserted into less disturbed bedrock to stabilize the slope mass. Soil nails are normally installed with perpendicular to the potential failure plane (Fig. 4).

Fig. 4
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Proposed conceptual design of a recommended method for slope stabilization showing soil nails orientation, wire mesh and retaining wall at section C of the studied slope

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4.2 Retaining Wall

A retaining wall is usually provided at the toe of a slope to prevent downslope movement. Guide to retaining wall design by GEO (1993), recommended a retaining wall with level backfill. The top 0.04 m layer of fill should be a suitable material of relatively low permeability and the ground surface should be formed with an adequate gradient in order to avoid ponding which can result in continuous infiltration into the backfill. For retaining wall less than 2 m high, drainage (weep hole) is usually vertically positioned against the back face of the wall.

4.3 Drainage Control System

Surface runoff is controlled by installing different types of drain; i.e. cut-off drain, cascading drain and toe drain. Cut-off drain intercept surface flow at the top of the slope, and are normally placed at the crest. Cascading drain connects the cut-off drain and toe drain. This drain network is constructed to intercept and divert surface runoff from flowing over the slope and to prevent surface erosion as well as to minimize infiltration into the slope mass.

Above all the planning and implementation of the proposed slope stabilization measures described above should be supervised by an experienced engineering geologists/geotechnical engineers.

5 Conclusion

Results of the discontinuity survey indicate phyllite and quartzite rock mass is moderately to heavily jointed, well foliated and is dissected by 2–5 sets of discontinuities. The slope failure geometry is largely controlled by discontinuities (joints) in the slope masses. The fallen blocks or debris are generally very small to small scale, measures at several tens of cm and generally less than 1.5 m in length or width because the joint spacings are relatively small (several cm to 0.5 m apart). Results of the kinematic stability analysis showed that the studied slope is unstable. The slope shows high potential for planar, wedge and/or combined modes of failure. Based on this study, the main causing factors for the landslide are unfavorable discontinuity orientation, lithology of slopes, highly weathered materials and heavy rainfall. The studied slope has stabilized with steel wire mesh coupled with active soil nails. The toe of the slope is further supported with retaining wall and surface drainage system. A hydroseeding method has also applied to the slope for soil improvement and to support vegetation growth on poor sub-soil conditions. This study showed a typical example of the importance of geological studies in identifying the root causes for a cut slope failure in weathered rocks and the recommended methods for slope stabilization and remediation of a failed weathered cut slope in a wet tropical country such as Malaysia.