Geosynthetics in the rehabilitation of failed slopes.

Ooi, T.A. ; Tee, C.H. ; Dobie, M.J.D. 等

This paper discusses the use of geogrids in the rehabilitation of failed slopes in restricted sites. Failed slopes are often associated with prolonged intense rainfall and lack of slope internal drainage. The consequences of a landslide in the case of a highway or railway can be very grave. The instant reinstatement of the same is almost demanded by the general public. In one particular failed slope measuring about 36m high, sheet piles were used as a temporary measure at the slope toe to stop the movement of the sliding mass. However, with sheet piling, the flow of groundwater was cut off and pore water pressure built up. This pore water pressure built-up caused the soil strength to decrease and the sheet piles began to rotate and tilt towards the railway lines. This steady movement of the sheet piles posed an immediate danger to the safe operation of the railway and daily movements of the goods train. Geogrids were used to mitigate this continuing ground movement and prevent further slope failure. The use of geogrids in the speedy rehabilitation works has been well established. However, the methodology in the application of the right technology is still not well understood, particularly for very high slopes. This paper reports case histories of landslide occurrences and the solutions successfully applied.

INTRODUCTION

The Tsunami that struck the Indian Ocean on 26 December was a global catastrophe that caught many in the world by surprise. The most devastated area happened at Bandar Acheh where the town was completely demolished. The coastal areas of the Indian Ocean were hit by Tsunami to varying degree. The world all over started the massive relief operation by way of sending aids and donations. The whole world was shocked when faced with such unexpected nature's calamities and steps are being taken to monitor such oceanic ground movements that triggered tsunami with devastating power so as to mitigate the damages and loss of lives. Landslide is one of nature's calamities that occur on land when there is excessive amount of water in the soil slopes. The excessive amount of water in soil slope occurs usually during the incessant period of rain. This paper reports case histories of slope failures rehabilitated by using Tensar Geogrids.

MOTEL DESA SLOPE FAILURE

In early 1980s, part of the Motel Desa slope in Terengganu collapsed during the heavy monsoon rainfall period and left the kitchen of the motel hanging precariously on top of the failed slope. The failed slope was about 30m high and 25m wide. It was important to ensure that the kitchen did not collapse. The access to site was difficult and speedy rehabilitation of the slope was necessary. The technology of geogrid reinforcement of slope was new at that time in Malaysia and careful instrumentation and testing was carried out during the construction stage. In view of the difficult access and the urgency to safeguard the motel kitchen, the upper half of the failed slope was stabilized by using a combined technology of geogrid reinforced soil slope of about 5m wide x 6.7m high supported on micro-piled platform while the lower half failed slope was reconstructed using traditional method of compacted fill without any soil reinforcement but to a gentle slope of not more than 1:2. Beyond the geogrid reinforced slope at the top similar unreinforced soil slope of not more than 1:2 was also adopted. Fig. 1 shows the general cross section of the rehabilitation scheme. The details of this landslide rehabilitation scheme were reported and discussed by Toh et al (1986) and Ooi & Tee (2004). The Motel Desa project has given designers an opportunity to make further improvement to the design of geogrid reinforced slope through in-situ monitoring of actual completed works. The rehabilitation work was completed more than 20 years ago and the slope has performed satisfactorily and remained stable.

[FIGURE 1 OMITTED]

RAILWAY LINE SLOPE FAILURE

A railway line cut slope in the east coast state of Malaysia measuring some 36m in height failed during a severe monsoon rain season. The failure threatened the safety of the railway line as the debris encroach the railway side drain and sub-ballast. Immediate action was taken to put a team of workers with plant and machinery on standby round the clock to remove any debris that come close to 4m from the centerline of the railway tracks. A speed restriction was imposed to enable the operator of the train to bring the train to an immediate stop upon sighting of any sign of further slope movement and at the same time a row of steel sheet piles was driven to refusal as an attempt to stop the flow of the debris and prevent the movement of the failed slope. Fig. 2 shows a general view of the landslide and the steel sheet piles at the slope toe near the railway line. It can be seen that the hill slope is too close for comfort to the railway line. An analysis shows that the landslide that had occurred was due to toe softening of the slope. The results of analysis show slope toe failure with factor of safety of 1.03. The provision of sheet piles driven to refusal however could not provide the embedment depth required and at the same time cut off the flow of ground water and thus caused built up of water pressure. Consequently, with accumulation of water and the loss of soil strength due to soaking of the slope toe, the sheet pile rotated and tilted forward towards the railway line. Fig. 3 shows the picture of tilted steel sheet piles and the railway line. Slope stability analysis show that the steel sheet piles are within the localized sliding circle with factor of safety of less than unity. In view of this critical situation, it was necessary to carry out urgent repair before the arrival of the next monsoon. Tensar geogrid reinforced block of 8m high was chosen as the solution to stabilize the toe area of the slope and at the same time provide support for the 36m high failed slope. Fig.4 shows the results of the slope stability analysis. In the rehabilitation of this type of high failed slope, it is very important that the correct technology with the right construction methodology is used so as to ensure successful implementation of the reconstruction of the slope. The provision of internal and external drainage system of the slope and the geogrid block remains as an important factor to be considered in design and construction of the rehabilitation works.

[FIGURES 2-4 OMITTED]

DESIGN METHODOLOGY

The design analysis must first start with the analysis of failure mechanism based on site information and relevant soil parameters. Having established the correct failure mechanism, it is then logical to proceed to find a solution that is most economic based on the local conditions and situations. Many designers often failed to consider the effect of ground water table on the stability of slope. This can give rise to wrong solution being prescribed and resulting in distress and/or failure of the slope when the rehabilitation work is completed or during service.

CONSTRUCTION METHODOLOGY

In the rehabilitation work, the landslide areas were divided into 3 sections namely, section A, B and C as shown in Fig.5. It is important to rehabilitate the slope in sections and to ensure failure of slope does not happen during construction. The site was first drained and the top 1m of the foundation soil for the base of the geogrid block was re-compacted with two layers of geogrid reinforcement to receive the geogrid block to be constructed. Suitable drainage layer is then installed with sand columns placed at the back slope and compacted in layers with geogrid reinforcement. The geogrid block is built to a safe height before excavation of the next section is started. This second section is similarly built to a safe height before the third section is excavated. The process is repeated until all the geogrid reinforcement blocks are completed. Surface drainage system of the slope is constructed as soon as it is possible and the completed slope is covered with closed turf. Figs.6 shows the completed slope.

[FIGURE 5-6 OMITTED]

CONCLUSIONS

From the case histories presented by Toh et al (1986) and Ooi & Tee (2004) as well as the railway project it can be concluded that:

1. Slope failures usually occurred during incessant raining season like that of the monsoon period.

2. Different solutions are used in different case histories even though the Geosynthetics of geogrid type were used, they were applied differently.

3. The need for foundation soil improvement was found necessary to support the geogrid reinforced block in order to reduce settlement and movement of slope during and after construction of the slope.

4. It is important to provide internal and external drainage system to ensure that groundwater table is lowered sufficiently to ensure long-term stability of slope.

5. Slope rehabilitation is often required when slope collapses threaten lives and or properties. We need to ensure the slopes are designed to withstand long-term slope stability for such cases and not wait for it to fail before something is done.

6. Combined technology is useful to provide economic solution to slope rehabilitation works.

REFERENCES

Ooi, T.A. and Tee, C.H. (2004). "Reconstruction of failed slopes using Geosynthetic Reinforcement in Malaysia", Proceedings Malaysia Geotechnical Conference, MGC 2004, Kuala Lumpur, P435-447.

Toh, C.T., Chee, S.K. and Ting, W.H. (1986). "Design, construction and performance of a geogrid reinforced high slope and unreinforced fill slopes", Proceedings Joint Symposium on Geotechnical Problems, Kuala Lumpur, P.90-111.

T.A. OOI

TAO Consult Sdn Bhd, 17A, Jalan Awan Hijau, Taman Overseas Union, Batu 5, Off Jalan Klang Lama, 58200 Kuala Lumpur, Malaysia

C. H. TEE

Mega Geoproducts And Services Sdn Bhd, 40-1, Jalan 2/109E, Desa Business Park, Taman Desa, Off Jalan Klang Lama, 58100 Kuala Lumpur, Malaysia

M. J. D. DOBIE

Tensar International Limited, Wisma Sejahtera, Room 407, Jalan Letjen S Parma, Kav 75, 11410 Jakarta, Indonesia