Landslide and Embankment Stability, Perspective and Demand in Brunei Darussalam
by
Hj Suhaimi bin Hj Gafar 1 and Dr. S. A. Sultan 2
ABSTRACT
Landslide is the perceptible displacement of mass of soil in which the center of gravity of the moving mass advances downslope. Slopes can be divided into three groups: (1) slopes with existing landslides; (2) slopes with potential landslides; and (3) slopes that are stable under existing conditions which include natural slopes as well as constructed slopes which do not fail during or following construction. Understanding the nature of factors which cause slope movement and the models that govern their relationship with each other is required for evaluating slope stability or designing economical preventive and corrective measures.
In this paper, a comprehensive analysis was carried out to identify the factors that are causing landslides in Brunei Darussalam and the models that govern the relationships among these factors using computer simulation on local data. New models were developed to be used for evaluating the stability of earth embankments and for economical design of preventive and corrective measures in Brunei Darussalam in the future.
1. INTRODUCTION
1.1 Background
A landslide is the perceptible displacement of a mass of rock or soil in which the center of gravity of the moving mass advances downslope, Deen, et. al. (1975).
The principles that pertain to the design of soil and rock cuts and fill embankments also apply to the prevention and correction of landslides. Landslides are often complex geotechnical problems requiring considerable experience for prevention and correction. For purposes of engineering studies, natural slopes can be subdivided into three groups:
(1) slopes with existing landslides ; (2) slopes with potential landslides ;and
(3) Slopes that are stable under existing condition.
Group 3 includes natural slopes as well as constructed slopes which do not fail during or following routine construction operations. Existing landslides are often recognized by examining the topography, either by ground reconnaissance or aerial photography. However, differentiation
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1
Acting Director of Roads Department, Public Works Department, Ministry of Development, Bandar Seri Begawan BB3510, Brunei Darussalam.
2
between potentially unstable slopes and stable slopes is difficult because of uncertainties in geologic conditions, shear strength parameters, and time effects. Evaluation of the geology and topography of a routine by geotechnical engineer with local experience can be very useful in delineating problem areas. Assessment of the stability of natural slope is based primarily on; (1) experience with similar slopes in the region; (2) field observations and measurements; and (3) engineering judgement, Deen, et. al. (1975).
1.2 Causes of Landslides
Landslides can result from both external and internal causes. Internal causes of sliding are those which result in a reduction of the shearing resistance of the materials within the slope. The most common causes of such a decrease are:
1. an increase of pore-water pressure; and
2. a progressive decrease of the cohesion of the materials within the slope.
The latter may be due to softening which results from clay swelling, a decrease in strength due to weathering, or to particle and/or block rearrangement during creep. External causes of landslides are those which produce an increase of the shearing stress on the slope, without changing the inherent strength of the materials within the slope. Principal external causes are:
(1) overloading the upper portions of the slope by surcharging it with a fill embankment ; (2) removal of lateral support at the base of the slope by grading or stream erosion ; and
(3) Seismic vibrations. Some slope instability involves both external and internal causes; examples are rapid drawdown, piping, and liquefaction.
Most landslides can be attributed to; (1) adding effective weight by placing a fill embankment on the slope; (2) removal of natural resistance by toe excavation; or (3) increasing the water within the slope by blocking drainage. Blockage of seeps and spring by a fill embankment, or changing the flow of surface or groundwater during construction, is common cause of landsliding. Surface and subsurface drainage improvements can be effective method of preventing or correcting slides.
Resistance to landsliding comes from the shearing strength of the materials within the slope. Most of this resistance is inherent to the materials; normally little can be done to economically improve these inherent factors. Understanding the factors which cause or resist slope movement is required for evaluating slope stability or designing economical corrective measures, Deen, R. C. et. al. (1975).
The ability to recognize the effects of slope strata, fractured rock, poor drainage etc., is requisite in assessing slope stability. If the natural resistance and the stresses acting on a slope can be estimated, several methods of correction can be theoretically examined and rational decisions can be made to produce the most effective and economical corrective measures, Sultan (1992a).
In this paper a comprehensive analysis was carried out to study the landslide problem in Brunei Darussalam. The properties of the prevailing natural soils were studied to assess their ability to resist landslides. A stability analysis was carried out to determine the strength characteristics of these soils in connection to landslides and to find the optimum design and construction procedures to alleviate the problem.
2. ANALYSIS
2.1 Input Data
In order to study the causes of landslides in Brunei, it is important to identify the properties of natural soils that prevail in the proximity of existing landslides. A comprehensive review has been carried out on tens of site investigation reports that had been prepared by local laboratories for different projects in Brunei Darussalam during the last 20 years.
The review of the site investigation reports revealed that the prevailing natural soil in Brunei is organic/inorganic silty clay soils. These silty clay soils have different consistency ranging from hard (shale) to very soft depending on their moisture content and their depth relative to the ground surface. The extent of the depth taken for the purpose of this paper is up to 50 meters below the surface level.
The layers of these silty clay soils are laminated with slices of silty sand and traces of decomposed roots and other organic materials. The percentage of sand in the composition of these soils varies but the in general this percentage is less than the clay and silt content. The color of these natural soils ranges from brownish to dark grey. The natural moisture content of these soils is very high; it can reach up to 256%.
A summary of the general engineering properties of these soils is as follows:
Dry Density (kN/m3) : Avg.=10.19, SD. = 3.34, Max.=17.32, Min.= 3.41. Moisture Content (%): Avg.=75.42, SD = 51.44, Max.=256.8, Min.=17.81. Liquid Limit (%) : Avg.=57.65, SD = 14.69, Max.=78.32, Min.=28.14. Plasticity Index (%) : Avg.=29.85, SD = 11.31, Max.=45.00, Min.=13.14. Cohesion (Cu , kN/m2) : Avg.= 9.49, SD = 10.73, Max.=39.00, Min.= 2.07.
Angle of friction (Φu , degrees) : Avg.= 2.92, SD = 4.66,Max.=20.50, Min. = 0.50.
Maximum Dry Density (kN/m3): Avg.=18.48, SD = 5.99, Max.=20.00, Min. =16.65. Optimum Moisture Content (%): Avg.=18.48, SD = 5.99, Max.=20.00, Min. =16.65.
Where; Avg. is average value, SD is standard deviation, Max. is the maximum value, and Min. is the minimum value.
Fig.1 is prepared to show the relationship between the maximum dry density, optimum moisture content, and plasticity index of the studied soils. This figure shows that soils which have higher plasticity index have smaller maximum dry density and higher moisture content. This relationship has a strong correlation factor with 95% confidence level.
Fig.2 to Fig.5 are prepared to show the strong relationships that exist between the undrained cohesion (Cu) of these natural soils and the rest of engineering properties. The cohesion of these
natural soils is strongly correlated to the liquid limit and the plasticity index as shown in Fig.2 to Fig.5.Increasing the liquid limit, plasticity index, or moisture content decreases sharply the undrained cohesion, Cu (or shearing strength) of the soil. These relationships have strong correlation
factors with 95% confidence level.
The stability analysis for natural slopes (or manmade slopes of the same natural soils) has been carried out using Bishop method (Lambe and Whitman, 1979) to find what is the optimum value for the height and length of slopes that can be built safely from these natural soils. Also this analysis will help in assessing the stability of existing natural slopes against landslide. The stability analysis was carried out on soils in natural condition (undisturbed samples) and the strength tests were based on undrained condition (Φu = 0.0) to simulate recently constructed slopes, Bowles (1988).
Fig.6 and Fig.7 are prepared to show the results of the stability analysis that was carried out on these natural silty clay soils. When the cohesion of soil is very small (less than 10 kN/m2) the failure is expected (factor of safety = 1.0) even at less steeply slopes. For a soil with undrained cohesion of 2.0 kN/m2, even a slope with 5.5 horizontal to 1.0 vertical will fail. Also for a soil with cohesion of 10 kN/m2 the slope of 1.0 horizontal to 1.0 vertical is not stable as shown in Fig.6.
When the factor of safety required is increased to 2.0 (or more stable slopes), the slope geometry becomes uneconomical for slopes built from these soils. A soil with cohesion less than 10.0 kN/m2 will need a slope of 3.0 horizontal to 1.0 vertical and for cohesion of 2.0 kN/m2 a stable slope needs a geometry of 8.5 horizontal to 1.0 vertical.
3. CONCLUSION AND RECOMMENDATIONS
The natural slopes gain its stability over a long time period by balancing the effects of natural source such as gravity forces, type of soil, moisture content, and/or erosion by rainfall or of manmade source such as cut and fill embankments Sultan (1992b).
There are many examples of stable, natural, high, and steep embankments in Brunei Darussalam inspite of the weak quality of their silty clay soils. This can be attributed to the existence of natural forest cover that protects the underlying soils from further exposure to erosion and keeps the rainfall water away.
water to expand and losing strength. There are also many examples of steep high embankments in Brunei Darussalam that are stable even without the existence of the natural forest cover , this can be attributed to the efficiency of these embankments to drain surface water quickly because of their steep slopes. The most important factor in controlling landslides is to prevent the contact between these soils and external water, Sultan (1993). The silty clay soils have high strength when they are dry. This is clear in black shale soil which has firm to hard constancy inspite of its composition of the same silty clay soils. The manmade embankments follow the same rules, but with one exception. The suitable soil should be used to construct fill embankments according to the standard specifications, this means that soils with plasticity index more than 30% and liquid limit more than 50% should not be used.
From the economical point of view, cut embankments which comprise of silty clay soils can be constructed without the use of retaining structures provided an efficient drainage system for surface and subsurface is provided.
3.1 Prevention and Correction of Landslides
Landslide prevention needs to be recognized during early stage in the planning stages of a highway. Evaluation of existing topographic, geologic and soil maps and air photos can be very useful in the initial planning stages. A field reconnaissance of the proposed route, preferably by a geotechnical engineer experienced in the region, can be very useful in identifying potential landslide problems.
Once preliminary study indicates potential landslide areas along a route, subsurface investigations can be conducted to define the probable limits of existing or potential landslides. Then, during design stage, studies can be conducted to make sure that the probability of construction-induced landslides is at low level. At this stage it is possible to change the alignment to avoid serious problems or alter the grade to minimize cutting and filling.
The prevention of a landslide is almost always less costly than the cost of correction if the landslide is permitted to occur. There are many methods for correcting landslides, the selection of the proper method is based upon its performance and cost effectiveness, Tomlinson (1995).
These methods are:
1. Relocation of highways.
2. Removal of anticipated landslides.
3. Excavation and replacement with compacted material. 4. Bridging.
5. Control of Drainage. -Surface drainage.
-Subsurface drainage techniques. 6. Retaining structures.
7. Chemical stabilization. 8. Soil Reinforcement.
9. Enforcement and Control Policy.
References
Al-Shakarchi, Y. J., and Sultan S.A., (1989), “ The Behavior of Instrumented Model”, Proc. 2nd Int. Conf. On Found. & Tunnels, London, UK, Vol.1, pp. 175-180.
Bowles, J. E., (1988), “ Foundation Analysis and Design”, Fourth Ed., McGraw-Hill Book Co., N.Y., USA.
Deen, R. C., et. al., (1975), “Subgrades, Foundations, Embankments, and Cutslopes”, Section 15, Handbook of Highway Engineering, Van Nostrand Reinhold Co., N.Y., USA, pp. 390-496.
Lambe, T. W., and Whitman, R. V., (1979), “ Soil Mechanics”, SI Version, John Wiley & Sons, N.Y., USA.
Sultan, S. A., (1992a), “ Winkler Model for The Prediction of Pile Group Behavior”, Proc. Of Jordanian Conf. On Civil Eng., Amman, Jordan, Vol. III, pp.974-984.
Sultan, S. A., (1992b), “Study of Mechanical and Mineral Properties of Silty Clay in Baghdad City”, Proc. 2nd Int. Conf. of Ministry of Housing & Construction, Baghdad, IRAQ, pp. 229-236.
Sultan, S. A., (1993), “ Foam Clay Blocks from Local Materials in Jordan”, Proc. 1st Int. Conf. on Implementing Local Materials in Industrial Application, Amman, Jordan, Vol. 1, pp.133-141.
