Study Area

Under an applied rainfall intensity of 22 mm/hr for 24 hours, the factor of safety of the slope without horizontal drain was initially 1.20 and decreased to a minimum value of 0.97 at elapsed time of 120 days as shown in Figure 6(b). The initial factor of safety of the slope with the length of horizontal drain extended to the middle of the slope was 1.35 and the minimum factor of safety was observed to be 1.33 at 26 days after the rainfall stopped. The initial factor of safety of the slope with the length of horizontal drain extended to a distance below the crest of the slope was 1.38 and gradually decreased due to rainthe length of the horizontal drain. Horizontal drains discharged a large amount of water as soon as they were installed as shown by the flux rate (i.e., 8.4x10" 6 m/s) for horizontal drains installed to a distance below the crest of the slope at and decreased gradually in a short period of time which was 10 days. The flux rate for horizontal drains extended to the middle of the slope was initially 3.4x10-6 m/s before decreasing to an equilibrium condition. Meanwhile, the flux rate for horizontal drains extended to the critical slip surface was initially 5.6x10-6 m/s before reaching an equilibrium condition. The flux rate of water flowing through the drains would increase significantly during the period of heavy rainfall and gradually decrease after the rainfall stopped (Fig. 7(b)). Under an applied rainfall intensity of 22 mm/hr for 24 hours the maximum flux was e as high as 7x10-5, 5,3x10- , and 3.4x10-5 m/s for the horizontal drains extended to a distance below the crest of slope, to the critical slip surface, and to the middle of the slope, respectively.

Elapsed Time (days) Elapsed Time (days)

Figure 7. Rate of water inflow to the drain for Bukit Timah Granite residual soil slope with different length of horizontal drains: (a) from the horizontal drains installation until reach equilibrium without rainfall to drawdown the GWT, (b) under 22 mm/hr rainfall intensity for 24 hours

 

Figure 8. Variation of factor of safety for Bukit Timah Granite residual soil slope with different length of horizontal drains: (a) from the horizontal drains installation until reach equilibrium without rainfall to drawdown the GWT, (b) under 22 mm/hr rainfall intensity for 24 hours

 

Variations in factor of safety with time for the different lengths of horizontal drains at the Bukit Timah Granite soil slope are shown in Figure 8. After installing the horizontal drains into the slope, the factor of safety of the slope increased slightly and reached equilibrium conditions in a considerably fast rate (see Fig. 8(a)). Only about 10 days was needed for the water to be drained out through the horizontal drain and for the slope to reach its equilibrium condition.

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Under an applied rainfall intensity of 22 mm/hr for 24 hours, the factor of safety of the slope without horizontal drain was initially 1.67 and decreased to a value of 1.16 when the rainfall stopped as shown in Figure 8(b).

The initial factor of safety of the slope with the length of horizontal drain extended to the middle of the slope was 1.85 and the minimum factor of safety of 1.40 when the rainfall stopped. The initial factor of safety of the slope with the length of horizontal drain extended to the critical slip surface was 1.87 and decreased rapidly due to rainfall until reaching the minimum factor of safety of 1.55 when the rainfall stopped. Meanwhile, the initial factor of safety of the slope with the length of horizontal drain extended to a distance below the crest of the slope was 1.88 and decreased rapidly due to the rainfall until reaching the minimum factor of safety of 1.63 when the rainfall stopped. The slowest recovery rate of factor of safety occurred in the slope with the length of horizontal drain extended to a distance below the crest of slope, followed by the slope with that to the critical slip surface, to the middle of the slope, and no drain condition. The rates of decreasing and recovery of factor of safety in the Bukit Timah Granite slope were faster than those in the sedimentary Ju- rong Formation slope due to the saturated permeability of the Bukit Timah Granite residual soil was higher than that of the sedimentary Jurong Formation soil.

Figure 9 presents the percentage of improvement of slope stability in the sedimentary Jurong Formation for the horizontal drain installed to the middle of the slope which is about 13% and to the critical slip surface is 15%. Less additional benefit can be derived from using drains extended beyond where the critical slip surface intersects the top of the slope (15.44%). Figure 9 also illustrates the improvement of slope stability in the Bukit Timah Granite for the horizontal drain extended to the middle of the slope which is about 10.5%, to the critical slip surface is 12%, and for the drains extended to a distance below the crest of slope is 12.5%. Figure 9 explains that a little attempt to extend the horizontal drain length in the residual soil slopes of the Bukit Timah Granite improves the slope stability at about the same percentages as compared to lengthen the longer horizontal drains in the sedimentary Jurong Formation.

5 CONCLUSIONS

The lower permeability of the residual soil of the sedimentary Jurong Formation resulted in a slower change in negative pore-water pressure during rainfall infiltration or lowering of ground water level and therefore, it would take a longer time for the rainwater to penetrate into greater depths as compared to that of the Bukit Timah granitic soils. This condition described why it took about two years to lower down the groundwater level until its equilibrium state in the residual soil slope in the sedimentary Jurong formation. Meanwhile, the higher permeability of the Bukit Timah Granite residual soil played an important role in the rapid development of pore-water pressure in the slopes due to horizontal drain installation and rainfall infiltration.

The factor of safety of the slope slightly increased after horizontal drains were installed into the slope and was significantly faster in reaching steady conditions. No more than 10 days was required for the water to be drained out trough horizontal drain and for the Bukit Timah Granite residual soil slope to reach its equilibrium condition.

It has been shown that the length of horizontal drain affects water flow through unsaturated soil slope and consequently stability of the slope under rainfall conditions. The effectiveness of horizontal drain to the slope stability should be also considered based on drain spacing, drain diameter, and drain location. Additionally, soil properties also influence the effectiveness of horizontal drains in maintaining stability of slopes.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Glossary

slope failures-оползание откоса.откосное разрушение

Rainfall-(гидрол).

жидкие осадки

soil-water-водонасыщенный горизонт

permeability functions-коэффициент фильтрации

permeability-проницаемость

unsaturated soil slope-водонасыщенный слой почвы склона

stability of the slope-устойчивость склона

geology affect-геологический разрез

groundwater  -грунтовые воды

weathering processes-выветривание.

pore-water pressures-давление поровой воды.

specific gravity- удельный вес

liquid limit-предел текучести

plastic limit-пластичность

toe slope-подошва склона

Finite element mes- Сетка конечных элементов

volumetric water content-объемное содержание воды

saturated volumetric water content -насыщенный объем содержания воды

correction factor-поправочный коэфициент

matric suction каркасное всасывающее давление

grain size-гранулометрическому составу

two-dimensional plane- двухмерная плоскость

strain problems-давление

dimensional computer codes- трехмерный код компьютера

surface runoff-поверхностный сток

Nodal flux-узловой поток

Flux boundary-граница потока

duration-продолжительность

the factor of safety-коэффициент устойчивости 

geo-filter-гео-фильтр

line of zero flux-линия нулевого потока.

slope height- высота склона

slope angle- угол наклона

rainfall intensity- интенсивность осадков

equilibrium state-стабильное состояние

Sedimentary-осадочный 

equilibrium condition-состояние равновесия

extended beyond-продолжаться за  пределами

permeability-проницаемость

infiltration-инфильтрации

condition-условие

unsaturated zone- ненасыщенные зоны

ground surface- поверхность грунта

seepage –фильтрация

water flow-поток воды

the critical slip surface- критическая поверхность скольжения

properties-своство

maintain-потдерживать

short duration-кратковременный

failures-разрушение

Preventive measures- профилактические  меры

Recognized-признанный

low-cost-маленькая стоимость

repair works-ремонтные работы

Old Alluvium- старый аллювий

hardened alluvium-закалённый алювий

igneous rocks-магматические породы

favorable- благоприятный

erosion-эрозия

weathering processes-процессы выветревания

two-thirds-две трети

in certain ranges- в некотором диапазоне

driest periods-сухой период

the wettest periods-очень влажный период

correspond-соответствует

the initial position-начальное положение

assumption-предположение

silt-ил

clayey silt- глинистый ил

mica flakes-блестки слюды

the water content-содержание воды

equation-уравнение

volumetric water content-объемное содержание вод

The measured saturated permeability-измеренная насыщенный проницаемость

pore sizes-размеры пор

quadrilateral elements - четырёхугольный элемент

Transient process-переходный процесс

the increment-приращение

Boundary-граница

equal to the desired-равный требованной

seepage flow-фильтрационные потоки

Power and Utilities Board-энергетики и коммунальное хозяйства