ABSTRACT:

Based on the limit-equilibrium methods, the design scheme of the slope in a lake tunnel project and the reason why choosing the ordinary method of slices have been given. Through comparing the effect of the important parameters influencing the slope stability and considering the economy and construction, an optimistic scheme is made.

I.
INTRODUCTION

Today, The limit-equilibrium method by the assumption of a circular slip surface is still the most common way to analyze the slope stability. Some researches have shown that although different theories of slope stability make different assumptions to satisfy the static balance, the safety coefficients being worked out are nearly the same when considering the rationality. Therefore, the limit-equilibrium method which often complies with actual failure surfaces and is simple to computation are widely used in engineering practice even it cannot involve the stress-stain behavior of soil. The method of draining the lake water and open excavating is adopted, because about 1.8 kilometer of the tunnel is under lake, part of which reinforcement is needed. The slope technology and economy must be synthesized to make an optimized scheme.

2.
THE ENGIEERING GEOLOGY

The under lake part or the tunnel are all located in the ancient fiver course, which increases complexity of the geological conditions. In the exploratory areas, the engineering soils mainly include the saturated powdery, sandy soil and the soil clay, which means low bearing capacity of foundation and high water-holding. 3.2 Choosing the method of calculation The ordinary method of slices is the simplest method on slope stability analysis because it assumes that the slip surface is circular and omits the action between the soil slices. There are many other methods like Bishop's and Janbu's etc. Which carry out a further research into the action between thc soil slice and the shape of the slip surface. But these methods have much more parameters and more complex to calculate, for which they are mainly used to check the slope stability. It has been pointed out by the reference (Zhang, 1987) that the location of the critical slip surface will almost be the same no matter how many the factor of safety has been worked out while using different calculating methods meanwhile the factor of safety will change little near the critical slip surface. Accordingly, the more accurate method may be used to calculate the factor of safety of the critical surface that has been found out by the ordinary method of slices and the other slip surfaces near it. In Tab.2, the ordinary method of slices, the Bishop's method and the Janbu's method are used to calculate the factor of safety to compare the result. There are six calculating sections and the slope is 1:1.5. It can be seen from the Tab.2 that it is safe to use ordinary method of slices, because the intrinsic friction angle Ф is big enough to avoid the factor of safety being too small caused by the normal stress acting.

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