SCHOOL OF CIVIL ENGINEERING AND SURVEYING

SCHOOL OF CIVIL ENGINEERING AND SURVEYING

A cut slope in soft clay has been constructed as part of a road alignment. The slope is 1 in 2.466 (or 2.466:1 as a horizontal:vertical ratio) and 10 m high. The unit weight of the soft clay is 18 kN/m3 . 

(a) At the time of construction the slope was designed based on undrained analysis parameters. An analysis using Taylors Charts yielded a factor of safety of 1.2 for the short term stability of the slope. Backcalculate the undrained shear strength (cu) of the soil assumed for the soft clay at the time.

(b) A walk over survey recently indicated signs of instability. Samples have been collected from the slope and the drained analysis parameters for the soil have been determined as follows: 

Groups 1&4: φ'= 25 º, c'=2.6kPa, d=17kN/m3 , sat=18kN/m3 

Groups 2&5: φ'= 24 º, c'=2.5kPa, d=18kN/m3 , sat=19kN/m3

Groups 3&6: φ'= 26 º, c'=2.3kPa, d=18.5kN/m3 , sat=19.5kN/m3

Based on the effective stress parameters given, perform a quick initial estimate of the factor of safety of this slope using Bishop and Morgernsterns charts. Assume an average pore water pressure ratio (ru) of 0.28 for the slope.

(c) Piezometers have now been installed to precisely monitor water levels and pore pressures and their fluctuations with the seasons. The maximum water levels occurred during the rainy season. The worst case water table position is given in Table 1 in the form of the mean height above the base of the 6 slices of the slope geometry shown in Figure 1. Using Table 1, estimate the drained factor of safety using the Swedish method of slices, accounting for pore water pressures. 

(d) There are plans to build an industrial steel framed building on the top of the slope with the closest footing to be positioned 3m from the top of the slope. The footing will be 0.7m width and the design load will be 90kN per metre run of footing. Calculate the long term factor of safety using Oasys Slope and Bishops variably inclined interface method, modelling the footing load as a surface load (neglecting any footing embedment). You will need to estimate the centre of the slip circle.

(e) Considering the factors of safety calculated in parts (b)-(d), critically evaluate the original design of this slope, its long term stability and the most important issues that it has. 

(f) Identify two viable remedial measures that could be taken to enhance the stability of this slope, explain in detail your rationale for choosing these solutions and how they would be implemented. Consider the soil type and the slope geometry carefully and consider some calculations to assess the effectiveness of your solutions in solving this problem

complete case study is attached below