Reference no: EM132734302

M23357 Soils and Materials - University of Portsmouth

Part 1A - Slope Stability Report

Objectives:

• Assess the stability of a cut slope in soft clay in the short and long term.
• Evaluate the impact of pore water pressures on slope stability.
• Identity suitable remedial techniques for unstable slopes

Task:

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:

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

Learning outcome 1: Apply standard procedures to assess the stability of slopes using stability charts and software
Learning outcome 2: Evaluate the important issues with regards to the remediation and maintenance of slopes
Learning outcome 3: Identity suitable remedial techniques for unstable slopes.

Part 1B - Cofferdam Report

Objectives:

To present calculations for the embedment depth of a sheet pile cofferdam and evaluate stability and construction issues.

Task:

The construction of pumping station very close to a river bank requires a long excavation, running parallel with the river, 25m in width and to a depth of 7m (Figure 1) To enable the foundations of the intake works to be constructed in the dry a steel sheet-pile cofferdam is to be constructed between the river and the excavation. The properties of the River bed soil are presented in Table 1.

(a) Using Geosolve Wallap and the strength factor design method, calculate the penetration depth (d) needed to achieve a factor of safety of 1.25 against overturning. Include a prop if you think it is necessary, and choose its position.
(b) Validate this design penetration depth (d) with hand calculations, again using the strength factor design approach, taking into account seepage forces and water pressures.

(c) The design penetration depth (d) will also impact the flow of water into the excavation under the sheet pile. Sketch a flow net for your chosen design penetration depth and use it to determine the quantity of flow into the excavation.
(d) Critically evaluate the design approach followed from part (a) to (c) considering any design requirements or construction aspects that have been overlooked, or assumptions that may not be reasonable.

Learning outcome 1: Use hand calculations and software to analyse the stability flexible retaining walls.

Learning outcome 2: Analyse ground water flow around flexible retaining walls

Learning outcome 3: Evaluate all construction and economic considerations for the design of flexible retaining walls.

Attachment:- Soils and Materials.rar