Conceptual Design of a Combined Retaining Wall and Foundation Engineering Assignment Help

Assignment Task

Conceptual Design of a Combined Retaining Wall and Foundation.
Briefing for Q1 and Q2: The development of a site requires the use of a retaining wall to stabilise the ground conditions. The natural soil (a weak clay overlying a stiff clay) is to be cut back (using a stepped excavation) and a coarse grained backfill is compacted into place behind a precast, reinforced gravity retaining wall. The weak clay soil extends 4 m below the proposed base of the wall, underlying this soil is a stiff clay layer.

A 1 m thick layer of the coarse-grained fill was to be compacted in front of the wall

Figure Q1 &Q2: Illustration of the cantilever gravity wall with small diameter piles (elevation and plan). Note that the plan view does not capture the length of the wall, just a small section to illustrate the location of the piles. Illustration is not to scale.

The initial design proposed the use of the reinforced concrete, cantilever gravity retaining wall: 6.5 m in height and 4.5 m wide, with the base and wall being 0.5 m thick. The initial design was analysed and found wanting, as overturning appeared to be a potential issue. Therefore, the design was modified to include small diameter piles (no more than 700 mm in diameter) that were installed 0.5 m from the toe and heel of the wall, every 3 m along
the length of the wall (Figure Q1 & Q2). The piles are tied into the base of the wall (the base effectively acting as a pile cap).

Q1 (a) Measurement of the compacted backfill indicated that before a groundwater table formed within it, the natural water content was 6%. This was less than the 12% optimum water content associated with the compaction curve derived from the laboratory. The stiffness and strength of the compacted backfill also proved greater than expected. What has caused this, and what is likely to happen to the backfill once the water table has formed and risen to the ground surface? Use sketches to illustrate your response

(b) Justify which shear strength parameters should be used in the analysis of the soil loads acting on the wall

Q2 The engineer in charge has suggested that the installation of piles will be sufficient to stabilise the ZDOOresistance to sliding and overturning. You have been tasked with designing these piles and identifying if the vertical displacement of these piles is within the threshold set. [Please note that in question Q2 it is assumed that the horizontal component of force need not be considered in the analysis of the piles. Therefore, for parts a-c, you only need to consider the vertical load. Furthermore, it is assumed that the vertical load is evenly distributed along the pile cap (for parts a-c) and for purposes of this calculation, the mean stress acting along the pile cap was found to be 190 kPa]. As such you are asked to undertake the following tasks.
(a) You should firstly consider the a pair of piles (transverse to the wall) and come up with appropriate dimensions showing that they have the capacity to sufficiently withstand the vertical loading from the retaining wall located above. Show all your calculations (stating any assumptions made) demonstrating that the single pile capacity is within an appropriate threshold ± show values achievable both with and without appropriate factors of safety. 

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