QUESTION $1,(25$ marks$)$

$($a$)$ A temporary trench is to be excavated and supported by sheet$-$pile walls for constructing a box culvert $($Figure $1).$ Given the high existing groundwater level, dewatering is required in the excavated area to provide a dry working environment. From calculation, it is found that hydraulic failure may occur as a result of the dewatering. Describe two possible solutions to avoid the hydraulic failure, and explain why these could improve the stability against hydraulic failure.

$[8$ marks$]$

Page $2$ of $9($total$)$

$($b$)$ A concrete retaining wall was originally designed based on the assumption of no water pressure $($Figure $2($a$)),$ i$.$e$.$ groundwater level being well below the wall base. However, due to changes in circumstances, the site is flooded, with a hydrostatic groundwater table raised to the level of the retained ground surface $($see Figure $2($b$)).$ Explain from first principles whether the factor of safety against sliding would increase, decrease, or remain unchanged as a result of the flooding.

$[8$ marks$]$

$($c$)$ Guidelines for design of gravity earth retaining structures are given in both the First Edition and Second Edition of Geoguide $1.$ Describe the main differences between the two editions of Geoguide $1$ in respect of the design approach for such structures and how factors of safety are to be applied.

$[9$ marks$]$