Soil Mechanics $2$

Seepage Coursework Exercise

A second road bridge crossing the Firth of Forth is under construction. One of the main bridge piers will be sited in the inter$-$tidal zone, and a sheetpile cofferdam is required in order to provide a dry working area, as shown in Figure $1.$ The width of the working area is to be $12m$ and the length in the perpendicular direction will be much greater than $12m.$ At spring high tides the sea level is $3\mathrm{.}5m$ above seabed level, and pumps must be provided to keep the working area dry.

The seabed consists of $6m$ of silty sand, overlying low permeability clay. The silty sand has a horizantal permeability of $3\mathrm{.}2{10}^{-4}\frac{m}{s}$ and a vertical permeability of $8{10}^{-5}\frac{m}{s}.$ The bulk unit weight $$ of the silty sand is $18k\frac{N}{m^3}.$

The cost $($per metre run$)$ of supplying $($for the required duration$),$ installing and removing a single line of sheetpiles varies with the depth $d$ of sheetpile penetration, as shown in Figure $2.$ The cost of supplying and operating pumps $($for the required duration$)$ is $1200$ for each litre$/$s of pump capacity.

Draw flownets for sheetpile penetrations of $1m,3m$ and $5m.$ Hence suggest an optimum depth of sheetpile penetration $d,$ by:

attempting to minimize the combined cost of pumping and sheetpile provision;

ensuring that the average upward hydraulic gradient in the zone of soil enclosed by the dashed line in Figure $1$ is less than $60\%$ of the critical value associated with piping.

Prepare a brief formal report, justifying your recommended depth of sheetpile penetration.

Low Pemeability Clay

Figure $1$

Figue $2$