Holdfast Training Services Trial Blog

Civil friends, I would appreciate your input on the following dewatering issue I am currently faced with.  This to ensure I am on the right path before I look into a suitable well point dewatering system, effects of drawdown, settlement and dealing with contaminates in the groundwater etc.

Design

Sheet piles have been installed as temporary works to a set depth to form a single skinned cofferdam.  A rail bridge will be subsequently constructed over the sheet piles footprint, prior to the cofferdams excavation to create an underpass.  To date the sheet piles have been installed but there is no set solution to dewater during excavation.

The rail bridge is at 8.5 mAHD, the sheet piles have been driven to a depth of -3.5 mAHD (12m long). To establish acceptable dry conditions for work, it has been proposed to dewater to 1.5 mAHD (the finished underpass road level is above the water table).

Conceptual Model of Groundwater Flow

There are two main groundwater systems beneath the site, a deeper confined groundwater system separated by the fractured Hawkesbury Sandstone and a shallow, unconfined / semi confined system within the Botany Sands. I have taken the groundwater level as RL 3.6 mAHD based on advice from Douglas Partners. Recharge of the Botany Sands Aquifer occurs through direct rainfall infiltration in the highly permeable sands as well as high and low tide. I have taken the average permeability across the Botany Sands as 1 x 10-4 m/s as per Douglas and Partners advice.

Estimation of Total Flowrate

I have used analytical methods to estimate the flow into the cofferdam which has produced a rather high daily flow rates of 1.9Ml per day.

Below is a draft flownet for the problem.

It has been deemed unfeasible to drive sheet piles to the clayey sand layer which may or may not be there or provide a cutoff.

Completed Rail Bridge and Underpass structure

Many thanks.

AHD – Australian Height Datum