The challenge
Mulholland Contracts, a civil engineering and groundworks contractor specialising in the house building sector, has been working on the Dalmore Mill housing development in Auchendinny, Midlothian for five years.
A stream runs alongside the Miller Homes development of 150 new homes and, with the increase in volume and weight of traffic, the local council recently requested that the current bridge over it be replaced.
That involved excavating an area on both sides of the stream for the supporting structure at each end of the bridge to sit in. At four metres deep, five metres wide and around 20 metres long, the cofferdams would be close to an occupied property and underground services. What’s more, the type of soil would require an Excavator Mounted Vibrator (EMV) to vibrate the trench sheets into the ground.
Our solution
With an occupied property sitting just three metres away from the second excavation, our customer product specialist worked extremely closely with Mulholland, both on and off site, to assist in the excavation.
Once the first cofferdam was installed, our engineers’ altered the design of the second Cofferdam to avoid existing services and work around the occupied property. Both cofferdams were approximately 20m x 5.0m x 4.5m deep, using 6.0m long M15 interlocking trench sheets, two levels of Multibrace frame with Heavy Duty mechanical bracing struts.
However, the second cofferdam had to be modified to accommodate existing foundations and services. The sequencing of construction of the cofferdams resulted in over 200 6m M15 piles having to be on site at the one time. The design was based on pre-driving the trench sheets before installing two levels of frame, the lower of which could be removed once a suitable concrete base had been cast. As the top frame had been positioned at ground level, this allowed Mulholland to construct the bridge abutments, below this frame.
