(with grateful thanks to Mrs Panter & family)
Retaining walls are poorly understood by property owners and builders alike - a good retaining wall will do its job completely unnoticed for decades, whereas one which has not been designed or constructed properly can fail suddenly and they have been known to kill or injure people when they collapse.
The majority of the structure of a retaining walls will be concealed beneath the ground; it may look like an ordinary wall to the casual observer, with only the drainage holes near the base to give a clue. The following case history may help to illustrate the ways in which Structural Engineers Cambridge Ltd can help with the design and construction of a safe and efficient structure.
Rapid Response to a Collapsed Wall
Back in February 2009, we were called by Rodger Harrison of Drain Surgeon UK, a contractor working for the Environment Agency to assist with a wall that had collapsed into a small river in Kent. At the time, the recent snow had melted rapidly, augmenting the rainfall and causing the river level to rise by a metre in a few hours. The rapid flow of water had caused the concrete wall at the side of a house to fall into the river and there was concern about the foundations of the house being undermined by the water.
The first visit was that evening, to witness the contractors working in the dark and freezing water to clear the blockage in the watercourse and allow the floodwater to escape downstream of the site. Proposals for removing the overhanging shed and banking the debris against the sloping surface that was once the side of the driveway were quickly agreed and arrangements made for contacting the Environment Agency and insurance company to gain approval for the replacement of the wall.
Shortly afterwards, Structural Engineers Cambridge Ltd conducted a measured survey of the site. The old wall was approximately 16 metres long and followed the line of the river along the boundary of the property. It was approximately 80 years old and made of unreinforced concrete with inadequate resistance to overturning under the effects of water pressure in saturated soil. Parts of the wall and its foundation remained in the riverbed and the remainder had been broken up.
The terms of riparian ownership of the riverbank meant that the owner of the house was liable for the upkeep of the bank and watercourse up to the centreline of the river. The insurers were initially reluctant to meet the claim, but an explanation of the position by Structural Engineers Cambridge Ltd enabled the elderly owner of the house to make a claim on her policy.
Underpinning and Piling
Restricted access in a suburban setting and the proximity of the house to the riverbank meant that the replacement of the wall would be a tricky operation. Working in partnership with piling specialists DJE Construction Ltd, the side wall of the house was first underpinned with driven piles and needle beams to provide stability before the excavation of the bank could begin in earnest. A further wall of steel-cased piles was driven along the side of the house and tied back with Platipus soil anchors, to form the functional retaining structure. Test boreholes were made before construction to verify the soil conditions and enable the correct choice of foundation design. Prior to piles being driven, all of the neighbouring houses were surveyed to ensure that any pre-existing flaws in their structure would not be exacerbated by vibration. In fact, the use of a compact mini-rig for pile driving meant that there was little disturbance and no damage even to the closest building. all of this work was agreed, designed and construction initiated before the insurance company had made the decision to accept the claim!
Reinforced Concrete Wall
As the piled wall could not follow the original contour of the riverbank and would not provide an acceptable appearance, a reinforced concrete fascia wall was specified so that the full area of the plot could be restored to level ground. This would need a foundation below the soft alluvium of the riverbed, requiring the contractor to temporarily dam the river and "overpump" the water beyond the working area. The new reinforced concrete wall would be erected in formwork and tied back to the piled wall, giving a compact arrangement that could be constructed within the restricted space and with minimal use of heavy plant and equipment.