An innovative method of underpinning is being used to save the historic Trinity Bridge in Crowland, Lincolnshire.
Thought to have been built around 1360 by the monks of Crowland Abbey, the stone triple abutment bridge had its river diverted in the 18th Century. Now the scheduled monument is a curiosity, a Venetian-like series of steps and arches with no water below.
Over the last two decades there has been a further gradual drying out and compression of an underlying peat layer. The resulting settlement has been evidenced by the appearance of a number of cracks of up to 8mm in width and almost 3m length.
The abutments are based in a band of dense sand and gravel, but the growing cracks emphasised the need to arrest the movement before the damage worsened.
Lincolnshire County Council engineer Richard Waters says:
'Historic monuments place many constraints on repair options. A method of stabilising the ground beneath the approach wall foundations was required which would not involve any damage to the structure itself, or disturbance of any adjacent archaeology.
The solution was the Uretek Deep Injection Method, which is based on the concept of introducing a strong, high density supportive solid into sub-soils while in a liquid, pre-hardened state through a system of small diameter injection boreholes. Compared to conventional underpinning methods it is almost keyhole surgery.
With the goal of preventing further movement of the soft clay and peat, Uretek UK injected its high density polyurethane resin at 1m centres over 28 linear metres until 1mm of lift was detected using a laser level mounted on the bridge.
With the underpinning completed successfully, a specialist stone conservator is set to start repair of the bridge in March 2003.