As part
of a flood alleviation scheme on the River Torridge in north
Devon for the National Rivers Authority, two sets of 500 year old
stone and cob cottages were lifted by 1.5m, clear of flood water.
Yeo Cottages and Dock Cottages comprise a
pair of dwellings and a terrace of six dwellings respectively.
The method used for lifting the cottages was as follows.
After breaking out the existing ground
floor slab, hand dug piles were installed down to bedrock at
pre-determined lifting positions. A reinforced concrete column
was constructed on each pile and the piles were connected by a
series of reinforced concrete ground beams.
A reinforced concrete raft was cast around
the columns, over the groundbeams and under the cottage walls.
The raft was connected by means of high-tensile steel lifting
bars to hydraulically operated jacking rigs, located on the top
of the concrete columns.
The raft and cottages were then lifted up
the columns in an incremental manual jacking operation. Jack
operators at each lifting position were directed via a
‘PA’ system by the lift controller. On command, each
operator would pump his jack hand pump a set number of strokes.
A water level was used to check that the raft was
being lifted by exactly the same amount at all jacking positions,
adjustments being made as necessary. This was vital in order to
avoid distortion of the raft and distress to the cottages.
Each set of cottages was lifted in increments of
3mm. After each increment, the lifting bars were supported
independently of the jacks as a safeguard against hydraulic
failure.
This simple manual system proved to be very
effective and each set of cottages was fully lifted in less than
three days without any problems.
Yeo Cottages, shown above, contain a number of
Jacobean plaster details. These were protected against accidental
damage and were unaffected by the lift.
On completion of jacking, permanent raft support walls were constructed on the ground beams and the jacking rigs were removed. The cottages were then refurbished and the surrounding gardens re-landscaped.
Client: NRA (South-West Region)
Designer: John Ropkins
Contractor: Jacked Structures Ltd
Scope of work: Design and construct
Contract period: 6 months
Completion date: December 1991
© John Ropkins Ltd