Historical research and site investigation have been combined by consultant CampbellReith to devise a remedial underpin for an historic building in Chatham, writes Adrian Greeman
Government investment and redevelopment of the dockyards at Chatham in north Kent have helped revive the area since the Navy left the site in the 1980s.
New roads and service infrastructure along with partnerships with private firms for commercial, retail and some housing have turned the fortunes of the area around.
Despite this investment, one of the most significant buildings in the docks has remained unused -the old steam pump house from mid-Victorian times.
The solid but elegant building is a popular locally and is a listed structure.
However, until recently the word “listing” would have meant something quite different, due to the building’s settlement and overloaded foundations.
The long brick built building has two high and large chambers inside, one formerly housing some nine steam boilers and the other two huge Victorian beam pumps, later replaced with electric pumps, which drew down the water in three new dry docks built in the 1860s.
The boilers also supplied power to the then new-fangled dockside steam cranes through underground brick culverts.
The South East England Economic Development Agency (SEEDA) has been working on the building for a decade to make repairs.
“We have put over £1M into it to renovate and clean it,” says SEEDA project director Julian Perry, who adds that the aim is to find a commercial developer once the building is in good shape.
“Early on, perhaps just after construction, the building seems to have broken its back, possibly due to a difference in the foundations in the middle”
One potential use would be as a banqueting and restaurant complex for wedding receptions, or similar social events.
Planning consent, and approval from English Heritage, has been obtained.
Much of this structure was examined and renovated around 2001 with many - around 27,000 or so, according to Perry - of the old bricks replaced.
They were a fairly soft type and had weathered badly over the years, but an unusual striped pattern of alternating light and dark colours is an important part of the building’s aesthetic.
A huge 1.8m deep water tank on the roof to feed the boilers was also renovated. An original chimney was removed, however, due to its very poor condition.
Consultant CampbellReith did the work assessing and designing the repairs at the time.
The old boilers were also taken out and the building cleaned down inside and out.
But the building had problems with cracking as well, says CampbellReith senior engineer Vernon Westwood, who has been involved since the beginning of the scheme.
The cracks were examined and monitored and it appeared that much of the cracking was historic.
Movement seemed to have stopped.
“Early on, perhaps just after construction, the building seems to have broken its back, possibly due to a difference in the foundations in the middle,” adds Westwood’s colleague, engineer Elizabeth Brown.
“Most of the structure is on a concrete encased timber grid supported by timber piles, while in the centre under the pumps it sits over deep culverts which bear directly on lower ground levels.”
The cracks appeared to be stable.
But after some years when the building was mothballed, a new assessment was made of the structure’s condition in 2009 and it was found that there had been significant movement.
“There were cracks in the brickwork you could put your hands in,” says Westwood.
At the southern end of the building in particular, monitoring indicates a slow progressive movement.
Three of the building’s main piers seem particularly in distress and some movement continues.
“If a developer is to take the building they would have to be confident that it was not going to cause huge problems in the future,” says Westwood.
“But at two points under the pump room the foundations use the steam and water culverts below to carry the loads directly”
A remedy was needed.
CampbellReith has undertaken extensive historic research and ground investigation to work out what is happening and from there to devise a solution.
“We are lucky to have found some excellent recorded lectures and drawings of the original construction, both of the docks here and the building,” says Brown.
From these records it is clear the building was part of a major development of the docks in the 1860s and 1870s when a muddy shoreline of the Medway estuary was developed for three new ship basins and several dry docks.
Marshy and alluvial ground was removed in large excavations and timber piles for a multi-celled concrete seawall were driven; the ground was built up again behind with imported “excellent loamy earth” from the nearby excavations for the London and Chatham Railway.
“For the pump house itself the ground conditions comprise made ground and alluvium to around 16m depth and below this is 3m of gravelled river terrace deposits, with chalk below,” says Westwood. “A perched ground water level was observed at around 3m.”
There is also groundwater in the gravel with a tidally affected piezometric level between 4.5 to 6m.
Like the seawalls the pump house was constructed by excavation into the estuarine silt, with piling into the underlying gravel.
It was a fairly long building at 60m and constructed as a single piece. “Modern building would have an expansion joint every 12m,” says Westwood.
Timber piles provide the main support via a grillage of timber elements about 5m down which is encased in concrete.
Above this are inverted brick arches to the piers and a solid brick wall above.
“But at two points under the pump room the foundations use the steam and water culverts below to carry the loads directly” says Westwood.
The culverts are more than 2m in diameter with an invert approximately 18m deep, bearing on gravel.
Some of the cracking appears to have happened through differential settlement at the interface of the timber and mass foundations shortly after construction.
Backfilling to raise the land in the area probably added to this, causing negative skin friction in the timber piles.
This might have remained stable but for a second phase of movement in recent times.
The Medway road tunnel which was built in the mid-1990s sits only 100m away.
It used considerable dewatering which resulted in groundwater levels dropping by 6m in the area of the pump house, which caused settlements of up to 90mm.
“It probably added to the negative friction on the piles,” says Brown.
“It is a bonded system rather than a needle and straddle with the pile passing through the underground structure and binding to it”
A back analysis of the piles shows that many of them would have been close to capacity anyway and so the additional loading was likely to have caused failure in some of them, possibly through shearing.
The result has been distress at the southern end of the building and continuing movement “although the rest of the building appears stable”, says Westwood.
To deal with this CampbellReith has devised a micropile underpinning system, based around the pali radice or “root pile” system used by Keller. “The design uses a series of raked piles which interlock to form a kind of cradle in the ground,” says Brown. “It is a bonded system rather than a needle and straddle with the pile passing through the underground structure and binding to it.”
The bonding is caused partly by the use of a triaxial drill bit for most of the pile formation which creates a serrated pattern in the hole and allows a strong bonding once the grout has gone off explains Westwood.
It causes less impact than other systems, which is important, especially considering the structure’s listed status.
The project uses 14 piles at carefully calculated angles to create three cradles around the most affected piers.
“I would like to have feathered the piers under one or two of the adjacent piers for extra certainty,” says Westwood.
Piles are 27m long and 220mm in diameter, drilling through the foundations and into the chalk below and then grouted with a cementitious grout.
A rotary triaxial head gets through the brickwork before a casing is introduced to go through the aluminium and water filled gravels below and into the chalk.
It is withdrawn once the reinforcing bar is inserted and the grout introduced.
Keller Geotechnique, which won the contract, has been using a Klemm 702 drill.
Keller site engineer Steve Hickey says the drill is particularly useful because besides its low headroom it had a capacity to set the drill head angle both forward and back and laterally, helping position the specific angles needed for the complex interlocking pattern of the underpinning.
Alongside the drilling there has been additional work in the brickwork, particularly using Cintec bars above ground.
The 8m long and 75mm diameter bars are inserted into 150mm diameter drilled holes and grouted to form a virtual “masonry beam”, which Westwood says helps distribute the loads on the piers to the next ones along.
Work for this has been done by Charterbuild.
“And there is also crack repair with Helifix bars bonded across using a resin mortar,” says Westwood.
Finally, conventional mortar pointing is carried out.
The main drilling work was completed in August and the next task is to find a private sector taker for the building to develop the interior.
The notional plan is for there to be a series of “floating” interlinked platforms for dining at various levels, a three-storey kitchen complex and banqueting space for larger functions.