The property is a semi-detached London town house in the Bolton’s Conservation area. In 2000 the property was significantly extended including a large basement, semi-detached Mews cottage and rear extension to the main house.
The basement extends under the full length of the rear garden linking part of the front main house and the rear mews. There is a swimming pool in the basement under the garden area. The rear garden roof slab of the basement is supported on a series of beams which in turn are supported on the party wall side columns and on the retaining wall on the other side.
The proposed works to the basement under the rear garden are to move the pool location and to increase its size by 8m2 , demolishing the existing reinforced Concrete(RC) columns in the pool area.
The water table level is 5.5m below ground level which means the existing swimming pool falls below this water table level. The existing columns which are taking the load off the one metre deep soil in the garden and the existing garden roof RC slab are keeping the basement pool in position against the uplift force caused by the water table level.
The existing RC beams are part of the neighbour’s structure too. In the neighbouring property (7 Tregunter Road), the RC beam is continuous on two intermediate columns, the party wall and the RC retaining wall. The top beam is supporting the garden roof slab and the soil.
The removal of the columns in 5 Tregunter Road will cause a huge increase in the bending and shear stresses within the beam.
To keep the same stresses as before, the analysis has been done applying a force in the existing column locations such that the applied force retains the same bending and shear stresses unaltered. This permanent force is going to be applied on site through flat jacks.
Flat jacks are installed in-situ and inflated using a high pressure injection system, epoxy resin or cementitious grout can be precisely injected. This process is primarily implemented whilst carrying out works for pre-loading of structural steelworks or part of other alterations.
After propping the existing RC beam, the existing columns will then be demolished. The load from the beam in the column location is going to be taken by a series of circular hollow sections(CHS). Holes with size equal to that of the CHS will be drilled into the RC beam. Because of the holes the existing shear links will be cut in the RC beam. In order to retain the same shear capacity a steel plate will be fitted on either side of the beam through epoxy resin and hilti bolts.
In order to apply jacking in-situ avoiding damage to the existing beam, a rectangular Hollow section (RHS) is going to be welded to the CHS sections to hold them in place. The RC beam is strengthened by steel beams to redistribute the forces in the changed configuration. A gap will be left between the strengthening beams and the RHS sections to accommodate the jacking equipment.
At basement level, U-shaped RC beams are designed to take the point loads of the new columns and the uplift forces. The pool base slab is designed in such a way that it spans between the U shaped beams. The pool walls are separately checked for lateral forces induced by earth pressure , surcharge and water pressures.
Developer: George James Associates LLP
Contractors: TR Construction