The site is located adjacent to the A403, a major route for
HGVs accessing the M5 motorway, consequently both the Client and Bristol City Council were interested in a scheme that ensured minimal impact on neighbouring sites and traffic flow during
construction. 250m of the alignment would be adjacent to the public highway, involving crossing the A403, several industrial estate access roads, a railway siding,
a number of large diameter culverts, as well as avoiding major services including; gas, water, telecoms, and electricity. 150m of the alignment would be within the confines of the plant.
Disruption was to be kept to an absolute minimum, with the existing pipeline remaining operational throughout. Access routes for emergency vehicles
were to be maintained at all times. Site Investigation
The maximum depth of the proposed pipeline was anticipated to be 3.0m below ground level. A basic ground investigation
comprising a number of trial pits was considered, however due to concerns regarding disruption to the works and the cost of disposing of potentially contaminated spoil, a
window sampler and dynamic probe rig was utilised.
The window sampler is a compact rig providing a rapid means of acquiring disturbed samples in soft ground. To avoid excavation within the works a
100mm diameter core was cut through the reinforced concrete road slab for each sample location The rig was then positioned over the hole for testing. The sampler comprises a 50mm
diameter hollow tube with a slot or window in one side. This produces a continuous profile of the ground conditions for soils and chemical testing. The same rig is adapted to carry out dynamic
probing to produce a continuous strength profile over the same interval.
The ground was found to consist of a layer of made ground (150mm of reinforced concrete within the plant), overlying a soft to firm silty clay,
of recent alluvial origin, with occasional peat layers at depth, with the water table 0.5m below ground level. Chemical testing found phosphate
contamination in one location within the plant although below threshold levels. Information on ground conditions and contamination was provided
to contractors as part of the tender documentation, and project Health and Safety Plan.
Design and Route Selection
With all of the constraints in
mind, SubTerra Engineering proposed the use of trenchless techniques throughout the contract. Immediate benefits of using trenchless
techniques were the rapid issue of a street works license from Bristol City Council as no excavation on the highway would be necessary, and negating the
need to trench through the reinforced concrete roadways within the plant, thus minimising the volume of potentially contaminated material for licensed disposal.
Following corrosion trials with various pipe materials 10bar MDPE pipe was specified, with electrofusion connections. Two excavations would be required within the
plant, one for the pipeline terminal adjacent to the existing pumping station, and one for a rodding point where the alignment required a 90o, 1.5m radius bend. Three rodding point
manholes were required in the verge adjacent to the A403 at approximately 100m intervals.
The existing connection to the public sewer is via a 2.5m deep,
restricted access manhole adjacent to a small traffic island. As the, however it was not possible to utilise this connection and it was considered found to be inadequate. Two new manholes
were designed, one acting as a pressure breaking manhole for the pumped effluent, with a 3.0m long gravity pipe connection to the a second manhole located on the 2.3m deep public sewer.
This manhole was subsequently adopted by Wessex Water.
The alignment selected was restricted to a 1.0m wide corridor to avoid two previously installed directional drilled sewers in addition to
services.
Construction
Thrustbore Contractors Ltd were awarded the lump sum contract with a value of £47,000. The work was carried out in spring 1999, over a period of
four weeks. After excavation of launch pits and manholes directional drilling and pipe installation proceeded rapidly, with the alluvial clays proving an almost ideal medium for this
method. The drill head was tracked using a hand-held detector on surface which continuously receives information from a transmitter sonde located immediately
behind the drill head. This allows the plan position and depth to be recorded as well as giving a forward prediction of the drills intended orientation prior to the next 3.0m length being drilled. This
information enabled the drill to be steered to avoid obstacles and ensure correct pipe alignment.
Some difficulties were experienced when passing below reinforced concrete, and
the railway sidings because the steel interfered with the signal from the sonde. However, these errors tended to average out over short distances, and in all cases the drill head emerged at the end of
the run within a few centimetres of its target position. After completing each run the drill head was removed and the end of the pipe
connected to the drill string to be pulled back through the hole, the ground being sufficiently soft to not require reaming to a larger diameter. Joints were made using electrofusion couplers and upon
completion the pipeline was pressure tested in accordance with WRC regulations.
The new pipeline was connected to the pumping station in the plant over a three hour period, after which the old line
was sealed off and abandoned. The Contract was completed ahead of the programmed time and on budget.
