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Ground stabilisation: Plugging the gap

Jet grouting has helped remedial works to a sewer collapse in a residential area of Stafford, in the West Midlands.

Following the collapse of a section of a foul sewer on Landstone Road in Stafford, main contractor Amey, working on behalf of Severn Trent Water, found itself in the middle of a geotechnical challenge.

Stafford sewer, Bachy

Work had to be programmed carefully on the small site, which was surrounded by residential properties

Located in the heart of a residential area, a new section of foul water pipe was needed to bypass the collapsed section but, due to underlying ground conditions of running sand, it was soon clear that specialist support would be needed to stabilise the area before remedial works could begin.

Bachy Soletanche’s Specialist Geotechnics division was brought in to develop Amey’s preliminary outline design and undertake the work in September this year. 

“Due to the
relatively shallow
depth of the
treatment drilling
tolerances were
critical”

Gavin Clifford,
Specialist Geotechnics

“The new sewer diversion runs through the back garden of an adjacent house, connecting with the existing wet well structure,” explains Gavin Clifford, contracts manager at Specialist Geotechnics. “A 5m square cofferdam was also needed around a manhole and a stretch of sewer beneath the road, to allow a connection to be built between the new section and the existing sewer.” 

The priority was to develop the most suitable solution to stabilise the running sands, preventing them flowing into the excavations for the new foul water pipes and associated shafts. By stabilising the running sands, potential settlement of nearby properties during the excavation works would also be minimised.

Stafford sewer, Bachy

A total of 62 jet grout columns were installed, creating more than 900t of spoil, which was disposed of by Bachy Specialist Geotechnics

Jet-grouted solution

Bachy proposed using jet grouting for ground treatment. Steve Hickey, contracts engineer at Specialist Geotechnics, explains: “The aim was to construct low permeability jet grouted blocks, made up of a total of 24 columns, to enable construction of two, 2.4m internal diameter, 9m deep, ring-lined shafts. The plan also included a jet grouted structural wall and low permeability plug for the cofferdam that was needed to build the new connection.”

Jet grouting was also used to form two blocks of stabilised ground to allow the launch and reception of the 300mm diameter tunnelling boring machine forming the new run of foul water pipework.

Clifford adds: “Due to the relatively shallow depth of the treatment (less than 10m below ground level), and the high energy and high pressures involved in breaking up the soil during jet grouting, drilling tolerances within the granular material were critical. This ensured treatment was successful in creating a strong, impermeable soil/grout material. A shape accel array (SAA) was used to survey 10% of the columns to confirm the tolerance requirements of the design were met. The cofferdam was formed by three sheet pile walls and a 5m long, 1m thick and 9m deep structural jet grout wall comprising five jet grout columns. A total of 34 jet grout columns were also installed between depths of 6m and 9m to form a plug to inhibit water and sand ingress, inclined beneath the adjacent foul sewer and storm water drain to prevent damage and to ensure the integrity of the manhole.

Stafford sewer, Bachy

About 2m of ground treatment was carried out between Shaft One and the existing wet well, comprising three overlapping columns, to ensure a homogenous block of improved ground strength and reduced permeability – enabling a safe tunnel drive from the new sewer system into the existing one

Jetting parameters were modified to take account of the incline while ensuring sufficient column overlap, Clifford explains, and construction of the jet grout wall was planned carefully to ensure costly and timeconsuming repairs were avoided.

A further, slightly unusual element of the project was that Specialist Geotechnics also managed the handling and disposal of the jet grout spoil, Clifford explains. “Due to the constraints of such a small site, plus the fact it was surrounded by residential properties, it was not possible to store significant volumes of spoil on site. With up to 40m³ [60t] of spoil being generated every day, a robust and efficient disposal solution was required.”

The answer was to remove the spoil with tankers on turnaround to ensure work could continue without interruption, he says. “Initially, we had some difficulty keeping the cementitious spoil sufficiently fluid to allow tipping at the end of a three hour trip to the nearest specialist waste treatment centre. We overcame this, completing all 62 columns in 17 shifts, disposing of approximately 600m3, more than 900 tonnes of jet grout spoil over the course of the contract,” Clifford adds.

Specialist Geotechnics completed the four-week, £300,000 ground stabilisation project in October.

Readers' comments (1)

  • aquafab

    Ground stabilisation and road recycling is now widely used as a way of rehabilitating roads in need of repairs. The range has broadened over time and now includes the RX-600 and RX-400 (all the mills are available in e and ex variants) but further models are due to plug the gap.
    http://www.aquafab.co.uk/formation-treatments.htm

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