A novel dry-driven drilling method is allowing environmentally sympathetic construction of anchored sheet pile walls on the River Sow in Stafford.
Stafford's picturesque and popular Victoria Park is the setting for a flood improvement project on the River Sow.
Public scrutiny of the work is likely to be intense. Jackson Civil Engineering, which won the £3.6M contract, was particularly concerned at the potential for environmental contamination resulting from the riverbank strengthening operations.
Consultant WS Atkins' anchored sheet-piled wall scheme was developed on the basis of using the highest achievable capacity anchors at the maximum permissible spacing to minimise the risk on the project.
Generally, ground anchors and other geotechnical processes require drilling with a form of flush system - air, water or sometimes grout - to transport the drilling spoil. Given the environmental sensitivity of the project, Jackson considered spoil management a critical risk item.
The solution proposed by anchor subcontractor Keller was to use a dry-driven drilling method to install the anchors.
This is a displacement system in which sections of strengthened drill casing are driven into the ground using top-driven rotary percussive drilling.
Once the casing has been driven to full depth, the anchor is installed in the borehole and pressure grouted over the fixed anchor length in the conventional way.
Because there is no flush medium, the risk of contaminating the watercourse is much reduced.
The technique has the benefit of avoiding the costs of spoil collection and disposal.
Keller developed the technique on the Millennium Dome project where it installed vertical anchors to hold down the structure's roof supports. Here the designer required a system that would not disturb the highly contaminated fill below the site on London's Greenwich Peninsula.
John Judge, senior geotechnical engineer with Keller, says: 'The key is knowing which ground the system will penetrate, and then demonstrating the anchors can sustain the specified loads.'
At Stafford, preliminary ground investigation carried out on behalf of WS Atkins and the Environment Agency showed a succession of made ground overlying various alluvial deposits.
The alluvium was identified as very soft clay and peat overlying granular deposits.
Keller based its anchor design on the assumption that it could achieve the fixed anchor length within the granular alluvial deposits - materials identified as variable, loose to medium dense mixed gravel, sand and silts.
'These ground conditions are extremely poor for the use of conventional multi-strand ground anchors, ' says Judge.
'We couldn't be confident that we could achieve the required 325kN capacity to the factors of safety set out by BS8081.' Instead Keller proposed using the single bore multiple anchor (SBMA) system - multiple anchors installed in a single borehole. Judge says it is suitable for all ground conditions, but works particularly well in mixed cohesive and granular soils where conventional anchors can only achieve a limited capacity.
'In the permanent SBMA system, the encapsulations of the unit anchors are located at staggered depths along the borehole fixed length, ' he says. 'The load is transferred from each unit anchor in a controlled manner to a discrete length of the borehole.
'The system ensures a uniform mode of load transfer to the ground over the entire fixed length and a dramatic increase in efficiency in the mobilisation of ground strength.' By overcoming the progressive debonding issues inherent in high capacity multi-strand anchors, SBMAs allow the use of 'an almost unlimited fixed length over which the load may be transferred', Judge says.
The system offers the flexibility to modify an individual anchorage during construction. If ground was found to be poor, an extra strand could be added at comparatively modest cost.
'We carried out a full risk analysis, based on a range of possible scenarios, which gave the client confidence that it had the risks covered. Essentially Jackson was made aware of possible variations and was comfortable with the resulting course of action should it be required, ' Judge says.
Keller's novel combination of SBMAs and dry-driven drilling provided a cost effective solution.
However, it had one disadvantage as Judge explains: 'Because it is a displacement system, the anchor installer does not see the spoil to confirm the ground conditions over the proposed fixed length.' Judge's solution - and a nod to his previous experience in site investigation - was based around extensive use of cone penetration tests behind the wall, to confirm the ground properties over the proposed fixed length.
Keller brought in CPT contractor Lankelma to carry out a series of tests before the anchoring works.
'This allowed rapid measurement of the ground properties and enabled comparison and interpolation of strata between existing conventional site investigation boreholes, ' says Judge.
Lankelma achieved up to 120m of testing a day with minimum disturbance and cost. This data was used to calibrate and substantiate the ground anchorage design.
Armed with this information, Keller installed and successfully tested a trial ground anchor. This demonstrated that the dry driven system operated satisfactorily to the required depth and that the anchors could safely support the 325kN working load.
The first phase of anchoring works took place in August.
Keller constructed 20 permanent anchors, inclined at 30° to the horizontal and up to 26m long.
To minimise disruption to the park and riverbank, drilling was carried out using a 'mast-off' unit mounted on a platform spanning the river. This provided an operational area for the drillers, casing handling and anchor tendon installation.
The platform was fully sealed and bunded with sand bags to contain any accidental oil and grout spillages.
To further reduce the potential for contaminating the watercourse, the power pack and grout mixing plant were positioned on the riverbank.
Judge says: 'All ground anchors in the first phase were successfully installed to programme and we received positive feedback from all parties on the environmentally friendly nature of the operation.
'In terms of geology, the phase one works were in the worst section for dry-driven anchor construction.' All augurs well for phase two, when a further 200 anchors are to be installed on adjacent sections of the River Sow, starting in April next year.