Holdfast Training Services Trial Blog

A cofferdam is required for the construction of an Underpass structure beneath a new Rail Bridge. The current rail line has been slewed 5m to the east of the piling platform to allow for the rail bridge piles and sheets for the cofferdam to be installed. The piling pad is at RL +8.0m, made of 800mm compacted DGB. RL 7.2m to RL -7.5m is dense to medium dense sand. Below RL -7.5m is clay. The GWT fluctuates between RL 1.5 to 3.5m, recharged by rainfall.

Difficult sheet pile driving conditions through coarse grained dense sand have meant pre-augering has been used to loosen the sand prior to driving 18m sheet piles for the Underpass cofferdam.  A phenomenon from CFA piling has occurred on the rail bridge piling platform where the predrilling auger has over-rotated and excessive sand has been removed, known as over-flighting.  In total, there are 29 augered predilling holes in the platform. the total volume of sand removed is 60m³.  Therefore on average 2m³ is removed from each hole.  Over-flighting of the auger is increasing the voids ratio within the ground leading to the ground around the auger to settle through vibration as the sheet pile is driven and an increased stress applied. Damian Warren experienced a similar issue at his project near the Thames in London.

Figure 1 – Tension Cracks in the Piling Platform due to Settlement from Over-Flighting

Where a coarse grained loose material overlays a stiff fine grained material issues in augering can occur. The stiff clay layer found at depth requires greater rotation of the auger due to its stiffness.  The greater rotations of the auger cause loose sands in the upper horizons to be transported up the flights leading to sands around the auger to fall into the flights creating a void.  The voids are being created under the sheet piling machines which may undermine their stability.  An augering machine with low torque will require a higher number of rotations to penetrate the stiffer, cohesive clay layers. This issue was not encountered while drilling the CFA piles for a previous task I conducted on the site as the piles for the Canal Bridge did not reach the cohesive clay layer.

Figure 2 – Settlement of the Ground around the Sheet Pile

The stress history of the soil has been changed by the predrilling.  The soil is exhibiting large volume changes after predrilling and during sheet piling suggesting it now has low stiffness compared to the surrounding strata. The stiffness of a soil is very difficult to assess. The differential settlement for across the pad could become an issue for the rigs stability and potentially effect the rail line.

The water table around the piling platform is recharge via infiltration of rain water.  Heavy rain over the past few days will have caused the ground water table to rise.  A high ground water table exacerbates over-fighting as the soil has increased fluidity in the weak submerged soil particles allowing it flow easier.

The predrilling is being conducted as if it were a secant piled wall, with overlapping bores.  The cumulative effect of the close spacing of drilling will increase the settlement issue.

Recommendations

To rectify the issue, geogrid or cement stabilised sand could have been used to reduce excessive ground movement.

Steel plates have already been utilised to support the 72T piling rigs during driving due to settlement issues on the piling platform.

Drill rig selections should be accessed in further piling works to ensure a drilling rig is selected with sufficent torque and crowd combinations that are compatible with auger rotation and penetration into the ground.

Verification

After inspecting the piling platform myself, I wrote a report to the John Holland SPE on my findings.  Subsequently a geotechnical engineer came out an observed the piling platform and concluded that the platform still met the design requirements so no further works are required. A rail inspector has also checked the alignment of the railway line, with no issues found.

Are there further issues which should be considered or is this issue normal on site?