A new high pressure gas pipeline is being construct- ed between Goxhill, on the south bank of the Humber Estuary in North Lincolnshire, and Paull, on the north bank in the East Riding of Yorkshire. The pipe- line route includes a new 5 km bored tunnel under the Humber Estuary through which the pipeline pass- es. A launch pit at Goxhill and a reception shaft at Paull were required for the TBM drive. This paper covers the design and performance of the temporary works groundwater control arrangements used to facilitate excavation of the launch pit and associated ramp structure. The site layout is shown in Figure 1.
The site was previously agricultural land with a ground level of approximately 2 mOD. The launch pit was to be primarily excavated in superficial glacial soils, which act to confine groundwater in the underlying chalk, although at its deepest point the excavation extended into the upper portion of the chalk. Groundwater levels monitored in the chalk prior to the works were typically around 1 mAOD. Preliminary studies suggested the likely presence of a thin horizon (up to 5 m) of potentially highly permeable chalk „bearings‟ (clast dominated frost shattered chalk), which are a feature of Northern Province Chalk Group, immediately below the superficial de- posits. The launch pit was 80 m long by 7 m wide, narrowing to 5 m, with an associated ramp structure 48 m long by 5 m wide. The maximum dig depth was 11.2 m, with a formation level between -3.8 mOD and -9.2 mOD. The launch pit had side support com- prising secant piles extending to 19 m AOD and sheet piles extending to 17.6 mAOD. Following excavation a base slab was cast to accept full hydrostatic loading, effectively isolating the launch pit from the groundwater regime and allowing cessation of pumping. A cross section through the launch pit is shown in Figure 2.
Figure 1. Site layout showing site investigation borehole locations and pumping test layout.
A groundwater control solution comprising a partial cut-off with internal pumping wells was developed to reduce upward pressure from the chalk and facilitate the excavation of the launch pit.
The site was subject to a number of environmental constraints that informed the design of the groundwater control system. The chalk in North Lincolnshire was classified as “fully committed” by the Environment Agency under their Catchment Abstraction Management Strategy meaning pumping to a surface discharge point was not viable. Additionally, the proximity to the Humber Estuary, which presented a risk of inland migration of the saline interface, and the presence of a nearby private supply borehole necessitated the use of external re-injection wells to minimise the drawdown in water levels outside the launch pit.
Figure 2. Cross-section through structure showing general soil profile and proposed cut-off.
A further consideration was the driveability of sheet piles in the relatively dense Northern Province Chalk Group. In order to achieve a suitable barrier to flow, the piles needed to be driven without causing disturbance at the pile toe, which would create preferential pathways. Vibratory driving of the sheet piles was deemed viable following review of the available ground investigation information. This proved to be the case, with only a small fraction of the piles refusing just short of the design depth and no indication of damage to the chalk below the toe of the pile leading to the development of preferential flow paths.
A pumping test programme was carried out to un- derstand the variability of hydraulic conductivity with depth. The pumping test results were used to de- termine the required target depth for the internal pumping wells, and the required drive depth for the piles to key into the competent chalk underlying the chalk bearings.