HST Shaft

Case Study

HST Shaft

Client: A E Yates Trenchless Solutions Ltd
Main Contractor J N Bentley/Mott MacDonald
Excavation SHumus Sedimentation Tank (HST)
Shaft Dimension: 12.5m Ø x 5.9m deep
Construction Method:Segmented Rings by driven caisson method (1m x 2m collar)
Existing Ground Level:105.0m AOD
Excavation Level:99.01m AOD (5.99m deep)

Project Overview: An 9no. deep well dewatering system for a 12.5m Ø shaft at Llanberis WwTW in Gwynedd. The shaft formed part of a new Humus Sedimentation Tank within the existing works and was constructed to 5.9m depth. Ground conditions consisted of Alluvium Fan Deposits with course Sand and Gravels overlying the Llanberis Slate Formation.

Groundwater level was at nominal 2.0m BEGL. Groundwater level was sensitive to both with hydraulic connection to the Llyn Padarn and Afon Seiont around 220m from the shaft location, which also acted as the discharge point for abstracted water; together with recharge from rainfall. This case study details the dewatering appraisal and design together with details on the methods used for a successful operation.

The Objectives of the Pumping Tests:

1. The 9no wells dewatering wells and internal monitoring points were drilled and installed over a 2 week period 1st November to 14th November 2016.

2. During this period dewatering wells were developed and BH pumps installed. The system was commissioned with phased pumping undertaken over a 1 week period from the 16th November 2016.

3. Dewatering was operational and the shaft successful completed over a 6 week period.

Borehole pumps suspended on 63mm Ø riser to a wellhead assembly. In turn the wells are connected 4no x 150mm Ø collection mains discharging via v-notch tank then to a collection MH with a 200mm Ø polybauer discharge line to river, with flotation strainer within the river yo mitigate environmental concerns with discharging into the river. Phased pumping was proposed to control the impact of flow and water quality with discharge samples taken and analysed during each phase, prior to starting the next phase.

Phase 1 – Initial commissioning of 9no BH pumps to discharge maximum 40 lts/sec until discharge water quality confirmed.

Phase 2 – Upon acceptance of water quality to increase the total discharge flow from the 9no dewatering wells in incremental increases of a maximum 20 lts/sec per day with a maximum increase of 5 lts/sec at any stage. Increasing to maximum flow with continuous pumping to enable construction of the shaft.