Instrumentation and Monitoring

The primary monitoring system comprised fourteen fast response vibrating wire piezometers (VWPs) and five standpipe piezometers to monitor the performance of the dewatering system (Figure 11.11). Retaining wall deformation was monitored by thirteen Shaped Accel Array inclinometers. Secondary instrumentation for base stability comprised four magnetic extensometers. The critical observations were groundwater pressure; these had to be supplemented by the observations summarised in Table 11.2 to facilitate interpretation of the groundwater pressure data.

In addition, to the instrumentation it was vital for the soil stratification to be carefully evaluated during the drilling of any boreholes for instrument or well installation, and as the shaft excavation progressed through the Lambeth Group. Therefore, a senior geologist was located on site throughout the OM implementation.

The installation of the dewatering wells and the instrumentation was planned to be carried out across several phases. This enabled the geology and hydrogeology to be better understood. It also enabled the effectiveness of the dewatering system to be assessed in a progressive manner. This phased approach enabled the dewatering design to be modified on the basis of observed performance. It also enabled safety to be assured, while minimising the risk of slowing down excavation.

The installation of the piezometers was carried out in two different phases, to facilitate faster excavation. Two VWPs were installed in the first phase and before box excavation. These piezometers were located in the sandy layers of the Lambeth Group. The holes for piezometer installation

Instrumentation locations for monitoring groundwater pressures and wall displacement

Figure 11.11 Instrumentation locations for monitoring groundwater pressures and wall displacement.

Table 11.2 Observations to support monitoring of groundwater pressure.

Parameter

Method

Comment

Soil

stratification

Logging during installation of all wells and instrument holes

Critical issue due to complex geology of Lambeth Group

Flow from individual wells

Flow metres

Important for selection of remedial options

Water quality and fines

pH/conductivity, check discharge tank

Water treatment necessary, removal of fines

Mechanical

performance

Pump pressure, power supply

System maintenance, back-up needed

were drilled using high-quality rotary coring (triple tube wire line with mud flush) and the continuous core was carefully logged by an experienced geologist. This provided important information for the dewatering system design. The piezometer monitoring data provided a set of baseline readings, prior to box excavation influencing groundwater pressures. In the second phase of piezometer installation, 12 VWPs were installed and five standpipes. As shown in Figure 11.11, some of the piezometers were installed behind the wall, by inclined drilling through the gaps between contiguous piles, when the excavation reached 83 m ATD. Limited space meant that monitoring instruments and any wells for dewatering behind the shaft walls had to installed by inclined drilling.

Traffic Light System for the О/И

The groundwater control flow chart for implementation of the OM, showing trigger levels and contingency measures, is shown on Figure 11.12. As discussed earlier, the primary focus for the traffic light system was groundwater control. If an amber trigger was breached, then more frequent monitoring was required, and the coverage of piezometers was also re-assessed. If considered to be inadequate, then additional instruments were to be installed. If the red trigger was breached, then excavation would be stopped, and the dewatering system would be enhanced by installing additional dewatering from a tool box of alternative systems (Figure 11.12). Selection of appropriate systems from the tool box mainly depended upon the anticipated local ground conditions and their assessed permeability (Figure 11.10). The ground model (summarising the 3D variations in ground conditions) was progressively developed by the designer’s site geologist on the basis of logging all excavation exposures and drill holes for instrument installations, wells and so on. The effectiveness of existing wells could also be enhanced (e.g. converting to vacuum enhanced wells). In the unlikely event of a red trigger for wall deformation being breached, then temporary props would be installed. A key component of the risk management was full time input by the OM design team, who were resident on site. This facilitated all monitoring data and

Groundwater control flow chart for the OM implementation

Figure 11.12 Groundwater control flow chart for the OM implementation.

Note: The dewatering toolbox comprised several systems, including deep wells; ejector wells; passive relief wells; trenching and sump pumps. Selection was based on locally assessed conditions. All equipment mobilised to site and available for installation.

site observations to be rapidly integrated and assessed in the context of the design requirements.

The installation of dewatering wells was carried out in two main phases: a first phase, with four deep wells installed; a second phase, with nine inclined and two vertical ejector wells (Figure 11.13).

 
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