Weymouth Relief Road Instrumentation
A 7.5km long relief road between Dorchester and Weymouth is currently nearing completion. The project required the construction of 18m high embankments and a new bridge over a main railway line. Monitoring of ground movements in such sensitive areas both during and following construction was needed to ensure that any problems could be identified at the earliest opportunity.
During pre-contract meetings between Skanska, Amey and Geotechnics Ltd, the design requirements were discussed and proposals for the desired monitoring arrangements and collation of data were explored. Amey’s primary criteria was for the installation of a number of Vibrating Wire Piezometers (VWP’s), Inclinometers and Hydrostatic Profile Gauges (HPG’s) to deliver real-time ground movement data to allow them to constantly analyse the overall construction performance. The priority of Skanska, the Main Contractor, was to complete the instrumentation installation without affecting the programming of the embankment works, and within a set budget.
The original specification was for the various instrument types to be manually monitored during construction works, which would have required an onsite team of up to 6 technicians each day. Initial meetings soon highlighted that not only would this entail considerable costs, the Health and Safety Risks associated with working amongst active construction plant were too great.
Geotechnics prepared a proposal based on the remote monitoring of all VWP’s and inclinometers and manual monitoring of just the two HPG’s, thus greatly reducing the number of man-hours required on site. This option was approved by all parties involved and Geotechnics Ltd were commissioned to carry out the works.
Remote monitoring was made possible by installing multi-channel dataloggers equipped with GSM modems which transmitted data to an off-site server. This processed the data and presented it on a website for real-time viewing by Amey at their offices in East Sussex. The dataloggers were situated outside the construction zones to minimise the risk of damage.
Thirty six Vibrating Wire Piezometers were installed to provide pore water pressure measurements during the building up of embankments.
Twenty three Uniaxial In-Place-Inclinometer (IPI) strings (comprising a total of 126 sensors) were installed to varying depths to allow monitoring of the lateral movement of large embankments and structures.
Two Hydrostatic Profile Gauges (HPG) were installed (197m and 121m in length) to monitor the settlement characteristics and response of the ground beneath embankments as they were built up.
Five multichannel datalogger boxes were installed, linking up all the remotely monitored instruments to an automated monitoring system. Each box comprised a watertight enclosure containing a multichannel datalogger, relay multiplexers (for increasing the number of channels), a cellular GSM modem with transmitter and a lead acid rechargeable battery. Geotechnics worked alongside Skanska to install instruments in several phases in order to fit around the construction programme and progress on site, which required Geotechnics to be flexible to adapt to changes in the programme.
Geotechnics Ltd’s knowledge of the ground conditions along the new road route (we undertook the main ground investigation in 2007, together with previous investigations along earlier proposed routes) enabled the most effective drilling methods to be adopted, ensuring the costs of installing instrumentation were kept to a minimum. Both Skanska and Amey were open to technical suggestions and advice from ourselves, which we consider was key to providing an effective monitoring scheme on time and to budget.