The relief road currently being built along Dorset’s Jurassic coastline for the 2012 Olympic sailing events required some careful montoring of the world-important strata. NCE reports.
The £87M, 7.5km long relief road between Dorchester and Weymouth along Dorset’s Jurassic coastline is currently nearing completion.
Built to accomodate visitors to Weymouth for the 2012 Olympics’ sailing events, the project requires the construction of 18m high embankments and a new bridge over a mainline railway.
Careful monitoring of ground movements in such sensitive areas both during and following construction is needed to ensure that problems can be identified at the earliest opportunity.
During pre-contract meetings between contractor Amey-Skanska and geotechnical specialists Geotechnics, the design requirements were discussed and proposals for the desired monitoring arrangements and collation of data were explored.
Detailed designer Amey’s primary criteria was for the installation of a number of vibrating wire piezometers (VWPs), inclinometers and hydrostatic profile gauges (HPGs) to deliver real-time ground movement data to allow them to constantly analyse the overall construction performance. The priority of main contractor Skanska 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 instruments to be monitored manually during construction works, requiring an onsite team of up to six technicians each day.
It quickly became clear that not only would this entail considerable costs, the health and safety risks of so many staff working amongst active construction plant were too great. Geotechnics decided to remotely monitor all VWPs and inclinometers, with manual monitoring of just the two HPGs. This greatly reduced the number of man-hours required on site.
Real-time ground movement data allowed the construction performance to be constantly analysed
emote 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 its offices in East Sussex. The dataloggers were strategically placed outside the construction zones, to minimise the risk of damage to delicate equipment.
Thirty-six VWPs were also 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 HPGs were then installed to monitor the settlement characteristics and response of the ground beneath embankments as they were built up.
Five multichannel datalogger boxes linked up all these 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 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.
Geotechnics’ knowledge of the ground conditions along the new road route - the company 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.
The monitoring scheme was delivered on time and to budget.