GEO-Instruments is providing a geotechnical monitoring scheme for a large housing development on the outskirts of Maidenhead in Berkshire.
Working with the ground works contractor who are undertaking surcharging works across the development, GEO-Instruments engineers have supported multiple phases of the project so far.
Surcharge monitoring is important for verifying the performance of the surcharging techniques and ensuring the ground is appropriately prepared prior to the construction of residential properties.
The monitoring scheme is comprised of two types of geotechnical instrumentation.
There have been 24 Settlement plates installed in several systems spread over multiple phases of works and five IP-Xs (extensometers), one for each surcharging area.
Settlement plates are large square plates fitted with vibrating wire pressure sensors interlinked with a common liquid circuit that calculates vertical movement using relative changes in pressure. Locations are chosen carefully to support monitoring and understanding ground settlement that may occur during and after the loading phase.
The plates, connecting liquid tubing, and signal cables are installed in pre-dug trenches leading to each location from the shared liquid reservoir. The system is then buried to protect the delicate elements during ground improvement works.
The reservoir and datalogging equipment are housed in a cabinet located at the edge of the monitored area, where no movement is anticipated.
Care is taken to ensure no air remains or is introduced into the liquid circuit to reduce the risk of inaccurate readings.
The Liquid Settlement System is pre-filled after manufacture and designed to need minimal maintenance after installation.
In-Place Extensometers (IP-X) are instruments designed to measure vertical movement of the ground. Unlike the settlement plates, IP-Xs are able to measure movement at multiple, greater depths. They are used to get more detail and better characterise the ground movement observed during the works.
The extensometers are installed to a depth of eight metres into predrilled boreholes. Each IP-X is comprised of casing, magnets and sensors attached to a rod inserted into the casing at the same depth as the magnets. These sensors are in fixed positions relative to the bottom of the borehole and detect movement of the external magnets as the ground responds to the surcharging.
After the installation of casing and magnets, the depths of magnets are confirmed to be at correct height using a reed switch probe and are then grouted in place to ensure good cohesion with the surrounding soil.
Each IPX is connected to a wireless node adjacent to the borehole location. A solar powered gateway system collects data from the nodes and relays readings to the QuickView online monitoring platform, where monitoring data can be accessed remotely for visualisation and analysis.
Mihhail Lonkin, Project Manager for the scheme said:
“It is an interesting ongoing project to be involved in. We faced a few challenges on site along the way, but it has been rewarding to come back to site for each phase, see the groundworks progress, and adapt our systems to the changing conditions.”
This is the latest project in a long-term collaboration with the client on several schemes with similar geotechnical monitoring requirements.
Read about another of our recent geotechnical monitoring schemes.