Disused quarries which often have been historically turned into landfills are a common sight across the British landscape. Left untouched, these infilled quarries can impact negatively on the environment with the risk chemical run-off and contamination hotspots. While many sites have been converted into water sports facilities or wildlife habitats, reclamation for commercial/industrial and/or residential end use is also a preferred option, with land for new development in short supply.
Depending on the historic use of the land and soil conditions however, rehabilitation can be expensive and time-consuming. In addition, securing planning consent requires detailed proof of the elimination of contamination and geotechnical risks prior to the commencement of construction. Obtaining specialist advice at an early stage can be highly cost-effective, helping to fast track the initial investigative and preparation stages and cutting costs through value engineering.
One such project is a site near Crawley, where an area of 132-hectare, previously used for inert waste landfill is being transformed into a mixed-use development comprising residential, retail and public amenity components including schools and a health centre. Ecologia’s involvement dates to 2016, with the work scheduled to continue for the next two years.
Working collaboratively with the principal contractor and engaging proactively with the client’s consultant, Ecologia has been able to provide high quality data during the various phases of site intrusive works associated with the proposed consolidations works and subsequently devised and promoted an innovative telemetric solution for ground surcharge monitoring to expedite the pre-construction works with the aim to reduce reclamation cost.
Gas Monitoring – the challenge
Initial site investigation work was carried out to identify gas concentrations levels within the former landfill site, including drilling, installation and monitoring of groundwater and ground gas monitoring boreholes. In parts of the landfill very high concentration of methane were encountered >95%. As such more frequent monitoring was required and this was achieved by the installation of continuous gas monitors (over a number of weeks). Initially this seriously hampered the progress of the drilling as the severity and extent of the problem was not fully acknowledged; however adequate planning in subsequent stages of the site investigation was successful in minimising disruption associated with ground gases.
Ground Improvement – the need of surcharging
This landfill near Crawley is a prime example of a site with variable ground conditions resulting from very heterogenous infill material consisting of metals, plastics, wood, concrete and other reworked soft soils imported over the years which do require consolidation prior to any development taking place.The required consolidation period can be lengthy, with construction only able to proceed once the ground has achieved a sufficient strength and density such that liquefaction potential is minimised and both primary and subsidiary settlement are predicted to be within acceptable parameters. Pre-loading or surcharge loading by subjecting the site to load in the form of added fill offers a simple way to shorten this process. However, to be successful, reliable and detailed monitoring data, prior to surcharge, during surcharge and post surcharge is essential in confirming site progress and establishing the required duration of treatment.
Ecologia’s involvement in the surcharging works consisted in the installation and monitoring of specialised equipment in the form of Vibrating Wire Piezometers (VWP’s) which are used for the measurement of pore water pressure and / or ground water levels, normal extensometers (which are used to monitor settlement) and more recently with Vibrating Wire Extensometers.
Surcharging – the advantages of telemetry
Ecologia was able to value engineer the monitoring process using the wireless sensor network to transmit and receive data remotely from the VWP’s and rod extensometers to reduce the requirement for visits to the site (as the data can be accessed 24/7), an added advantage with extremely challenging conditions prevailing at times in the form of very soft clay.
Further cost advantages are provided by the system which minimises cabling requirements, enabling nodes to be placed directly above the installation without the need for trench cabling from each VWP to a centralised location. The provision of continuous monitoring has improved settlement data as variance in site data generally associated with manual readings taken by different staff has been removed. Data are now elaborated daily and can be timely sent to the contractor who can accelerate / decelerate the consolidation works.Through its ongoing investigation processes, Ecologia is providing detailed reporting to assist the client in assessing the expected construction timeframe and to support the planning process through documentation of post construction tolerances, surcharge fill and site material properties. The project is currently subdivided into several phases scheduled to completed over the next 2 years
If you have a development scheme where remediation and consolidation expert advice is required advice, contact us