Case Studies
| Project: | Thames Water – Craven Road |
| Overview: | Murphy Surveys UK Limited was contracted by Thames Water Limited (TWL) to carry out this survey. The sewer on the South side of Craven Road West London currently has no access. Records show a MH (4919) located outside number 45 Craven Road, but this either does not exist or is buried. If it is buried, it needs to be located so it can be rebuilt. If it does not exist, then a new manhole will need to be constructed. In order to do this all other services in the area will need to be mapped to establish where there is room to gain access to the sewer. |
| Client Profile: | Thames Water We is the UK's largest water and wastewater services company. Every day, they supply 2,600 million litres of tap water to 8.5 million customers across London and the Thames Valley. They also remove and treat 2,800 million litres of sewage for an area covering 13.6 million customers. |
| Survey Requirements: | The objective of this geophysical survey is to locate and map all underground services and anomalies within the survey area. In particular, Manhole 4819, if it exists, needs to be located. The extent of the underground railway tunnel needs to be established. The size and location of the sewers needs to be established A range of geophysical techniques were applied on the site in order to best locate the utilities, buried anomalies and the underground tunnel. Initially a desktop study was carried out to obtain as full a picture as possible of all existing utilities in the area. Contact was made with all relevant utility companies as well as the local authorities, London Underground etc. A full topographic survey was carried out of the site before the utility survey commenced. A total of 33 covers were lifted and recorded. Each Manhole and Inspection Cover on site was individually numbered and inspected. A copy of each individual Manhole Sheet can be found in Appendix 6. A record of cover size, cover and invert levels, chamber dimensions, type of service, material, diameter, flow direction, top of pipe level, number of ducts etc is recorded. Photographs were also taken of each chamber, which are included on the CD with this report. Hydraulic Manhole Lifters and appropriate signing and guarding were used. A range of radio frequency techniques were used to trace utilities from each Manhole including direct induction, cable clamping, sondes and passive radio frequency surveys. A multi-channel and multi-frequencies RIS-MF system mounted on a cart was used for the GPR data acquisition. An odometer is mounted on the cart for distance calibration. The grid spacing was 2 metres in both directions and the trace spacing is set up to 2.5 cm for each profile. The GPR data collected is composed of 8 channels and 2 frequencies of 200 and 600 MHz for shallower and deeper investigations. The RIS-MF system is 1.8m in width, resulting in full coverage of the area. The time window was set up to 100 nanoseconds with a wave velocity of 10 cm/ns. The exact location of each GPR file is located via a survey baseline, with all files parallel or perpendicular to this line. The GPR data was stored digitally and transferred back to the Murphy Survey offices for processing using specialist software. Post site processing is done using the specialist software, Gred/3D Utilities. A number of processing stages are involved, including start time correction, amplitude gain adjustments, gaussian filtering, dynamic correction and noise removal. Once the raw data is processed individual targets are identified on each survey line and linear features mapped out over the survey areas. On top of this thorough utility survey, a MicroGravity survey was carried out over the site. The Gravity technique is based on measuring localised variations in the Earth’s gravitational field, which are generated by materials of different densities. The presence of an anomalously high (or low) density body in the subsurface causes a localised high (or low) anomaly in the measured gravitational field. The gravity effects are extremely small; however, modern instruments and exhaustive data processing technique enable the detection of both geological and artificial structures. A grid of 3m spacing was marked out across the site and each point surveyed in accurately, allowing post site processing of the gravity data to correct for level and terrain. This also enables the results to be overlain onto results from all other techniques. A base station was set up and monitored throughout the survey. This allows for corrections to be made for the diurnal drift in the Gravity data throughout the length of the survey. At each point the gravimeter was precisely leveled and left to settle in order to give consistent readings. The data was logged both digitally and by hand and all the data was taken back to Murphy’s offices for processing. |
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