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Detecting voids and soft ground with geophysics

George Tuckwell
George Tuckwell

Voids are amongst the most common buried hazards construction projects encounter. They may be man made or naturally occurring; mine workings or solution features. Mitigating the risk requires a detailed site investigation. Including a geophysical survey in the planned SI often provides a cheaper and more robust solution. There are a number of geophysical techniques available to detect voids. Microgravity, if practical, is the most effective. Others that might be quicker and cheaper to deploy may also get the information needed, especially for man-made voids. The trick, as ever, is to get the right advice.

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RSK Geophysics 18 Frogmore Road Hemel Hempstead Hertfordshire HP3 9RT VOIDS AND SOFT GROUND Tel: 01442 416656 geophysics@rsk.co.uk www.environmental-geophysics.co.uk Geophysical Techniques Available ] Penetrating Radar (GPR) Ground ] Microgravity ] Electromagnetic Mapping (EM) ] Wave Ground Stiffness (SWGS) Surface ] Resistivity Imaging (ERI) Electrical Subsurface voids (whether naturally occurring or Geophysical techniques provide a suite of site manmade) and associated areas of soft ground reconnaissance tools that enable site characterisation present a significant risk to future, and existing, and provide total site coverage. The examples below infrastructure and buildings. Unknown voids can be illustrate how relatively simple geophysical surveys discovered during construction and can cause can be applied to a site in order to plan and design a hazards and expensive delays to a construction targeted intrusive investigation and subsequent project. remedial works to problem areas of ground. The use of geophysics reduces the associated risks and saves the developer time and money during the project. Survey examples Ground Penetrating Radar (GPR) 1m A primary school was experiencing localised settlement in the playground. Historical maps showed the presence of former buildings at the site which may have had basements. A GPR survey was undertaken to identify the location of any Depth (m) remaining basements present, and the possible presence of voids or poorly compacted backfill material. The GPR survey was completed in a single day and provided total site coverage around the school grounds in the shallow sub-surface. The data showed a number of discrete areas exhibiting anomalies indicative of the presence of basements. Secondary to the GPR survey a targeted dynamic probing Where the backfill is poorly compacted, investigation was implemented in order to seek to validate or contains voids, the GPR signal the findings of the GPR survey. Over the anomalous areas reverberates, generating large amplitude recorded in the GPR data the dynamic probe results were reflections in the radargram found to be very low thus confirming the presence of very loose backfill material, or possible voided areas. DP11 (m) 0 20 40 60 l Schoo 0.20 0.50 0.80 1.10 1.40 DP14 1.70 (m) 0 20 40 2.00 0.20 0.50 2.30 0.80 2.60 1.10 1.40 High-amplitude 1.70 reflections 2.00 indicating void 2.30 2.60 High-amplitude reflections indicating void 0 25 50 75 100 Scale (metres)
RSK Geophysics 18 Frogmore Road Hemel Hempstead Hertfordshire HP3 9RT VOIDS AND SOFT GROUND Tel: 01442 416656 geophysics@rsk.co.uk www.environmental-geophysics.co.uk Microgravity Mapping Solution features were identified within chalk bedrock Corrections were applied to the observed gravity to during drilling of a site prior to redevelopment of a produce a final microgravity map (below left). The sports centre. A microgravity survey was corrections applied included drift, latitude, free-air, and commissioned to seek to determine the lateral extent of the Bouger correction. The residual gravity map solution features and voids within the chalk bedrock in displays three areas of broad gravity lows (coloured order to provide a reliable interpretation of the sub- blue) across the survey area. The results were used to surface at the site. The gravity survey was conducted target further boreholes to prove the anomalies. Very on a 5x5m staggered grid over the proposed footprint of weak to weak low denisty chalk or voided ground was the building, ensuring comprehensive cover. found present in the gravity low areas. Based on the findings of the investigation, important changes were made to the design and location of the development POLYTUNNE L Grass Barrier h 1.0 POLYTUNNE L Floodlight Yew Hedge Ht=1.70 Grass The anomalies are interpreted to CLF h 2.9 represent possible solution features in the chalk bedrock (above). Floodlight Blue areas in the gravity map delineate areas of low density or possibly voided ground 0 25 50 75 = high number of dynamic probe Scale (metres) blow counts indicating competent ground. = low dynamic probe blow counts indicating poorly consolidated ground. 50m EM Ground Conductivity and Resistivity Imaging Exploratory holes across a derelict site earmarked for EM-31 data. redevelopment identified the presence of solution features Oranges and reds show and voids in the underlying chalk bedrock. An integrated higher ground geophysical survey including EM31&34 ground conductivity. conductivity, and resistivity imaging was undertaken to determine whether further voiding was present. Due to the D. large size of the site, and the presence of chalk at shallow ROA (<5m) depth, the EM technique was chosen to rapidly Anomaly indicative of a image the variation in ground conductivity to a depth of possible solution feature ~10m across the entire site. Chalk solution features are present in both datasets likely to manifest themselves as conductivity highs due to an increased thickness of overburden or a change in N S ground conditions such as increased groundwater saturation. A number of resistivity lines (right) and 10m targeted dynamic probing were deployed over the EM anomalies to confirm the presence of soft/loose ground in Top of chalk Resistivity data showing blues as areas bedrock the subsurface associated with potential voids at depth. of lower resistivity (more conductive).

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Detecting voids and soft ground with geophysics

  • 1. RSK Geophysics 18 Frogmore Road Hemel Hempstead Hertfordshire HP3 9RT VOIDS AND SOFT GROUND Tel: 01442 416656 geophysics@rsk.co.uk www.environmental-geophysics.co.uk Geophysical Techniques Available ] Penetrating Radar (GPR) Ground ] Microgravity ] Electromagnetic Mapping (EM) ] Wave Ground Stiffness (SWGS) Surface ] Resistivity Imaging (ERI) Electrical Subsurface voids (whether naturally occurring or Geophysical techniques provide a suite of site manmade) and associated areas of soft ground reconnaissance tools that enable site characterisation present a significant risk to future, and existing, and provide total site coverage. The examples below infrastructure and buildings. Unknown voids can be illustrate how relatively simple geophysical surveys discovered during construction and can cause can be applied to a site in order to plan and design a hazards and expensive delays to a construction targeted intrusive investigation and subsequent project. remedial works to problem areas of ground. The use of geophysics reduces the associated risks and saves the developer time and money during the project. Survey examples Ground Penetrating Radar (GPR) 1m A primary school was experiencing localised settlement in the playground. Historical maps showed the presence of former buildings at the site which may have had basements. A GPR survey was undertaken to identify the location of any Depth (m) remaining basements present, and the possible presence of voids or poorly compacted backfill material. The GPR survey was completed in a single day and provided total site coverage around the school grounds in the shallow sub-surface. The data showed a number of discrete areas exhibiting anomalies indicative of the presence of basements. Secondary to the GPR survey a targeted dynamic probing Where the backfill is poorly compacted, investigation was implemented in order to seek to validate or contains voids, the GPR signal the findings of the GPR survey. Over the anomalous areas reverberates, generating large amplitude recorded in the GPR data the dynamic probe results were reflections in the radargram found to be very low thus confirming the presence of very loose backfill material, or possible voided areas. DP11 (m) 0 20 40 60 l Schoo 0.20 0.50 0.80 1.10 1.40 DP14 1.70 (m) 0 20 40 2.00 0.20 0.50 2.30 0.80 2.60 1.10 1.40 High-amplitude 1.70 reflections 2.00 indicating void 2.30 2.60 High-amplitude reflections indicating void 0 25 50 75 100 Scale (metres)
  • 2. RSK Geophysics 18 Frogmore Road Hemel Hempstead Hertfordshire HP3 9RT VOIDS AND SOFT GROUND Tel: 01442 416656 geophysics@rsk.co.uk www.environmental-geophysics.co.uk Microgravity Mapping Solution features were identified within chalk bedrock Corrections were applied to the observed gravity to during drilling of a site prior to redevelopment of a produce a final microgravity map (below left). The sports centre. A microgravity survey was corrections applied included drift, latitude, free-air, and commissioned to seek to determine the lateral extent of the Bouger correction. The residual gravity map solution features and voids within the chalk bedrock in displays three areas of broad gravity lows (coloured order to provide a reliable interpretation of the sub- blue) across the survey area. The results were used to surface at the site. The gravity survey was conducted target further boreholes to prove the anomalies. Very on a 5x5m staggered grid over the proposed footprint of weak to weak low denisty chalk or voided ground was the building, ensuring comprehensive cover. found present in the gravity low areas. Based on the findings of the investigation, important changes were made to the design and location of the development POLYTUNNE L Grass Barrier h 1.0 POLYTUNNE L Floodlight Yew Hedge Ht=1.70 Grass The anomalies are interpreted to CLF h 2.9 represent possible solution features in the chalk bedrock (above). Floodlight Blue areas in the gravity map delineate areas of low density or possibly voided ground 0 25 50 75 = high number of dynamic probe Scale (metres) blow counts indicating competent ground. = low dynamic probe blow counts indicating poorly consolidated ground. 50m EM Ground Conductivity and Resistivity Imaging Exploratory holes across a derelict site earmarked for EM-31 data. redevelopment identified the presence of solution features Oranges and reds show and voids in the underlying chalk bedrock. An integrated higher ground geophysical survey including EM31&34 ground conductivity. conductivity, and resistivity imaging was undertaken to determine whether further voiding was present. Due to the D. large size of the site, and the presence of chalk at shallow ROA (<5m) depth, the EM technique was chosen to rapidly Anomaly indicative of a image the variation in ground conductivity to a depth of possible solution feature ~10m across the entire site. Chalk solution features are present in both datasets likely to manifest themselves as conductivity highs due to an increased thickness of overburden or a change in N S ground conditions such as increased groundwater saturation. A number of resistivity lines (right) and 10m targeted dynamic probing were deployed over the EM anomalies to confirm the presence of soft/loose ground in Top of chalk Resistivity data showing blues as areas bedrock the subsurface associated with potential voids at depth. of lower resistivity (more conductive).