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.

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).