GSS has completed another testing scheme for HS2, this time for pipeline diversions in Oxfordshire - 12 tests giving profiles up to15m depth are all in a morning's work for ACSW.
Continuous Surface Wave (CSW) testing measures ground stiffness from surface down to depths of 6 to 10m or more.
Testing is undertaken from a self-contained 4WD test vehicle wherever access is possible to within 10m of test locations to ensure the fastest production rates. However as test equipment is portable testing can be undertaken remotely from the test vehicle where 4WD access is not possible.
Our advanced C-DAS data acquisition software completes each test in around 5 minutes, with preliminary results immediately displayed. On favourable sites with 4WD access 12 or more tests per shift are usually possible, allowing rapid site characterisation.
Continuous Surface Wave (CSW) testing provides in-situ stiffness profiles without boreholes or sampling and the associated potential for sample disturbance. Stiffness is averaged over the test length, normally 3m, so gives a representative bulk sample, avoiding the problems of very local variation in ground conditions.
Unlike many other stiffness measurements CSW provides a small-strain stiffness (G0), a 'baseline' measurement at a known strain which can then be readily adjusted ('strain softened') to actual design strains. GSS provides stiffness data in an easy-to-use spreadsheet, allowing the user to input design strains. Data is also ideal for 3D ground modelling or BIM applications.
There is excellent published data to prove the equivalence of CSW data with other more expensive techniques such as downhole seismic testing or large scale load tests.
Continuous Surface Wave (CSW) testing does not require boreholes or the use of additional plant. Combined with the speed of testing (typically 12 per shift) this makes it an extremely cost-effective way of providing high quality stiffness data. CSW also provides non-intrusive shear wave velocity (Vs) profiles for seismic design, liquefaction assessment and preliminary ground charachterisation using published correlations with soil parameters. In rock Vs profiles can be used to assess rock quality and excavatability.
The non-intrusive nature of CSW also means that it is sustainable and low risk, hence it can be used in many situations where the presence of sensitive services or structures would make other types of testing impossible. The portable nature of equipment and speed of testing means it is ideal for use in difficult to access, time-restricted situations such as the rail environment.
The cost of each CSW test is only around that of a conventional plate load test, but provides a stiffness profile to depths of 6 to 10m or more of equivalent quality to the most expensive stiffness measurement techniques.