ACSW and Ground Investigation

ACSW Test Set Up

Designed and manufactured by GSS, the ACSW system comprises the following bespoke equipment and software:

• Three sizes of robust, water and mud resistant, bespoke shakers with a welded steel casing to provide sinusoidal sources (standard source 10Hz to 90Hz)

• Armoured cables with external grade plugs (connecting to a standard 230v AC generator power supply normally mounted in the test vehicle but removable if required)

• A vehicle crane which allows shakers to be safely taken in and out of the test vehicle by a single technician without manual handling.

• A wheelbarrow cradle which allows all shakers to be moved safely around site by a single test technician.

• A ruggedized, water & mud resistant, portable data acquisition box.

• A 6 No. 4.5Hz geophone array and shielded take-out cable (with steel mounts for working on hardstanding).

• C-DAS data acquisition and analysis software (running on a ruggedized Windows tablet).

Reliability and Repeatability

Each ACSW test comprises data acquisition during a sweep across the full range of shaker frequencies (Sweep test) and a series of discrete constant frequencies (Monotonic test).

This provides two complete sets of dispersion curve data. During data capture, the C-DAS software automatically checks the quality of data being recorded and the calibration of the geophones and shaker by:

• Comparing the frequency being measured by the geophones against that being generated by the source.

• Comparing the phase difference across each pair of geophones to ensure the assumption of cross-isotropy (constant lateral stiffness) is within acceptable limits.

This process is then repeated.

ACSW tests can be undertaken on any reasonably level and stable surface – there is no requirement for levelling to set up shakers.

Testing can be undertaken in any location where there is space to set up a test (approx. 1m x 6m level area) and safe person access. All equipment is designed to be transported by a standard 4WD test vehicle but can be fully demounted and transported by site vehicle or rail trolley.

ACSW testing is a low risk operation can be undertaken most site conditions including along rail tracks and directly pavements and slabs without impact to services. All equipment be can be safely operated in muddy and wet conditions with minimal precautions.

Test can be undertaken on active construction sites and with line open roads or rail lines. The low intensity, rapid operation involving a single operator with low noise and vibration provides minimum disruption on active or sensitive sites.

Limited Access Investigations? No problem

Mineworking's Assessment

Using ACSW for Mining Investigations
Mineworkings are often associated with zones of loosened ground due to crown collapse and bulking of soils or areas of surface backfilling around shafts. These loosened materials typically have lower Vs and G0 values than undisturbed material. Permitting their detection by ACSW profiles where the stiffness contrasts presented are sufficient and their depth of burial not excessive.

Where voids are present, then ACSW profiles identify a deterioration in data quality below the crown of the void, due to the complicated seismic interactions generated at this free surface.

Contextualising CSW
Continuous Surface Wave (CSW) testing permits geotechnical profiles to be determined non-intrusively by measurement of the velocity of surface Rayleigh waves (Vr) over a range of frequencies (f).

Inversion of the Vr versus f data (the ‘dispersion curve’) provides a shear-wave velocity (Vs) profile which can be simply converted to a stiffness (G0) against depth profile along with a number of other geotechnical parameters including undrained shear strength (cu) and Rock Quality Designation (RQD).

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