Downhole Seismic Testing within Existing Steel Cased Sonic Boreholes

Downhole seismic (DHS) data was reliably recorded in steel cased sonic drilled boreholes without the usual additional time or materials required to grout and prepare the boreholes. DHS results compared well with adjacent seismic cone penetration test (SCPT) results (sonic holes were located within a few metres (m) of existing SCPTs). This technique works well in uncemented granular and fine-grained soils when sufficient time is allowed for the induced dynamic effects to dissipate. However, a drawback of this technique is that a rapid assessment of the data is necessary to ensure that sufficient records have been collected for a complete profile. Once the casing has been pulled the opportunity to collect data is lost, unlike a grouted casing installation that can be revisited to complete additional testing, if necessary. As sonic drilling is becoming more common in geotechnical investigations, this paper demonstrates how sonic holes can be utilized without the added expense and time of a grouted installation to obtain shear wave velocity (V-s) measurements.