Tunnelling-Induced Ground Movements

The Canadian Geotechnical Society (CGS) is hosting a seminar on this topic on April 28.  You can access the information by clicking here.

Feature Article

Kodiak Drilling is routinely involved in drilling projects to solve non-routine drilling problems.  As the company specializes in limited access areas, solving a problem of how to get the drill to the borehole location and complete the required work program is a daily challenge.

A recent project was presented that involved drilling in an underground parking garage.  The location initially did not present a problem as many projects have been completed in parking garages.  The combination of the additional constraints presented below however, looked like they were going to be impossible to overcome:

• Borehole depth requirement of 40-50ft (~12-15m),

• Overhead clearance of approximately 7ft (2.1m),

• Requirement for Standard Penetration Tests (SPTs),

• Firm silt till materials, overlying unstable flowing sands below the water table, and

• Use of hollow stem augers was not permitted due to the potential for structural disturbance of the sand formation.

Together with the geotechnical engineering consultant and colleagues at Walker Drilling (who worked together with Kodiak on this project), a method was developed to overcome these obstacles.  The primary components of the developed method include the following:

• A Big Beaver drill was used with the tower removed from the drill to allow access to areas with restricted headroom.  Extended hydraulic hoses were used to allow the machine to operate without exhaust fumes within the confines of an indoor environment.

• To overcome the issues of depth and a requirement to limit the disturbance to the sand formation, the drill head was used to rotate BW size drilling casing, with a cutting shoe at the base of the string.  Given the short stroke available, fabrication of custom casing lengths was required before starting the project.

• In order to prevent blow-back of the heaving sands into the casing, the interior of the casing was pressurized (through a swivel), with a dense, viscous drilling mud.

• As a traditional recirculation mud pit could not be accommodated within the low overhead environment, a vacuum extraction system was employed for retrieving the expended mud/cuttings mixture.  A mud pump, operating from an auxiliary hydraulic power unit, provided the mud solution to the casing string, while the vacuum system returned it to a recirculation tank.

• The remaining problem to be overcome was the requirement for SPTs in an area lower than the 8ft minimum requirement for this drill.  This issue was overcome by using a manual slide hammer to advance standard split spoons through the hollow casing.  The  requirement of 300-400 blows per foot in some areas made this aspect particularly challenging for the site crew.

Operation of the casing, the manual hammer, the mud pumping system and the vacuum recirculation component required a large crew, working in off-hours to minimize the impacts to the ongoing garage operations.  By the end of the project, several boreholes along with piezometer installations were completed to a depth of 48ft.
Although the casing/washboring method is commonly used by larger rigs in unlimited access areas, the confines of this site made this seemingly impossible project possible with unique limited access equipment and teamwork.