Environmentally stringent, geotechnical marine site investigation on Lake Michigan.
The City of Chicago's Department of Water needed a site investigaton to help evaluate their proposed Jardine fresh water intake tunnel project. Designed by consulting engineer Montgomery Watson Harza, the new tunnel extends beneath Lake Mitchigan and up to 15.2 km beyond the Chicago shoreline. Seacore mobilised the jack-up Skate 3, which was container freighted to Chicago, where it was modified to ensure the project went smoothly.
Lake Michigan's waters can be tricky at times - Chicago is know as the windy city after all - wave heights can exceed 3.5m and with jack-up leg penetration of up to 10m and water depths of up to 24m, the leg length was increased to 41.5m. The drill mast was modified for quick conversion from vertical to angled drilling with minimal time and effort. Seacore operated two 12 hour shifts, with a RIB safety boat for crew changes to the worksite which was up to 15 km from the shore.
Eight boreholes up to 125m were drilled to evaluate the subsurface conditions along the proposed tunnel alignment - 3 of them at an angle of 25 degrees from vertical to assess bedrock properties. The Geobor-S triple wireline system was used to penetrate up to 21m of lacustrian clays, sand and glacial till overlying the dolomite bedrock, recovery averaging around 83%. The underlying dolomite bedrock was cored to depth using the HQ triple tube wireline system, which achieved excellent average core recovery of around 95%.
Fifteen boreholes to depths of 39m using Geobor-S and NQ coring systems, were also drilled to appraise the subsurface conditions in the vicinity of the proposed cofferdam and head house facility. Extensive soil sampling was performed during the drilling process using a variety of methods:
- Piston sampling in the softer lake sediments
- Pushed Shelby tubes
- Driven standard penetration tests in the harder clays and dense sands
Bedrock permeability/fluid migration was evaluated using a combination of single and double packer tests - 117 in total. Single packer tests were typically performed at about 15m intervals during borehole advancement, double packer tests at 7m intervals were performed after total depth was achieved.
Following the packer tests borehole, deviation and inclinations were recorded with a Tropari downhole survey tool in the deeper boreholes. Drilling parameters such as mud pressure, torque, depth, penetration rate and rpm were recorded during drilling using the Jean de Lutz system, which while developed to monitor equipment parameters and performance proved to be very beneficial as a tool to aid the evaluation of subsurface conditions.
Strict environmental conditions on Lake Michigan required a zero discharge policy - achieved by using an efficient drilling fluid recirculation and separation system designed by Seacore. Drill cuttings were removed from the drilling fluid during recirculation, containerised onboard Skate 3 and later removed and properly disposed of onshore by a local authorised waste contractor.
"We at MWH Energy & Infrastructure have been favourably impressed by Seacore." said senior vice president James Lindell. "Their jack-up rig was well suited for work in the 7-25m water depths and 3.5m waves that occur in Lake Michigan, while coring equipment for soil and rock was exceptional and used successfully in vertical and inclined holes to depths of 125m. Their drill crews and supervisors, from site managers to drillers' helpers, were all professional, helpful, and enthusiastic for the assignment" James continued.