By Robert J Slyh, P.E., GE, Northwest Soil Stabilization, Inc. and Joe Patterson, Vice President, TEI Rock Drills
(reprinted from ADSC Foundation Drilling Magazine)
Geotechnical drilling has been described as “waiting for something to go wrong.” Since drillers experience this much too frequently, it has become a fact of life in our industry. The “waiting” rule is frequently exacerbated by a lack of quality geotechnical data which in turn hinders bid preparation and plan drilling operations. Adaptability, as always, is the key to a successful drilling operation.
ADSC Contractor Northwest Soil (NW Soil) recently completed work on a 9,000 square
foot temporary soil nail shoring wall on a 28 foot deep excavation for a replacement potable water supply reservoir structure. Subsurface conditions in the geotechnical data were described as Loess over Silty Gravel, Gravel, and finally variable Bedrock. The “geotechnical” descriptions provided in the data report were limited, with effectively little professional interpretation. All of the data available in the bid documents, including the wall stability analysis completed for the design, indicated the coarse soil units as having limited cobble or boulder size particles and cohesion – that magical property that keeps bore holes open and speeds drilling operations.
Drilling for the initial lift in the Loess went very smoothly using air rotary methods. While a few soft boulders were encountered, nothing indicated significant drilling issues. Optimism and spirits were high looking at a potentially very successful drilling operation. However, the second lift began to encounter the “Silty Gravel” and the “Waiting” was over. Something was wrong.
Air circulation loss became an issue almost immediately within the silty gravel soil unit that was reportedly cohesive. Not just a little air, but 750 CFM disappeared down the bore hole with 1 foot of penetration into the Silty gravel formation, below / behind the Loess. An 1800 CFM compressor was utilized with only slightly better success and we were only on row 3 of a 6 row nailing job. In addition, soil units described as medium dense to dense were drilling in a manner more
consistent with a loose formation. Augers were also attempted but the holes began caving in the Silty Gravel soil unit and the hole quality was not suitable for soil nails.
Drilling operations were suspended and the infamous meeting with the design professionals was called. Options – including both casing and Hollow Bar Soil Nail (HBSN) drilling methods – were discussed. Both of these options involved a significant increase in cost. The HBSN carried with it a potential for large grout takes, however, the apparent porosity of the formation posed a similar issue for a cased drilling system and air circulation loss in general was still a concern.
The morning of the meeting NW Soil switched drill heads to ADSC Associate Member TEI Rock Drill’s TEI 560 drifter and while the meeting was in progress had installed two HBSN test nails in the Silty Gravel formation. While grout takes were above normal, the nails were installed rapidly with minimal resistance. The HBSNs were tested two days later and passed successfully. A change order was negotiated and the drilling was re-started. Waiting for a problem was never an issue again as the drilling conditions continued to deteriorate with depth and the operation struggled. Numerous boulders and rock outcrops(?) were encountered well above the estimated contact with bedrock. Grout takes were extremely variable ranging from 4 cubic feet up to 40 cubic feet per hole and drilling resistance continued to decrease as well.
The amazing thing was the HBSN holes were installed without significant issue. The TEI 560 drifter had more than ample power to push the 38 mm Williams Form (an ADSC Associate Member) hollow bar fitted with 4 inch diameter carbide bits easily through the highly variable drilling ranging from loose cobbles and gravels to boulders and rock, without hole relocation or change in methods. Other than waiting on grout circulation, the HBSNs were installed quickly enough to make up for much of the lost time in the schedule. A total of nearly 1.5 miles of drilling was completed in conditions that would normally be judged unsuitable for soil nail construction.
About TEI Rock Drills, Inc.
The TEI headquarters and manufacturing facility are located in Montrose, Colorado. TEI rock drills are hard at work in solid and hollow bar soil nailing, micropile, mining and tunneling jobsites worldwide. Our TEI team members take great pride in building real American-made quality into every TEI rock drill and rock drill component.
TEI Rock Drills contact: Joe Patterson – 1.970.209.1778 or email@example.com