Located in western Washington near Tacoma, Holocene Drilling Inc. specializes in complex geotechnical subsurface investigations, including the installation of geotechnical instrumentation and monitoring wells. The company also is a full-service environmental investigation drilling contractor.
Inclinometers and ExtensometersEarlier this year, Holocene was awarded the first phase of the Sound Transit U-220 TBM Tunnel Project at the University of Washington Campus in Seattle. The general contractor for this $309,175,274 project is a Traylor Brothers/Frontier-Kemper joint venture. Holocene’s client, Group Delta Consultants Inc. of Irvine, Calif., contracted with Holocene to provide drilling services during the installation of slope inclinometers and extensometers used to monitor the geotechnical conditions during advancement of the tunnel boring equipment. This project presented several complexities that Holocene addressed based on its previous geotechnical instrumentation installation experience. These hurdles included coordination of difficult boring locations, and site access limitations due to on-going construction activities by other subcontractors. Additionally, two of the inclinometer borings were located in the environmentally-sensitive Montlake Cut, which passed from Lake Union to Lake Washington.
Holocene retained the services of Northwest Marine Salvage Inc.’s barge and tug, named “The Seahorse,” to sail up the Montlake Cut, and move Holocene’s track-mounted Diedrich D-50 to onshore drilling pads on both sides of the Montlake Cut to perform these installations. Once the D-50 was in position, it was discovered that the proposed boring locations had been sited too close to an extensive concrete retaining wall (about 20 ft. thick) running the entire length of the cut. This forced Holocene’s driller to stand upslope on a 30-degree angle, and make cuts into the slope to support a safe operating location for the borings.
In order to accomplish this, Holocene was required to consult with both the general contractor, Group Delta Consultants Inc., and the U.S. Army Corps of Engineers (USACE), which oversees the waterways in and around the Montlake Cut. The extensive environmental requirements of the USACE dictated that Holocene crews use visqueen and vacuums to ensure that there was no runoff of drilling fluids or mud during drilling and installation. This presented a unique challenge, since this clean-up was required all day during drilling operations, and at the end of shift each day. As most of you are aware, Seattle is not exactly known for its dry climate in the spring months. This past spring was even wetter than most, so Holocene crews had to really stay on top of this requirement when it was raining to meet drilling production schedules. The general contractor and USACE monitored every move while the firm was on-site at this high-profile landmark, and ensured that every aspect of the environmental requirements to specification was met.
Other challenges included deep mud rotary borings between 200 feet and 250 feet during the installation of some of the vertical inclinometers. Specifications required that these units all be tested and calibrated upon installation. The vertical inclinometers for this portion of the project are specified to monitor stress relief, ground movements, and possible displacement of rock blocks during the construction of tunnels and shafts, and advancement of the boring machine. These vertical incline meters also are used to verify the adequacy of ground supports, detect potential flaws in the construction approach, monitor tunnel alignment, and serve as a warning system for potential ground failure.
The type of inclinometer casing and backfill material for these vertical installations were selected to conform to the surrounding ground to reflect ground deformations accurately. Conformance to this specification was accomplished by selecting materials that are similar in shear and bending.
Installed by mud rotary drilling methods, the borehole diameter needed to be sufficiently large (6 in.) to allow insertion of the inclinometer casing, and to ensure that grout or backfill was accomplished around the entire annulus, without leaving voids or soft zones. Insertion of the inclinometer casing was centered in the borehole to allow the grout backfill to completely surround and support the casing.
The borehole and inclinometer casing need to be made as vertical as possible to minimize reading errors. Selection of the type of coupling and method of attachment was modified by Group Delta Consultants to include a welded L-joint on the side of the monument covers to allow connection of wiring to a centralized data-acquisition system. These were specially fabricated in Holocene’s shop facility.
Since ground water or drilling fluids were present within the borehole, the inclinometer casing was anchored to counteract buoyancy. This alleviates applying forces or twisting motions to the top of the casing, which could cause adverse spiraling and snaking of the casing, leading to erroneous data and interpretations. This alternative decision by Group Delta Consultants to attach an anchor to the bottom of the casing allowed the anchor to embed itself into the borehole when activated. This approach also applies when Holocene’s crews were grouting, which also causes temporary buoyancy.
A series of extensometers also were installed in boreholes to monitor settlement and heave associated with construction, mining and tunneling operations at the project site. The relevant data obtained from these extensometer installations were used by Group Delta Consultants to indicate possible settlement zones, as well as total displacement during the construction process.
Each extensometer system used a probe, a steel measuring tape, a tape reel with built-in light and buzzer, and a number of magnets positioned along the length of the borehole access pipe. The magnets then are connected to the surrounding soil, and move up or down as heave or settlement occurs. Readings then were obtained by Group Delta Consultants by drawing the probe through the access pipe to find the depth of the magnets. When the probe enters a magnetic field, a reed switch closes, activating the light and buzzer. The field operator then refers to the 1 millimeter or 0.01 foot graduations on the tape, and notes the depth of the magnet. When the pipe is anchored in stable ground, the depth of each magnet is referenced to a datum magnet that is fixed to the bottom of the borehole access pipe. Settlement and heave are determined by comparing the current position of each magnet to its initial position.
Subsurface InvestigationHayward-Baker Inc., North America’s leader in specialty geotechnical construction, contracted with Holocene to provide four mud-rotary borings and NX rock coring on the Sound Transit U-220 TBM project. Holocene’s scope included verification of the jet grout block, a block of cement stabilized soil for the start of the tunnel project. These borings were approximately 500 total lineal feet or approximately 125 lineal feet. Hayward-Baker. provided Holocene with proposed exploration data, which was used to support the subsurface investigation plan and an up-to-date understanding of likely site surface and subsurface conditions. Holocene used a track-mounted CME 850 drill to accomplish these tasks for Hayward-Baker.
Each boring was drilled using mud-rotary methods to approximately 75 lineal feet below ground surface (bgs), and targeted the approximate jet grout block interface that the drillers were expecting to encounter. Drilling methods and equipment then were switched over for NX rock coring, and the borings were advanced another 50 lineal feet bgs to prove the approximate end of the jet groutblock. This allowed Hayward-Baker’s engineers to map to the end of the jet grout block, and to ascertain its depth and position in the subsurface formation. No samples were advanced or extracted other than the contents of the NX rock core barrels for the purpose of the investigation.