June 15, 2005
Crews arrived at the Hoko borehole site first thing Tuesday morning ready to begin the testing of the strainmeter and continue preparing for the installation.
First, crews tested the “trip mechanism” - a device that is used to dispense the grout once the 30’ dump bailer touches the bottom of the borehole. A couple of dry runs to the bottom of the borehole proved successful activation of the trip mechanism upon reaching the floor of the hole.
Meanwhile, Mick Gladwin performed over 60 different tests on the strainmeter itself to ensure every part is working properly. Once Mick declared the pre-installation testing a success, a 700-pound spool of strainmeter data cable was put into position behind a capstan. Attached to a hydraulic motor, the capstan will help guide the data cable as the strainmeter is lowered into the hole. Additionally, crews used a unique “Strainmeter Delivery System”, developed by Senior Strainmeter Engineer Robert Mueller, to guide the instrument steadily and evenly down the hole. For extra measure, crews used a long rope in place of the data cable to test the angles and positioning to ensure a straight line down into the borehole. Next, the data cable was hooked up to the strainmeter, and crews checked the internal compass for accuracy.
At this point, the clock started ticking quickly for the rest of the strainmeter installation steps. Six bags of grout were mixed with carfefully-measured amounts of water to obtain the right consistency, taking into account the intermittent rain and quickly changing –temperatures. Getting the right consistency is extremely important and sometimes very hard to achieve due to atmospheric changes.
Next, the 30’ dump bailer was lowered into the hole, keeping just a few feet above ground and using a large funnel to pour buckets of the mixed grout into the bailer. In a prime example of field ingenuity, crews used a basic tennis racquet as a strainer when pouring the grout to stop any large chunks from getting into the hole. Once filled, the bailer was lowered to the bottom of the borehole where the trip mechanism released, allowing the grout to fill the bottom of the hole, after which the dump bailer was slowly pulled back out of the hole. Unfortunately, when the bailer was pulled out of the hole, the engineering crew found that the trip mechanism did not release after hitting bottom due to a jammed o-ring. A spare trip mechanism solved the minor (but repairable!) problem. More grout was mixed and the second attempt at pouring grout at the bottom of the hole was a success.
The crew worked quickly before the grout began to dry and thicken inside the borehole. With all the cable guiding and lowering devices in place, the strainmeter was carefully and slowly lowered down the borehole. Once safely in the borehole, the instrument was re-tested to ensure no damage was caused while being deployed into the hole. Around 3 PM, heavy sighs of relief and large congratulatory smiles were exchanged when Mick Gladwin announced that the installation was a success and the strainmeter was performing as expected!
After cleaning up the site and putting away the strainmeter equipment, the crew immediately began working on the next step: installing the seismometer. Attached to 20-foot pieces of 2” PVC, the seismometer was lowered into the hole above the strainmeter and grout, reaching a depth of about 512’. About 40 feet above the seismometer, a slotted “screen” piece of PVC was inserted to allow the measurement of pore pressure near the strainmeter.
After the crews left the site about 7 PM, they learned from the management at their motel that there was a tsunami warning in effect for Neah Bay, a small town about 15 miles from the motel in Sekiu. Fortunately, the warning was cancelled by 9 PM, thus leaving the installation of the strainmeter as the most exciting part of the day!
Last modified: 2020-01-28 22:54:11 America/Denver