San Jose Site Prep.
Published in: California Builder and Engineer
Date: 10/17/2005
By: Larry Trojak
Contractor’s skills and equipment help avoid serious setbacks.
With nearly 2,700 students, Oak Grove High School is one of San Jose, California’s larger secondary schools. Bond money, made available for campus improvements, was channeled to a project to reconstruct the school’s music building, taking it from 6,000 square feet to 10,000 square feet. But the $3.1-million project hit a stumbling block early on when on-site soil conditions were found to have changed significantly.
Problem
“An exceptionally wet winter and spring brought the water table up far higher than it was at the time of the (soils) report,” said Joe Ascolese, project manager for Blach Construction Co., overseeing the construction. “That report showed water at 17.5 feet but when we started drilling, we hit very moist clay at 10 feet. Given that, the soils engineer who was on-site at the time felt that we would hit loose ground sand at a depth of about 12 feet. At that point, being just 5 feet above the water table — and already seeing indications of water — we all felt the risk of a cave-in was just too great.”
Ascolese discussed options with Bruce Smith, foreman for Casey-Fogli, the prime contractor for the Oak Grove project, and modified their plans to include the use of caissons to stabilize each hole prior to concrete. They spoke to Darryl Mueller Construction, drilling contractor, to set up caisson delivery for the following Monday, and opted to drill the remaining 9 feet at that time.
Solution
“An experienced, resourceful drilling contractor is key, particularly in a project like this,” said Bill Evans, Blach’s project superintendent. “One of the features of this project is a roof from the previous building that had to be maintained in order to meet bond issue criteria. The roof is 13 feet high which means a traditional drill rig — usually with a mast that is 20 feet to 25 feet tall — wouldn’t work. Mueller powers his drills and other tools with a Case 680L backhoe which, while it doesn’t make access easy, at least makes it possible.”
Mueller’s responsibilities at Oak Grove include drilling fifteen 24-inch-diameter piers, each of which will be capped by as much as 160 yards of reinforced concrete in support of the building’s K-brace structural steel design. While the piers to the outside perimeter of the structure are relatively accessible, Mueller said those set back 6 feet or so were directly under the existing roof and presented the real challenges.
“Digging a 19.5-foot hole with 13 feet of overhead clearance has made this interesting, to say the least,” he said. “There were several areas in which I literally only had inches to spare when maneuvering my boom to insert or remove a drill bit, the auger bucket or a caisson.”
Tools
Mueller was able to meet Blach’s needs at Oak Grove with a broad range of drilling equipment including a 20-inch-diameter auger, a 24-inch-diameter auger and a 24-inch-diameter heavy-duty drilling bucket, all manufactured by Pengo (Laurens, Iowa), a Paladin Company.
“The drilling bucket was particularly helpful in getting material from the very bottom of each caisson just prior to insertion of the rebar cage,” says Mueller.
In addition to augers with diameters as large as 72 inches, Mueller’s inventory includes Pengo drill heads which range from 5,000 foot-pounds of torque up to 20,000 foot-pounds — with a 30,000-pound head on order. “I’m using a 15,000-foot-pound head on the Case 680 at Oak Grove and it’s providing more than enough torque to get the job done,” he said.
Once each caisson is drilled, de-watered and fitted with a rebar cage, a tremie pipe is inserted and concrete is pumped from the bottom. Doing so forces any residual mud and water to the top of the concrete. When 9 feet or 10 feet of concrete has been poured, the caisson is raised for removal. Material that was previously at risk of collapsing is now held in place by the newly poured concrete. Near the top of the pier, the pour is stopped and the caisson totally removed. At that point the tremie pipe is reinserted and the pour resumes until fresh concrete can be seen overflowing the hole. Additional rebar is then added in preparation for subsequent pouring of the pier caps.
Oak Grove’s Music Building is set for occupancy in mid December.
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