Subway Work Moves South Of Interstate-380By Alice Barnes
BART work on Huntington Avenue began in earnest this week with pile driving and excavation work started just south of Forrest Lane/Interstate 380.
Although the line work will actually be done on the east side of the old Huntington Avenue this is the nearest Tutor-Saliba/Slattery will actually come to San Bruno residences during this phase of construction.
For the first time, San Brunans are seeing the typical subway-construction operation beginning with the sheet piling and excavation. Similar work has been ongoing through South San Francisco and Colma for nearly two years.
Since late in 1998 work has been progressing, except for a four-month delay, on construction of the shell for San Bruno's BART station. When completed, it will be one of four new stations in the 8.7 alignment extending service from Colma to the San Francisco International Airport and to Millbrae's new intermodal station.
Construction of a typical "cut-and-cover" subway segment consists of the following stages:
- sheet piling or some other means of walling in the area
- soil excavation down to approximately 45 feet
- installing rebar for the tunnel flooring
- pouring the concrete floor
- installing rebar for the tunnel walls and roof
- pouring concrete for the tunnel walls and roof
- backfilling dirt on top of the tunnel
- removal of the sheet piles
- making surface improvements
Thus far, Tutor-Saliba/Slattery has been averaging fifty feet of tunnel progress per week with the entire process taking several weeks to pass by any one point. That means, if the work did not leapfrog down the alignment, it would move down Huntington approximately 40-50 feet per day but any one spot would witness all the steps to take up to a couple months.
According to BART engineers, the noisiest portions of the process are the installation and removal of the piles. This is because a large vibrating hammer is used to secure the piles into the ground and then, to shake them lose upon removal.
Tutor-Saliba uses a couple of mitigating measures to lessen both the vibrating and noise impacts. First, an augur drills a hole nearly forty feet deep so that a pile will have place to go without overly vibrating the area. Then the hammer pounds down the pile in channels connecting the separate piles. Depending on soil conditions, these piles are driven sometimes in sets varying from two to five sheets.
Another way to cut down on noise, if it is expected to exceed standards pre-established for the construction, is to shroud the area with a giant structure. On the inside are sound dampening materials while on the outside is a shinny waterproof covering.
Sidewalk engineers frequently mistake this as a necessary part of the pile driving process. It is used only in those circumstances where it is known in advance that noise limitations may be exceeded. This was done consistently behind the Kaiser Permanente Medical facility in South San Francisco last year but was not necessary though most of South San Francisco.
On Wednesday of this week, BART engineers and San Bruno City monitors were observed monitoring the noise levels and amount of vibrations right at the house lines of homes on Huntington. If either the noise or vibrations exceed established parameters the contractor may be required to stop work completely in that location. This has happened elsewhere along the line when unforeseen ground conditions were encountered.
BART officials are quick to note that this does not mean the remainder of the work will be quiet. Jim Tousey, a BART project manager, tells San bruno B.A.R.T. that the contractor has scheduled his work so as to have the loudest noise happen during daylight hours. The contractor was recently given City approval to work on weekends and overnight.
Persons with specific questions may submit them to this website by clicking here or by calling Kassie Wilner at 650-689-8465. She is the Community Relations Liaison to the City of San Bruno for the BART-SFO Extension. Residents wishing to take a tour of the construction site can also contact Wilner.
Copyright ©2000 San Bruno B.A.R.T. Newsroom. Published 03/03/2000.