peat, concrete rubble, buried concrete retaining walls, and pea gravel backfill overlying a Glacial till formation. All of the upper soils had been infiltrated over the last several decades with hydrocarbons and lead from various contamination sources. Additionally, the ground was saturated with perched water and had a water table located approximately 10 to 15 ft (3 to 4.6 m) below existing grade. To contend with existing ground conditions, a secant pile wall designed by Ground Support PLLC of Redmond, Wash., was utilized for the eastern two-thirds of the site while the shallow western portion was supported by conventional shoring designed by Coughlin Porter Lundeen of Seattle. The west side of the site, aside from the dewatering, was somewhat typical for SLU shoring projects. Secant Wall Installation and Project Schedule The secant wall installation began with the construction of a guide wall 5 ft (1.5 m) wide and 3 ft (.9 m) in depth with 4,000 psi (27.6 MPa) concrete and limited reinforcing. Block outs consisting of sonotubes were utilized to allow room for the sectional casing to penetrate the guide wall. The guide wall was utilized for both layout control and maintaining secant shaft location. It was instrumental in dealing with the timber piles located beneath the site. Some of the more shallow piles were removed with a vibratory hammer, as they were exposed during the guide wall excavation, but the majority of the piles were up to 15 ft (4.6 m) below existing grade and could not be removed or located. With the help of the guide wall and sectional casing, Malcolm used a BG-40 top drive drill to core through the vertical piles while maintaining the shaft alignment critical for water cutoff. Casing with a diameter of 3.3 ft (1 m) and a wall thickness of 1.5 in (38.1 mm) withstood the drill forces imposed while coring through the wood piles. The secant walls were built in 16 shaft runs beginning with every 4th pile, then every 2nd pile — these were the primary shafts. Next the secondary shafts were constructed as the 1st and 3rd piles were installed. The shafts were approximately 46 ft (14 m) in length, and the goal was to complete about 200 linear ft (61 m) or 4 shafts per day per rig. On Block 43 a total of 157 primary piles and 155 secondary piles were installed. The many obstructions encountered on site took a toll on the project schedule. Although Malcolm had two drills installing the secant piles, every day the crews slipped further from the planned completion date. To help remedy the situation, Malcolm Superintendent John Muzzall coordinated closely with the earthwork contractor, Northwest Construction, and began excavation of the western third of the site ahead of schedule. With an early start on the excavation, the project gained valuable time considering the reduced daily yardage accepted by contaminated soil dump sites. Given the abundance of contaminated soil sites presently under construction in Seattle, the few available dump sites put limitations on how many yards would be accepted from each site per day. The limitations associated with contaminated soil export were an ongoing burden and eventually restricted excavation and tieback installation later in the project. As a result of Installation of strand anchors through primary secant shafts Dewatering Approach Dewatering played a critical role in effectively excavating the site. Prior to the bid, Vulcan performed a test drill and pump test that indicated the presence of an aquitard separating an upper and lower aquifer. The test drill also found that the lower aquifer contained a substantial amount of pressure as a result of its confinement. As mentioned previously, the dewatering design, developed by Robert Middour LHG, was intended to dewater the shallow aquifer while depressurizing the deeper formation. In order to perform these functions, Middour Consulting, LLC DEEP FOUNDATIONS • JULY/AUG 2014 • 13 A deep dewatering system was used to drain the upper fill and reduce the impact of the pressurized aquifer. accelerating the schedule, Malcolm had to install lagging and tiebacks on the west portion of the site while at the same time constructing secant piles using two separate drilling operations on the east side. Malcolm then added a 4th element: compaction grouting, which was also performed on the eastern portion of the project further adding to the congestion, but unfortunately necessary to maintain the project schedule. Ground anchors were utilized for both the conventional soldier pile wall and the secant pile wall. A total of 520 anchors were installed with an average length of 60 ft (18.3 m), resulting in 31,000 linear ft (9,449 m) of drilling. The anchors consisted of two 270 ksi (1860 MPa) strands with a design load of 75-80 kips (334-356 kN). Anchors were battered between 20 and 35 degrees from horizontal in order to avoid shallow surface utilities as well as a 72 in (1.8 m) diameter city sewer line approximately 40 ft (12.2 m) below existing grade.
