NEW HAVEN SCHOOL DISTRICT
New Haven, Connecticut
 Secant wall construction is common for soil retention systems; but the application at the New Haven School District was, perhaps unique. Secant pile technology was selected to expedite the schedule and improve on the cost of the project. The school district had 10 months to build a 6-million gallon stormwater run-off holding tank. The 140-foot by 240-foot tank was to have a bottom elevation 25 feet below ground surface (bgs) and be adjacent to the property line. Additionally, the water table was at 10.0 feet.
Drilling Rig
Remedial Construction Services, L.P. (RECON) used a “state-of-the-art” Delmag RH-190, top head drive drill rig, the first of its kind in the United States. The drill rig has torque capacity of 190,000 foot-pounds, digging depth capability of 120 feet (with standard Kelly and casing) and diameter widths up to 11.5-feet (with Kelly and no casing). This same machine has the capability of being converted to a continuous flight auger (CFA) unit, drilling 24-inch diameter holes to depths of 52 feet.
Secant Pile Installation
Secant piles are drilled shafts that interlock to form a continuous wall. The wall is constructed by drilling alternate shafts and then “back stepping” to drill the intervening shafts in order to interlock the two adjacent shafts. Every second shaft is reinforced usually with a wide flanged steel section or reinforcing steel cage. The reinforced shafts are called “primaries” or “king” piles. The alternate shafts, which are not reinforced, are called “intermediates” or “secondaries”. The drilling sequence typically calls for the intermediates to be drilled first, so the reinforcing of the primary piles will not be compromised by subsequent drilling. The concrete used for the secondary piles is usually lean concrete; to remain soft enough for the drilling and interlocking of the primary shafts. The primaries are usually poured with structural concrete.
Due to high pressures – both lateral and vertical – structural concrete was used in all the shafts. When drilling 60-feet into concrete that is near 2,500-psi, it poses quite a challenge. RECON was up to this challenge. To ensure and facilitate alignment and spacing control at the working platform elevation, RECON constructed guide templates. The shafts were installed through these concrete guide walls to maintain proper alignment and pile overlap.
RECON installed the 3-foot diameter secant piles on typical 2-foot, six-inch centers, which produced a 6-inch overlap to form a full 30-inch wide pile for the structural exterior wall of the holding tank. The perimeter of the rectangular tank was 760 feet (306 secant piles) with depths to 60 feet. In order to meet the aggressive schedule, RECON used two Delmag and one CFA secant pile rigs.
 Anchors and Tiebacks
Excavation for the holding tank began after completion of the drilling operation. After removing the first 7 feet of soil, a row of temporary anchors or tiebacks were installed in the primary piles through the steel reinforcement. This continued every 7 to 8 feet until all the soil was removed from the holding tank.
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