Secant Pile Walls

Secant pile walls are an innovative way to build retaining walls. They are formed by a series of interlocking drilled shafts used primarily where there is a high water tables or unsuitable ground conditions. Because of the lack of noise and vibration, these walls are attractive near buildings, roads, and other sensitive structures. Secant walls can be used to form a continuous watertight wall, an added benefit on environmental projects.

Secant walls have been used for:

• Intake structures
• Below grade holding tanks
• Cut and cover tunnels
• Basement construction

Secant pile walls are constructed so the piles overlap each other. The common practice is to alternate the placement of the piles along the wall alignment, leaving a space of a little less than the diameter of the intermediate pile. The exact spacing is based on construction tolerances. The intermediate piles are usually formed using a heavy temporary casing that cuts through the concrete piles on either side. RECON has the ability to design and construct all types of pile walls using our Delmag RH-190 drilling rig. Typically, 36-inch diameter shafts are installed at 30-inch centers; although this may vary depending on retained height and soil properties. The shafts are usually installed through a concrete guide wall that serves as a template to maintain proper alignment and pile overlap. For additional technical information on secant pile walls, see www.secantpile.com

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Case Studies IPSCO WARM WATER BASIN Mobile, Alabama Remedial Construction Services, L.P. (RECON) was contracted to install a secant pile wall in order to create an underground warm water basin for the client´s production system. The 50-foot diameter basin reached to a depth of 41 feet below the ground and 25 feet below the groundwater table. This new construction project selected secant pile technology because the walls can be used as both retention and finished walls. Design The design of the secant piles was developed for long-term performance. Conditions were based on having a 4-foot thick floor that would be tied into the wall piles by doweling into the primary shafts. With the groundwater table at 14 feet, the floor and the perimeter wall would be subjected to full hydrostatic uplift. For this condition, it was determined that the penetration of the secant pile wall to a depth of 40 feet below the ground surface. This depth would be sufficient to provide uplift resistance based on the buoyant weight of the floor and the secant piles. The uplift resistance of the secant pile-floor system assumed that the mat would be structurally connected to the secant piles. In addition to the weight resistance, the secant pile wall would also mobilize friction along the embedded perimeter area, which would be effective in resisting uplift pressures. Construction The 3-foot diameter piles were installed by drilling through concrete guidewalls. Guidewalls were poured in place on the ground and served as a template to maintain proper alignment and pile overlap. After installing the piles, the guidewalls were removed and disposed of as recyclable concrete. The 66 pile wall was constructed by drilling alternate shafts, 1, 3, 5, 7, etc., to a depth of 41 feet below ground surface, and then back-stepping to drill the intervening shafts to the same depth, in order to interlock the two adjacent shafts. Every second shaft was reinforced with a reinforcing steel cage. The reinforced shafts are called "primaries". The intervening piles that are not reinforced are called "secondaries." This technology calls for the secondaries to be drilled first. This is done so that the reinforcing of the primary piles will not be compromised by subsequent drilling. The concrete used for the secondary piles was 2,500 psi strength concrete. Lean concrete is used so it will remain soft enough for the drilling and interlocking of the primaries. The primaries were poured with 4,000 psi structural concrete. After installation of the secant pile wall, the inside and outside perimeters were dewatered in order to install the floor. Once dewatered, the inside pit was excavated to 36 feet below the ground surface and a mud mat was poured. This allowed workers to safely enter the excavation to install dowel rods into the primary piles. Afterwards, a 4-foot concrete floor was poured to complete the warm water basin. Using secant pile wall technology, RECON constructed a warm water basin, 50 feet in diameter and 41 feet in depth Back to Top 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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