Secant Pile Case Studies

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.

This 1-1/2 acre industrial site handles the characterization, transportation, and disposal of various by-products for the local industries. To keep up with market demands, the facility needs to be renovated and expanded. However, the soils were impacted with chlorinated solvents, hydrocarbons and trace metals.

Project Goals

The facility goals for expansion include:

• Remediating all soils surrounding the building footprint;
• Installing 9 injection trenches, approximately 30 to 70 feet long and 27 feet deep;
• Completing the remediation and new construction without impacting current on-going operations.

These goals are being accomplished by working two shifts with two oversight companies at this active facility.

Scope of Work

The original remediation approach was to perform a mass excavation and replace the area with suitable backfill. However, the cost was not effective since the impacted material would need to be incinerated. Deep Soil Mixing was then selected as an alternative solution. This approach will remediate the impacted soils while at the same time achieve greater soil strengths for the new construction.

Prior to arriving on site, RECON developed a mix design to determine additive rates necessary to achieve the performance criteria, which includes a minimum unconfined compressive strength (UCS) of 50 psi in 14 days and a maximum permeability of 1 x 10-7 cm/sec. The mix design chosen includes adding 10% cement.

RECON is performing in-situ solidification/stabilization to approximately 15,000 cubic yards of impacted soil using our own Deep Soil Mixing Rig. The depth of the stabilization will extend to 14.5 feet below ground surface. To account for a swell factor, 7,500 cubic yards will be removed for off site disposal.

RECON is also installing approximately 10,800 square feet of injection trench using the open cut method and the bio-polymer slurry trench method of construction. These trenches will include pipe and risers for the groundwater remediation effort.

Work is being completed in three phases in order to minimize disruption to new construction and ongoing facility operations.

Ancillary Tasks

Additional work to complete the project includes:

• Grading the work area for management of the surface water;
• Demolishing various below grade structures inside the footprint, including sumps, culverts, and drainage piping;
• Excavating surface soils and grading stabilized soils to manage the swell and maintain the original grade; and
• Controlling vapors by using suppressant foam and plastic sheeting.