Braintree, MA – Helical Drilling Inc, a geotechnical design-build company, announced the completion of three recently completed projects. The following is the summary of those completed projects:
Helical Drilling completed work on an existing New Bedford, Mass., warehouse in preparation for a 50,000sf addition that needed to remain in service during construction. The addition required column loads exceeding 300 kips and wall footing loads that surpassed 10 kips per linear foot.
The design team evaluated different foundation support, and Geopier RAP ground improvement was chosen as the most cost-effective solution as it eliminated premium costs associated with the excavate/replace option, including offsite soil disposal, dewatering, potential shoring, and potential underpinning of the existing structure.
Drilled micropiles – Stratford Bridge
Helical assisted replacing a single-span bridge along the Route 110 Corridor leading into downtown Stratford, Conn., for the Connecticut Department of Transportation (DOT).
Helical installed the 60 drilled micropiles per the DOT’s design using compressed air percussive drilling methods.
Interior installation of ductile iron piles – Mystic Avenue
- –Helical Drilling provided deep foundation services for a commercial project at 291 Mystic Avenue, Medford, Mass. The Helical team completed the installation of a low-headroom, deep foundation system within an existing high-bay building to support the new concrete slab while limiting installation vibrations. Helical was successfully awarded the project based on the specified micropile solution. However, Helical approached the project team with a potential value engineering option to save time and money. After completing an additional boring to identify the depth of the underlying glacial till/rock layers, the ductile iron pile alternative was selected for floor slab support based on cost savings and, more importantly, more than two weeks of schedule savings.
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Drilled Micropiles Support New Bridge in Connecticut
The Connecticut Department of Transportation (DOT) needed to replace a single-span bridge along the Route 110 Corridor leading into downtown Stratford. The project was broken into two phases, ensuring that half of the bridge would remain open to traffic at all times. The first phase of work focused on the southbound side, and once complete, work transitioned to the northbound side.
Subsurface soil conditions consisted of approximately 5 to 15 feet of sand overlying sloping bedrock. Due to relatively high vertical and lateral loads, the DOT required that the bridge abutments be supported on a total of 60 drilled micropiles (DMPs), 21 of which were designed as battered piles to resist lateral loads. The DMPs had a design capacity of 163 kips. Given the shallow depth to bedrock, the DMPs were designed to derive their capacity via bedrock sockets (“rock sockets”).
DMP installation needed to occur from within a depressed open-cut excavation such that the working subgrade was near the top-of-pile design elevation. Working within an excavated area while the other half of the roadway remained open created relatively tight (but manageable!) working conditions for the crew. For the second phase of work, the piles were installed around a live gas main. Working around the gas main required careful coordination and precise construction methods to ensure safe working conditions and uninterrupted gas service. Furthermore, the Freeman Brook was as close as 10 feet to the work area, so potential impacts to the brook had to be managed prudently. The work area was contained using tarps, haybales, and other protective measures. Drilling and grouting spoils were 100% contained within small local excavations (“spoil pits”).
Pile Installation:
Helical installed the 60 drilled micropiles per the DOT’s design using compressed air percussive drilling methods. A specialty winged drill bit was used to seat 7-5/8-inch diameter permanent steel casing into bedrock. After seating the casing, our crew drilled an additional 9 feet into the rock with a conventional drill bit to form a 6-inch diameter rock socket bond zone between the grout and surrounding bedrock. The rock sockets and cased hole were then tremie grouted and the threaded core steel was installed down the full length of the pile. Since the bedrock depth varied, final pile lengths ranged from about 12 to 18 feet.
Prior to DMP production work, full-scale load tests were completed on two non-production piles. The test results indicated less than about ½-inch of deflection at 100% design load and confirmed that the DMPs would perform as designed.
Drilled Micropile Advantages:
- Installation within a constrained site
- Helped permit phased construction and maintain traffic over the bridge
- Work was performed safely around an active gas line
- No vibrations or environmental impacts to the surrounding area
- High load carrying capacity for relatively short piles
- Pile lengths were easily adjusted based on varying bedrock depth
291 Mystic Avenue
Renovations to the single-story building on Mystic Avenue in Medford, MA included conversion of the former lumber yard high-bay storage area into new commercial space. The original building foundations were supported on caissons and would remain in place to support the existing structure. However, a new pile-supported interior concrete slab was needed to replace the existing asphalt floor.
Soil conditions encountered during explorations at the site consisted of 9 feet of urban fill followed by up to 9 feet of peat. The fill and organics were underlain by stiff to very soft clay extending to about 74 feet below grade. Below the clay, medium dense to very dense sand and gravel (glacial till) was encountered to the maximum explored depth of 84 feet. Groundwater was recorded at a depth of about 6 feet below grade during drilling.
Installation of a low-headroom, deep foundation system within an existing high-bay building to support the new concrete slab while limiting installation vibrations.
The construction of the new concrete slab required installation of a deep foundation system within the existing building interior for support. Headroom clearance was limited to about 20 feet from the working grade level to the underside of the roof structure. Traditional low-overhead options to support the slab included low-capacity helical piles and higher-capacity micropiles terminating in the clay. A total of 42 micropiles were designed with 20 ton working capacities. The micropiles were designed to be 40 feet in length and develop bonding within the clay after 2 rounds of post-grouting.
Helical was successfully awarded the project based on the specified micropile solution. However, Helical approached the project team with a potential Value Engineering option to save time and money. After completing an additional boring to identify the depth of the underlying glacial till / rock layers, a Ductile Iron Pile alternate solution was also proposed. The Ductile Iron Pile (DIP) option was designed to penetrate the fill, peat and clay to terminate in end-bearing on glacial till or rock.
The Ductile Iron Pile alternative was selected for floor slab support based on cost savings and, more importantly, more than 2 weeks of schedule savings. DIPs are installed using low-vibration driving energy with an excavator-mounted hydraulic hammer. The excavator-hammer combination is selected to work within the limited headroom situation. Accommodations for the limited headroom during installation included pre-excavation at grade beam locations. Additionally, the 5 meter long modular pile sections were cut in half and connected with couplers to assist in the tight working conditions.
The Ductile Iron Piles were installed on a 1:1 basis to replace the 42 micropiles. The DIPs were installed to depths of over 85 feet to refusal. Even with greater than twice the length of piles, the DIP solution provided cost savings and was completed in only 7 working days – saving over 2 weeks in the construction schedule.
Ductile Iron Pile Advantages:
- Rapid installation
- Low overhead installation
- Low vibrations for interior work
- High capacity
Geopier Ground Improvement – Warehouse
Geopier® Rammed Aggregate Pier Elements Installed Within Five Feet of Existing Structure
An existing warehouse required a 50,000sf addition and needed to remain in-service during construction. The addition included column loads exceeding 300 kips and wall footing loads that surpassed 10 kips per linear foot.
The primary geotechnical challenges included the presence of fill up to about 7-feet-thick, soft organic soils up to about 3-feet-thick, and a shallow groundwater table. The unsuitable fill and soft organic soils were underlain by relatively dense glacial soils. Constructing conventional footings and slabs-on-grade directly on the existing unimproved fill and organic soils would have led to unacceptable settlement.
- Substantial cost savings compared to the excavate/replace option
- Permitted conventional shallow footing and slab-on-grade construction
- Displacement technique eliminated dewatering and excess spoils
- Fast installation that allowed the existing warehouse to remain operational
Geopier Ground Improvement – Warehouse
The design team evaluated different foundation support solutions including: 1) shallow footings and slabs-on-grade after excavation and replacement of the unsuitable soils, and 2) shallow footings and slabs-on-grade after Geopier Rammed Aggregate Pier (RAP) ground improvement. Geopier RAP ground improvement was chosen as the most cost effective solution as it eliminated premium costs associated with the excavate/replace option, including off-site soil disposal, dewatering, potential shoring, and potential underpinning of the existing structure.
Geopier Rammed Aggregate Pier ground improvement allowed for conventional shallow footing and slab-on-grade construction and helped expedite the construction schedule. The RAP elements were designed to limit the total and differential settlement to less than 1 inch and ½ inch respectively, and provided a maximum allowable footing bearing pressure of five (5) kips per square foot.
Geopier ground improvement advantages:
- Helical installed more than 750 grouted Rammed Aggregate Pier (“GAP”) elements in about two weeks while the existing warehouse remained operational. GAP elements were installed using a displacement technique that did not generate excess spoils, further reducing premium off-site soil disposal costs. The use of grouted elements helped provide enhanced stiffness and settlement control through the unsuitable fill and soft organic soil layers. Below slabs, the GAP element shafts were grouted through the soft organic layer and the remainder of the shaft was constructed with ungrouted aggregate. Below footings, the GAP element shafts were fully grouted to form a “rigid inclusion” ground improvement element. An engineered granular fill “footing pad” was installed between the footings and the tops of the GAP rigid inclusion elements to help transfer footing stresses down into the elements and surrounding matrix soil. The footing pads also helped to provide a stable working subgrade for footing construction.Helical’s crew included a full-time Quality Control person to oversee all testing and installation procedures. An individual GAP modulus test was performed to 150% of the GAP element design stress. The testing results showed deflections of less than about ½-inch at the design stress level indicating excellent performance of the Geopier ground improvement system.
