- Provided design for rigid and in some cases impermeable or effectively dewatered shaft excavations to support microtunnel boring machine (MTBM) operations and enable deep manhole construction
- Elements include interlocking secant pile shafts, soldier pile/micro-pile and lagging shafts, sheet pile shafts, concrete ring beams, jacking thrust blocks, jet grout, tremie concrete, jet grout invert slabs, and custom junction chamber connections
- Successful installation of 20 shafts and completion of approximately 11,800 linear feet (over 2 miles) of pipeline installation by MTBM methods
GZA provided structural and geotechnical engineering to Barletta Heavy Division (BHD) for the micro-tunnel pipeline and deep shaft construction portions of a sewer interceptor relief project and also designed reinforced concrete thrust blocks used for pipe jacking operations.
Challenge
The new deep shafts required special considerations in their design and construction due to several issues, including: contaminated groundwater, existing utilities, headroom restrictions, tie-ins to existing sewer structures, difficult ground conditions, wide variations in the soil strata, and stability of nearby residential and commercial structures located, in some cases, immediately adjacent to the proposed construction.
Solution
The support of excavation systems designed for these shafts utilized a range of different innovative construction methods to take into account the limiting factors. GZA designed 20 shafts that were utilized as jacking and/or receiving shafts for the micro-tunnel operations, 11 of which were required to provide impermeable excavation support to prevent intrusion of contaminated groundwater. This was accomplished using a combination of interlocking drilled secant piles, steel sheet piles, jet grout invert seals, and jet grout ground improvement for MTBM entry and exit. These techniques allowed BHD to minimize the amount of groundwater treatment required and resulted in a major cost savings both in dewatering and water treatment, as well as a time savings to the project schedule created by allowing the shaft excavation and construction to continue unimpeded by water seepage.
GZA designed 6 water-tight shafts, using an interlocking circular secant pile system. This deep shaft construction technique minimized the need for contaminated groundwater treatment and eliminated the need for internal bracing due to thecircular arching properties of the shaft itself by effectively transmitting the hoop stresses incurred by the support of adjacent soils around the perimeter of the shaft.
One of the other major obstacles that BHD needed to overcome was low headroom clearances and close proximity to existing critical utilities at 6 of the required shafts. To deal with these two issues simultaneously, GZA proposed using a drilled micro-pile and lagging system. This allowed Barletta to address the headroom restriction by using a lower profile drill rig, locate and install the drilled micro-piles, and avoid utility support and relocations. This resulted in major cost and schedule savings. In addition, the use of drilled micro-piles at these shaft locations minimized vibrations on the nearby residential and commercial structures. This method mitigated the risk to BHD of potential physical damage to the structures as well as minimized the impact to the local residents, allowing shaft construction to advance efficiently and eliminated potential delays due to work hour restrictions.
Benefit
: GZA’s shaft design provided a significant cost savings to BHD. The economy realized in the design and construction of these shafts was a direct product of the elimination of internal bracing, which allowed for a reduction in shaft diameter and construction material and excavation quantity, and resulted in a continuous excavation process since excavation work did not have to be stopped in order to install the bracing levels. This not only saved BHD the costs of potential additional excavation, soil treatment and disposal, but also saved them the costs associated with the materials and labor involved with bracing installation itself. For each potential level of bracing that was eliminated, an estimated one day was saved. As these were deep shafts, an estimated 3 levels of bracing would have been required for each, so a typical installation that would have taken 4 days would now only take one day. This increased efficiency on the construction of these 6 shafts and resulted in significant cost and schedule savings to BHD.