The proposed 51,000 square foot expansion of Colgate Universitys Case Library involved an excavation which varied in depth from 24 to 27 feet into an adjacent slope to create space for the new addition to new footing subgrade. The subsurface profile at the site consisted of 25 feet of fine to medium coarse sand and gravel with some cobbles and large boulders, overlying fine to coarse sand. Groundwater was encountered at 10 to 12 feet above the proposed new subgrade. Moretrench was retained by general contractor J.J. Lane Construction Inc of Liverpool, NY to design and install a temporary, tied back, soldier pile and lagging system to support the cut, and to design and install a wellpoint system to maintain groundwater level below excavation grade for lagging installation and subgrade preparation.
The project presented several challenges:
- Numerous critical utilities ran parallel to and through the retaining wall alignment.
- A streambed, consisting of seams of gravel and coarse sand and swollen by a very wet summer, was encountered in the northeast corner of the excavation.
- Considerable groundwater runoff from the slope resulted in saturated ground conditions.
- Limited site ingress/egress for haulage of excavated material.
- A very limited construction timeframe, requiring the work to be completed during the summer break.
Given the single site exit, installation of the soldier pile and lagging system was closely sequenced with excavation operations. Mud rotary drilling was selected for the soldier pile installation due to the proximity of the existing buildings large, plate glass windows. During the work, immediate engineered adjustment to the original wall design was necessary to accommodate large boulders that were encountered within the soldier pile wall alignment without compromising the integrity of the retention system. As wall construction and excavation progressed, a single row of tieback anchors, with a design load of 114 kips, was installed to provide additional lateral support. All anchors were proof-tested to 133% of design capacity.
When excavation and installation of the earth retention reached groundwater level, Moretrench installed the wellpoint system in front of the wall alignment to draw the groundwater down below proposed footing subgrade. The wellpoints were connected to an 8-inch diameter header pipe which was in turn supported by the soldier pile andlagging wall. Piezometers were installed to monitor water levels during dewatering operations. Given the wet summer and unanticipated stream bed, four 20-inch sumps were installed in addition to the wellpoint system to ensure that the northeast corner of the excavation remained dry. With the dewatering system fully operational, excavation to subgrade then resumed.