Construction is underway on the $57 million, 103,000 square-foot Connecticut River Academy that will serve as a science and environmental magnet high school in East Hartford, Connecticut. GeoDesign served as the project's geotechnical engineer for design and construction of the foundations for the four-story academic building and precast concrete parking garage on Goodwin College's campus.
Wick drains were used to quickly stabilize the site, enabling students to benefit from this new school facility sooner than otherwise. The method saved time and money.
The terrain situated along the east bank of the Connecticut River is comprised of a 20 foot sand cap over a 150 foot stratum of soft compressible, varved (layered) silt and clay, over bedrock. This varved clay presented two special challenges. New building loads could induce large time-dependent settlement of the soils below the foundations and adversely affect the serviceability of the facility. In addition, with recent seismic standards in place in Connecticut, GeoDesign also had to address the typically low seismic site class associated with such weak soil deposits.
To address these challenges, GeoDesign performed field and laboratory testing, engineering analyses, and developed recommendations which resulted in more than $1 million in construction cost savings for the project over the more conventional solutions (deep piles).
Subsurface explorations into the clay included cone penetration testing. These results were correlated with laboratory consolidation and triaxial shear strength tests, which allowed GeoDesign to calculate the average shear strength of the upper 100 feet of the soil profile. This approach supported the selection of seismic Site Class D, versus Site Class E that would have resulted based on more commonly used split-spoon standard penetration testing. A classification of seismic Site Class E would have resulted in a more costly structure and foundation due to the need to resist larger lateral shear loads.
Due to the heavy building loads, preliminary calculations indicated that excessive settlement would result for structures supported on spread footings or a mat foundation. In particular, we predicted that excessive differential settlement would occur between the school and the garage which have differing loading conditions.
Commonly used deep foundation solutions to support the structures in the vicinity (such as end-bearing piles driven to bedrock), while technically feasible, would have been very costly due to the average depth to bedrock of about 170 feet.
GeoDesign recommended settlement control measures to allow both structures to be supported on normal shallow, spread-footing foundations.
The settlement control measures included installation of wick drains (also termed prefabricated vertical drains) and geotechnical instrumentation, placement of an earth surcharge, and a waiting period to allow a portion of the settlement to occur prior to foundation construction. Wick drains provide a quicker drainage path for water being squeezed out of the clay under the weight of the new fill. Without the wick drains, it would have taken several years to attain the required settlement. The wick drains shortened this time period to four weeks.
Wick drains have also been used by GeoDesign engineers in the Connecticut Valley to expedite consolidation to support filled embankments to highway approaches in the I-91 corridor and for the New-Britain Busway at the Flatbush Avenue crossing in Hartford.
GeoDesign value-engineered the design by specifying 110-foot long drains, which penetrated only the upper two-thirds of the 170-foot thick soil deposits at the site. Using partially penetrating foot-drains cut in third the construction costs compared to the use of full-penetrating wick drains, which also require a much taller and larger rig and stronger drain materials. We believe the Goodwin College project is the first use of partially penetrating wick drains in this part of the country.
GeoDesign designed and installed the geotechnical instrumentation, including settlement platforms, Borros-anchors, and piezometers. We monitored the instruments during the waiting period and compared actual settlement performance to predictions. After the desired settlement (up to about one and a half inches) had occurred, and we were satisfied that field observations reasonably matched our theoretical model, the contractor removed the surcharge and began construction.
The construction of the building foundations is now complete, and the buildings are expected to open in December 2013.
GeoDesign thrives on providing our clients with cost-effective geotechnical engineering solutions. Contact us if you are facing challenging soils, site or foundation issues.