Abstract:
Wings B and D of the Somerville High School in Somerville, MA were required to be underpinned to allow for a large expansion of this facility. A combination of micropiles and soil nailing was designed by Earthwork and Hub to provide both structural underpinning of the existing foundations and lateral support for both the structures and the soils below the foundations. The depth of the excavation was up to 25 feet.
The original start date was July 2018, which would be close to ideal weather conditions for the shotcreting portion of the work. However, after the adjacent Wing C was demolished, exploratory test pits discovered the presence of contaminated soils and the presence of a dry rubble stone foundation in the section that had to be underpinned. The original micropile/soil nailing design could not be used to support the rubble stone foundation. A drilled soldier pile – shotcrete – tieback system was designed to provide support to the structure in this section. Delays resulting from this changed condition extended the start date to December 20, 2018. Hence, construction of the soil nail and soldier pile walls would be done in severe winter conditions.
Several photos are attached to show the flow of the work which now included drilled soldier piles very close to the existing structures, micropiles drilled through the existing concrete foundations (where it existed), tiebacks, soil nailing and shotcreting. The latter items were highly affected by the cold winter temperatures.
The site is located on what is known as the Spring Hill drumlin. A very dense ablation glacial till underlies the entire site. The till consisted of sand and silt with gravel and cobbles. Drilling was extremely difficult. For this reason, hollow core bars were selected for the micropiles, soil nails and tiebacks, which proved to be a very effective solution.
The original completion date was scheduled to be March 25, 2019. Hub was directed by the Owner to accelerate this work with both additional crews and longer work hours. The latter proved to be very difficult due to the cold temperatures and short days. Nonetheless, Hub was able to complete this entire scope on April 4, 2019.
Extensive instrumentation was included to monitor both vertical and lateral movements of the existing structures and the lateral support systems. The maximum vertical movement of the adjacent existing structures was only 1/8” which was well below the project requirement of 1.0” . The maximum lateral movement of the excavation support systems was 0.4” which was less than the 0.6” project requirement.
This presentation will discuss all the design aspects of both support systems, the means and methods utilized to install the systems in a harsh environment and the results of the monitoring programs.