July/August 2024 - 15

the present. As of 2018, the student population of the high
school was about 1,200.
Following the demolition of the C Wing, the foundations of
the adjacent B and D Wings were required to be underpinned
to accommodate the 25 ft (7.6 m) deep excavation for a large
expansion of this facility. A composite underpinning system
comprising micropiles and soil nailing was designed by
Earthwork Engineering and Hub Foundation to provide both
structural underpinning of the existing foundations and
lateral support for both the structures and the soils below the
foundations.
Originally scheduled to start in July 2018, construction of
the composite underpinning system was anticipated to have
ideal weather conditions especially for the shotcreting portion
of the work. However, after demolition of the C Wing, exploratory
test pits revealed the presence of contaminated soils and
a dry rubble stone foundation in the section of building 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 and tieback system
was designed to provide the overall support to the structure
and excavation in the area of the stone foundation. Delays
resulting from these unknown conditions postponed the start
date from July to the middle of December and carried the
construction through the entire winter season.
Somerville High School wings with upper and lower sublevel soil nail walls
Existing Site Conditions
The school is situated at the top of Spring Hill, a drumlin in
Somerville, located approximately 2.5 mi (4km) northwest of
Boston. Drumlin fields commonly are characterized by their
elongated hills oriented in a common direction. The Boston
drumlins are unusual in that their patterns and shapes vary
drastically from elongated to circular with adjacent drumlins
diverging in orientation up to 70 degrees. It is commonly
accepted that the Boston drumlins were created from glacial
ice since they are largely comprised of till. Their varying
patterns are most likely a result of the complex glacial history
and the associated glacial flows.
The existing site grade at Somerville High School is at
about 108 ft (33 m) with a 2H:1V or steeper slope on the
backside of the school, which drops down to about elevation
40 ft (12 m) along a Massachusetts Bay Transportation
Authority rail corridor on the northern side of the site. A small
number of borings and test pits were conducted around the
site. In general, the subsurface conditions are comprised of a
3-12 ft (1-4 m) thick layer of fill, underlain by an 80 ft (24 m)
thick layer of very dense glacial till, resting on a moderately
hard, weathered, fractured Argillite bedrock.
It is noteworthy to mention that the glacial till on the
Spring Hill drumlin is considered an abalation glacial till with
DEEP FOUNDATIONS * JULY/AUG 2024 * 15

July/August 2024

Table of Contents for the Digital Edition of July/August 2024

TOC
July/August 2024 - Intro
July/August 2024 - 1
July/August 2024 - 2
July/August 2024 - TOC
July/August 2024 - 4
July/August 2024 - 5
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