July/August 2024 - 16

no clay content. Therefore, no cohesion was considered in the
soil parameters for the support of excavation design. When
disturbed and exposed to water, the strength of the soil was
severely weakened. Proper compaction of this disturbed soil
was unachievable, resulting in equipment becoming stuck.
Workers also had to watch where they stepped. One wrong
misstep could result in sinking halfway up their leg in mud.
Groundwater observation wells were installed in several
locations. The wells indicated water from 5.5-32 ft (1.7-9.7 m)
below ground surface. During excavation, groundwater was
not encountered. However, thin silty sand lenses produced
slight amounts of water, which was repeatedly debated to be
the groundwater table.
Composite Underpinning Design Methodology
The excavation support required along the existing 1895
building, 1929 building and 1914 classroom addition for
construction of the new addition consisted of micropile
underpinning with a permanent soil nail wall for lateral
support. In addition, a portion of the existing foundation at the
1895 building consists of a stone foundation instead of a
concrete foundation. In this area, micropiles could not be
installed and the lateral support consisted of permanent
drilled in soldier piles with shotcrete lagging and tieback
anchors for lateral support.
Support for the 1895 building
The micropile underpinning was designed to support the
wall footing load provided in the contract drawings. The
specified wall loading included a dead load of 13 kip/lft (190
kN/m) and a live load of 5 kip/lft (73 kN/m). The micropiles
were to be installed at 4.5 ft (1.4 m) spacing and therefore the
piles were designed for a compressive loading of 82 kip (365
kN) per pile. The micropiles were designed in accordance with
the Massachusetts Building Code 9th Edition, which adopts
the 2015 International Building Code with amendments. The
micropile bond length was determined based on an estimated
allowable bond stress for the grout to glacial till soils. The piles
were designed to derive their entire capacity in adhesion in
the glacial till below final subgrade, ignoring end bearing. The
micropiles were designed to use the hollow bar system. For
this permanent condition, the diameter of the hollow bar
tendon was reduced for potential corrosion.
The permanent soil nail walls were designed to support
both the soil and building surcharge loadings. Like the micropiles,
the soil nails were designed to use the hollow bar system.
The diameter of the soil nail tendons was reduced to account
for potential corrosion. The height of support varied from
18-23 ft (5.5-7 m). The design of the soil nail wall was done
using the Snail design software developed by the California
Department of Transportation. The results of this computer
analysis were used to determine the required nail size and
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July/August 2024

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

TOC
July/August 2024 - Intro
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