May/June 2024 - 102

subcontractor was tasked wi th
providing a design-build solution for
the required shoring and SOE walls for
the rema ining demo l i t i on and
excavation to reach the recreation
center subgrade.
The alignment of the existing
foundation walls for the former LRDC
and O'Hara structures did not encroach
on the limits of the proposed recreation
center's building walls. These walls
were visually inspected to assess their
condition during and after the demolition
phase. Based on visual survey,
they appeared to be performing
satisfactorily after significant demolition
of the horizontal support systems
(floor slabs, buttresses and shear walls).
Soil and Rock Conditions
Civil & Environmental Consultants performed
a geotechnical investigation
including 14 borings, which showed the
soil and rock at the site consist of surface
fill materials overlying residual soils and
weathered to competent rock. The bedrock
materials are characterized as the
Pittsburgh formation of the Monongahela
Group, which consists of interbedded
layers of claystone, limestone,
shale, siltstone and sandstone with
occasional coal seams. At the O'Hara and
LRDC walls, a steep rock cut face extends
above the existing walls. Above elevation
980 ft (300 m), the primary rock
type is sandstone/siltstone and below
elevation 980 ft (300 m), the rock type is
mostly claystone. The sampled rock
properties are summarized in Table 1.
Table 1. Summary of rock sample tests
No. of
Average
Rock Type
Siltstone
Sandstone
Claystone
Limestone
Shale
Recovery (%)
95
97
95
94
98
102 * DEEP FOUNDATIONS * MAY/JUNE 2024
Average
RQD (%)
58
43
10
49
5
REC/RQD
Samples
Collected
24
15
34
4
2
Average
UCS (psi)
9115
14763
1120
-
-
No. of UCS
Samples
Tested
10
6
4
Exposed existing walls, looking west
Existing walls, looking east
Tight site access on the east side and
a steep rock slope on the north side
limited the areas where borings could
be ob t a i ned . No bo r i ngs we r e
performed directly behind the locations
of the north (O'Hara wall) and east
(LRDC wall) sides of the recreation
center. The type and extent of the soil
and rock materials behind the walls
were inferred from the as-built
drawings for the LRDC and O'Hara
structures. The type of backfill used
between the foundation walls and the
rock cut was assumed conservatively to
be a loose, granular material.
As t he demo l i t i on and s i t e
excavation progressed toward the
northwest corner of the WPIC wall and
the area became accessible, the project
t eam per formed an add i t i ona l
exploratory boring to confirm soil and
rock conditions. Test pits were also dug
within the excavation along the WPIC
wall to delineate the extent of its
foundation. The additional exploration
provided a better understanding of the
ground and structure conditions and
helped refine the design for the most
efficient earth retention systems. The
soil and rock conditions along the WPIC
wall generally consisted of residual
soils and claystone bedrock. The rock
elevation dips from the north to the
south near O'Hara Street. The Nspt
values of the residual soils are
consistently 50+ blows per foot (0.3 m).
May/June 2024

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