DFI Magazine March/April 2020 - 19

sieve. The apparent cohesion of the soil was expected to range from
100 psf to 200 psf (4.8 kPa to 9.6 kPa), based on the FHWA Soil Nail
Walls Reference Manual (2015). Although this temporary property
was not used to consider the overall design of the active soil nail
wall, this characteristic of the soil would be expected to provide
sufficient cohesion of the ground during excavation.
Other advantages of using an active soil nail wall to shore up the
Carnegie Building included:
Reduced foundation disturbance: An active soil nail wall
significantly reduced the risk of excavating boulders. All of the
excavating could be done by an excavator adjacent to the building
foundation footprint. The soils directly underneath the building
would be minimally disturbed since the soil nail wall excavation
limits would remain outside of the structure's foundation. This
would significantly limit disturbance induced to the foundation. In
addition, the soil nail wall did not raise the risk of disintegrating
parts of the building foundation that were made of cemented stones.
Improved soil stability: Among the benefits of an active soil nail
wall compared to the passive soil nail systems common in the
industry is the use of prestressed anchor components. These
anchors resist soil movement and effectively limit settlement of the
soil and of the structure being contained.
Safety: By decreasing the risk of boulders falling during pit
excavation or of the foundation disintegrating, on-site safety was
improved.
Efficiency: All of the excavation work for soil nailing could be
performed by machines. This approach shortened the shoring
construction schedule.

Preparation of Carnegie shotcrete panels

failure plane of the wedge. Nails on the top row and the second row,
where reducing movement was most critical, were tested and
locked off. The implementation of these measures ensured that the
soil underneath the building foundation was retained with limited
movement. This in turn helped the existing foundation to perform
as intended prior to excavation.
Percent Passing No. 200
Sieve by Weight (%)

Maximum Apparent
Cohesion, psf (kPa)

Active Soil Nail Wall Design

5-15

50 (2.4)

A conventional soil nail wall is a passive earth-retaining structure,
where movement is required to mobilize resistance from the
reinforcing elements. However, the project performance
specifications limited the allowable vertical movement of the
Carnegie Building to 0.25 in (6.35 mm) to prevent damage.
Conventional soil nail walls would exceed this threshold in the
process of generating resistance. In order to take advantage of the
benefits provided by soil nailing, several key modifications in the
design and implementation approach were needed to make the
project successful.
First, the soil nail design was analyzed as an active earthretention system with surcharge loading from the Carnegie
Building directly above the wall. The surcharge was estimated using
historical plans and data, and the soil nail wall was modeled as a
stiff wall element using Boussinesq's method to calculate increases
in lateral earth pressure. The design section and stiffness of the
shotcrete were considered to reduce flexural movement of the nail
wall during excavation. Active loading on the soil nails was
considered similarly to anchors in a retention system. Prestressing
loads for the soil nails were determined by the equilibrium force
method. Flexural deflection of the reinforcement shotcrete section
was designed to be minimal under full excavation conditions.
Second, the active soil wall design incorporated knowledge that
the nails in the field each would have a free stressing length and a
bond length just like a tieback. Nails were also bonded beyond the

15-25

100 (4.8)

25-50

200 (9.6)

Extrapolated building soil values of apparent cohesion

Modified Nail Wall Construction
In addition to deviations made from traditional soil nail design, the
contractor also made various constructability changes to the
sequencing typically seen for soil nail wall installation. In soil nail
construction, a vertical lift 4 ft to 5 ft (1.4 m to 1.5 m) deep is
excavated along the wall alignment with the face of the cut fully
stabilized before excavation of the next level. The horizontal length
of the excavation is directly related to the area of soil face that can be
completed in one working day. Closely spaced reinforcing bars
(nails) are installed across the cut face and a structural shotcrete
fascia bridges the nails and supports the soil/rock between the nails.
For the Gateway Academic Center project, the nail wall
constructability considerations implemented worked to minimize
the risk of the Carnegie Building settling. First, a berm was placed in
front of the wall on a 1:1 slope with a 3 ft (1 m) wide bench at the top.
Next, the soil nails (which were 35 ft (10.67 m) in the top row and 22
ft (6.7 m) in the bottom two rows) were installed through the berm,
grouted in place and the grout allowed to cure. Finally, the berm was
excavated to the finished slope and the shotcrete was installed.
DEEP FOUNDATIONS * MAR/APR 2020 * 19

DFI Magazine March/April 2020

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