July/August 2021 - 17

Design Constraints
With the project site in a dense urban area involving
numerous stakeholders, the excavation had to address the
following constraints:
* Installing anchors beneath adjacent buildings, several of
which contained critical state infrastructure, such as data
centers. At one adjacent building, anchors had to be
installed between existing drilled shafts, and at two
others, directly beneath shallow spread footings.
* Accommodating for the presence of existing utilities,
which often required relocation or temporary support to
span the excavation.
* Accommodating foundations for a temporary utility
support structure immediately adjacent to the excavation.
* Allowing for a potential utility tunnel and deep foundation
elements of the new structures within the retention system
anchor envelope. Design of these elements was not advanced
past the concept level until after excavation began.
* Incorporating a " blindside " waterproofing system, which
required tight excavation tolerances to accommodate a
thick exterior basement wall.
* Installing anchors in severely limited space in the upper
lifts of the retention system in Phase 1A due to the
permitting schedule; existing structures and utilities also
constrained the top rows of anchors to varying degrees in
Phase 1B and 1C.
* Allowing for future Phase 2 construction, which requires
excavation up to the existing Phase 1 below-grade structure
where 16th Street and Congress Avenue intersect.
Geologic Setting
The site is characterized by clayey surficial soils underlain by
limestone of the Austin/Atco Group, which, in turn, is
underlain by the Eagle Ford Shale. The excavation section is
entirely within the Austin Group; however, the lowest
anchors in some locations were bonded into the Eagle Ford
formation. The surficial soils are typically less than 6 ft (2 m)
thick, but are locally up to 12 ft (4 m), are stiff to very stiff fill,
alluvium, and completely weathered limestone. Austin/Atco
limestone is a low-strength rock with an average compressive
strength of 2,900 psi (20 MPa) and generally good-to-excellent
rock quality. Although discontinuities in the rock mass are
generally widely spaced, numerous joints and slicken-sided
fractures were noted during geotechnical investigation. More
importantly, a mapped displacement fault that crosses the
site approximately parallel to Congress Avenue was also
inferred by the subsurface investigation, but its location and
dip direction could not be precisely defined, and it was
therefore estimated from predominant regional geology.
Groundwater is generally perched atop the harder, underlying
Phase 1 excavation limits (background credit, Page/Texas Facilities Commission)
slightly weathered/fresh limestone strata and accumulates
within fractures or depressions at the contact between soil
and rock units. The rate of flow into the excavation was
generally correlated with precipitation events and proved to
be manageable with grading and sump pumping.
Retention System Components
The large dimensions of the excavation - 125 ft (38 m) to
400 ft (120 m) wide - meant internal bracing to restrain
excavation surfaces was considered impractical. Subsurface
license agreements were obtainable when required from
owners of adjacent buildings, favoring an anchored system
approach. Due to the phased installation and construction,
anchors were in service for an average of 18 months, and were
installed from July 2018 to August 2020. Final abandonment
of the remaining temporary anchors will occur in August
2021 as the entire below-grade structure is completed.
In general, the anchored system consists of two levels of
short nails in the surficial soils for local face stability, and five
levels of longer rock anchors bonded in the underlying Austin
limestone for global stability. Up to seven rows of rock anchors
were required in more critical areas. The horizontal and
vertical spacing of anchor rows was varied to match anchor
DEEP FOUNDATIONS * JULY/AUG 2021 * 17
July/August 2021

Table of Contents for the Digital Edition of July/August 2021
