DFI Magazine January/February 2020 - 97

Pressure (AEP) diagram. Schnabel
Foundation Company (Schnabel) used an
AEP diagram for this project that was
developed based on its 60 years of
company experience. Experience gained
from using the Schnabel AEP, which was
based on thousands of constructed ERS
projects, has proven the Terzaghi & Peck
(T&P) diagrams to be slightly conservative
in certain situations. The main differences
between the Schnabel T&P AEP diagrams
are the magnitude of the overall load and
the concentration of load around the
tieback or strut. The concept that the load
is transferred to the stiffer parts of the
structure has also been proven through
documented monitoring of constructed
ERS, where arching of the soils behind the
wall allows for this to happen in relatively
flexible systems. A hinge was assumed at
subgrade, as is typical with other AEP
diagrams, which results in zero moment
below the bottom of the excavation.
The additional load (i.e., surcharge)
from the existing structure was also
considered in the design of the jet grout
wall. The surcharge was transferred directly
to the jet grout columns, which were
installed so that the columns extended
directly beneath the existing footings and
were in intimate contact with the bottom of
the existing footings. Using this process,
the load was transferred directly from the
existing structure and into the jet grout
columns, allowing for the loads from the
footings to be transferred to the soils below
the subgrade of the adjacent excavation.
The application of the load directly
onto the jet grout columns required the
design analysis to consider the effects of
combined bending and axial loading. The
internal stresses of the jet grout columns
were analyzed per ACI-318 for unreinforced concrete given the loading conditions. Based on the magnitude of the
earth pressure that exerted the bending
stress and the axial loading from the
existing building, it was determined that
the jet grout columns could be installed
without the need for reinforcement within
the jet grout columns. In addition, based
on the results of the analyses, the minimum
required unconfined compressive strength
of the jet grout columns was established as
1,000 psi (6,895 kPa).

Apparent Earth Pressure (AEP) diagrams (Schnabel et al, 2002): (a) Schnabel,
(b) Terzaghi & Peck sand and (c) Terzaghi & Peck clay

Jet grouting beneath an existing footing
is a method of underpinning. As such, it
requires careful sequencing because there
is a temporary condition during which the
jet grouting is still fluid and there is littleto-no support directly beneath part of an
existing footing. Before the jet grout
material sets and hardens, there is a
temporary reduction in the factor of safety
for bearing capacity. This reduction (and
increased risk exposure) is managed by
working below only a small portion of the
footing at a time. Jet grout underpinning
was performed on this project beneath
both continuous strip footings and isolated
spread footings.

Quality Control and Construction
On almost every project, a test program is
performed prior to the start of the
production work to verify the many
different parameters to be used and to
confirm the achievable diameter and
strength of a soil-cement column. To
confirm the diameter of the column, it is
best to excavate around a test column and
to visually examine and measure the
constructed geometry. However, this is
usually difficult, if not impossible, when
working in limited space conditions and
deep columns. As an alternative, one
method of verifying column diameter

would be to drill and retrieve samples from
the centroid of a three-column layout. In
this scenario, three individual test
columns are installed in a triangular layout
in which all three columns overlap slightly
in the middle.
A borehole can be drilled in the
centroid of this three-column layout and
continuous split spoon samples can be
retrieved. Standard penetration testing
(SPT) blow counts obtained from driving
the split spoon can be compared with soil
borings taken prior to jetting to confirm
that the ground has been strengthened. In
addition, phenylethane indicator can be
sprayed onto the retrieved samples to
indicate the presence of cement by turning
a bright color if present. These are but two
ways of confirming that the column
diameter has been achieved.
Coring is sometimes used to retrieve
samples for visual observation and lab
testing. However, coring of relatively low
strength material has proven to be unsuccessful in obtaining intact samples,
especially in granular material containing
gravel and/or cobbles. The rotary-wash
action of the coring tends to erode low
strength soil-cement, and larger pieces of
sand or gravel, if present, can get caught
on the core barrel teeth causing the sample
to crumble.
DEEP FOUNDATIONS * JAN/FEB 2020 * 97

DFI Magazine January/February 2020

Table of Contents for the Digital Edition of DFI Magazine January/February 2020