DFI Magazine January/February 2020 - 96

to hold the sand grains together (apparent
cohesion), and these soils are known for
"running" during excavation. The excavation was positioned directly adjacent to
existing shallow footings that are founded
on these loose sands, which presented a risk
of encountering loss of ground and settlement of the structure. Clearly, there was a
significant concern when evaluating how to
support and underpin the existing structure.
On a positive note, these types of sands
are ideal for jet grouting. The low silt
content and uniform grain size enable
erosion, mixing and jet penetration to
occur during the jetting process. Furthermore, sandy soils yield a high rate of
strength gain and high overall strength
since the in-situ granular material acts as
the aggregate when it is mixed with the
injected cement.

Jet Grout Columns
The purpose of installing the jet grout
columns beneath the existing footings was
three-fold to achieve the following:
1. Serve as an earth retention system for
the proposed excavation
2. Prevent ground loss and associated
settlement of the ground and existing
structure during the excavation process
3. Structurally support the existing building by permanently transferring the
load from the existing structure to the
soil directly below subgrade of the
proposed excavation

Single, double or triple fluid jetting can
be performed. Single fluid jetting injects a
single fluid, cement grout into the ground,
whereas double fluid jetting adds a shroud
of compressed air around the grout
injection point. Triple fluid jetting utilizes
double fluid injection of high-pressure
water and compressed air, followed by
injection of cement grout at a different
position and at lower pressure. For this
project, single fluid jet grouting was
performed because it provided the most
control over column diameter and spoil
return compared to methods utilizing
compressed air.
The concentration of cement in the
injected grout is controlled by the mix
design, which, for neat cement grout, is
essentially the water/cement (w/c) ratio.
The w/c ratio is the amount of water in the
mix divided by the amount of cement per
unit volume, as measured by weight. A 1:1
w/c ratio has equal portions (by weight) of
water and cement. Typical w/c ratios used
for jet grouting range between 0.8 and 1.8.
The amount of cement injected into the
ground is controlled by two other factors:
the flow rate and the lift rate. The flow rate is
usually measured in gallons or liters per
minute (gpm or lpm) and is a function of the
pump capacity. The lift rate is usually
measured in inch or mm per minute (in/min
or mm/min) and is controlled at the drill rig.

A zone of soil that is treated for a longer time
with a slower lift rate will result in more
cement being injected as well as a potentially greater radius of grout penetration into
the soil.
The rotation rate during jetting is another
variable that can be adjusted. To create
cylindrical soil-cement columns, the rotation rate is generally calibrated to the lift rate
to ensure the full circumference is treated is
achieved. A column can be achieved using a
single nozzle and full rotation before lifting;
alternatively, two nozzles located 180
degrees apart can be used where a half
rotation is made before lifting.
The w/c ratio, lift rate and flow rate will
control the amount of cement injected into
the ground; whereas, the lift rate, flow rate
and jetting pressure control the diameter of
the soil-cement column. The selection of
these parameters is based on the existing soil
conditions and the experience of the
specialty contractor. The fines content,
relative density of the in-situ soil and particle
size are all factors that have an impact on the
column diameter that can be achieved.

Design Basis
Since groundwater was below the bottom
of the excavation and would not need to be
retained, the earth retention portion of the
jet grout design could be designed and
analyzed using a simple Apparent Earth

Jet Grouting Concept
Jet grouting is a process that utilizes high
pressure, high velocity erosive jets of fluid
grout (with/without the use of water and/or
air) to mix the in-situ soil with a cementbased grout to produce a soil-cement
composite material. The resulting column of
jet grouted material is stronger and less
permeable than the in-situ soil. As grout is
injected, a mixture of excess soil, water and
cement (collectively, spoils) travels to the
surface through the annulus between the
drill rods and the borehole wall that was
created by the drill bit. The in-situ soilcement strength, hydraulic conductivity and
column diameter resulting from jet grouting
are functions of the rate and concentration of
cement injected as well as to the time spent
jetting a certain area.
96 * DEEP FOUNDATIONS * JAN/FEB 2020

Effects of different jet grouting methods: (a) single fluid, (b) double fluid and
(c) triple fluid (Croce et al, 2014)

DFI Magazine January/February 2020

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