Pressure grouting has gained popularity as a soil reinforcement method. However, the behavior of the interface between rock and grout is not well known. This study investigates the interaction of pressure grouting and rock, through a series of laboratory tests performed on specially designed and fabricated equipment and using standard testing methods. The test measures the density, compressional strength, and frictional resistance of grout relative to the applied pressure and curing time. Simultaneously, the velocities of the elastic wave traveling through the grout are obtained to develop correlations between the physical properties of the grout and the test conditions. The results of the tests show that the density, compressional strength, and frictional resistance of the grout increase with applied pressure and curing time. The strengths of the influencing factors are seen to be correlated within the range of the test conditions. Using the results of these tests, the potential development of a new method that requires less cement was discussed.

1. 1
Keywords: compressional strength, elastic wave velocity, frictional resistance, grouting pressure
ABSTRACT: Pressure grouting has gained popularity as a soil reinforcement method. However, the
behavior of the interface between rock and grout is not well known. This study investigates the interaction
of pressure grouting and rock, through a series of laboratory tests performed on specially designed and
fabricated equipment and using standard testing methods. The test measures the density, compressional
strength, and frictional resistance of grout relative to the applied pressure and curing time.
Simultaneously, the velocities of the elastic wave traveling through the grout are obtained to develop
correlations between the physical properties of the grout and the test conditions. The results of the tests
show that the density, compressional strength, and frictional resistance of the grout increase with applied
pressure and curing time. The strengths of the influencing factors are seen to be correlated within the
range of the test conditions. Using the results of these tests, the potential development of a new method
that requires less cement was discussed.
1. INTRODUCTION
Pressure grouting, a method in which grout is
injected into soils using pressure, is widely used
in reinforcement applications, such as ground
anchors, soil nailing, and micro-piles. Compared
to normal grouting, the advantage of pressure
grouting is that the body has a higher interface
friction. Pressurized grout also has higher density
and a confined pressure due to its increase in
volume. Although the pressure and volume of the
injected grout can be measured, there is no direct
correlation between the increased density and the
performance of the treated area. A common
problem is how to verify the improvement. With
other techniques engineers would rely on
localized strength testing to deduce the overall
improvement, but the effect of grouting pressure
on the interface shear resistance is still not well
understood.
Among the influencing factors that affect the
performance upon improvement of the technique,
the most important is the strength variation of the
pressure grouted body with pressure. However, it is
not easy to prepare specimens that eliminate the
possible development of a negative entry, due to
leakage of free water and pressure applied to the
grout. Bhasin and Gothall and Stille showed the
importance of the water/cement ratio and the
grouting pressure on the performance of a grouted
body, even in the case of a rock mass. Also, the
direct measurement of a pressure grouted body in
the field presents additional difficulties. Ohtsu and
Watanabe and Shiotani et al. show that elastic waves
provide a non-destructive method to generate data
that can be used to evaluate the degree of
improvement with relatively accuracy and within
the economic constrains of most projects.
Laboratory experimental study on physical properties of pressure
grouting and elastic wave velocity
Hoang Tien Trung
Institute of Foundation and Underground Engineering, Hanoi, Vietnam. E-mail: trunght@fecon.com.vn
Geotechnics for Sustainable Development - Geotec Hanoi 2013, Phung (edt). Construction Publisher. ISBN 978-604-82-0013-8

2. 2
The behaviors of ground anchors and micropiles
in soils are generally well known. However, the
performance of these systems in rock remains
relatively unknown due to the limited case studies
available; consequently, curren design practives of
these systems are generally overconservative. A
careful study was conducted to examine the
characteristic behavior of pressure grouting.
Although pressure grouting is known to improve
bond quality and load-carrying capacity over
gravity grouting in rock based on field
observation, the machanism for this improved
capacity is not fully explored in geotechnical
literature. This paper presents an investigation
into the variation of important parameters, such as
the uniaxial compressional strength, frictional
resistance, and elastic wave velocity, according to
the applied pressure, the water/cement ratio, and
curing times. Based on the test results, regression
analyses were performed to predict the strength of
the pressure grouting body as means of non-
destructive testing.
2. LABORATORY EXPERIMENTS
Laboratory testing was used to examine the
uniaxial compressional strength, frictional
resistance, and elastic wave velocity. The test
specimens were prepared using specially designed
and fabricated test equipment (Fig. 1). An air
compressor (Dragon air compressor, HL-2.5, max
pressure of 1MPa) was used to apply pressure to
the grout container used for pouring grout into the
mold with pressure. A pressure regulator connected
to the compressor and the container controlled the
grout pressure. The mold used to build the
specimens had a diameter of 50 mm and height of
100 mm; it included two valves, one for grout-in at
the bottom and the other for air-out at the top. The
air-valve was closed immediately after the
pressurized grout reached the top of the mold. The
mold can be split in four to make samples for the
uniaxial compressional tests. Preliminary testing
was used to investigate a possible leakage of the
mold at the water tank; the mold was subsequently
found to be pressure-proof.
For the elastic wave velocity tests, the entire
apparatus simply consisted of a function generator
(Tabor Electronics, 2MHz FG 8020), an
oscilloscope (Tektronix, TPS 2024), and two
transducers (MKC ndt, MK-9545). The generator
provides a step wave to a transducer, and the
oscilloscope measures the traveling time of the
wave through the specimen (Fig. 2).
(a) Compressor, pressure regulator and container
(b) Assembled Mold
(c) Disassembled Mold
Figure 1. Test set-up of four parts
Figure 2. Set up for wave test

3. 3
The programs for the experiments are
summarized in Table 1. All measurements were
done in triplicate under the same conditions, and
their average values were used in the subsequent
analysis. Tests were initiated using carefully
preparing specimens under the desired conditions.
The samples were then submerged in a water
curing tank at 20°C. After curing, the specimens
were located within a universal testing machine
(UTM, YUL-5T) to determine the wave velocity,
the uniaxial compressional strength, and the
frictional resistance.
The standard test procedures ASTM C597-09
and ASTM C39 were closely followed to
determine the wave velocity and compressive
strength of each specimen, respectively. The
frictional resistance measurement of the grout
sample was obtained using UTM while the
machine pushed out the specimen in the mold.
Table 1. Test Programs
Pressure (kPa)
Curing date
(days)
Water/cement
ration (%)
0, 100, 300, 500 2, 4, 7 50, 60
Beside, the laboratory testing also involves
unconfined axial compression tests of six grout
specimens prepared in custom-made molds using
two grouting methods to investigate the effect of
grouting method on the mechanical properties of
grout. Cement paste with a water-content ratio of
0.5 was injected into a custom-made cylindrical
mold through gravity grouting or pressure grouting
and cured for seven days. For pressure-grouted
specimens, the cemen paste was first injected into
the mold under a pressure of 0.5 Mpa; when the air
inside the mold was completely bleed out, the inlet
and outlet valves were then closed and the pressure
inside the mold was monitored through a pressure
gauge. The mobilized skin friction resistance τult
corresponding to the ultimate pullout capacity can
be calculated as:
τult = Qu/πDbLb (1)
where, Db = effective grout column diameter
(approximately the borehole diameter); Lb = bond
length and Qs = ultimate pullout capacity in kN.
3. RESULTS AND DISCUSSION
Table 2 summarizes the initial average specimen
weight immediately before curing. There is an
increase of the weight with pressure, which can be
attributed to the densification of the micro-pore
space due to pressure. Fig. 3 shows the pore space
variation of the grout, using microscope images
with 230X magnification. It is seen that the pore
space of the grout is reduced owing to pressure.
Gravity grouting may result in insufficient skin
friction resistance due to the cavities and poor
filling that frequently occur during installation. The
pressure-grounted specimens had higher density
and compressive strength, which is contributed by
their denser microstructure with significantly fewer
and smaller voids from air bubble inclussions than
the gravity-grouted specimens.
Table 2. Specimen Weights (g)
Applied pressure (kPa)
0 100 300 500
Water/cement
50% 413.5 415.9 421.7 429.6
60% 385.7 390.4 392.1 395.0
(a) Without Pressure (b) With Pressure
Figure 3. Pore space with variation with applied pressure.
Table 3. Test Results
w/c ratio
(%)
Pressure
(kPa)
Curing
ratio
(days)
Velocity
(km/s)
Compressing
Strength
(kPa)
50
0
2 3.23 4219.3
4 3.32 5608.7
7 3.74 9448.0
100
2 3.29 4929.1
4 3.69 9030.6
7 3.84 10525.3
300
2 3.64 7541.9
4 3.81 10382.7
7 4.00 12600.0
500
2 3.69 8342.8
4 3.91 12372.9
7 4.16 17341.8
60
0
2 3.21 3216.6
4 3.28 5117.5
7 3.53 6234.6
100
2 3.25 4299.4
4 3.52 5946.3
7 3.67 8596.3
300
2 3.29 4578.9
4 3.56 7953.1
7 3.80 10198.9
500
2 3.52 6002.1
4 3.72 8862.6
7 3.84 11259.4

4. 4
Table 3 shows an increase in wave velocity and
compressive strength with the amount of cement,
the applied pressure, and the curing time of grout
at each test condition. Fig. 4 illustrates a significant
increase of frictional resistance with pressure
grout. It is seen that the effect of pressure of grout
on the physical properties is significant. With
adding pressure, there is a remarkable increase in
strength whereas the densification of the grout is
rather small. Therefore, it could say that it has a
potential use of the method with efficiency, and it
could minimize the amount of cement.
From Table 3, the relationship between the
wave velocity and compressive strength can be
inferred using parameters such as the applied
pressure, curing times, and water/cement ratio.
Strain (%)
0 5 10 15
Stress(kPa)
0
1000
2000
3000
4000
grout
pressure grout
(a) w/c = 50 %, 3 days curing
Figure 4. Frictional resistance (cont.).
Strain (%)
0 5 10 15
Stress(kPa)
0
1000
2000
3000
4000
grout
pressure grout
(b) w/c = 60 %, 3 days curing
Figure 4. Frictional resistance.
A regression analysis was used to determine the
dependency of the elastic wave velocity on the
applied pressure and curing dates. Fig. 5 provides a
graphical illustration of the highly correlated
results of the analysis, showing a strong
relationship between the parameters. Fig. 6 also
shows a direct relationship between the elastic
wave velocity and the compressive strength of
grout. Within the range of test conditions, these
analysis results show that the compressive strength
can be predicted by the wave velocity as obtained
by direct measurements or from applied pressure
and curing dates. It can be postulated that the
applied pressure, curing dates, and water/cement
ratio could be used to determine the compressive
strength of pressure grout.
3000
3200
3400
3600
3800
4000
4200
4400
0
100
200
300
400
500
0123456
velocity(m/s)
pressure(kPa)
curing days(days)
(a) w/c = 50 % (R2 = 95.5%)
3000
3200
3400
3600
3800
4000
4200
4400
0
100
200
300
400
500
0123456
velocity(m/s)
pressure(kPa)
curing days(days)
(b) w/c = 60 % (R2 = 93.9%)
Figure 5. Regression analysis.
velocity(km/s)
3.0 3.5 4.0 4.5
Strength(kPa)
0
5000
10000
15000
20000
0 kPa
100 kPa
300 kPa
500 kPa
Figure 6. Correlation between wave velocity and
compressional strength. (R2 = 96.1%)

5. 5
Fig. 7 shows the monitored pressure versus time
for the three pressure-grouted concerns by
unconfined axial compression tests. The decrease
in pressure is likely due to a combination of
leakage and the shrinkage of cement paste during
hydration. Fig. 7 shows that pressure grouting
results in a locked-in residual pressure inside the
mold. For gravity-grouted specimens, similar
precedures were adoped except that no grouting
pressure was applied.
Figure 7. Residual pressure inside mold
Figure 8. Results of unconfined axial compression tests.
Fig. 8 shows the results of the unconfined axial
compression tests performed on the grout
specimens, which demonstrate excellent
repeatability. The peak compressive strength and
density of the specimens are reported in table 4.
Table 4. Result of unconfined axial compression test
Tests
Test
#
Uniaxial
Compression
Strength (Mpa)
Density
(g/cm3
)
Laboratory
Tests
1 9.53 2.14
2 9.68 2.13
3 9.69 2.12
4 17.80 2.36
5 17.42 2.36
6 17.83 2.37
The pressure-grouted specimens have an
average peak unconfined compressive strength of
17.68 Mpa and average density of 2.36 g/cm3
.
The corresponding values for the gravity-grouted
specimens are approximately 9.63 Mpa and
2.13 g/cm3
, respectively.
4. CONCLUSIONS
This study uses laboratory experiments to examine
the variation of the compressive strength of
pressure grout. The specimens were prepared using
a specially designed mold, and standard test
procedures were followed to obtain the
characteristics of the pressure grout, including the
density, the velocities of the elastic wave traveling
through the grout, the compressional strength, and
the frictional resistance of the grout relative to the
applied pressure and curing time. The results of the
study show that the density, compressional
strength, and frictional resistance of grout increase
significantly with applied pressure and curing time.
It can be concluded that simply adding pressure to
grout might increase the efficiency of the method
and minimize the amount of cement material. The
aforementioned factors were correlated by a
regression analysis within the range of the test
conditions. The analysis resulted in higher values
of the correlation coefficients, demonstrating the
possibility of predicting the compressive strength
of pressure grout to relatively good effect.
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Foundation under static axial Tensile Load. D
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Conshohocken, PA.
Bhasin, R., Johansen, P.M., Barton, N. and
Makurat, A., (2002).. Rock Joint Sealing
Experiments using an Ultra Fine Grout,
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Borden, R. H., Holtz, R. O. and Juran, I., Editors
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Gothall, R., and Stille, H., (2008). Rock Mass
Response during High Pressure Grouting. The
42nd U.S. Rock Mechanics Symposium, San
Fransisco, CA, USA.

6. 6
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