Pages 177–186 http://dx.doi.org/10.1680/grim.12.00016
materials
Ground Improvement
sands
Markou and Droudakis
Shear strength of microfine cement grouted sands j1 Ioannis N. Markou  PhD
Assistant Professor, Department of Civil Engineering, Democritus University of Thrace, Xanthi, Greece
j2 Alexandros I. Droudakis PhD Department of Civil Engineering, Democritus University of Thrace, Xanthi, Greece
j1 j2
Unconsolidated–undrained (single and multi-stage) triaxial compression tests were conducted to evaluate the shear
strength of microfine cement grouted sands. Microfine cements of three different types were obtained by pulverising
ordinary cements produced in Greece. Multi-stage triaxial compression tests can be used dependably for determina-
tion of the shear strength parameters of cement grouted sands. It has been observed that the Mohr–Coulomb failure
criterion represents adequately the behaviour of the grouted sands. Grouting with microfine cement suspensions
improves the strength of sands significantly, and the improvement is primarily controlled by the water-to-cement
(W/C) ratio of the suspensions. The positive effect of microfine cement grouting on the shear strength of sands is
mainly the addition of cohesion, which is substantial even at a distance of 1.2 m from the injection point. Grouting
with suspension, using W/C 1 provides the sand with cohesion of about 2.6 MPa. The shear strength parameters
vary with axial strain, and cohesion attains a maximum value well before failure.
Notation C u   uniformity coefficient
c   cohesion (total stresses)
d i   i% of grains finer than this grain size
d max   nominal maximum grain size of cements
emax   maximum void ratio
emin   minimum void ratio
 K f -line failure envelope resulting from  pf , qf  values
 p, q   stress path coordinates (total stresses)
 pf ,  qf    values of  p  and  q  at failure (total stresses)
 R2 correlation coefficient
 3   confining pressure
j   angle of internal friction (total stresses)
1. Introduction Microfine cement grouts have been used in the last few decades
to extend the application range of ordinary cement grouts in
 permeation grouting for ground improvement and to reduce the
use of harmful chemical solutions. A variety of projects through-
out the world, in which different microfine cement grouts were
utilised, was reported by  Henn and Soule (2010). The design of 
structural grouting projects is mostly based on the evaluation of 
mechanical properties of the grouted mass on the basis of results
obtained from unconfined compression tests, although it is
generally accepted that the triaxial compression test best simu-
lates field conditions. The available, relatively limited in number,
laboratory investigations of the mechanical behaviour of sands
grouted with microfine cement suspensions include results
obtained from consolidated–drained (Clarke  et al., 1993;  Dano
et al., 2004;   Krizek    et al., 1992), consolidated–undrained 
(Krizek  et al., 1986,  1992;  Naeini and Ziaie-Moayed, 2003) and 
unconsolidated–undrained (Maalej  et al., 2007) triaxial compres-
sion tests. Numerical simulations of triaxial compression test
results were also performed for the modelling of the mechanical
 behaviour of grouted sand (Hicher  et al., 2008).
The experimental investigation reported herein is part of an
extensive research effort aimed toward the development of a
relatively fine-grained material, suitable for permeation grouting,
obtained by pulverisation of ordinary cements produced in
Greece. Suspensions of three different cement types, each at three
different gradations, were tested. The aim of this study was to
quantify the improvement of the shear strength parameters of 
sands by grouting with these coarse- and fine-grained cements, to
177
document the effect of cement type and fineness, grout water-to-
cement (W/C) ratio, sand gradation and distance from the injec-
tion point and to evaluate the development of shear strength
 parameters with axial strain. Multi-stage unconsolidated– 
undrained (M-UU) and unconsolidated–undrained (UU) triaxial
compression tests were conducted on grouted sand specimens
 produced using a specially constructed grouting apparatus.
2. Materials For the purpose of this investigation, three cement types (Port-
land, Portland-composite and pozzolanic cement, code-named 
CEM I, CEM II/B-M and CEM IV/B, respectively, according to
European Standard EN 197-1 (BSI, 2000)) were utilised. The
amount of clinker used for production of the CEM I cement
(90%) is significantly higher in comparison with 63% and 58%
for CEM II/B-M and CEM IV/B cements, respectively, and the
 pozzolan content increases from 0% (CEM I) to 23.5% (CEM II/
B-M) and 38% (CEM IV/B). Consequently, the selected cement
types reflect the tendency of the cement industry to reduce the
clinker percentage, by replacing a part of it with pozzolan, in
order to produce more economical cements. Each ordinary
cement (nominal  d max ¼ 100  m) was pulverised, by performing
dry grinding in a special laboratory mill, to produce additional
cements with nominal maximum grain sizes (d max) of 20 and  
10   m. Characteristic grain sizes and Blaine specific surface
values for all cements used in this research effort were reported 
 by Pantazopoulos et al.  (2012). In terms of gradation, all cements
with nominal   d max ¼ 10  m can be considered as ‘microfine’
since they satisfy the requirements of Standard EN 12715
(d 95 , 20  m and specific surface over 800 m2/kg;  BSI (2001)),
as well as definitions adopted by the International Society for 
Rock Mechanics (ISRM), the American Concrete Institute (ACI)
Committee 552 and the Portland Cement Association (PCA)
(Henn and Soule, 2010). Furthermore, cements with nominal
d max ¼ 20   m have adequately small characteristic grain sizes to
 be considered, marginally, as ‘microfine’.
All suspensions tested during this investigation were prepared 
using potable water since it is considered appropriate for prepar-
ing cement-based suspension grouts (Eriksson   et al., 2003;
Littlejohn, 1982). The W/C ratio of the suspensions was set equal
to 1, 2 or 3 by weight because suspensions with a W/C. 3
would have prohibitively large bleeding, long setting times and 
low strengths, whereas suspensions with a W/C , 1 would have
 prohibitively high viscosity (Bruce  et al., 1997;  Littlejohn, 1982;
Lombardi, 2003). A superplasticiser (patented new generation of 
admixture based on polycarboxylate chemistry), at a dosage of 
1.4% by weight of dry cement, was used to improve the
suspension properties of the microfine cements. This fixed super-
 plasticiser dosage was determined following a laboratory evalua-
tion of the effect of various dosages on the apparent viscosity and 
the rheological characteristics of the pulverised cement suspen-
sions (Pantazopoulos   et al., 2012). Suspension preparation
required a total mixing time of 10 min in high-speed mixers, of 
the type used for the preparation of soil specimens for hydro-
meter testing, with a speed of 10000 rpm at no load. The
experimental documentation of the suspension properties and 
groutability of the cements used in this investigation indicates
that microfine cement suspensions, enhanced with superplastici-
ser, have acceptable apparent viscosity, behave as Bingham fluids,
that is, they present linear rheological (shear stress– shear rate)
curves with constant slope defined as the plastic viscosity and 
intersecting the shear stress axis at a value defined as the yield 
stress, are stable for W/C ¼ 1, have reasonable setting times for 
field applications and can be injected into medium-to-fine sands
(Pantazopoulos et al., 2012).
The soils used were clean, uniform, limestone sands with angular 
grains and were grouted at a dense (mean value of relative
density,  Dr , 98 1%) and dry state. Four different sand grada-
tions were used with grain sizes limited between ASTM sieve
sizes 5 and 10, 10 and 14, 14 and 25, and 25 and 50. The
 properties of the sands, designated using the aforementioned 
sieve, are presented in   Table 1. The values of the angle of  
internal friction of the sands were obtained from UU triaxial
compression tests conducted on dense and dry specimens, under 
confining pressures equal to 100, 200 and 400 kPa. Dense sands
tested under a wide range of confining pressures are expected to
exhibit curved failure envelopes and, as a result, have friction
angle values that decrease with increasing stress level. However,
this behaviour was not observed in the present investigation,
 possibly due to the range of confining pressures used in the
Sand
friction,a j:  o
a Sands in dense and dry condition.
Table 1. Properties of sands
178
grouted sands
triaxial compression tests. More specifically, the fitting of the
experimental data with a linear failure envelope was satisfactory
in all sands, resulting in the constant friction angle values shown
in Table 1.
3. Experimental procedures The special apparatus shown in  Figure 1   was constructed and 
used for injecting sand columns with cement suspensions. It
allows adequate laboratory simulation of the injection process
and investigation of the influence of the distance from injection
 point on the properties of the grouted sand. The grouting column
was made of a thick PVC tube with an internal diameter of 
75 mm and a height of 1440 mm, and was formed by placing at
each end a 50 mm thick gravel layer, between two screens of 
suitable aperture, and filling the remaining length (1340 mm)
with dense, dry sand. The rate of discharge of the pump was
regulated to be constant and equal to 60 l/h. Injection was
stopped when either the volume of the injected grout was equal
to two void volumes of the sand in the column or when the
injection pressure became equal to 700 kPa. After injection, the
grouted column remained on its base for 24 h, then its ends were
sealed with plastic and it was stored in a vertical position. After 
curing for 28 days, the grouted columns were cut and some of the
resulting specimens, with a length of 160 mm, were utilised for 
triaxial compression testing. Prior to testing, the loading surfaces
of each specimen were capped using a cement-based low-strength
compound.
content in multi-stage (three-stage) unconsolidated–undrained 
(M-UU) triaxial compression according to the procedures de-
scribed by Head (1982), using conventional laboratory equipment
without modifications. As typically shown in Figure 2(a), after 
applying the confining pressure for the first stage (100 kPa), a
constant rate of axial strain, equal to 0.05%/min, was applied 
Grout outflow
Pressure sensors
0
1000
2000
3000
4000
0 2 4 6 8
D e v ia t o r s t re s s ,
: k P a
     q   f : k P a
 pf: kPa
compression test and (b) failure envelopes for grouted sand from
UU and M-UU triaxial compression tests
179
grouted sands
 
until failure of the specimen was imminent; that is, the stress– 
strain curve was in the curved (plastic) area and approached the
 peak deviator stress value. The decision to terminate the loading
stage was facilitated by the use of an automatic data acquisition
system, which provided real-time observation and full control of 
the test progress. Then, the axial load was reduced to zero, and 
the confining pressure for the second stage (200 kPa) was applied.
The loading and unloading sequence was repeated, the confining
 pressure for the third stage (400 kPa) was applied, and axial load 
was applied until the specimen failed (point of maximum deviator 
stress). The M-UU triaxial compression tests were preferred to
conventional single-stage UU tests for material economy, since
the latter would require the preparation of several (at least three)
identical grouted sand columns and the testing of specimens with
equal distances from the injection point for the reliable study of 
this parameter.
the past to determine the shear strength parameters of pulverised 
fly ash grouted sands (Markou, 2001). A method for conducting
multi-stage drained triaxial compression tests on weakly ce-
mented sands and estimating the resulting shear strength para-
meters was proposed recently (Sharma  et al., 2011). However,
this type of test is more satisfactory for plastic soils than for 
 brittle soils (Head, 1982). Accordingly, it was necessary to
document its trustworthiness, by comparing with the results
obtained from conventional UU tests on specimens of identical
composition. For this purpose, grouted sand columns with proper-
ties unaffected by distance from the injection point were selected,
and UU tests were conducted on specimens adjacent to the
M-UU specimens. UU triaxial compression tests were conducted 
under confining pressures of 100, 200 and 400 kPa and at a
constant axial strain rate equal to 0.05%/min. Failure was defined 
as the point of maximum deviator stress. Typical stress– strain
curves of grouted sand from M-UU tests and failure envelopes
( K f -lines) obtained for the same grouted sand from UU and 
M-UU tests (total stresses) are presented in Figure 2. The shape
of the stress–strain curves obtained from the M-UU test (Figure
2(a)) indicates that the specimen behaviour during testing was
normal for tests of this type (Head, 1982). As typically shown in
Figure 2(b), the Mohr– Coulomb failure criterion represents
adequately the behaviour of the grouted sand, and the two types
of test give equivalent shear strength parameters. More specifi-
cally, the comparison of UU and M-UU tests results in three
cases leads to identical values for the angle of internal friction
and comparable values for cohesion, since the observed differ-
ences of 5.5, 8.5 and 10.3% between cohesion values were low.
Therefore, the performance of M-UU triaxial compression tests
on cement grouted sand specimens was considered as practicable,
and the values of the shear strength parameters obtained were
considered as credible.
reported in the literature, the research effort reported herein was
limited to one-dimensional grout flow (a simplification of the
actual three-dimensional field conditions) and did not address
shear strength behaviour of the sands grouted in saturated 
conditions. An idea of the effect of the saturation of sands prior 
to grouting, on the strength of them after cement grouting, can be
given on the basis of the very limited information reported in the
literature. From the unconfined compression test results published 
 by  Schwarz and Krizek (2006), it is evident that the strength of 
initially saturated, microfine cement grouted sands is, on average,
lower by 15% than the strength of initially dry, grouted sands.
However, the injection process is adequately simulated on a
laboratory scale, and the results of the present investigation can
 be used for the evaluation of the mechanical behaviour of 
microfine cement grouted sands. Moreover, the M-UU triaxial
compression tests can be particularly useful for the study of the
effect of the distance from the injection point on the shear 
strength parameters of grouted sands.
4. Strength improvement The effect that grouting has on the strength of the sands is
 presented in  Figure 3, in terms of the strength ratio,   S R , a s a
function of the confining pressure used in the tests. The strength
ratio is defined as the ratio of the maximum deviator stress (stress
at failure) of the grouted sand, ( 1  3)g, to the maximum
deviator stress, ( 1  3)s, obtained for the clean sand at the same
confining pressure. The values of the strength ratio are always
higher than unity, are often higher than 2 and can be as high as 15,
indicating that the grouted sands have higher strength and, in many
cases, considerably higher strength than the clean sands. Signifi-
cant improvement of sand strength after grouting has also been
reported by several researchers (e.g.  Clarke  et al., 1993;  Dano  et 
al., 2004;  Krizek   et al., 1992). The strength ratio and, conse-
quently, the positive effect of grouting on the strength of sands
increases as the confining pressure decreases. This behaviour is
attributed to the fact that the failure envelopes of the correspond-
ing clean and grouted sands are, as concluded in the next section,
approximately parallel, straight lines intersecting the shear stress
axis at a value equal to zero or equal to cohesion, respectively.
Accordingly, the difference between the diameters (maximum
deviator stresses) of the two Mohr semicircles, which are tangent
to the failure envelopes of grouted and clean sand at the same
confining pressure, increases with decreasing confining pressure.
The type of cement (Figure 3(a)) appears to have some effect
only at the lower confining pressure applied (100 kPa), with CEM
I (pure Portland cement) suspension grouted sand yielding a
higher strength ratio value than sand grouted with suspensions of 
the other two cement types containing pozzolans. As shown in
Figure 3(b), strength improvement increased slightly with increas-
ing cement fineness. Fine-grained suspensions (d max ¼ 10 and 
20   m) yielded a range of strength ratios from 1.6 to 2.4 (average
1.9) and from 1.5 t o 2.3 (average 1.8), respectively, while the
range for coarse-grained suspensions (d max ¼ 100   m) was from
1.4 to 2.1 (average 1.7). The strength ratio ranged from 7.1 to
15.7 (average 11.0), from 1.5 to 2.3 (average 1.8) and from 1.1 to
1.2 (average 1.1) when grouting with suspensions of W/C¼ 1, 2
180
grouted sands
and 3, respectively (Figure 3(c)). Therefore, grouting with suspen-
sions of W/C¼ 1 resulted in significantly higher strength im-
 provement than grouting with suspensions of W/C¼ 2 o r 3 .
Average values of strength ratio equal to 1.82, 1.85, 2.39 and 2.42
were obtained for sands 5–10, 10–14, 14–25 and 25–50, respec-
tively (Figure 3(d)). These values indicate that sand gradation has
an effect on the strength improvement of the grouted sands, which
increases with decreasing sand grain size. From all the aforemen-
tioned observations, it is also evident that the suspension W/C
ratio affects the strength improvement of the grouted sands more
drastically than any of the other investigated parameters.
5. Shear strength parameters As typically shown in  Figure 2(b), all triaxial compression tests
conducted during this investigation yielded linear failure envel-
opes ( K f -lines) with exceptionally high correlation coefficients,
 R2, ranging from 0.997 to 1. These results indicate that the
Mohr–Coulomb failure criterion represents the behaviour of the
grouted sands adequately, as also observed by other researchers
(Dano   et al., 2004;  Krizek   et al., 1982,   1986,   1992;  Maalej,
2007). Accordingly, the shear strength of the grouted sands is
quantified in terms of the angle of internal friction,  j, and the
cohesion, c, after total stress analysis. The resulting friction angle
and cohesion values, obtained from M-UU triaxial compression
tests, are shown in  Tables 2  and  3, respectively, as a function of 
the distance of the grouted sand specimens from the injection
 point. It can be observed that the distance from the injection point
had no consistent effect on the shear strength parameter values of 
the grouted sands. However, the differences between the measured 
values and the average value of the shear strength parameters,
obtained for each grouted sand column, were lower than 4% for 
the angle of internal friction and 12.5% (valid for 80% of  
columns) for the cohesion. Therefore, distance from the injection
 point can be considered to have an insignificant effect on the
1
2
3
4
S t re n g t h
r a t io ,         S  R
Confining pressure, : kPa (a)
S t re n g t h
r a t io ,         S  R
Confining pressure, : kPa (b)
S t re n g t h
r a t io ,         S  R
Confining pressure, : kPa (c)
S t re n g t h
r a t io ,         S  R
Confining pressure, : kPa (d)
d max   µ 
Figure 3. Effect of (a) cement type, (b) cement fineness, (c) grout
W/C ratio and (d) sand gradation on the improvement of sand
strength due to microfine cement grouting
181
grouted sands
 
values of the shear strength parameters of the grouted sands, even
at a distance of about 1.2 m from the injection point.
The average values of the angle of internal friction obtained for 
each grouted sand column are shown in  Table 2. The values of 
the internal friction angle, based on M-UU triaxial compression
tests, ranged from 40.78   to 45.78. Although the internal friction
angles of grouted sands (Table 2) were up to 5.58  higher than
those of the clean sands (Table 1), the average increase was
considered as low since it was equal to 1.78  or 4%. Due to the
fact that grouting had a positive, but not pronounced, effect on
the angle of internal friction of the sands, it was confirmed that
the improvement of the shear strength of the sands consists
 primarily of the development of cohesion and not of an increase
in the angle of internal friction. This general beneficial effect of 
grouting has also been documented for sodium silicate solutions
Cement W/C ratio Sand Angle of internal friction,  j:  o Differencea
Type   d max:   m Distance from injection point Average
value
o %
I 20 2 14–25 – 40.4 41.0 40.7   1.9   4.5
II/B-M 20 2 10–14 43.2 45.0 46.2 44.8 +2.6 +6.2
IV/B 20 2 14–25 43.1 43.8 45.2 44.0 +1.4 +3.3
II/B-M 100 2 10–14 42.5 42.9 42.7 42.7 +0.5 +1.2
II/B-M 10 2 10–14 44.2 44.6 45.8 44.9 +2.7 +6.4
II/B-M 20 1 10–14 – 42.0 40.2 41.1   1.1   2.6
II/B-M 20 3 10–14 44.2 41.3 43.4 43.0 +0.8 +1.9
II/B-M 20 2 5–10 44.7 45.3 47.0 45.7 +5.4 +13.4
II/B-M 20 2 14–25 42.9 44.2 45.4 44.2 +1.6 +3.8
II/B-M 20 2 25–50 44.8 43.9 44.4 44.4 +1.8 +4.2
a Difference between angles of internal friction of grouted and clean sand.
Table 2. Friction angle values of grouted sands from multi-stage
unconsolidated–undrained triaxial compression tests
Cement W/C ratio Sand Cohesion,  c : kPa Variationa: % Examined
parameterb
value
I 20 2 14–25 – 455 543 499 +20.5 1
II/B-M 20 2 10–14 245 274 273 264 0.0 (C), 2, 3, 4
IV/B 20 2 14–25 407 454 380 414 0.0 (C), 1
II/B-M 100 2 10–14 187 235 345 256   3.0 2
II/B-M 10 2 10–14 253 309 263 275 +4.2 2
II/B-M 20 1 10–14 – 2421 2907 2664 +909.1 3
II/B-M 20 3 10–14 96 76 148 107   59.5 3
II/B-M 20 2 5–10 210 266 237 238   9.8 4
II/B-M 20 2 14–25 398 433 365 399   3.6/+51.1 1, 4
II/B-M 20 2 25–50 381 416 426 408 +54.5 4
a Relative to the average cohesion value of equivalent control column. b (C), control column; 1, cement type; 2, cement  d max; 3, W/C ratio; 4, sand gradation.
Table 3. Cohesion values of grouted sands from multi-stage
unconsolidated–undrained triaxial compression tests
182
grouted sands
(Krizek  et al., 1982), microfine cement suspensions (Dano  et al.,
2004;  Krizek  et al., 1986) and microfine cement–sodium silicate
mixtures (Krizek  et al., 1992).
Also presented in  Table 3  are the average cohesion values of the
grouted sands, obtained for each grouted sand column. The
utilisation of the grouted sand columns in the subsequent
 parametric analysis is clarified in the last column of  Table 3, by
referring to the relevant parameters investigated using a particular 
column, and by defining the control columns used to quantify the
effect of parameter variations. It can be observed that the
cohesion of the grouted sand increased with increasing cement
fineness and with decreasing suspension W/C ratio and sand grain
size. It can also be observed that grouting with CEM I suspension
 provided higher cohesion than those provided by the other two
cement types. The superiority of CEM I suspension can be
 justified by the composition of the cements, since CEM I is a
 pure Portland cement consisting of a larger proportion of clinker 
in comparison with the other two cement types and does not
contain pozzolanic materials. The observed increase of cohesion
with decreasing sand grain size was attributed to the increased 
number of grain-to-grain contact points in a finer soil and, as a
result, to the increased number of points available for cementa-
tion (Dano et al., 2004; Zebovitz et al., 1989).
A review of the variations of cohesion values of grouted sand 
columns relative to the cohesion of equivalent control columns,
 presented in Table 3, indicates that the suspension W/C ratio was
the most important of the investigated parameters, since it yielded 
the largest variations in cohesion values. Grout W/C ratio was
followed, in order of decreased cohesion variations, by sand grain
size, cement type and cement fineness. The average cohesion
values of sands grouted with microfine cement suspensions were
as high as 2664 kPa for W/C ¼ 1, ranged from 238 to 499 kPa
for W/C ¼ 2 and were equal to 107 kPa for W/C¼ 3. The
significant increase of the cohesion of grouted sands with de-
creasing suspension W/C ratio has also been documented by
other researchers (Dano  et al., 2004;  Krizek  et al., 1992). The
very small effect of cement fineness on grouted sand cohesion is
also attributed to the relatively high suspension W/C ratio
(W/C¼ 2) used in the injections. Using suspensions of the same
cements,   Pantazopoulos and Atmatzidis (2011) observed that
grouting with microfine cements of W/C¼ 1 provides, in general,
double the cohesion provided by ordinary cements.
6. Shear strength development It has been documented in the literature that the cohesion and the
angle of internal friction of grouted sands are functions of the
axial strain and that the sum of their contribution to shear 
strength is maximum at failure (Krizek  et al., 1982,  1986). For 
saturated cohesive soils, the development of shear strength with
axial strain can be quantified, and the effective shear strength
 parameters can be evaluated as functions of axial strain (Schmert-
mann and Osterberg, 1960). The application of this principle in
the investigation reported herein is based solely on the results of 
UU triaxial compression tests, analysed in terms of total stresses.
Although, as pointed out earlier, M-UU tests result in trustworthy
values of stresses and shear strength parameters for cement
grouted sands, they may lead to questionable values of axial
strain because, as is typically shown in Figure 2(a): (a) a residual
axial strain exists at the end of the first and second loading stages
and (b) the axial strain at failure can be determined accurately
only for the third (last) loading stage. Consequently, the results of 
the M-UU tests were not utilised herein for quantifying the shear 
strength development of microfine cement grouted sands.
Based on the stress–strain curves obtained from UU tests (Figure
4), it can be observed that the deformation at failure (point of 
maximum deviator stress),   f , of 14–25 sand grouted with
microfine cement suspension of W/C¼ 2, increased with increas-
ing confining pressure. More specifically, the average strain at
failure of the grouted sand was equal to 2.5, 3.8 and 4.8% for 
tests conducted with confining pressures equal to 100, 200 and 
400 kPa, respectively. The strain at failure of the clean sand was
significantly higher and presented similar behaviour, as it was
equal to 5.7, 6.4 and 9.3% for confining pressures equal to 100,
200 and 400 kPa, respectively. This means that the three speci-
mens of grouted or clean sand, tested under different confining
 pressures, were in different states of strength development for 
each specific strain value. For this reason, the shear strength
 parameters of grouted sand were determined for specific percen-
tages of failure strain of each specimen. Selected percentages of 
failure strain are shown on the stress–strain curves of  Figure 4. It
is evident that, for each one of these percentages, all specimens
were in the same state of strength development, regardless of the
confining pressures and that percentage equal to 100% corre-
sponds to the failure strain of each specimen.
0
1000
2000
3000
4000
0 2 4 6 8
D e v ia t o r s t re s s ,
: k P a
grouted sand at various failure strain percentages
183
grouted sands
The methodology for the quantification of shear strength develop-
ment was applied to the grouted sand by determining, for all UU
tests, the deviator stresses corresponding to each percentage of 
failure strain, plotting the resulting  p – q  envelopes ( K -lines), as
shown in  Figure 5(a)   for all percentages of failure strain, and 
evaluating the shear strength parameters, as presented in  Figure
6, as functions of the percentage of failure strain. Both cohesion
and angle of internal friction exhibit a high rate of increase
during the initial stages of axial loading. Cohesion (Figure 6(a))
attains a maximum value for an axial strain ranging between 20
and 30% of the failure strain. This point also corresponds to the
transition of the stress–strain curves from the initial linear part to
the curved (plastic) area, as shown in  Figure 4. The maximum
cohesion value, which is 35% higher than the cohesion obtained 
at failure, decreases, thereafter, at a progressively decreasing rate
until failure (point of maximum deviator stress). The angle of 
internal friction (Figure 6(b)) attains a value approximately equal
to 90% of the value obtained at failure, for an axial strain
corresponding to 40% of the failure strain and increases, there-
after, at a lower rate until failure. These observations are in good 
agreement with the behaviour observed for sands grouted with
sodium silicate solutions (Krizek   et al., 1982) or microfine
cement suspensions (Krizek  et al., 1986). More specifically, it has
 been reported that cohesion increases rapidly, reaches a maximum
value at low strain levels, then decreases and maintains a constant
value until failure; the angle of internal friction increases
gradually as axial strain increases and becomes maximum at
failure.
Application of this methodology to the data obtained from UU
tests conducted on clean sand specimens, yielded the   p – q
envelopes ( K -lines) shown in  Figure 5(b). These envelopes were
 plotted using stress values for the clean sand corresponding to the
strain percentages used for grouted sand. In this manner, the
development of the internal friction angle of the clean sand was
quantified at the same compression levels as for grouted sand,
0
500
1000
1500
2000
     q  
(a)
0
500
1000
30%
5%
Figure 5.  K -lines for (a) microfine cement grouted sand and
(b) clean 14–25 sand at various failure strain percentages
0
100
200
300
400
500
C o h e s io n ,
: k P a
a n g le ,
Clean 14 25 sand
Grouted 14 25 sand
Failure of grouted sand
Failure of clean sand
Figure 6. Development of (a) cohesion and (b) angle of internal
friction of microfine cement grouted sand as a function of failure
strain percentage
grouted sands
internal friction angle development of clean and grouted sand are
compared in  Figure 6(b). It can be observed that the angle of 
internal friction of the clean sand increases rapidly up to an axial
strain approximately equal to 25% of the failure strain of the
grouted sand, and continues to increase at a substantially reduced 
rate, until failure. A very similar behaviour has been observed for 
other clean sands (Krizek  et al., 1982). It can also be observed 
that at the failure point of the grouted sand (failure strain
 percentage¼ 100%), the internal friction angle of the clean sand 
reaches 95% of its maximum (failure) value, attained at consider-
ably higher axial strains, and that the grouted sand exhibits the
same form of internal friction angle development and system-
atically higher values than the clean sand, for axial strains up to
the failure strain of the grouted sand. Grouting is most effective
when the sand voids are filled with solidified grout material,
which also adheres to the surfaces of the sand grains. Apart from
 providing cohesion to the grouted sand, the combined effect of 
cementation and void filling restricts the relative movement
 between sand grains during shear, and is quantified as an increase
in the value of the angle of internal friction.
The aforementioned quantitative and qualitative observations
indicate that there is a sequence of axial strain ranges where
distinct behavioural patterns are exhibited by the microfine
cement grouted sands: (a) as shown in Figure 6(a), for axial strain
values between 20 and 30% of the failure strain, the cohesion
reaches a peak value that is 35% higher than the value at failure,
(b) for axial strains over 20 to 30% and up to 60 to 70% of the
failure strain, it is postulated that a breakdown of bonds between
sand grains and grout occurs, followed by a decrease of the
cohesion values and (c) beyond an axial strain of approximately
60 to 70% of the failure strain, both cohesion and angle of 
internal friction vary at a very low rate, and the grouted sand 
appears to behave as very dense sand with voids filled by another 
fine-grained material.
7. Conclusions Based on the results obtained and the observations made during
this laboratory investigation and within the limitations of the
range of parameters investigated, the following conclusions may
 be made.
(a) Multi-stage UU triaxial compression tests on specimens
obtained from a single grouted sand column can be utilised to
determine the shear strength parameters of the grouted sands
and to quantify the effect of distance from the injection point.
(b) Grouting with microfine cements, produced by pulverising
ordinary cements, improves the shear strength of sands
significantly, even at a distance of about 1.2 m from the
injection point. Grouting is most effective when thick cement
suspensions (stable, W/C¼ 1) are injected, when pure
Portland cement suspensions (no pozzolan) are used and 
when finer sands are grouted.
(c) The Mohr– Coulomb failure criterion represents the behaviour 
of microfine cement grouted sands that obtain cohesion values
ranging from 240 to 500 kPa for W/C¼ 2 and reach 2650 kPa
for W/C ¼ 1 and angle of internal friction values higher by up
to 5.58 compared to that of the clean sands.
(d ) The activated cohesion of the grouted sands reaches a
maximum value for an axial strain of about 25% of the
failure strain and decreases thereafter, until failure.
(e) The activated angle of internal friction of the grouted sands
increases with increasing axial strain, as for clean sands, but
it attains higher values than clean sand, possibly due to the
restriction of the relative movement between sand grains
during shear, caused by the cementation and void filling
effect of grouting.
Acknowledgements The research effort reported herein is part of the research project
PENED-03ED527 that was co-financed by the EU–European
Social Fund (75%) and the Greek Ministry of Development– 
GSRT (25%). The contribution of TITAN Cement Company S.A.
was substantial for the selection, chemical analysis, pulverisation
and grain size analysis of the cements.
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grouted sands