RESEARCH PAPER

Development of an empirical criterion for predicting the hydraulicfracturing in the core of earth dams

Ali Ghanbari • Shima Shams Rad

Received: 6 February 2011 / Accepted: 11 June 2013

� Springer-Verlag Berlin Heidelberg 2013

Abstract In this research, based on the laboratory stud-

ies, a new empirical criterion was developed to predict the

hydraulic fracturing pressure in the core of earth dams. To

simulate the core condition in the laboratory, a special cell

was designed and assembled based on advanced consoli-

dation cell (Rowe cell). The hydraulic fracturing tests were

performed in unconsolidated and unsaturated conditions on

the materials of an under-construction dam and the results

were used according to critical conditions in which the

hydraulic fracturing is initiated in the embankment dams. It

can be concluded that for fine-grained soils and also

coarse-grained soils containing considerable percent of fine

particle, the hydraulic fracturing initiation pressure is

dependent on the minor principal stress of the soil and

increase linearly with the increase in mentioned stress. In

addition, an empirical equation is introduced to estimate

the hydraulic fracturing initiation pressure based on shear-

strength properties of the soil, and also the effect of com-

paction energy on the pressure is discussed. Afterward, the

numerical analysis has been carried out on the Madani

Earth dam considering three types of soil for the core of the

dam. Furthermore, by using several empirical criteria, the

districts of the core which are susceptible to hydraulic

fracturing were identified for each soil. Results of numer-

ical study show that among three selected soils for the core

of the dam, the CL which is susceptible to hydraulic

fracturing is identified as critical soil and the GM-GC as

the recommended one.

Keywords Clay � Core material � Earth dams �Hydraulic fracturing � Rowe cell

1 Introduction

The core of the earth- and rock-fill dams acts as sealing

element of the dam and any cracking in it may lead to total

failure of the dam. Hydraulic fracturing is one of the major

problems in the core of earth-fill dams, because it plays an

important role in the initiation of cracks in the core of earth

dam. Therefore, hydraulic fracturing has attracted a lot of

attention and many experimental and analytical researches

have been performed. Hydraulic fracturing is initiated due to

overcoming of the water pressure of reservoir on the effec-

tive stress of the core soil. In this process, water pressure

makes the stress state to move toward failure envelope in one

point with shear or tension mechanism and consequently the

cracks are initiated in the soil mass. These cracks are called

hydraulic cracks and the water pressure by which the cracks

are initiated is called hydraulic fracturing pressure.

The oldest study about hydraulic fracturing was done in

the first decade of the second half of twentieth century.

Nobari et al. [16], by using the modified triaxial test

apparatus, studied the hydraulic fracturing in compacted

sandy clay soils. The modified apparatus, in addition to

applying axial stress, injects water pressure inside and

around the specimen. They indicated that in tested speci-

mens, the fracturing mechanism is in tension mode and the

cracks have been initiated in the plane of maximum tension

stress. On the other hand, shear fracturing mechanism has

not been reported in these specimens.

After failure of Teton dam in 1975, a special committee

was founded to investigate the reasons of total failure of

this great dam in the first impounding of reservoir [9].

A. Ghanbari (&) � S. Shams Rad

Faculty of Engineering, Kharazmi (Tarbiat Moallem) University,

No. 49 Mofatteh Ave, Tehran, Islamic Republic of Iran

e-mail: Ghanbari@tmu.ac.ir

S. Shams Rad

e-mail: sh.shams@tmu.ac.ir

123

Acta Geotechnica

DOI 10.1007/s11440-013-0263-2

Finally, occurrence of hydraulic cracking was reported as

one of the main reasons of dam failure. Laboratory studies

of hydraulic fracturing were conducted on re-compacted

materials of Teton dam by Jaworski et al. [8] and by

invention of a special apparatus. These researchers used

cubic specimens of dimensions 203 mm, containing an 8.4-

mm diameter hole in the middle. The results indicate that

for the fine-grained tested soil, hydraulic fracturing pres-

sure is the linear function of primary total horizontal stress

and is expressed by the following equation:

Pf ¼ mrh þ rta ð1Þ

where rta is the apparent tensile strength of soil for

preventing crack initiation in fracturing pressure Pf.

Furthermore, m is the slope of linear relation between

fracturing pressure and rh is the horizontal stress (in the

state of total stress) before creation of the hole. Mori and

Tamaru [14] conducted hydraulic fracturing tests on

hollow cylindrical specimens with 5 cm diameter and

15 cm height, and finally suggested the following equation

for hydraulic fracturing pressure in terms of total stress.

Pf ¼ rmin þ qu ð2Þ

where qu is the unconfined compressive strength.

Fukushima [2] conducted hydraulic fracturing test on

compacted cohesive soils. He assumed that the stress dis-

tribution in radial direction is linear and by comparing the

total and effective stresses concluded that the tensile stress

of compacted cohesive soil is negligible in terms of

effective stress. Also by using Mohr–coulomb criterion, he

developed the following hydraulic fracturing equation:

Pf ¼ mrc ð3Þ

where rc is the confining pressure and m is a constant

ranging from 1.3 to 1.6. The studies done on fine-grained

soils by Yanagisawa and KomakPanah [22] confirmed the

existence of linear relation between hydraulic fracturing

pressure and main stresses of the soil. The laboratory tests

were carried out by the corrected triaxial test apparatus

using hollow cylindrical specimens. Their suggested

equation is expressed as follows:

Pf ¼ ð1:5r3 � 0:5r2Þð1þ sin /uÞ þ Cu cos /u ð4Þ

where Pf is the hydraulic fracturing pressure, r2 and r3 are

the total principal stresses, Cu and uu are the undrained

cohesion and undrained friction angle of the soil,

respectively. Furthermore, these researchers have

emphasized on shearing mechanism of specimens failure.

Satoh and Yamaguchi [17] used the core material of an

under-construction rock-fill dam as the material of

laboratory tests. They used large and medium size hollow

cylindrical specimens with the external diameter of 0.3 and

0.15 m and investigated the influences of Dmax and

confining pressure on the hydraulic fracturing pressure.

Satoh and Yamaguchi [17] suggested the following

equation for hydraulic fracturing pressure:

Pf ¼ mr3 þ n ð5Þ

The values of m and n derived from the hydraulic frac-

turing test results by Satoh and Yamaguchi [17] are shown

in Table 1.

In recent years, various research methods have been

applied to the study of fracturing and cracking of soils and

concrete materials [5, 7, 11, 16, 20, 21]. Considering all the

past studies indicated that, generally in fine-grained soils

and fine-grained clay soils containing sand, the hydraulic

fracturing pressure is the linear function of horizontal

stress. Furthermore, failure mechanism of specimens in the

fine-grained soils is in shear mode and in the fine-grained

sandy soils is probably in tension mode.

In the laboratory studies made by the authors, the

hydraulic fracturing pressure has been investigated for the

fine- and coarse-grained soils. Therefore, a special labo-

ratory cell and its new sampler have been designed and

made accordingly. The new cell has approximately large

diameter according to maximum size of the selected soils.

The present study has the special characteristics that con-

sidered both fine- and coarse-grained soils and compared

the hydraulic fracturing pressure of them. However, in the

past researches of hydraulic fracturing, the soils containing

a significant percent of coarse particles are not observed.

2 Advanced consolidation cell

Advanced consolidation cell is the corrected type of

ordinary consolidation cell, which has been invented to

improve the odeometer apparatus in British Standard [1].

The advance cell has special advantages including capa-

bility of continuum loading on the soil specimens, capa-

bility of measuring consolidation properties and soil

permeability in radial form. This cell can drain soil spec-

imens horizontally or horizontally applies pressure from

the middle of sample toward the shell. Loading control,

measurement of the discharging water volume and apply-

ing of backpressure are some of the other advantages of

Table 1 The values of m and n derived from the hydraulic fracturing

test results (Satoh and Yamaguchi [17])

Size of specimen Dmax (mm) m n (kN/m2)

Large size 2 1.72 -14

Large size 19 1.29 34

Medium size 19 1.17 -0.8

Medium size 4.75 1.18 4.6

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this apparatus. According to different available sizes of this

cell and possibility of using the large-scale cells, the test

results seem to be more close to real conditions of the soil.

In this research, to study the correlation between soil

properties and hydraulic fracturing pressure, some changes

have been made in advanced consolidation cell and con-

sequently the hydraulic fracturing test apparatus was

designed and made. The designed cell has 25 cm internal

diameter, and in addition to radial drainage, it is capable of

applying the lateral pressure on specimens. The new cell,

which is shown in Fig. 1, includes different parts such as

cell body, valves, nut and bolts, sealing rubbers, metal and

porous planes, rubbery diaphragm, etc. Also a special

sampler has been designed and made for this cell [3].

3 Hydraulic fracturing tests apparatus

For conducting hydraulic fracturing tests, a system con-

sisting of air-pressure generation source, pressure regula-

tor, discharge control valves, data reading and measuring

equipment and the set of testing cell has been designed.

The mentioned system is indicated in Fig. 2. Air com-

pressor applies the pressure and then the air pressure is

converted to water pressure by water reservoirs. The sys-

tem consists of:

– Air compressor

– Transformer reservoirs of air pressure to water pressure

(S1, S2, S3)

– Pressure control regulators (R1, R2, R3)

– Water-pressure measurement pointers (H1, H2, H3)

– Water flow control valves (V1–V9)

– Hydraulic fracturing test cell (which valves G1–G5 and

several other tools are connected together)

– Sampling devices (sampler mold, 5-kg hammer, etc.)

The hydraulic fracturing tests are conducted in seven

main stages, respectively: soil grading, supplying moisture

of sample, compaction, recovery of sample from the sam-

pler, block out the sample in the cell, loading, initiation of

hydraulic crack and recording data.

General procedure of the tests is presented in this

section; first, the required amount of soil is prepared

based on the selected sieve-analysis curve and then the

soil is watered to reach the required moisture. After

normalizing the soil-water mixture, the soil is compacted

by dropping a hammer of 5-kg mass on the sample.

Finally, hollow cylindrical specimens are recovered out of

the specially designed sampler and are blocked out in the

test cell.

Bentonite has been used for sealing the sample and to

increase the efficiency of the sample, glass plates have

been utilized in top and bottom of the sample. When the

cell is prepared, loading is started on the sample and test

data are also recorded. When the test is conducted in sat-

urated conditions, after placing the sample into the sam-

pler, water flow should be established in radial form, from

the outside to the inside of the sample. Full saturation is

reached when the internal and external pressures of the

specimen become equal. Hereunder, the stages of the test

are described briefly.

4 Sample preparation

Hollow cylindrical specimens are used in the test. There-

fore, special sampler was designed and manufactured to be

used to prepare them. For this purpose, a rod was installed

in the center of a cylindrical mold with an inner diameter of

40 mm, external diameter of 150 mm and the height of

90 mm. The sampler is capable of producing the com-

pacted hollow cylindrical specimens. The sampler has a

5-kg mass, which drops on the soil sample parallel to the

middle bar of the mold. The number of sample layers,

number of drops and the drop height are determined

according to the type of equivalent compaction. Specifi-

cations of the compaction methods have been presented in

Table 2. As can be seen, the number of drops of the

sampler hammer is different from that of standard hammer.

When the sample reaches the optimum moisture and the

compaction is done, the sample is ready to be inserted to

the cell. To simplify the drainage and ensure the even

pressure distribution, around and the hollow center of the

specimen is filled by sandy materials. Besides, the distance

between top and bottom surfaces of the sample and also

cell plugs are coated by bentonite to avoid any seepage

between these two surfaces.

The entire process of hydraulic fracturing test can be

divided into three major stages which are presentedFig. 1 Exterior view of the designed test device

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hereunder. The following stages are conducted consecu-

tively when the wind pressure is produced behind the

regulators.

When the G2, V1 and V2 valves are kept open and R3

regulator is adjusted, vertical pressure (20 kN/m2) is

inserted into the sample. To ensure that the air is dis-

charged completely from the diaphragm, the air-discharg-

ing valve (G1) is opened and kept open till water is lifted

out of it.

To make sure that the rv and P0 pressures are constant

during the hydraulic fracturing test, the V2 and V5 valves

are closed and disconnected from their sources.

Then, the G4, V7 and V8 valves are opened and the R1

regulator is adjusted to increase the internal pressure of the

sample slowly. The increase rate should be equal to the

assumed rate of loading. The time is recorded at the

beginning of this stage, and H1, H2 and H3 gauges are read

carefully in specific intervals and the data are recorded in

special forms.

Immediately, when the crack is initiated in the sample,

water would flow from the inside to the outside of the

sample and consequently the confining pressure is

increased. The changes in pressure can be measured by H2

gauge. Besides, by considering the increase rate of the H2

gauge, sudden or slow failure of the sample is also rec-

ognized. The P1 gauge at the moment of failure indicates

the hydraulic fracturing pressure (Pf). During the test, for

saturated specimens, the changes of pore water pressure are

measured by the G5 valve.

At the end of the test, pressure is slowly removed from

the specimens and then the upper cap of the cell is taken

off. After discharging the water out of the cell, the crack

path is identified which is usually clear and observable.

5 The results of laboratory studies

The past hydraulic fracturing studies discussed in the

Introduction were all done on the fine-grained soils, and

the coarse-grained soils containing considerable percent-

age of fines are not considered in most of them but

Satoh and Yamaguchi [17]. Although the hydraulic

fracturing pressure and its mechanism have been sur-

veyed by many other researchers, their results are varied

to some extent and prevailing mechanism of cracking

has not been definitely identified in fine-grained soils. In

Fig. 2 General plan of hydraulic fracturing tests system

Table 2 Specifications of compaction used in special sampler of

hydraulic fracturing tests

Method of

compaction

Standard No. Number

of layers

Number

of drops

Falling

height

Standard proctor ASTM-D698 3 13 18

Modified proctor ASTM-D1557 5 35 18

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the present study, the following subjects have been

considered specifically:

A. Evaluate the hydraulic fracturing pressure for coarse-

grained soils containing clay, and compare them with

the results of fine-grained soils.

B. Considering the special conditions of hydraulic frac-

turing that may occur in the core of earth-fill dams.

C. Determination of the fracturing mechanism, position

and direction of the cracks created in the specimens.

First impounding of dam reservoir is the most probable

stage in the lifetime of the dam, for occurrence of hydraulic

fracturing. In this condition, the core of dam has not been

completely saturated and consequently the soil would show

unsaturated behavior during the loading. Accordingly, the

specimens are loaded in unsaturated conditions and

because the test is a short-term one, the specimens are not

saturated during the test. Figure 3 indicates one of the

tested specimens after failure. Physical properties of the

soils used in the tests have been estimated by conducting

several soil mechanics tests and are presented in Table 3

briefly.

The hydraulic fracturing was studied using 40 soil

specimens. One of the resulting diagrams is shown in

Fig. 4. The relation of hydraulic fracturing pressure and

confining pressure is also presented in Fig. 5 for the first

type of soil (GM-GC) in two light and heavy compaction

conditions. As observed, there is an almost linear relation

between fracturing pressure and confining pressure of the

specimen. Furthermore, considering the shape of created

cracks in each sample indicates that the all created cracks

can be classified into four main groups, as shown in Fig. 6.

All four types of cracks have been initiated from inside of

the specimens and are propagated in one of the shear,

tension or shear-tension mechanisms. Crack propagation is

dependent on various factors such as unbalanced geometry

or heterogeneousness of materials; therefore, created

cracks have different shapes.

6 Developing an empirical equation for hydraulic

fracturing pressure

Laboratory results indicate that with a very good approxi-

mation the relation between hydraulic fracturing pressure

and confining pressure of the specimens is linear.

Accordingly, the empirical equation was expressed by

general formula of a line. Eq. 5 indicates the hydraulic

fracturing pressure as a function of confining pressure.

Pf ¼ mrh þ n ð6Þ

where m and n are the constant parameters varies in

accordance with the type of soil, saturation condition and

compaction of the specimens. The current study indicates

that for unsaturated compacted soils, m is ranging from 1.0

to 1.2 and n is ranging from 0.2 to 0.4. Exact values of

these parameters for each soil are dependent on different

factors, such as grading, plastic properties of the soil, the

friction angle, cohesion and the rate of compaction.

According to the test results and considering the past

studies of researches, Table 4 is presented the m and

n parameters for different soils. It should be noted that the

aforementioned criteria are just to determine the fracture

initiation.

7 Sensitivity analysis of the hydraulic fracturing

pressure

According to the variation of soil properties, the sensitivity

analysis of hydraulic fracturing pressure to different factors

such as mechanical properties of the soil was performed

and is presented hereunder briefly.

7.1 Confining pressure

The tests results indicate a linear relation between confin-

ing pressure and the hydraulic fracturing pressure. There-

fore, the hydraulic fracturing pressure increases with the

increase in confining pressure. For some of the tested soils,

the test results indicate less standard deviation toward the

fitted line.

7.2 Shear strength

When the tensile fracture mechanism governs on the soil

failure the cohesion and friction angel of the soil have

approximately no effect on hydraulic fracturing pressure.Fig. 3 Picture of one of the tested specimens after failure

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Nevertheless, the experimental results indicate that shear-

failure mechanism or shear-tensile mechanism has more

adaptation to the test results comparing to the tensile

mechanism. Therefore, the cohesion and the friction angle

would influence on the hydraulic fracturing pressure, par-

ticularly when the pressure exceeds 50 kN/m2.

7.3 Tensile strength, compaction rate and moisture

content

Considering the tensile strengths of the tested soils as

shown in Table 5 demonstrates that the m and n parameters

are not dependent on the tensile strength. The other

important point, which should be mentioned, is the

decrease in the tensile strength with the increase in com-

paction degree. It also has been considered in the studies of

other researchers [6, 12, 13], but the varied results and

sometimes, inconsistent results of different researchers

indicate that the variation of tensile strength with the

compaction energy follows no specific procedure.

In this laboratory study, changes of hydraulic fracturing

pressure via optimum water content of compaction have

not been considered. But studies of Medeiros [12] indicate

that the increase in initial water content of the specimens

leads to decrease in hydraulic fracturing pressure. It seems

that this conclusion is true only in limited range of moisture

Table 3 Properties of the soils used in the study

Classification Plasticity

index

Max. size of

particles (mm)

Optimum

moisture (%)

Friction

angle

Cohesion

(kN/m2)

Tension strength

(kN/m2)

GM-GC 6 50 10 4 76 130

SC 19 19 15 5 26 190

CL 21 5 18 6 40 250

Fig. 4 Variation of stresses versus time for one of the specimens

Fig. 5 The results of hydraulic fracturing pressure of the specimens

in two states of light and heavy compactions

Fig. 6 The shape of different types of cracks created in the

specimens

Table 4 The values of m and n derived from the hydraulic fracturing

test results

Type of soils m n

Coarse mixed soil, well graded, containing clay 1.2 40

Gravely soils containing silt and clay 1.15 25

Sandy soils, well graded, containing silt and clay 1.05 25

Sandy soils, poor graded, containing silt and clay 1.0 20

(CH) Fine particle soils with high plasticity 1.0 40

(CL) Fine particle soils with low plasticity 1.05 30

If light compaction (ASTM-D698) is carried out, the amounts of

m and n can be reduced about 0.05

If the specimens are tested in saturated condition, the amounts of

m and n can be reduced about 0.1–0.2

Table 5 Variation of m and n with tensile strength in tested

specimens

Granular soil with

light compaction

Granular soil with

heavy compaction

rt (kN/m2) 13 18.6

N 25.2 21.5

M 1.164 1.137

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Fig. 7 Madani dam geometry

Fig. 8 Middle and upstream sections of the core

Table 6 The summary of the hydraulic fracturing equations

The equation n m

Satoh and Yamaguchi [17] Pf = mr3 ? n Ranging from -14 to 34 Ranging from 1.18 to 1.82

Fukushima [2] Pf = mrc – Ranging from 1.3 to 1.6

KomakPanah and Yanagisawa [10] Pf ¼ ðmr3 � nr2Þð1þ SinuuÞ þ CuCosuu 1.5 0.5

Mori and Tamura [14] Pf = rmin ? qu – –

Current study Pf = mrh ? n Ranging from 20 to 40 Ranging from 1 to 1. 2

Table 7 The soil properties used in analysis for three types of soils

Parameter CL GM-GC SC

Unsaturated friction angle uus 16� 30� 26�Saturated friction angle uuu 6� 4� 5�

u0=uCD

22 28 29

Unsaturated cohesion Cus 0.3 kg/cm2 0.3 kg/cm2 0.13 kg/cm2

CCD = c’ 0.21 kg/cm2 0.18 kg/cm2 0.09 kg/cm2

Saturated cohesion Cuu 0.4 kg/cm2 0.76 kg/cm2 0.26 kg/cm2

Soil permeability K 3E - 7 cm/sec 1E - 8 cm/sec 1E - 6 cm/sec

Maximum dry density cdry 1,720 kg/m3 2,020 kg/m3 1,670 kg/m3

Wet density cwet 1,780 kg/m3 21 kg/m3 1,720 kg/m3

Saturated density csat 1,830 kg/m3 2,220 kg/m3 1,790 kg/m3

Elasticity modulus 120 kg/cm2 450 kg/cm2 230 kg/cm2

Poisson’s ratio 0.35 0.35 0.35

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contents. Because the higher is the moisture content, the

higher is the flexibility of the sample and accordingly the

tensile strength. Consequently, the hydraulic fracturing

pressure increases with the increase in moisture content.

Nevertheless, the reverse relation of moisture content and

internal friction angle should be considered. The variation

of m and n with tensile strength of soil in tested specimens

is presented in Table 5.

In addition, the soils with higher plasticity index indi-

cate more flexibility and so are more resistant against

cracking. Nevertheless, plastic soils may have less shear

strength and therefore be more susceptible to hydraulic

fracturing. But, it can be accepted as a general rule that

increase in plasticity index causes the improvement of

strength against cracking. According to the test results,

n would increase with the increase in plasticity index.

In addition to the discussed factors, there are some other

effective parameters, which are not in the scope of this

paper but have been studied by other researchers such as

Medeiros [12], Lo and Kaniaru [11], Haimson [4], Jaworski

et al. [8], Mori and Tamura [14] and Mhatch [13]. The

aforementioned factors are the soil loading rate, degree of

consolidation, the soil permeability and the size of sample.

8 Numerical analysis of hydraulic fracturing

in the core of Madani dam

In this section, the consistency of the empirical criterion

that was proposed in current study was compared to other

hydraulic fracturing studies, by numerical analysis of the

Madani dam. The Madani dam is a clay core, heteroge-

neous zoned-dam which is located in 1,458 m height above

sea level and is to be constructed on the Talkheh-Rood

River in the north west of Iran. The dam geometry is

indicated in Fig. 7.

The Geo-studio software was utilized in numerical

analysis, which is finite element geotechnical software. The

modeling was done in the 16-layer fill to consider the stage

construction of the dam, using fixed stress/strain boundary

conditions in both directions. Based on the dam stress

analysis, the probability of hydraulic fracturing initiation in

the middle points of the core (A-A section) and upstream

boundary of the core (B-B section), as indicated in Fig. 8,

is investigated after first impounding of the reservoir.

Section A-A is selected because lowest minimum stress

(lowest r3) and maximum arching occur at this section.

Also, section B-B is selected because previous experiments

show that starting points of hydraulic fracture are located in

this section [3].

Table 8 Soil properties for other zones of the dam

Alluvium Disposal Shell Transition

zone

Parameter

30 20 38 36 Friction angle

20 30 0 0 Cohesion kN/m2

51 30 83 78 Elasticity modulus MPa

0.3 0.35 0.3 0.3 Poisson’s ratio

20.5 19.2 19 18 Density kN/m3

Fig. 9 Comparing the hydraulic fracturing pressures for GM-GC soil in A-A section

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The stresses needed for each criterion, in the both

mentioned sections, were derived after the numerical

analysis in different depths of the dam core and the cor-

responding pore water pressure in each point was derived

from the software accordingly. In addition, the other cri-

teria such as Fukushima [2], Mori and Tamaru [14], Ya-

nagisawa and KomakPanah [22] and Satoh and Yamaguchi

[17] were considered to calculate the hydraulic fracturing

initiation pressure. Using these criteria and the stresses that

was derived from the software in desired points, the Pf was

computed and was compared to the criterion introduced in

this paper. Aforementioned criteria, which were used in

verification, are summarized in Table 6.

In this section, the results of three sets of simulations are

reported. The first set of simulations is performed to sim-

ulate the core with GM-GC soil and the two other

Fig. 10 Comparing the hydraulic fracturing pressures for GM-GC soil in B-B section

Fig. 11 Comparing the hydraulic fracturing pressures for SC soil in A-A section

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simulations were done for SC and CL soils. Three different

soil properties (GM-GC, SC and CL) used for the simu-

lation of the core soil are summarized in Table 7. As well,

the soil properties for other zones of the dam are presented

in Table 8. It should be noted that the soil properties used

for analysis was based on the laboratory soil mechanic tests

done on the samples used in hydraulic fracturing tests.

Figures 9 and 10 indicate the hydraulic fracture initia-

tion pressures for the core with GM-GC soil, in middle and

upstream sections of the core, respectively.

Fig. 12 Comparing the hydraulic fracturing pressures for SC soil in B-B section

Fig. 13 Comparing the hydraulic fracturing pressures for CL soil in A-A section

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As can be seen in Figs. 10 and 11, the predicted

hydraulic fracture pressure is always higher than pore

water pressure. Consequently, it can be concluded that the

core with GM-GC soil is not susceptible to hydraulic

fracturing in both A-A and B-B sections because the pore

pressure never exceeds the hydraulic facture initiation

pressure. Also, the same results for SC and CL soils are

demonstrated in Figs. 11, 12, 13 and 14.

As can be seen, the pore water pressure has exceeded the

hydraulic fracturing pressure in some points of the core

with CL and SC soil. But the number of these points is

much more in CL soil. So, CL is identified as the most

susceptible soil to hydraulic fracturing among three selec-

ted types of soil and is not recommended to be used in core

of Madani earth dam.

In addition, comparing these criteria indicate that

Fukushima [2] is the most optimistic criterion which

almost has higher values and Yangisawa and KomakPanah

[22] always have the less values in predicting the hydraulic

fracturing initiation pressure. Also in the middle section of

the core, there is an acceptable adoption between proposed

empirical criterion and Satoh and Yamaguchi [17], but in

upstream section, the predicted values are close to Fuku-

shima [2].

To prevent the occurrence of hydraulic fracturing in

earth dams, it is strongly suggested that by creating a wide

transition region between the core and the shell and also by

removing roughness of foundation, prevent the occurrence

of under tension regions in the core [19]. To predict the

occurrence of hydraulic fracturing, the empirical relation

presented in this paper can be used. Nevertheless, it is

recommended that hydraulic fracturing pressure be con-

servatively considered equal to the confining pressure.

9 Conclusion

To perform laboratory study of hydraulic fracturing phe-

nomenon in the core of earth dams, and by the rearrange-

ment of advanced consolidation cell (Rowe cell), a new

research cell and its special sampler have been invented.

The tested soils have been selected from three different

types, which have been selected from the soils of an under-

construction dam.

The test results indicate a linear relation between

hydraulic fracturing pressure and confining pressure.

Based on this fact, an empirical equation for predicting

the hydraulic fracturing pressure of the soil was devel-

oped. Furthermore, the results indicate that saturated

specimens in comparison with unsaturated ones have less

hydraulic fracturing pressure. Also, increase in soil com-

paction energy leads to increase in hydraulic fracturing

pressure.

Considering the shape of cracks and also comparison of

laboratory and theoretical hydraulic fracturing pressures in

both shear- and tension-failure mechanisms indicates that

the specimens’ failure happened in different tension, shear

and shear-tension modes. Nevertheless, by increase in

confining pressure, failure mechanism is converted from

tension to shear mode.

Fig. 14 Comparing the hydraulic fracturing pressures for CL soil in B-B section

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Three sets of numerical simulations were performed for

the Madani dam. The analysis indicated the consistency of

the proposed criterion to the other criteria especially in

middle section with Satoh and Yamaguchi criterion and in

upstream section with Fukushima criterion. Furthermore,

among three selected soils for the core of the dam, the CL

which is susceptible to hydraulic fracturing is identified as

critical soil and the GM-GC as the recommended one.

References

1. BS 1377-6 (1990) Method of test for soils for civil engineering

purposes. Consolidation and permeability tests in hydraulic cells

with the pore pressure measurement. British standard institution

2. Fukushima S (1986) Hydraulic fracturing criterion in the core of

fill dams. Rep Fujita Kogyo Tech Inst 22:131–136

3. Ghanbari A (2003) Laboratory study of the hydraulic fracturing

in the core of earth dams, PhD thesis, Amirkabir University of

Technology, Tehran

4. Haimson BC (1968) Hydraulic fracturing in porous and nonpo-

rous rock and its potential for determining in situ stresses at great

depth, PhD. Thesis, University of Minnesota

5. Hassani AW, Singh B, Saini SS (1983) Experimental investiga-

tion of hydraulic fracturing. Indian J Power River Valley Dev

12:181–187

6. Holcomb D, Rudnicki JW, Issen AK, Sternlof K (2007) Com-

paction localization in the Earth and the laboratory: state of the

research and research directions. Acta Geotechnica 2(1):1–15

7. Independent Panel to Review Cause of Teton Dam Failure (1976)

Report to United States department of the interior and the State of

Idaho on failure of Teton dam. United States Bureau of Recla-

mation, Denver, CO

8. Jaworski GW, Duncan JM, Seed HB (1981) Laboratory study of

hydraulic fracturing. J Geotech Eng Div ASCE 107(6):713–732

9. Kim H, Wangoner MP, Buttlar W (2009) Numerical fracture

analysis on the specimen size dependency of asphalt concrete

using a cohesive softening model. Constr Build Mater 23(5):

2112–2120

10. KomakPanah A, Yanagisawa E (1989) Laboratory studies on

hydraulic fracturing criteria in soil. Soils Found 29(4):14–22

11. Lo KY, Kaniaru K (1990) Hydraulic fracture in earth and rockfill

dams. Can Geotech J 27(4):496–506

12. Medeiros CH de AC, Moffat AIB (1996) A hydraulic fracturing

test based on radial seepage in the Rowe consolidation cell. In:

Advanced in site investigation practice. Thomas Telford, London

13. Mhatch HK (1991) An experimental study of hydraulic fracture

and erosion, Ph.D. thesis, City University, London

14. Mori A, Tamura M (1987) Hydrofracturing pressure of cohesive

soils. Soil Found Jpn Soc Soil Mech Found Eng 27(1):14–22

15. Murdoch LC (1993) Hydraulic fracturing of soil during labora-

tory experiments: methods and observations. Geotechnique 43(2):

255–265

16. Nobari ES, Lee KL, Duncan JM (1973) Hydraulic fracturing in

zoned earth and Rockfill dams, contract report TE-73-1. U.S.

Army engineers waterways experimental station, Vickburg MS

17. Satoh H, Yamaguchi Y (2008) Laboratory hydraulic fracturing

tests for core materials using large size hollow cylindrical spec-

imens. In: The 1st international symposium on Rockfill Dams,

Chengdu, China

18. Shams Rad S (2010) Numerical study of the hydraulic fracturing

in the core of earth dams, MSc thesis, Tarbiat Moallem University,

Tehran

19. Sherard JL (1986) Hydraulic fracturing in embankment dam.

J Geotech Geoenviron Eng ASCE 112(10):905–927

20. Wang SY, Sun L, Au ASK, Yang TH, Tang CA (2009) 2D-

numerical analysis of hydraulic fracturing in heterogeneous geo-

materials. Constr Build Mater 23(6):2196–2206

21. Wood DM, Maeda K (2008) Changing grading of soil: effect on

critical states. Acta Geotech 3(1):3–14

22. Yanagisawa E, KomakPanah A (1994) Two dimensional study of

hydraulic fracturing criteria cohesive soil. Soil Found 34(1):1–14

Acta Geotechnica

123