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Proceedings of the Institution of Civil Engineers

Geotechnical Engineering 166 February 2013 Issue GE1

Pages 49–55 http://dx.doi.org/10.1680/geng.10.00038

Paper 1000038

Received 17/03/2010 Accepted 21/01/2011

Published online 09/05/2012

Keywords: foundations/rock mechanics/site investigation

ICE Publishing: All rights reserved

Geotechnical Engineering

Volume 166 Issue GE1

Deformation parameters of bedrock at a

nuclear reactor site

Harikumar, Sivathanu Pillai and Chetal

Deformation parameters ofbedrock at a nuclear reactor siteC. Harikumar MTechScientific Officer, Civil Engineering Group, Indira Gandhi Center forAtomic Research, Kalpakkam, Tamilnadu, India

C. Sivathanu Pillai BE, MS, ADAssociate Director, Civil Engineering Group, Indira Gandhi Center forAtomic Research, Kalpakkam, Tamilnadu, India

S. C. Chetal DSDirector, Reactor Engineering Group, Indira Gandhi Center for AtomicResearch, Kalpakkam, Tamilnadu, India

Many engineering structures have to be founded deep into rock or on rocks, especially in a nuclear reactor

site. Numerical finite-element and boundary-element analyses for studies of the stress and displacement

distribution of any rock engineering projects are based on deformation parameters. Empirical relations are

available to relate the rock mass deformation modulus to index properties, but the equations include some

uncertainties relating to the variability of rock type and the heterogeneous nature of the rock masses. For

important structures, site-specific parameters have to be evaluated to assess strains and stresses in rock due to

various load combinations. Pressuremeter tests, cyclic plate load tests, uniaxial jack tests and seismic crosshole

tests were conducted to evaluate in situ design parameters. The objective of this paper is to evaluate

deformation parameters of a prototype fast breeder reactor from these in situ tests. For high strain

deformation modulus, the pressuremeter modulus is comparable with the rock mass modulus obtained from

laboratory results in association with rock quality designation. Crosshole tests results are used for evaluation of

low strain modulus.

1. IntroductionThe deformation parameter is the important engineering property

governing the behaviour of rock masses. Deformation modulus

and Poisson ratio are the representation of mechanical behaviour

of a rock mass. This is why most numerical finite-element and

boundary-element analyses for studies of the stress and displace-

ment distribution of any rock engineering projects are based on

these parameters (Palmstrom and Singh, 2001). Among these, the

deformation modulus has vital importance for the design and

successful execution of projects because the deformation modulus

is the best representative parameter of the prefailure mechanical behaviour of the rock material and of a rock mass (Kayabasi et

al., 2003). Many engineering structures have to be founded deep

into rock or on rocks, especially in a nuclear reactor site.

Empirical relations are available to relate the rock mass deforma-

tion modulus to index properties such as rock quality designation

(RQD) (Coon and Merritt, 1970) or to rock mass classifications

such as rock mass rating (RMR) (Bieniawski, 1978; Seram and

Pereira, 1983) and Q-system (Barton et al., 1974). Although the

empirical equations for the indirect estimation of the deformation

modulus are simple and cost effective, the equations include

some uncertainties relating to the variability of rock type and the

heterogeneous nature of the rock masses (Kayabasi et al., 2003).

For safety-related structures, site-specific parameters have to be

evaluated to assess strains and stresses in rock due to various load

combinations. Site-specific deformation parameters of a prototype

fast breeder reactor (PFBR) site are presented below from differ-

ent types of in situ tests.

2. Study areaKalpakkam is located about 65 km south of Chennai city, on the

east coast of peninsular India, which is a major nuclear complex

for India (Kannan et al., 2002). The PFBR is a 500 MW reactor.

The reactor is being constructed at Kalpakkam, close to the

existing pressurised heavy water reactors (PHWRs) (Chetal et

al., 2006). The PFBR is the first breeder reactor in India

intended for power generation on a commercial scale. The

nuclear island connected buildings (NICB) is one of the main

nuclear safety-related structures of the PFBR. It is a conglomer-

ate of eight structures, namely, the reactor containment building,two steam generator buildings, the fuel building, control build-

ing, radiation waste building and two electrical buildings, resting

on a common base raft. The size of the raft is 101 .5 m 3 93 m.

The base raft is unique in the NICB complex, owing to its

massive size and the complicated loading environment. Owing

to the functional requirements, the raft was founded about 18–

20 m below the existing ground level. After excavating up to the

desired founding level, it was established that the floor area

consists of medium- to coarse-grained, greyish-blue-coloured

charnockite. Charnockite rock is rich in the mafic (dark)

mineral, hypersthene. The name charnockite was defined by T.

H. Holland and it is an orthopyroxene-bearing granite, found in

southern India, of plutonic igneous origin (Holland, 1900). The

rock samples indicated that they are inter-layered and contain

garnet (at times phenocrysts), biotite, and muscovite and

phlogopite mica. Generally these charnockite rocks are in shades

of blue, grey and black. Spacing of joints is in the range of

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0.6–2 m and most of the joints are slightly weathered and rough. The rock’s depth persistence and lateral prevalence has

been established from deep boreholes (Harikumar et al., 2010).

The rock below the foundation floor exhibits massive structure,

although there are a few shallow fractures and joints. The entire

floor rock mass is 100% weathering grade I to III (BS 5930;

BSI, 1999). Average uniaxial compressive strength of the rock

mass is 106 MPa and RMR is 65.

3. Materials and methodsFigure 1 shows the soil profile of the site from geotechnical

borehole investigations. Finished grade level (FGL) is assigned as

elevation (EL) 30.00 m. The top 8–10 m comprises coarse to

medium dense sand, followed by 1–2 m thick, highly compres-

sible clay, then weathered rock and then bedrock. Bedrock is at

15–18 m below the existing ground level. Control blasting

(charges and delay time were controlled so that peak particle

velocity was restricted within allowable limits) was carried out

for the excavation of the rock to safeguard the nearby safety

structures from rock excavation vibrations. Some weathered rock

pockets were present at the founding level.

Weathered rock pockets were removed by further excavation and

refilling with plain cement concrete (PCC) of M15 grade

(characteristic compressive strength 15 N/mm2) after confirma-tory geotechnical investigations. After excavation, confirmatory

geotechnical investigations were carried out to confirm/modify

foundation design parameters assumed from preliminary investi-

gations. Investigations included boreholes 100 m deep, crosshole

seismic tests, static plate load tests, cyclic plate load tests, static

and cyclic permeability tests, concrete rock interface shear tests,

uniaxial jacking tests, block vibration tests and various laboratory

tests on soil/rock samples. In order to idealise soil for static and dynamic analysis, deformation modulus was established through

in situ tests.

Elastic and inelastic behaviour of the rock mass is defined

through the modulus of deformation (ISRM, 1975). The usage of

the term modulus of deformation, rather than modulus of

elasticity or Young’s modulus, is because of the coupled inelastic

and elastic behaviour of jointed rock masses. Four types of in situ

tests were carried out to evaluate deformation parameters.

3.1 Pressuremeter tests (PMTs)

The TRI-MOD_S pressuremeter was used for the borehole

determination of the modulus of deformation. NX size (76 mm)

boreholes were used for testing. Tests were carried out up to

20 MPa. Deformation modulus ( E m) was calculated over the

pseudo-elastic portion of the first loading cycle. The calculation

of the modulus is based on Lamé’s equation on the expansion of

the cylindrical cavity in an elastic medium, ASTM D 4719

(ASTM, 2007)

E m ¼ 1 þ ð Þ ̃P

̃R= Rð Þ1:

where E m is the deformation modulus, is Poisson ratio, ̃P isincrease in pressure and ̃R/ R is relative change of radius.

The probe was placed at the test depth in a predrilled borehole. A

stress-control mode was used to run the test. Equal increments of

pressure were applied to the probe and held constant for 1 min.

The diametric changes were logged 30 s and 60 s after each

pressure step was reached. In situ stress– strain curves were

obtained by plotting the changes in each of the three instrumented

diameters or their average against pressure.

The deformation modulus results derived from pressuremeter

tests are presented in Figure 2. The mean value is 8480 MPa and the 95% confidence range is also shown in Figure 2. The mean

value of modulus does not alter significantly with depth.

3.2 Cyclic plate load tests (CPLTs)

A 25 mm thick, 600 mm diameter plate was used on an initially

cleaned plane surface of charnockite for these tests. 450 mm and

300 mm stiffener plates were placed above the 600 mm diameter

plate. The jack base was placed above the stiffener plate. Loading

column and packer were placed on the top of the ram so that the

gap between the main girders and the ram was neatly packed.

The load was applied by means of an hydraulic jack against a

kentledge of suitable height.

The kentledge had cribs in the form of stacked sand bags, over

which a platform was built up with rolled steel beams in such a

manner that its centre of gravity was coaxial with the centre of

the plate. The overall weight was about 150 t. The maximum test

0 20 40 60 80 100 1200

2

4

6

8

10

12

14

16

18

20

22

2426

28

30

Existing ground level

FGL

Rock

Weathered rock

Clay

Sand

DepthEL:m

East–West direction: m

Figure 1. Subsurface cross-section of the site

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pressure was 320 t/m2 and this was reached in five cycles; during

each cycle the load increment was one-eighth of the peak load of

that cycle. For each loading settlement the peak load was recorded

at an interval of 1, 2.25, 4, 6.25, 9.15, 25 and 60 min, and

thereafter at hourly intervals up to a time when the rate of

settlement was recorded at a value of 0.2 mm/min. Settlements

were observed by means of four dial gauges with 25 mm travel

and least count of 0.01 mm. Settlement gauges were held down by

datum bars resting on immovable supports at a distance of 1 .5 m

from the plate. The deformation modulus was calculated (ASTM

D 4394; ASTM, 2004) as

E m ¼ 1 2

P

2 R2:

where is the Poisson ratio of the rock, P is the total load on the

rigid plate, is the average deflection of the rigid plate and R is

the radius of the rigid plate.

The stress deformation pattern from the CPLT is shown in Figure

3. The deformation modulus is shown Figure 4.

3.3 Uniaxial jack test (UJT)

For conducting a uniaxial jacking test on the rock mass, the

size of the drift or gallery or niche in which the test is to be

carried out should be kept as small as required for testing, so

as to reduce the number of packing plates. A niche of size

1.25 m 3 1.25 m 3 2.2 m was made on the NICB area. During

the excavation, the disturbance of the rock was minimised.

The final trimming of the rock provided two parallel, con-

centric and smooth rock faces by means of chisel and

hammer.

Two 25 mm thick, circular steel plates of 600 mm diameter were

placed on either side of the pit. The test location was saturated

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

100

80

60

40

20

0

0

Pressuremeter modulus: MPa

Lower 95% confidence limit

Upper 95% confidence limit

Linear fit

Depthfrom

floor:m

Figure 2. Deformation modulus from PMTs

0·5 0 0·5 1·0 1·5 2·0 2·5 3·0 3·5 4·0

0·5

0

0·5

1·0

1·5

2·0

2·5

3·0

3·5

Stress:MPa

Deformation: mm

% UJT% CPLT

Figure 3. Stress deformation curve from UJT and CPLT

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with water to simulate seepage at the site. A hydraulic jack of

200 t capacity was used. The test was conducted in five cycles.

Each cycle consisted of 20% of the of maximum test stress. Ineach cycle of loading the load increment was one-fifth to one-

tenth of the test load involved in each cycle. Unloading was done

in three steps. Equation 2 was used for evaluating the deforma-

tion modulus. The deformation pattern of rock under UJT is

shown in Figure 3 and the deformation modulus is shown in

Figure 4.

3.4 Seismic crosshole test

A 24-channel signal enhancement seismograph was used for this

study. The main accessories were three component borehole

geophones, land geophones and geophone cables. The three

geophones were mutually perpendicular: one geophone was placed in the vertical direction and the other two geophones

were placed horizontally. Instantaneous electrical charges were

used to generate seismic waves in the crosshole studies at every

depth. Boreholes were 76 mm (NX size) and 60 m deep and

consisted of one source borehole and four receiver boreholes

(RBH1, RBH2 and RBH3, RBH4), two each in orthogonal

directions to obtain wave velocities in both directions, as shown

in Figure 5.

The distance between boreholes was 3 m. Since the seismic

geophysical tests induce shear strains lower than about

3 3 104%, the measured shear wave velocity can be used to

compute the largest value of shear modulus G max as (Kramer,

2005)

G max ¼ rV 2s3:

E dm ¼ 2 1 þ ð ÞrV 2s4:

where G max is maximum shear modulus, V s is shear wave

velocity, r is total mass density, is Poisson ratio and E dm is

dynamic modulus.

The results of shear wave velocity (V s) and compression wave

velocity (V p) are given in Table 1. The dynamic deformation

0

100

200

300

400

500

600

UJT: MPa 415·626 194·762 152·754 153·869 183·314

CPLT: MPa 561·647 356·089 250·194 265·935 261·104

Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5

Figure 4. Deformation modulus from UJT and CPLT

SourceBH

3·0 mRB4

3·0 mRB5

3·0 m

RB1

3·0 m

RB2

Figure 5. Borehole set-up for crosshole test

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modulus was calculated using Equations 3 and 4, and the average

value was 55 459 MPa.

3.5 Laboratory investigations

Deformation modulus and Poisson ratio were evaluated from

uniaxial compression tests by measuring circumferential and axial

strains. The tangent modulus of the stress–strain curve was taken

for evaluating deformation modulus and Poisson ratio at 50% of

ultimate stress. This test was also a high-strain test. The Young’s

modulus ranges from 72 000 MPa to 92 000 MPa. The average

Poisson ratio evaluated from crosshole test and laboratory test is

0.26. This value was used in the interpretation of in situ tests.

4. DiscussionThe deformation modulus results derived from the CPLT and UJT

were found to be 15–20 times lower when compared with the

PMTs. This is due to the microcracks formation during the

control blasting for rock excavation and also to the lesser

confinement of rock mass. The PMT also shows lower modulus at

the upper region.

The dynamic deformation modulus was calculated from crosshole

test results. This is a low strain modulus. At the lower strain

levels, the stress–strain behaviour will be linear, but at the higher strain levels, behaviour will be non-linear by experiencing plastic

deformation. Thus, the modulus value is higher than the deforma-

tion modulus from PMTs. This value was used for modelling

soil/rock under dynamic excitation cases, whereas the PMTs and

the laboratory tests for modulus of elasticity gave high-strain rock

properties, which were used for static analysis.

Table 2 shows laboratory test results of the deformation modulus.

The deformation modulus was evaluated in the laboratory using

rock core samples and the value is obviously higher than the

PMT results because of non-representation of actual field condi-

tions. It was noted that to obtain deformation properties of the

intact rock mass from laboratory test results, the rock mass

inherent properties had to be used. RQD is the commonly used

parameter to represent rock mass continuity, so RQD is widely

used for estimating rock mass deformation modulus (Zhang and

Einstein, 2004). The average RQD of the rock below the

foundation floor is 85%. Gardner (1987) proposed the following

relation (Equation 5) for estimating the rock mass deformation

modulus E m from the intact rock modulus E r by using a reduction

factor ÆE, which is related to RQD

E m ¼ ÆE E r

Æ E ¼ 0:0231 RQD 1:32 > 0:155:

Zhang and Einstein (2004) provides a modified relation (Equation

6) recommended for estimating the lower bound rock mass

deformation modulus

E m

E r ¼ 0:2 3 10 0

:0186 RQD1:91ð Þ

6:

Since the rock mass evaluation was conducted for the foundation

Depth: m V p: m/s V s: m/s

1 2934 1786

10 5044 2902

20 5934 3005

30 5782 3106

40 5176 2918

50 4643 2783

60 4455 2725

Table 1. Shear wave velocity and compression wave velocity from

floor level

Depth:

m

Young’s

modulus, E r:

MPa

ÆE (Gardner,

1987)

E m:

MPa

ÆE (Zhang and

Einstein,

2004)

E m:

MPa

28.92 92 000.00 0.64 59 202.00 0.09 8626.17

31.20 84 000.00 0.64 54 054.00 0.09 7876.06

37.05 92 000.00 0.64 59 202.00 0.09 8626.17

49.90 80 000.00 0.64 51 480.00 0.09 7501.01

50.20 87 000.00 0.64 55 984.50 0.09 8157.35

66.75 88 000.00 0.64 56 628.00 0.09 8251.12

78.79 70 000.00 0.64 45 045.00 0.09 6563.39

78.82 89 000.00 0.64 57 271.50 0.09 8344.8888.38 89 000.00 0.64 57 271.50 0.09 8344.88

90.55 78 000.00 0.64 50 193.00 0.09 7313.49

Table 2. Laboratory results for Young’s modulus

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of a nuclear reactor, the lower bound value is considered for design. Also, it can be seen from Figure 2 and Table 2 that the

deformation values from corrected laboratory test results are in

the range of pressuremeter modulus. These results also indicate

that the values obtained from PMTs are reliable.

4.1 Strain dependence on modulus

Strain dependency on modulus was studied. Since the CPLT and

UJT were conducted at the excavated floor level, results from

PMTs and crosshole tests near the floor (at 1.5 m depth) only

were compared. The crosshole test is a low-strain test (106) and

the PMT is a high-strain test (0.03–0.055). Low-strain tests show

higher modulus (Figure 6) compared to high-strain tests.

Figure 6 also shows variation of deformation modulus with strain

level for UJT and CPLT (decrease in modulus with increase in

strain level). Figure 7 shows that the plastic deformation was far

larger than the elastic deformation for both CPLT and UJT. This

excess plastic deformation also indicated that microcracks were

formed during rock blasting. Pressuremeter modulus at shallow

depth (nearly 1 m from the floor) indicated that this microcracks

effect extended only less than 1 m.

4.2 Deformation modulus for design

From the above discussion it was concluded that pressuremeter

modulus was more appropriate for this type of massive rock. TheRQD value of the rock mass also shows that the PMT results

were more reliable. The loading areas for plate load tests and

UJTs were very small compared to the raft foundation. Addition-

ally, the influence depth of the stress arising from applied

pressure depends on the size of the loading area. Microcracks of

the very top layer only were represented in these two tests. The

massive rock stratum which was to carry the foundation load was

not represented in the surface tests, so pressuremeter modulus

was considered for high strain modulus and shear wave velocity

modulus was considered for low strain modulus.

5. ConclusionThe deformation parameter is the important engineering property

governing the behaviour of rock masses. Although the empirical

equations for the indirect estimation of the deformation modulus

are simple and cost effective, the equations include uncertainties.

For safety-related structures, site-specific parameters have to be

1 10 3

0·01

1 10 7

1 10 6 1 10

51 10

41 10

31 10

2 1 10 1

0

10000

20000

30000

40000

50000

0

100

200

300

400

500

600

Deformationmodulus,

M

Pa

Em:

Strain

UJT

CPLT

Pressuremeter test

Crosshole test

Figure 6. Strain plotted against deformation modulus

00·51·01·52·02·53·03·54·0

Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 50

0·51·01·52·02·53·03·54·0

(b)

Deformation:mm

(a)

Elastic deformationTotal deformation

Figure 7. Elastic and plastic deformation of (a) CPLT and (b) UJT

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evaluated to assess strains and stress in rock due to various load combinations. Owing to the formation of microcracks from the

control blasting for rock excavation and also because of the lesser

confinement of the rock mass, lesser values were reported from

CPLTs and UJTs. PMTs indicate reasonable values of deforma-

tion modulus. This is comparable with rock mass modulus

obtained from laboratory results in association with RQD. Since

the crosshole test is a low strain test, the modulus obtained from

it is higher than from the PMTs. The pressuremeter modulus was

used in modelling soil for static analysis and the crosshole test

modulus was used for dynamic analysis.

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