Rock Structure in Kemang Dist of AP

Abstract

In the present study, heterogeneous rock masses strength characterization for brittle deformation regime are

analyzed from Hoek-Brown Failure criterion by using a simulated model with suitable RMR values. To reduce the

material constants óci and mi, for Hoek - Brown system for rock masses is accomplished through the Geological

Strength Index (GSI). Using these values failure envelope range for ó3max is around 33 MPais estimated for

general geotechnical applications, with 2.0341 MPa c (cohesion intercept) and 12.4186 degrees ö (friction angle

for the failure envelope) for Mohr-Coulomb envelope. Other parameters associated with Hoek-Brown Failure

criterion like ót (-0.00494332MPa), ó

c (0.181273MPa), ó

cm (5.06151MPa) and E

rm (917.074MPa) are also

determined.

Key words: stress, rock strength parameters, GSI.

INTRODUCTION

The study area is located in Sub Himalayas in Bhalukpong-Tipi-Pinjoli section West KamengDistrict, Arunachal Pradesh, India. It lies between 27o00'- 27o10' N and 92o30'-92o40' E (Figure

1). This fold-thrust belt has uplifted the Neogene[1] sediments deposited in the foreland basin inthe southern frontal part of the orogeny. Siwaliks are consisting of sandstone, siltstones, clay and

boulder beds of Mio-Pliocene age[2,3] and are considered as thin-skinned tectonic fold-thrust beltsandwiched between Main Boundary Thrust (MBT) and Himalayan Frontal Thrust (HFT), driving

by late HOM (Himalayan Orogenic Movement) activities. The rock groups exposed in this partof the area exhibits brittle deformational regime in massive, hard, bedded sandstones of Middle

Siwalik rocks as called Dafla Formation[1] within a lateral distance of about 6km along. Totalwidth of Siwalik group of rock in this part of Arunachal Himalaya is about 9km. Siwalik Group is

10-20km in width in East Bhutan and in Arunachal Pradesh[2]. This fold-thrust belt is featuredby many major and sub thrust systems of MBT, Tipi Thrust and HFT. These thrust are sub splays

of a major detachment present about 5km[3] below surface and this fold-thrust belt have beenshortened at a rate of about 16- 20 mm/yr[3,4]. Middle and Upper Siwalik rocks exist in between

these thrust zones. Middle Siwaliks are known as Dafla Formation and Upper Siwaliks are Subansirand Kimin Formation[1]. Lithological constituents of Dafla Formation are mainly hard well bedded

ESTIMATION OF ROCKS STRENGTH PARAMETERS FOR MIDDLE

SIWALIK, WEST KAMENG DISTRICT, ARUNACHAL PRADESH

MANASH PRATIM GOGOI

Department of Geology, Sibsagar College, Joysagar-785665, Assam

(E-mail: [email protected])

ISSN: 2249-9903

Journal of Frontline Researchin Arts and Science

Vol. 01 (2011) : 94-99, Research Paper

94 Journal of Forntline Research/01/94-99

sandstones of variable thickness with alternating shale and clay layers. Fracture surfaces and jointsare found associated with these rocks throughout the area. Variation of joint spacing and pattern

etc. (Figure 2.b) depends on various factors like nature of stress field, types of lithologies, andstrength and deformation characteristics of rock masses. The layer competency contrast existing

between Dafla sandstones and shale layers exhibiting a differential accommodation of stress andshows brittle and brittle-ductile deformation (Figure 2.a). These rock groups have been deformed

into small poorly interlocked seams of angular and irregular fragments in trust zone area. In someplaces rocks have developed cataclastic flow and shear fabrics due shear parallel slips in rock

bodies. However, sheared shale and clay layers with deformed sandstone layers are a commonscenario in the vicinity of the study area.

Figure 1: SRTM-DTEM (Digital Terrain Elevation Model)[12]

showing the locations of the study area. Highlighted area

showing Middle Siwalik rocks and their distribution

marked by two major thrust zones, MBT-Main Boundary

Thrust and Tipi Thrust.METHODOLOGY

Geotechnical analysis is important for many respects in geological constraints that reflectedin the geological information about estimates of the strength and deformation characteristics of

rock masses. As the regional geological settings of the area are mentioned earlier, hence someapplied prospects are also added in the study to take account of the information in civil and

general engineering expeditions. In the year 1980, Hoek and Brown proposed a method toestimatestrength of jointed rock mass[5,6]. This criterion is utilized to assess the strength for the

M. P. Gogoi


Dafla formation under differential stress field in a computer simulated model whose inputrequirements were selected accordingly as per the laboratory analysis. The relationship between

Mohr envelope for Hoek Brown Criterion[5] and fitted linear relationship (Equation 2) trendswere graphically reproduced (Figure 4).

The static equilibrium state under confining overburden stress P, is the state of non-deviatoric

(i.e. P = σ1= σ2=σ3=0) [7] should be overcome.

P = ρgz[8] (1)

Where, ñ is the mean density, g is the acceleration due to gravity and z is the depth of burial.

However, approximate relationship can be determined by a RocScience (www.rocscience.com)

module RockLab v1.0, which works on principles of Hoek-Brown Failure Criterion[5] and Empiricalestimation of rock mass modulus[6]. The programme is used for understanding the the Generalized

Hoek-Brown[5] strength parameters of a rock mass (mb, s and a)[5] practiced for geotechnical,mining and civil engineering applications. It is a simple and intuitive implementation of criterions.

There is an uncertainty in determining rock strength of in situ condition, hence we applied thissimulated approach as it is applicable considering required input data for the lithology and extent

of deformation exhibiting by the Dafla sandstones successively accomplishes the considerations.Mohr-Coulomb criterion[5] is a two-parametric criterion, assumes that a shear failure plane is

developed in the rock material. It takes shearing into account. It considers the major and minorprincipal stresses only (the two principal stresses making the largest difference). Plot the Hoek-

Brown failure envelope in principal and/or shear-normal stress space requires consideration ofchange ó

ci, GSI, mi, D, to see how the failure envelope changes with each parameter. However, 5

input parameters[5] (óci, mi, GSI, D and Ei), can be conveniently estimated from built-in charts

(Figure 3), based on rock type, geological conditions. The shear strength can be defined as[5] :

s =c+σtanφ (2)

Where, c and ö = cohesion intercept and friction angle for the failure envelope and ó = total

normal stress on the failure plane. The Hoek-Brown criterion is introduced to provide input datafor the software base rock strength analyses, derived for brittle failure of rock mass. Although the

criterion initiated from intact rock but implemented for deformed rock mass by introducingappropriate reducing values of properties (Figure 3). Selection of input data depends on the

geological observations by means of one of the available RMR classification[8]. Generalised equationof the Hork-Brown criterion is[5] :

σ1` = σ3` + σci (mb + s)0.5 (3)

Where ?1` and ?3` are the major and minor effective principal stress at failure. Determinationis mb is defined by formula[5] -

mb = miexp ( ) (4)

Where, GSI is Geological Strength Index[9], s and a are constants.

Est. of Roc. Stre. Para. for Middle Siwalik, West Kameng District, Arunachal Pradesh


Figure 2: Dafla Formation consisting of alternating bedded sandstones with shale/clay layers.

Rock strenght of sandstone decreases due to presence of incompitent layers (shales),

however, extend of deformation in the rocks severely affects the rock properties: (a)

development of flexlural folds with numerous orthogonal joint sets due to layer

parallel slips and extension in alternating sandstone and shale bands, (b) variation

in joint spacing is clearing observablefor different thickness of sandstone beds.

Figure 3: RocScience module RockLab v1.0[13], built-in charts for Geological Strenght Index (GSI) values[9] have

distinction from Intact rock strength (IRS) values. These reduced value of GSI capable of analyzing rock

strength from brittle deformation regime.

M. P. Gogoi


Figure 4: Graphical relationship between ó1 - ó

3 (Max-Min Principal Stress) and ó

s - ó

n (Shear and Normal Stress).

MC envelope for Hoek-Brown criterion of brittle failure is over lined with fitted linear equation. ci is the

tensile strength of the material also called the instantaneous cohesion.

The modulus of deformation[5] Em is expressed as -

Em (GPa) = (1 - ) √( ).10 ((GSI-10)/40)

(5)The relationship between determination of ó`max and Mohr-Coulomb parameters varies

upon the specific applications and geotechnical approach. Rock mass strength is also dependsupon the "Disturbance Factor"[5] varies for lateral confinement for slop plan w.r.t. their height forwhich frictional angle (ö) and cohesive strength (c`) is considered.

RESULTS AND DISCUSSION

The confining pressure of sediments deposited in this foredeep at a depth range of 2km to5km, i.e. the probable location of MFT detachment[3] may indicate a range from 0.52 to 1.32GPa.Considering standard geological strength index (GSI)[9], disturbance factor (D)[5]; deformationmodulus (Ei)[5], and other material constants ó

ci and mi shows that the cohesion intercept (c) for

fitted linear relationship (equation 2) is 2.0341 MPa and the angle of friction (ö) for the failureenvelope12.4186 degrees. However the ö value may increase to 19.36 degrees if GSI (of about 66)value for 'blocky' well interlocked undisturbed rock mass is considered. Lower value of ö is becauseof lithological contrast and selection of reduced GSI value. The best-fit Mohr-Coulomb strengthenvelope is related with the cohesion and friction angle. Depending upon lithology of the studyarea, cohesive strength is variable of layer parallel friction. Mohr-Coulomb strength and it fitted

Est. of Roc. Stre. Para. for Middle Siwalik, West Kameng District, Arunachal Pradesh


linear trend (Figure 4) shows a linear incremental variation between ó1 and ó

3. However the ratio

between the shear stress and normal stress also suggest a linear relationship for the material. Theintercept ci (also known as instantaneous cohesion) is the transitional-tensile regime required toattain tensile stress necessary to induce tensile failure may be represented by the point, the tensilestrength (Figure 4). The envelope is the plane of orientation on which shear fractures can developwithin the rock in a state of stress. Instantaneous points on envelope gives c and ö values for specificó

1 and ó

3 from which the orientations of differently oriented conjugate shear planes can be determined.

CONCLUSION

Rock strength characterization for determination of specific parameters can be analysed bysimulation method. These strength parameters provides the relationship between shear and normalstress field and essential in engineering and geotechnical investigations for the construction ofbridge, dam, roads, houses, retaining walls. However, Bhalukpong-Bomdila road section connectsAssam and Tawang of Arunachal Pradesh[11]. Habitation practice, construction of roads and smallbridges in numerous places etc. in this part required proper geotechnical analysis of the rocks,especially in construction of earthquake retaining buildings. Taking in consideration of intensebrittle deformation in sedimentary rock groups of in this part eventually required proper geologicalmapping, structural and lithological demarcation and determination of rock strengths so that riskof failure of the objects can be estimated.

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and prospects, Himalayan Geology. 15: 3-21.3. Kelty T.K., Yin A., and Dubey C.S. 2004. Structure and crustal shortening of the Subhimalayan fold and

thrust belt, Western Arunachal Pradesh, NE India, Himalayan Journal of Sciences. 2 (4): p. 175.4. Powers P.M., Lillie R.J. and Yeats R.S. 1998. Structure and shortening of the Kangra and Dehra Dun

reentrants, Sub-Himalaya, India, Geological Society of America Bulletin, 110(8): 1010-1027.5. Hoek E., Carranza-Torres C.T. and Corkum B. 2002. Hoek-Brown failure criterion, Proceeding North

American Rock Mechanics Society meeting in Toronto. (http://www.rocscience.com)6. Hoek E. and Diederichs M.S. 2006. Empirical estimation of rock mass modulus, International Journal of

Rock Mechanics and Mining Sciences. 43: 203-215.7. Ragan D.M. 1973. Structural Geology: An Introduction to Geometrical Techniques, Wiley, New York. Pp.

208.8. Bieniawski Z.T. 1989. Engineering rock mass classification. Wiley, New York. Pp. 251.9. Hoek E., Marinos P. and Benissi M. 1998. Applicability of the Geological Strength Index (GSI) classification

for very weak and sheared rock masses. The case of the Athens Schist Formation. Bulletin of EngineeringGeology and Envionment. 57(2):151-160.

10. Marinos P. and Hoek E. 2001. Estimating the geotechnical properties of heterogeneous rock masses such asflysch. Bulletin of Engineering Geology and the Environment. 60: 85-92.

11. Bhattacharjee S. and Nandy S. 2008. Geology of the western Arunachal Himalaya in parts of Tawang andwest Kameng districts, Arunachal Pradesh. Journal of Geological society of India. 72: 199-207.

12. Jarvis A., Reuter H.I., Nelson A. and Guevara E. 2008. Hole-filled SRTM for the globe Version4, availablefrom the CGIAR-CSI SRTM 90m Database. (http://srtm.csi.cgiar.org)

13. RocScience module RockLab v1.0. (www.rocscience.com)

Manuscript Accepted : 18 Nov., 2011

M. P. Gogoi


Rock Structure in Kemang Dist of AP

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