Optimization of Support System for Desilting Chambers of NJPC using Discontinuum Modeling

Optimization of support system for desilting chambers of

Nathpa Jhakri hydroelectric project using discontinuum

modeling

R. Venugopala Rao, GD Raju, R N Gupta, National Institute of Rock Mechanics

R. S. Chauhan, GM, SJVN

ABSTRACT: Stress analysis using numerical modeling is widely accepted for design

of underground excavations due to their ability to incorporate most of the

complexities and their reliability to produce realistic information. This paper

presents the results of stress analysis carried out to bring out the effect of Adit-II. The

effect of discontinuities and the intersection of Adit -II with chambers were brought

out in these studies. Actual jointing details like dip, dip direction and locations

supplied by the project authorities were simulated in the analysis explicitly. The

shotcrete and Rock bolts were able to reduce the deformations to a small extent only

as the existing support system was inadequate. Strengthening of the support systems

in the walls of the chambers may be required near the Adit -II junctions to an extent

of 20m on either side of the Adit -II walls. As the additional support system

consisting of cable anchors has little effect, longer bolts of 12m length were

recommended. As seen from the performance of the bolts, majority of the yielded

bolts exist in the vicinity of Adit -II region. The spacing of the bolts may be reduced

to 1.2 m on either direction to reduce their percentage of failure.

1. INTRODUCTION The desilting chamber arrangement at NJPC project consists of 4 parallel chambers

of 525m long with cross section of 17x27m (modified horse shoe shape) separated by

46.5m center to center. The rock cover varies from 150 to 490m. The rock is

predominantly augen gneiss with 5 sets of joints and several shear seams. These

chambers have access through two numbers of D shaped Adits -I & II

The final support system for desilting chambers consists of 6m long 32mm

diameter fully grouted rock bolts at 1.6x2m spacing in the roof and at 2x2m in the

walls along with shotcrete of 50mm in the roof and 100-150mm in the walls. These

support measures were simulated (Dasgupta, 1999) in stress analysis using FLAC

(Fast Lagrangian Analysis of Continuum) software. He reported that the failure

zones would extend up to 1m in the roof and 4-5m in the walls and the forces

induced in the support system are with in their safe limits

During October 1999, 100m3 of rock fall was observed at the junction of Adit 2

with Chamber II. Similar rock falls were observed at the junctions of Chamber III

and IV during May 2005. The Panel of Experts for the projects recommended a fail

safe system of supports consisting of 3 rows of 20m long cable anchors of 60T

capacity at 7.5m spacing in the walls. National Institute of Rock Mechanics was

requested to review the support system suggested by POE and its adequacy.

This paper presents the results of for analysis carried out to bring out the

effect of Adit-II on the walls of chambers 2 and 3. The effect of discontinuities and

the intersection of Adit -II with chambers were brought out in these studies. Actual

jointing details like dip, dip direction and locations supplied by the project

authorities were simulated in the analysis explicitly. The joint samples were

collected and tested in the laboratory for their stiffness and strength properties.

2. COMPUTATIONAL MODEL A large model consisting of 150mx150x150m (shown in Figure 1) was considered for

simulation. Since the area of interest was quite large and finer mesh was needed, two

chambers were modeled at each instant. The model was truncated at the top, with

rock cover of 100m above the cavern considered for discretization, and rest of the

topography was simulated as applied stress. In the longitudinal direction, chambers

of 150m long (around Adit 2) were simulated, but the results were considered

representative for middle 100m only, as the boundaries of the model were

restrained.

The effect of discontinuities and the intersection of Adit -II with chambers are

brought out in these studies. Actual jointing details like dip, dip direction and

locations supplied by the project authorities are simulated in the analysis explicitly.

The joint samples were collected and tested in the laboratory for their stiffness and

strength properties.

The excavations were simulated in two stages. The analysis was carried out

without support system and with support system consisting of shotcrete of 150mm

and rock bolts of 32mm diameter of 6m length at 1.5m spacing and 12m long rock

bolts near adit-2 region. Cable anchors of 20m long and 50t capacity at 7.5m spacing

were simulated.

2.1 Physico-mechanical parameters The physico-mechanical properties of the rock mass and the shear zones are

Youngs Modulus = 13.3 GPa

Poissons ratio = 0.23

Density = 2650 kg/m3

Mohr - Coulomb Parameters

C of rockmass = 1.3 MPa

f of rock mass = 45

Joint Properties

Average Shear Stiffness JKs = 1.0 GPa/m

Normal Stiffness JKn = 40.0 GPa/m

Cohesion = 0.3 MPa

Friction = 25

2.2 Insitu Stresses The insitu stresses (principal stresses) were determined from hydraulic fracturing

test. The in-situ stresses, in terms of vertical (Sv) and in the longitudinal (SH) and

transverse (Sh) direction of the chambers, and at different depths were given below.

Cover Sv MPa Sh MPa SH MPa

310 8.21 6.11 8.89 410 10.86 7.69 10.77 490 12.97 8.94 12.27

3. THE RESULTS AND DISCUSSION The results in terms of displacements and factor of safety values at different

elevations and cross sections are presented in the following sections. The analysis

will yield the results in terms of displacement and stresses at each tetrahedral zones.

The factor of safety can be computed at any point as follows

Let s1, s3 are the major and minor principal stresses then major principal

stress at failure can be computed as

s1f = s3 tan 2 (45 + f/2) + 2c tan (45 + f/2)

Where c and f are Mohr-Coulomb parameters for rock mass.

Then factor of safety can be computed as FOS = s1f/s1

The principal stress vectors, colored by the values of factor of safety (of less

than 1.2), are shown in the figures.

3.1 Convention used in Figures The results displayed in figures in the following sections depict the displacement

vectors or stress vectors, colored factor of safety values over a plan view, or over a

cross section or over a longitudinal section. The geometrical scale indicates the

geometry and the value gives in meters. The vector scale indicates the scale for

measuring the displacement vectors and is given in meters. The stress scale indicates

the scale for measuring the stress magnitude in MPa. The color contours indicate the

range of values. The dip and dip direction indicate the orientation of the plane (cross

section) being viewed. The center of the plane passes through the coordinates given.

The magnification of the figure is indicated by the value mag. The maximum

stress or displacement observed over the cross section is shown at the bottom.

The results in terms of displacements, factor of safety values at different cross

sections were presented in the following sections. To understand the rock mass

behavior due to further excavation, two stages of excavation were considered, the

present stage and complete excavation.

3.2 Displacement Vectors The displacement vectors over cross section near the wall of Adit 2 without support

and with rock bolts and with rock bolts and cable anchors are shown in Figures 2, 3

and 4 respectively. Maximum displacements of the order of 76mm are observed at

this location in all three cases. The supports have had little effect on the rock mass

behavior.

3.3 Factor of Safety Values The factor of safety values in the rock mass over the cross sections through Adit 2

center and at 10m from the walls of Adit 2 are shown in Figures 5 and 6 respectively.

The rock mass failure was observed through out the pillar near the Adit 2. The extent

failure of the rock mass in the pillar between the chambers was less than 10m from

the walls of the chambers in the reaches away (15m from either wall) from the Adit -

II walls.

3.4 Efficacy of Support System The efficacy of the support system can be gauged by the forces induced in the

supporting elements. The following tables depict the summary performance of the

rock bolts with incorporation cable anchors. It can be seen from the table only 9% of

bolts exceeded the 30T capacity and 20% of bolts exceeded 25T for entire reach. The

concentration of failed supports can be gauged from the Table 2. It can be seen that

62% of yielded bolts exist in the vicinity (5m on either side) of Adit-II and 80% of

bolts exceeding 25T capacity exist in the vicinity (5m on either side ) of Adit-II.

Table 1. Performance of Rock Bolts

% Total bolts reached 30T Capacity % Total bolts exceeded 25T Capacity

9.8 20.6

Table 2. Locations of yielded bolts

% yielded bolts near Adit II region % bolts exceeding 25T near Adit II

region

62 80

4. CONCLUSIONS Based on the stress analysis with and without support systems, following

observations can be made

1. The rock bolts and cable anchors have little effect in reducing the

deformation around the excavation

2. The entire rock mass between the chambers II and III near the Adit -2

reached failure.

3. The extent failure of the rock mass in the pillar between the chambers was

less than 10m from the walls of the chambers in the reaches away (15m

from either wall) from the Adit -II walls.

4. The majority of rock bolts yielded even in the presence of cable anchors in

Adit 2 region, and the entire pillar reached failure, longer rock bolts of

12m length at 1.2m spacing are recommended.

5. ACKNOWLEDGMENTS The authors wish to thank the project authorities, particularly, Mr Y. N. Apparao,

CMD, Mr. Ranjodh Singh (design consultant), Mr. R. S. Chuahan, AGM, We wish to

thank Mr. S. P. S. Chauan, Director, CWC and officers of GSI for their advice and

suggestions during numerous discussions. The advice given by POE members is

gratefully acknowledged.

Figure 2. Displacement Vectors over Cross section near Adit Wall without Support System

3DEC (Version 2.00)

3D-Discontinuum Analysis of Desilting Chambers of NJPC with Actual Geomtry

dip= 40.00 abovedd = 150.00center 1.250E+02 5.000E+01 3.250E+02cut-pl. 0.000E+00mag = 2.00 cycle 0

25-Apr-04 11:41

Figure 1. Model Showing Chambers II and III and Adit 2

3DEC (Version 2.00)Three Dimensional Discontinuum Analysis of Desilting Complex - with 13m Trace Length & without Supports

Numerical Modeling DivisionNational Institute of Rock MechanicsKolar Gold Fields - 563 117

Maximum Displacement = 7.583E-02

vector scale

0 5E-01

0.000E+00 1.500E-02 1.500E-02 3.000E-02 3.000E-02 4.500E-02 4.500E-02 6.000E-02 6.000E-02 7.500E-02 7.500E-02 9.000E-02 min maxinterval = 1.500E-02 Color by Disp mag.


2-Sep-01 21:05

geometric scale

Cross section plot:

0 2E+01

Figure 3. Displacement Vectors over Cross section near Adit Wall with Rock Bolts

Figure 4. Displacement Vectors over Cross section near Adit Wall with Rock Bolts and Cable Anchors

3DEC (Version 2.00)Three Dimensional Discontinuum Analysis of Desilting Complex - with 13m Trace Length & Support System



vector scale

0 5E-01



3-Sep-01 10:14

geometric scale

Cross section plot:

0 2E+01

3DEC (Version 2.00)Three Dimensional Discontinuum Analysis of Desilting Complex - with 13m Trace Length & Cable Anchors



vector scale

0 5E-01



7-Feb-** 15:50

geometric scale

Cross section plot:

0 2E+01

Figure 5. Factor of Safety Values over Cross section through centre of Adit with Rock Bolts

Figure 6. Factor of Safety Values over Cross section at 15m from Adit Wall with Rock Bolts



Max Compressive Stress = -8.996E+01

vector scale

0 5E+02

0.000E+00 2.400E-01 2.400E-01 4.800E-01 4.800E-01 7.200E-01 7.200E-01 9.600E-01 9.600E-01 1.200E+00tension = 0.000E+00 cohesion = 1.300E+00 phi = 42.00 Color by Mohr-C FS


13-Jul-01 11:18

geometric scale

Cross section plot:

0 2E+01



Max Compressive Stress = -3.635E+02

vector scale

0 2E+03

0.000E+00 2.400E-01 2.400E-01 4.800E-01 4.800E-01 7.200E-01 7.200E-01 9.600E-01 9.600E-01 1.200E+00tension = 0.000E+00 cohesion = 1.300E+00 phi = 42.00 Color by Mohr-C FS


3-Sep-01 9:37

geometric scale

Cross section plot:

0 2E+01

Documents

Optimization of Support System for Desilting Chambers of NJPC using Discontinuum Modeling