Blasting Problems in Underground Constructions through Deccan Trap Formation: Some Experiences at Koyna Hydro-Electric Project, Stage IV
A. K. Chakraborty. V. M. S. R. Murthy and J. L. Jethwa
Abstract--The underground excavations of Koyna Hydro-electric Project, Stage IV were carried out by drilling and blasting through Deccan trap formations cc,nsistiag mainly of basalts and volcanic braccia. Overbreak in the basaltic formation and underbeak in the volcanic braccia increased the cost and time of construction considerably. Modifications in the sequence and method of exeavation and implementation of ~zmooth blasting practice improved the produotivity substantially. The paper reports in details the rock mass condition- nd the blastlng problems in the machine hall, the head race turnnelm and fhe surge shaft, the reasons behind such problems in retrospect Of rock mass-explosive interaction, the modifications made in the blasting practice and the improvements obtained thereby. The observations and the results reported herein can help set guidelines for the planning and execution of future construction work in the similar formations.
1.0 Introduction
C onsidering the importance of power sector in the accelerating ag.ricultural and industrial production and socio economic developments, it was decided to
construct Koyna Hydro-electric Project (KHEP), Stage IV to convert the existing Koyna Stage I and II power stations in Maharashtra, India, which currently serve as base load stations, into peak statiLons by installing an additional four turbo generators, each with a capacity of 250 MW each. Stage IV is being constructed in the adjoining valley almost as a replica of Koyna Sl;ages I and II. The total tunnelling involved in Stage IV is eLbout 16 kin. Alocation map of Stage IV is shown in Figure £',a); Figure l(b) shows the schematic layout of Stages III and IV of the project.
The underground excavation of the project was carried out by conventional d~illing and blasting, mainly through three types of formations:
• compact basalt; • amygdoloidal basalt; and • volcanic braccia.
The contrasting nature of the rocks posed two kinds of severe blasting problems during underground construc- tion on the project:
Present address: Centra]L Mining Research Institute, Regional Centre, 54 B, Shankar Nagar, Nagpur 440010 India.
I. Overbreak problems are greater in the compact basalt than in the amygdoloidal basalt, resulting in more time and higher costs for the support and concrete lining; and
2. Underbreak and poor fragmentation in the volcanic braccia lead to low pull and secondary blasting in the floor.
The overall cost of construction and rate of progress were adversely affected by these factors.
This paper reports in detail problems in the machine hall, head race tunnel and surge shaft; their effects on project productivity; remedial measures adopted; and the associated improvements resulting from these measures.
2.0 Geology During the Upper Cretaceous to Lower Eocene period,
tremendous volcanic activity occurred in the Indian Penin- sula, resulting in an outpouring from a series of lava flows, which obliterated previous topographic features and built up an extensive volcanic plateau. The volcanic formation thus developed is called the Deccan Trap in the Indian geological parlance. This formation covers a 5,000-kin 2 portion of the Indian peninsula (Fig. 2). Isolated outliers scattered between Rajmahal to the east and Sind hill to- wards the west indicate that the original Deccan Trap formations might have extended over a 1.5-million km 2 area, including an unknown segment under the Arabian Sea in the west of Bombay. Though flows of the Deccan Trap are mostly basic in characteristic, some occurrences of acidic flows are noticed around Bombay (Bhave and Tandale 1992).
Figure l(a). Locational map of Koyna Hydro-electric Plant, Stage IV (Huddar et al. 1995)
The Deccan Trap formation has an almost uniform mineralogical composition comprising plagioclase and aug- ite in almost equal amounts and, occasionally, olivine in large amounts. Based on lithologic characteristics, the formation is grouped into the following varieties:
• aphanitic to porphyritic compact basalt; • amygdoloidal basalt; • volcanic braccia; and • red bole• Based on different field characteristics, flows of flood
basalts are classified into three categories: (i) flows predominantly of aphanitic to porphy÷itic com-
pact basalt (Fig. 3a); (ii) flows predominantly of amygdoloidal basalt, which
is further divided into (a) amygdoloidal basalt (Fig. 3b) and (b) compound flow of amygdoloidal basalt (Fig. 3c); and
(iii)mixed flows of type (i) and type (ii) (Fig. 3d).
In addition to the above rock types, heterogeneous com- plex structures observed here are the volcanic breccia and conduits (Fig. 4). They were created when lava poured out through one or more pipe-like structures, which are pre- served in the form of conduits consisting of zeolitised rock mass or volcanic breccia with conspicuous hydrothermal alterations. These structures are distributed unevenly throughout the terrain.
Of the two main types of basalts, compact basalts are well jointed. Because joints provide passageways for water, compact basalts arelikelytobewaterbearing. Amygdoloidal basalts, in contrast, are less jointed and therefore are quite impervious when freshly exposed. Because of the absence of divisional planes, they are more stable and are prone to less overbreak.
312 TUNNELLINQ AND UNDERGROUND SPACE TECHNOLOGY Volume 11, Number 3, 1995
STAGE ]][ I STAGE ~ KOYNA DAM ~" J -- E V1" SURGE INTAKE STRUCTURE \
Bus PASSAGE PowER HOUSe ~,'PPROACH.T. \ ~ ' - ~ ' ~ " ( . . . . . . . \ "TAIL RACE TUNNEL ~ ' - -
: " ( "~ . . PRESSURE SHAI ~ T ~ : f _ "POWER House
\ -
~.TAIL RACE CI-PkNNEL
JSCHEMATIC LAYOUTJ
Figure l(b). Schematic layout of Stage III and IV of Koyna Hydro-electric Plant, Stage IV (Huddar et al. 1995).
The main properties of the Deccan Trap formations observed at different locations at the Koyna project are listed in Table 1.
The higher values in Table 1 represent the properties of compact basalt; the lower values pertain to volcanic braccia. Barton's rock mass quality (Q) (Barton et al. 1974) of the above formations was determined at various locations,
Accordingly, it was observed that the Q value of the compact basalt varied from 15 to 67.5, and that the Q values of the amyydoloidal basalt and volcanic braccia were 10 and 1.25, respectively (CMRI Report, 1994-95). The compact basalt formations had two major vertical to subvertical joint sets spaced at 0.4 m on average, at angles of 60-150 degrees with the excavation wall. Gupta et al. (1988) found that the
C'ANOA • r . . , " " • . . . . . . . . . ' .
0 0 ~ • i l l •
Qc~ ¢ ' . RAJAIdA I tAL
Figure 2. Basalt flow in Indian peninsula (Bhave and Tandale 1992).
Volume 11, Number ~,, c' 1996 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 313
Figure 3(a): Flow of aphanitic to porphyritic compact basalt (Bhave and Tandale 1992). Figure 3(b): Flow of simple amygdoloidal basalt (Bhave and Tandale 1992). Figure 3(c): Compound flow of amygdoloidal basalt (Bhave and Tandale 1992). Figure 3(d): Mixed flow of amygdoloidal and compact basalt (Bhave and Tandale 1992).
RADIAL LY ARRANG E JOZ NI"S
] i /
Figure 4. Volcanic vents and conduits (Bhave and Tandale 1992).
overbreak in tunnels was highly affected if the joint orien- tation with the tunnel wall was between 1 and 30 degrees.
3 . 0 O b s e r v a t i o n s o n P r e v i o u s C o n s t r u c t i o n s t h r o u g h D e c c a n T r a p
3.1 Tunnels Driven in Amygdoloidal Basalt Twenty-five railway tunnels have been driven through
amydoloidal basalt in Bor Ghat on the Bombay-Pune line of the Central Railway. Of these 25 tunnels, only 6 were fully or partially lined because of the presence of fractures and hydrothermal alteration of rock; the other 19 tunnels were left unlined.
The head race tunnel (HRT) in stage III of Koyna Hydro- electric project, the Malshej Ghat tunnel on the Murbad- Malshej Road of State Highway no. 2, and a tunnel of the Dimbhe Left Bank Canal Project in Pune are also driven through amygdoloidal basalt.
Another tunnel through this formation is in the Hanakhurd-Belapur line of the Central Railway near Vashi
314 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume II, Number 3, 1995
Table 1. Engineering properties of Deccan Trap formation at Koyna Hydro-electric Project, Stage IV (Nawathe etal. 1994).
in New Bombay. No major strata control or overbreak problems were observed in driving these tunnels.
3.2 Compact Basalt The tail race tunnel in stage III of the Koyna Hydro-
electric Project was driven through closely jointed columnar compact basalt for a length of over 200 m. Heavy overbreak and leakage of water through the joints and blast-induced cracks made tunnelling very difficult and expensive.
The Panchdhara tmmel near Koynanagar passes through jointed compact basalt, where overbreak in the form of wedges was found and[ roof falls were common.
Tunnels nos. 3, 4 a~.d 5 of the above-mentioned Bor Ghat of the Central Railway pass through compact basalt forma- tion also, but these joiLnts were irregular and inconsistent and the rock was dry. In spite of the overbreak, no strata control problems were encountered here.
3.3 Volcanic Braccia The Bhivpuri tunnel of Tata Electric Companies, con-
structed to carry water of the Thokarwadi dam to the Bhivpuri power generating station, passes through volcanic breccia. A black layer of 1-m-thick tachylyte at the roof deteriorated on exposure, resulting in roof fall. No overbreak was observed at the sides.
4.0 Experiences at Koyna Hydro-electric Project, Stage IV
Overbreak problems in the basaltic formations, caused by vertical and subvertical joints, and underbreak and low pull problems in the volcanic braccia zones were encoun- tered in different parts of the underground excavations of the Koyna Hydro-electric Project, Stage IV. These prob- lems slowed the rate of progress and increased the cost of the tunnel lining. Modifications, either in the sequence of excavation or in the blast pattern, significantly improved productivity of the project. The problems and modifica- tions are discussed in more detail below.
4.1 Construction of Niches in Machine Hall (Chakraborty et al. 1995)
Construction of narrow niches of the desired shape in underground works i,,~ very delicate work that requires a carefulblast operation. Overbreak, acommon phenomenon in such narrow excavations, leads to extra expenditures for filling the overbroken zone with concrete. Forty-six num- bers of vertical niches 0.8 m wide and 1 m deep, with a spacing of 10-m centre to centre, were required on both sides of the machine hall cavern of dimensions 144m x 23m x 50 m, under an average rock cover of 290 m for housing the concrete column to support the gantry beams (Fig. 5). The required height of the niches was same as the height of the
vertical wall of the cavern: 43.75 m. The niches were intended to minimise the extra excavation required for erection of the concrete columns and to prevent the bending of these columns towards wall side due to load of the gantry beam. Because of overbreak in the niches, the width of the machine hall was increased by about 1 m in most of the sections.
The rock formations in the machine hall cavern consisted of compact and amygdoloidal basalt with occasional hori- zontal to nearly horizontal bands ofzeoloite and breccia. A survey of the rock masses in the cavern and the intercon- necting openings showed the presence of one joint set with a few random joints as well as fractures resulting from previous blasting. The open joints observable in some cases were due to delayed support and deformation of the walls. The values of the Schmidt hammer number obtained in severallocations ranged between 18 and 35. The study ofin- situ stress determination revealed that the vertical stress in the cavern was 0.0146 times the overburden (m).
The sequence and method of excavation followed by the contractor for niche excavation are described below.
Step 1: A large number of holes were blasted one at a time in a benching operation for excavation of rock up to the cavern walls. To obtain a smooth profile, two blast holes were kept uncharged in between two charged holes along the Wall. The spacing between a charged and an uncharged hole was 0.3 m.
! ¢St. 75 m.
. _ . . . . . . ~ + HI ¢ hes
L j . l U n L I- 20m
Section alon~l Z-Z
Niche dimension " I m x 0 .S in
13.5m
~'-~ Z ~Niches .L
--,t ~'--- O.Om
I L.~Z
Plan
4
Figure 5. Vertical niches in machine hall cavern (Chakraborty etal. 1995).
Volume 11, Number 3, 1996 ~ L L I N O AND UNDERGROUND SPACE TECHNOLOGY 315
i
--4 t.5 ml,--
P L A H
I st. $:ep
~ for conlrolled Molting
i~ / /~ : A Vfz:~ "--j, t
!:l .oc~ he,~
I 2rid. Step
--,4 tSrnl---
Niche ~ I , Cavern well
,,, Olesl hole
Cenleur hobs
SFCTIOH ALONG A-A
Figure 6. Modified sequence of excavation (Chakraborty et al. 1995).
Step 2: Smooth wall blasting was conducted in the cavern wall for niche construction. The blast holes were horizontal. A strong explosive such as Special Gelatine 80%, which
has a density of 1.40 T/m 3 and velocity of detonation (VOD)
of 5,000 ndsec, was used in the above blasting operations. Thirty to fifty percent overbreak at the sides of the niches was obtained.
Pre-splitting was tried with a detonating fuse (composi- tion: PETN + TNT) with a charge density of 10-60 gms/m. The confinement in the blast holes was high because of the non-availability of any free face and high in-situ stress. As a result, distinct separation in the rock mass could not be obtained along the pre-split holes, and the nearby rock masses were damaged.
When a weaker explosive (Indomite-60, with a density of 1.15 T/m 3 and VOD of 3,700m/sec) was used in place of the strong Special Gelatine 80% in the niche blasting, it yielded a better result and the overbreak was restricted to 20-30 percent. Moreover, it was observed that the overbreak in the niches occurred considerably less frequently in the bottom portion of the wall, compared to the upper portion. The tightness of the rock mass there was mainly due to low wall deformation and blast-induced fractures.
Thus, it was revealed that for a better niche configura- tion, the rock mass in the cavern walls needed to be least disturbed before the niche blasting. Accordingly, a modi- fied method of excavation in the cavern, with a view to minimising the wall rock mass deterioration in bench blast- ing and the wall deformation prior to niche excavation was followed (Fig. 6). It comprised the following steps:
Step 1: Siskind and Fumanti (1974) and Holmberg and Persson (1978) found that the conventional small-diam- eter hole bulk blasting may create and induce cracking up to a distance of 1-2 m beyond the blasting zone. Accordingly, the rock in the cavern was excavated in 3- to 4-m-high benches keeping a rock bark of 1.5 m towards the walls of both the sides. It was assumed that the blast- induced crack in benching would not advance beyond the bark and that the rock mass of the walls where niches were to be constructed later would be least disturbed by bench blasting. Step 2: The barks were removed using the smooth blasting technique. The bark portion required very little
T[ h. OIIm I | : ®
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L~ "Z it''~'l"~. Geloline ; S Ge;ellne : 8
Specer : I Specer : I
Bles l ln l peremeters
Delay no : Z l e !
Torsi ~ : 6.S k |
Trio| veivme : l 8 Im m.
Powder fet ter : O.36~r k|/cu.m. Oeplh d hole : • m
Figure Z Blast pattern for removal of rock bark (Chakraborty et al. 1995). Not to scale.
316 TUNNELLZNO AND UNDERGROUND SPACE TECHNOLOGY Volume 11, Number 3, 1995
charge, as it had already been fractured in previous bench blasting. Use of a low powder factor and smooth blasting resulted i:a very little damage to the cavern walls. Step 3: Construction of niches in the cavern walls by controlled blasting before further lowering of the bench were carried out. The blast pat terns ibr removal of the rock bark and niche
excavation are shown in Figures 7 and 8. As a result, the overbreak was reduced by 15 percent and niches with good profile were obtained.
4.2 Head Race Tunnel The cross-sectional area of the horseshoe-shaped 4.224-
kin-long head race tunnel (HRT) is 90 m 3. The excavation of the head race tunnel was carried out from three different faces by the heading and bench method. During benching, horizontal drilling was preferred to vertical drilling in order to obtain the horse shoe shape. The tunnel passed through compact basal t and vo]canic braccia formations. The follow- ing problems were encountered during the excavations:
1. Because of the vertical and subvertical joints in the compact basal t zones, the overbreak at the sides was as great as 35-50 percent. This overbreak zone was to be filled by concrete during lining. Additional cost and time were also spent due to the undesired excavations in the form of overbreak. The total cost of the head race tunnel construc- tion was increased by about 19 percent because of these problems.
2. A severe underbreak problem was encountered in the volcanic braccia formations. This resulted mainly from the low modulus of elasticity of the rock where the shock energy was mostly absorbed and poorly utilised in fragmentation (Singh 1991), thereby lowering the pull. Moreover, several rounds of secondary blasting were required at the floor to obtain the desired gradient. These combined factors re- sulted in high explosive consumption and a slow rate of progress. It was observed that the peripheral holes in the bench blasting were blasted with different delays. This resulted in torsion in the rock mass during blasting (Langefors and Kihlstrom 1973), which, because of the unfavorable joint orientation in the compact basalt zones, caused huge overbreak.
The following modifications were made in the existing blast pattern:
1. The peripheral holes were blasted simultaneously with same delay detonator.
2. The peripheral holes were spaced closely and the burden with the immediate next row of holes was increased. As a result, the burden to spacing ratio was increased from 1 to 2. By increasing the peripheral rock mass confinement during blasting, this restricted overbreak.
3. The explosive charges at the peripheral holes, instead of being concentrated at the bottom of the holes, were spread over a length of the holes using bamboo spacers. Thus, the stress due to explosion at the periphery was distributed which brought more uniform breakage at the sides.
let
_L /
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BLAST DESIGN PARAMETERS
SNs PARAIdI[T[RS UND[R CUT III[NCH I~AST
|. N*. ef helee :
• Lee~led : II' II 0 U.l*e4wl : S I0
2. a~..~ cvt ,~.. : co" so"
3. Lee4mll : '
• I q r i i ~ y ~ : 112 Cerh'i4~ie ( 1 ~ ) 112 C~rt r l# le( l~ lml
Figure 8. Blast pattei,n for niche excavation using Indomite 60% as explosive (Chakraborty et al. 1995).
Volume 11, Number ~;, 1996 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 317
This modified blast pattern, shown in Figure 9, brought about a significant improvement and reduced the overbreak by 10-15 percent.
Bhanda (1975) found that the fragmentation in blasting can be substantially improved if the spacing to burden ratio is kept at greater than 1; and that the value of this ratio depends upon the amount of burden, which in turn depends on the blast hole diameter, rock type and charge density. For larger burdens, this value should fall between 1 and 2. In smaller burdens, the ratio can be increased. Accordingly, the spacing to burden ratio of the production holes in the volcanic braccia, wherein proper fragmentation could not be obtained, was increased from 1 to 1.3-1.4. As a result, the rock beams consisting of same delay detonator number were more flexible. This resulted in improved breakage in spite of the reduction in the number of blast holes per round from 84 to 62 (see Fig. 10). In this way the underbreak problem was almost eliminated. Monitoring of the blast results for five months revealed improvements in the productivity at the volcanic braccia zone (see Table 2).
4.3 Surge Shaft A 22-m-wide, 140-m-deep sha_& was sunk to accommo-
date a sudden surge of water during closing of the gates at the dam. The shaft-sinking operation was done in two stages. First a 4-m-wide pilot shaft was sunkto the full 140- m depth. The pilot shaft then was widened from 4 m to 24 m in two steps: from 4 m to 14 m, and then from 14 m to 22 m. Rock bolts (2-m deep) were used to protect side falls in the first step of widening. Shotcrete (0.05 m thick) and 4-m- deep rock bolts were used in the second stage of widening. Finally, the surge shaft was lined with reinforced concrete.
The shaft was sunk mainly through volcanic braccia and compact basalt formations. The cross-section of the strata through which the shaft was sunk is shown in Figure 11. Many overbreak problems were encountered in the compact basalt formations because of the vertical joint pattern. Moreover, since some of the joints widened as a result of blasting, several side falls from the compact basalt zones occurred, causing panic among the workers that affected
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LOADING DETAIS
Delay Delonotor No
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2 2
3 3
4 4
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7
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9 20
Charge/hole ( Spl. Gel + ANFO
carlridges )
74"9
7 + 9
7 + 9
8 + 9 9"1'9 4 + 6
(Charge distri buted by spacers of IOcm each )
Scale : Nol !o scale
Figure 9. Smooth blasting pattern in head race tunnel through compact basalt (CMR11994-95 ).
318 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 11, Number 3, 1995
the progress of the 'works and delayed its completion. Further, the additional cost of filling the overbroken zone with concrete increased the time and cost of construction.
A smooth blasting ]?attern (Fig. 12(b)) was suggested to minimise damage to t:he rock mass around the shaft. This procedure showed very good results. Half drill-hole marks
were visible at several places on the periphery, revealing that the blast-induced damage in the rock mass was re- duced. As a consequence, the overbreak was reduced from 10 percent to 6.5 percent of the total excavation. The gradual reduction in the overbreak is shown in Figure 12. Because less blast-induced damage occurred, the original
L 10.3m
foce
el
I I
\ 5 \ , ! 3/ 4 j e
8 8 6 6 6' 7 7 7 6 6 6 8 e
k l 7 . 6 m '
!-r Im
0 6 m
0.6m -t
0 5 m
O.5m -t
05m
lm
Blast P a r a m e t e r s :
Volume excovoled = 143'36 cu.m
Tolol explosive = 97'5kg
Sp. chorge = 0"6B kg
Depth of hole = 4 m
( ! Corlridge of SG 80%-" 130gm )
( ! Cortridge of ANFO = tOOgm)
Load ing D e t a i l s "
Deloy No, No. of holes Chorge/hole Gel ANFO
!
2 5 4
2 4 8 11
5 6
5 7 6 7 7 8
5 6
7 8
14
6
3 14
7 9
8 9 8 9 4 6
Tolol 62 holes .50 61
Figure 10. Modified bi!ast pattern in head race tunnel through volcanic braccia (CMRI, 1994--95).
Volume 11, Number 3, 1996 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 319
Table 2. Im ~rovements in the volcanic braccia zone of H R T (CMRI Report 1994-95).
Sl. No.
1
2
Parameter
Average pull(m)
Earlier
2.44
After modification
3.2
Improvement (%)
31
Average powder factor (Kg/m 3) 1.09 0.76 43
3 Average number of holes per round 84 62 26
4 Average cycle time (hrs.) per metre of advance 4.816 4.76 5.6
5 Average monthly progress (m) 131.25 153.7 17
R.L.(m)
/ ~ Surfoce ( 706"57 m ) \ \ \ \ \ \ X I / / / / X \ \ \ \ \ ~ ( / / / / X \ \ \ \ ~( / / / / / / /
I . I 7 0 4 . 4 5
.oo ',','J • - I .oo
~'A,J'~I 6 5 5 . 0 0
• ' , ' , 1 ~ 5 . 0 0
AAAA.j 6f9 "00
" • •1 6 0 9 . 0 0
Index -
Overburden
Compoc! Borolt
~ Volconic Broceio
Figure 11. Geological cross-section of the surge shaft.
rock mass properties around the opening were retained to a great extent even after blasting.
As a result, the final widening work was continued without the need for shotcreting at the deeper levels of the shaft. In earlier times, the production work typically was held up during shotcreting at the immediate higher eleva- tions because of the reduced work space and the generation of heavy dust in shotcreting. Elimination of shotcrete not only reduced the cost and time of support, but also increased the availability of the production time.
Moreover, the sequence of excavation was designed so that all the unit operations, e.g., drilling, blasting, mucking (slashing of muck through the pilot shaft), and rock bolting were carried out simultaneously in the four sectors of the shaft. This arrangmenet greatly reduced the crews' idle time. The time cycle and the manpower in any sector was fixed in such a way that the activity in that sector was least hampered by that in the other sectors. As a result, the average monthly excavation was increased from 1,972 m 3 to 3,038 m 3 (see Fig. 13)--an improvement of 54 percent.
5.0 Discussions and Conclusions The Deccan Trap formation consists of compact basalt,
amygdoloidal basalt, and volcanic braccia. It was observed in the Koyna Hydro-electric Project (KHEP), Stage IV that though Barton's rock mass quality (Q) of the compact basalt varied from 15 to 67.5, the rock also was characterised by two sets of prominent vertical to subvertical joints at an angle of 60-150 degrees to the excavation wall. Such joint patterns were responsible for overbreak and control prob- lems in much of the tunnelling work through compact basalt formation. However, the amygdoloidal basalt and the volcanic braccia, which had a comparatively low rock mass quality (Q) rating of 10 and 1.25, respectively, were devoid of any consistant joint pattern. During blasting for the underground constructions in KHEP Stage IV, severe overbreak problems were encountered in the basaltic for- mations; these adversely affected the time and cost in excavation and support. The experience was just opposite in the volcanic braccia. The formation, having a low modu- lus of elasticity, absorbed much of the shock energy, and thus fragmentation was adversely affected. Low pull and underbreak at the bottom of the excavations were obtained regularly in this formation in spite of a higher powder factor. Therefore, it was very difficult to obtain a healthy rate of progress in the Deccan Trap formations and the cost and time of excavation were increased.
Such problems were encountered in the niches at the machine hall, in the head race tunnel, and in the surge shaft. For niche construction, the sequence of excavation was modified to prevent deterioration of wall rock in bench blasting. Smooth blasting, using distributed charge with low density and velocity of detonation and simultaneous
320 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 11, Number 3, 1995
0 PIIol shall (~) Primary widening
~ ~ . (~) Final widening
!
~ 4 -.,i
Final periphery
I • 2 ; ! m '1
Suppor ling Mucking
Pilol s h o f !
• ' g Blosling
,S,urge shaft widening Suggesled cycle of operations
( secfor wise)
---fl I ~ t -*k-t.2-H
Blasl paltern
h ~ Final periphery
Figure 12(a). Modified ,~equence of excavation and smooth blast pattern for surge shaft widening work in KHEP, Stage IV (CMRI, 1994-95).
Volume 11, N u m b e r 3, ].996 TUNNELLn~G AND UNDERGROUND SPACE TECHNOLOGY 321
detonation of the periphery holes, was adopted in the niches of the machine hall, surge shaft, and head race tunnel. The result was a reduction in overbreak of 25, 10 and 28 percent, respectively. In the volcanic braccia zone of the head race tunnel, the spacing to burden ratio was increased from 1 to 4 to reduce the number of holes per round from 84 to 62 and to obtain better fragmentation. Consequently, the pull per
round, powder factor, cycle time, and monthly rate of progress were improved by 31, 43, 5.6, and 17 percent, respectively.
The smooth blasting pat tern and the suggested sequence of excavation in the surge shaft were responsible for reduc- ing the overbreak by about 4 percent and shattering of the rock mass. As a result, the excavation could be continued without shotcreting. These modifications reduced support
2.1m
For rows
. : t '.'.-I ° , .
. . . , .
~::: '... -. Slemming , % , .
ee l
,~, ANFO
~ ~ G e l - S G 80"/. ~ . ~ Air gop 0.2
:;, • - ' ANFO
~ - - Gel- SG 80"/.
( I ) , (2 ) E~(3)
:. , : :
. . . ; .
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For per iphery row
Blast - P o r a m e l e r s ( per seclor )
Volume excovoled • 113 cu.m
Tolol explosive = 49'16 k9
Sp. charge (kg/cu.m) = 0"43 kg
Tolal no. of holes = 70
Loading Deta i ls
Spacing Burden Chmge/holo No. of holes Charge/row Delays used Max.charge/delay (m) (m) (kg) (kg) (holes/delay) (kg)
t"3 H 1"I2 10 1t.2 5"6
Row
2 1"2
3 1"2
Peripher~ 0"5 row
I'1
I '0
0"65
1'12
1'12
0 '26
Total
12
14
34
70 holes
13"44
1568
8"64
49,16 kg
Z I (5) , ( 5 )
2 3 4 (4 ) , ( 4 ) , ( 4 )
5 6 7 (5) , (4), (5 )
8 9 10 (l 1 ), (12), ( 11 )
4.48
5'6
3"12
Figure 12(b). Charging pattern of blast holes in the surge shaft (CMRI, 1994-95).
329. TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume Ii, Number 3, 1995
Figure 13. Overbreak at different reduced levels and average monthly excavation before and after smooth blasting practice in the surge shaft.
costs, improved the tizzm cycle, and improved the monthly rate of excavation by 54 percent.
The case study sets guidelines for planning and blast design in the forthcoming underground constructions in similar ground conditions. The excavation engineers must consider the underbreak and overbreak problems while estimating the cost and time of excavation. An optimum blast pattern including smooth blasting should be evolved for such formations in order to save considerable time and money in the construction work.
6.0 Acknowledgments The authors express their thanks to the Director, Cen-
tral Mining Research Institute, Dhanbad, for permission to publish the paper. Thanks are due to the Koyna Project authority for making useful arrangements for the investi, gations conducted by the authors.
7.0 References Barton, N.; Lien, L.; and Lunde, J. 1974. Analysis of rock mass
quality and support lc,ractice in tunnelling and a guide for estimating support reqruiements. Internal Report of the Norwegian Geotechnical Institute, Oslo, 6-9.
Bhave, P. and Tandale, T. D. 1992. Geologic constraints in blasting technology for underground openings in Deccan trap basalts, Proc. of Workshop on Blasting Technology for Civil Engineering Projects, New Delhi, 16-18 Nov. 1992, 63-76. Indian Society of Rock Mechanics and TunneUing Technology.
Bhandari, S. 1975. Improved fragmentation by reduced burden and more spacing in blasting. Mining Magazine (March 1975),187-195.
Chakraborty, A. K.; Murthy, V. M. S. R.; and Jethwa, J.L. 1995. Blasting technique for niche construction in underground caveru--a case study. Proc. of a Conf. on Design and Construction of Underground Structures, New Delhi, 409-417.
Chakraborty, A. K. and Jethwa J. L. 1994. Tunnel blasting techniques in difficult ground conditions. Geotechnical and Geological Engineering 12, 219-239.
Central Mining Research Institute. "CMRI Report of Investigations (1994-95), Interim Reports on Koyna Stage IV Excavation Monitoring." Dhanbad: CMRI.
Gupta, R. N.; Singh, R. B.; Adhikari, G. R.; and Singh, B. 1988. Controlled blasting for underground excavation. Int. Syrup. on Underground Engineering, April 14-17, New Delhi, India, 449- 460.
Hagan, T. N. 1984. Blast design considerations for underground mining and construction operations. Proc. of ISRM Syrup. on Design and Performance of Underground Excavation, 255-262.
Holmberg, R. and Persson, P.A. 1978. The Swedish approach to contour blasting. Proc. of 4th. Conf. on Explosives and Blasting Techniques, 113-127. Montville, Ohio: Society of Explosives Engineers.
Huddar S. N.; Kulkarui, S. D.; and Inamdar, A.A. 1995. Head race tunnel of Koyna Hydro-electric project stage IV---a case study, Proc. of Conf. on Design and Construction on Underground Structures, New Delhi, 685-704.
Langefors, U. and Kihlstrom, B. 1973. The Modern Technique of Rock Blasting, 180-256. New York: John Wiley and Sons.
Kulkarni, S. R.; Karmarkar, B. M.; Marathe, S. S.; and Gupta, R.
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B. Geological conditions and tunnelling in Deccan traps. Proc. of a Workshop on Tunnelling India, 23-25 Feb. 1994, Pune, 127-131.
Nawathe, G. S.; Huddar, S. N.; Tandale, T. D.; and Mehendale, S. 1994. Some aspects of design and construction of tunnels and large caverns of Koyna Hydro-electric Project, Stage W--case study. Proc. of a Workshop on Tunnelling in India, 23-25 Feb. 1994, Pune, 232-242.
Siskind, D. E. and Fumanti, R. 1974. Blast-produced fracture in Lithonia granite. U.S. Bureau of Mines Report of Investigations No. 7901, 38.
Singh, D. P. 1991. Effect of physico-mechanical properties of rocks on drilling and blasting operations in underground drivage, Proc. of Workshop on Tunnels, Mine Roadways and Caverns, Ooty, September, 1991, IV.63-IV,68.
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