sanjiv shah underground excavations at chep

8/17/2019 Sanjiv Shah Underground Excavations at CHEP

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UNDERGROUND EXCAVATIONS AT

CHAMELIYA HYRDOELECTRIC PROJECT:

CASE STUDY

DR. SANJIV SHAH

SHAH CONSULT INTERNATIONAL (P.) LTD.


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PROJECT DETAILS


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LOCATION

Hydroelectric project:

Darchula District,

Far-western Development

Region

960 km from Kathmandu

Transmission line:

Darchula, Baitadi, Dadeldhura,

Doti & Kailali Districts

131 km long, 132 kV


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SALIENT FEATURES

Installed capacity: 30 MW

Scheme: PROR, with six hours daily peaking

Design discharge: 36 m3/s

Gross head: 103.7 m

Net head: 94 m

Average annual energy: 184.2 GWh


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PROJECT LAYOUT


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PRINCIPAL COMPONENTS

Dam Concrete gravity, 54 m high, 2 radial gatesStilling Basin Horizontal, 54 m long, 17 m wide

Diversion Tunnel 210 m long, 4.2 m dia., inverted D-shaped

Intake Two side intakes, orifice type, 3 x 8 m each

Desanding Basin Two underground basins, 105 x 12 x 25 m each

Headrace Tunnel 4.067 km, horseshoe shape, 5.2 and 4.2 m dia.

Surge Shaft Restricted orifice 8 m dia., 48 m high

Vertical Shaft 72 m high, 3.9 m dia., concrete lined

Penstock Tunnel Concrete/steel lined, 384 m long, 3.7~2.5~1.8 m dia.

Powerhouse Semi-underground, 37.5 x 23.5 x 27.4 m

Tailrace Box culvert, 703 m

Turbine Francis, 2 x 15 MW

Switchyard Outdoor conventional, 57 m x 47 m

Transmission Line 132 kV, 131 km single phase to Attariya


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PROJECT REALITIES

CHEP: 30 MW project, but complicated civil structures

Highest concrete gravity dam in Nepal (54 m high vs. 43 m at

KGAA and 52 m at MMHEP)

Two underground desanding basins (vs. three at MMHEP)

4 km headrace tunnel through probably the poorest ground

conditions encountered in Nepali hydropower projects

Several underground structures (surge shaft, vertical shaft,horizontal penstock tunnel) in poor rock

162 m high powerhouse back-slope (only other example in

Nepal: desanding basin slope at KGAA)


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CHRONOLOGY

1991 Project identified by NEA

1993 Potential confirmed by JICA’s master plan study

1996 Feasibility study through SADF/NIDC

1997 - 98 Upgraded feasibility study by NEADec 2001 Detailed design and tender documents

Jan 10, 2007 Mobilization of civil contractor

May 2007 Start of major civil works

Jun 1, 2008 Start of headrace tunnel excavationDec 2, 2009 First river diversion

Apr 27, 2010 First concrete pour in dam

Jun 15, 2010 First concrete pour in powerhouse

May 12, 2012 Headrace tunnel breakthrough


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PRESENT STATUS


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DIVERSION DAM


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DIVERSION DAM


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INTAKE


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DESANDING BASIN


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HRT CONFLUENCE


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HRT CONCRETE LINING


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HEADRACE TUNNEL


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POWERHOUSE


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POWERHOUSE


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TAILRACE


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SWITCHYARD


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UNDERGROUND STRUCTURES


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INVESTIGATIONS

Geological investigations between 1996 and 2001

Headrace tunnel: surface geological investigations

Limited core drilling and seismic resistivity surveys

Structure Core Drilling SeismicSurvey (m)Planned (m) Actual (m) Angle

Desanding Basin 115 50 Horizontal -

Headrace Tunnel - 35 Vertical 295

Surge Shaft 150 39.5/35.5 Vertical 115

Penstock - - - 805


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TUNNELS

Structure Shape L (m) Dia. (m) Lining

Diversion tunnel Inverted D 203 4.0 S

Connecting tunnel No. 1 Inverted D 127 3.4 C

Connecting tunnel No. 2 Inverted D 116 3.4 CAccess tunnel to DB Inverted D 105 4.5 S

Flushing tunnel Inverted D 132 3.0 S/C

Headrace tunnel Horse-shoe 4,067 5.2/4.6 S/C

Adit No. 1 Inverted D 305 3.5 S



Aeration tunnel Inverted D 156 3.0 S

Penstock tunnel Horse-shoe 372 3.9/3.7 C/St


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CAVERNS & SHAFTS

Structure L (m) W/D (m) H (m) Lining

Desanding basin No. 1 105 12 25 Concrete

Desanding basin No. 2 105 12 25 Concrete

Surge shaft 8.0 48 ConcreteVertical shaft 3.9 72 Concrete

Total linear length of excavations 6,749 m


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PREDICTED VS ACTUAL GEOLOGY

Geological conditions during construction significantly

different from those predicted in design

Rock mass substantially weaker, especially in lower half of

project area

Several large shear zones and faults, mostly unpredicted or

incorrectly predicted

Orientations of bedding planes and joints incorrect

Squeezing rock mass between Adit Nos. 2 and 3 not

anticipated


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STRUCTURES IN HEADWORKS


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DESANDING BASIN


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HEADRACE TUNNEL


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HEADRACE TUNNEL


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HEADRACE TUNNEL SUPPORTS


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UG STRUCTURES AROUND PH


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HEADRACE TUNNEL GEOLOGY


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ROCK TYPES

SN Region Rock

1 Connecting tunnel desanding basin,

bifurcation & initial part of HRT

Silicious dolomite

2 Adit No. 1 area Diamictite/ meta sandstone

3 Adit No. 1 D/S Black slate

4 Adit No. 1 D/S & Adit No. 2 U/S Gray dolomite

5 Adit No. 2 D/S Phyllite-Dolomite inter-bedded

6 Adit No. 2 D/S & Adit No. 3 U/S Talcosic dolomite

7 Adit No. 3 U/S Talcosic phyllite

8 Surge shaft, vertical shaft &

penstock tunnel

Limestone and gray phyllite


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IMPACTS OF POOR GEOLOGY

Consequences:

Higher levels of supports installed

Frequent collapses and debris flows

Large water inflows

Squeezing in 843 m

Frequent stoppages

Reduced rate of advancement

Planned: 2 – 2.5 m/day

Actual: 0.6 – 1.4 m/day at different faces, 0.37 m/day in

squeezing section

Delays and claims


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SQUEEZING IN HEADRACETUNNEL


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HRT EXCAVATION

Commencement of HRT excavation:

From Adit No. 2: June 2008

From Adit No. 3: August 2008

From Adit No. 1: March 2009

Drill and blast method, with full-face excavation

Initial 22 months of construction: no major problems barring a

few incidences of collapses, heavy water ingress and minor

squeezing


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TUNNEL SQUEEZING

Squeezing in 843 m of headrace tunnel between Adits 2 & 3

Adit No. 2:

Severe to extremely severe squeezing (20 to 40%)

Deformation of steel ribs, cracking of shotcrete

Large mud flows

Work halted for over 9 months due to monsoon and mud flow

clearance

Adit No. 3: Large deformations in crown and sidewalls

Severe to extremely severe squeezing (10 to 30%)


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TUNNEL SQUEEZING – ADIT 2 D/S


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TUNNEL SQUEEZING – ADIT 3 U/S


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TUNNEL SQUEEZING – ADIT 3 U/S


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PROBLEMS IN TUNNELING

Debris Flow Rock fall


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DEBRIS FLOW

Water & gravel flow Mud flow through portal


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MAJOR CAUSES

Overburden only between 174 and 279 m, but poor geology

Ch. 3+102.5 and 3+592.7 m

Fault zone with highly crushed rock mass of grey to white talcosic

dolomite with phyllite intercalation Bedding planes nearly parallel to tunnel (deviation ~ 11°)

Large ground water inflow

Ch. 3+659.4 and 3+945 m

Mainly composed of highly weathered talcosic phyllite with veryrare intercalation of white to grey, thinly bedded dolomite or

quartzite

Bedding planes vertical, nearly parallel to tunnel (deviation ~ 11°)

Large ground water inflow


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GEOLOGY BETWEEN ADITS 2 & 3


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GEOLOGY BETWEEN ADITS 2 & 3


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SEVERITY OF SQUEEZING


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INITIAL ATTEMPTS FOR SOLUTION

Various methods for solution attempted

Top heading and benching

Manual excavation

Fore poling

Placement of closely spaced steel ribs

Drainage

Invert strengthening (invert strut and concreting)

Implementation difficult due to poor ground conditions

Sequential excavation: misalignment of steel rib crown & legs

Invert strengthening: Not possible due to slushy conditions,

immediate sidewall squeezing & floor heaving following invert

excavation, severe buckling


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OPTIONS EVALUATED

• Over-excavation

• To gain sufficient time to install supports

• Led to further collapses

•

Tunnel realignment• Large lateral extent of poor rock mass

• Diversion would run parallel to bedding planes under increasingoverburden, creating even greater problems in excavation


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ENGINEER’S DECISION

Proceed further with tunnel excavation along designed

alignment & repair tunnel after breakthrough

Reasons:

Tunnel excavation & deformation would result in significant stressrelaxation in surrounding rock mass

Breakthrough would eliminate all tunnel faces that had

repeatedly been serving as sources of large debris flows

Breakthrough would drain surrounding rock mass


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EXCAVATION PROCEDURE

Fore-poling to create protective shield

Sequential excavation (heading & benching)

Sealing tunnel faces after to prevent collapses or debris flows

R5 and R6 supports with steel ribs but no rock bolts

Locations of excessive squeezing: double layer of 100 mmsteel ribs with invert struts

Temporary supports

Weep holes to drain out groundwater behind crown & wall

Limited grouting at locations with crushed rock mass


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SQUEEZING TREATMENT DESIGN

• Various alternatives considered

• Steel lining

• 4.9 m steel liners embedded in plain or reinforced concrete

•

Could fit within deformed tunnel profile at most places, requiringminimal reshaping at places with excessive squeezing

• Transportation, assembly and quality control extremely difficult

• Exorbitant cost (approx. Rs. 2.05 billion)

• Concrete lining

• Concrete lining after reshaping deformed• Performed sequentially, extremely slow (almost two years

provided no major stoppages)


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Engineer designed treatment plan based on case histories

(e.g. Yacambu-Quibor tunnel) and information gained through

trial sections

Tunnel categorized into three typical zones based on observed

maximum strains Circular cross-section with 5.4 m finished diameter

Supports for each zone designed using Phase 2

Reinforced shotcrete lining selected because of shorter

installation time

Steel ribs with yielding joints in zones of excessive squeezing topermit stress relaxation and ensure safe working conditions


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PHASE 2 ANALYSIS

Initial values of rock mass properties (GSI, mi and sci) selected from

published charts

Back analysis performed to adjust the material parameters so as to

match the computed and observed deformations

Rock mass was modeled as an elastic-perfectly plastic material

Analyses showed extensive stress relaxation around tunnel up to

distance of at least 2 to 2.5 times tunnel diameter

Supports for each zone designed using the various support facilitiesavailable in Phase 2

Stage-wise modeling performed to simulate pre-existing

deformations and supports were installed at appropriate stages


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DESIGN PROFILE

SQUEEZED PROFILE

TREATED PROFILE

BASIC APPROACH


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Zone A: Squeezing extremely severe, on-going

Total length: 418 m

Over-excavation to 6.6 m dia. circular shape

Steel ribs with yielding joint

60 mm reinforced shotcrete

Zone B: Squeezing extremely severe but stable

Total length: 226 m


40 mm thick reinforced shotcrete

Zone C: Squeezing very severe but stable

Total length: 198 m


35 mm thick reinforced shotcrete


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TYPICAL DESIGNS


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TYPICAL YIELDING SUPPORT


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POE RECOMMENDATIONS

Visited site from December 8 to 11, 2013

Observations:

Tunneling performed under extremely poor geological conditions

Observed squeezing was not “classical” squeezing Engineer’s approach to complete tunnel excavation and then

treat the squeezed section correct

Closed circular section with shotcrete looked promising and

should be maintained

Yielding steel sets were a very meaningful ground supportmeasure for squeezing rock conditions

Recommendation

Added Type D support (Type A without steel rib)


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ORIGINAL CONDITION


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RE-EXCAVATION (MANUAL)


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RE-EXCAVATION (MACHINE)


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STEEL RIB INSTALLATION


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FIRST STAGE SHOTCRETE


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STEEL BAR PLACEMENT


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SHOTCRETING


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FINAL SECTION


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IMPLEMENTATION OF TREATMENT

• Treatment started in March 2013

• Difficult & slow process, but no major problems encountered

• 67.5% treatment complete

Zone Total (m) Complete (m) Remaining (m)

A 283.2 248.1 31.1

B 277.2 223.1 54.1

C 181.9 97.2 84.7

D 100.0 0.00 100.0

Total 842.3 569.4 273.9


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PROBLEMS ENCOUNTERED


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PROBLEMS ENCOUNTERED


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VERTICAL SHAFT &PENSTOCK TUNNEL


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HORIZONTAL PENSTOCK TUNNEL

Tunnel length: 300 m

Only 160 m provided

with steel lining

Large debris flow, about

1,700 cu. m

Indication of very poor

geological conditions


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VERTICAL SHAFT

Large cavity around shaft

Approx. 25 x 8 x 5 m

Crushed material

Concrete lininginadequate

Leakage

Slope instabilities

Steel lining proposed


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VERTICAL SHAFT


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PENSTOCK & SHAFT LINING

• Original design: penstock concrete and steel lined, verticalshaft concrete lined

• Revised design: steel lining required over entire length due topoor geological conditions (large cavities)

• Concrete lining without adequate rock support may burst underwater hammer

• Water leakage through cavities

• Further revision based on POE recommendations:

•

Concrete lining (“water tight”)• Geo-membrane layer to prevent water leakage


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THANK YOU!

sanjiv shah underground excavations at chep

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