Dam Design 663 674

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Hosted by

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GEI Consultants, Inc.

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Parsons Water and Infrastructure Inc.

URS Corporation

21st Century Dam Design

Advances and Adaptations

31st Annual USSD Conference

San Diego, California, April 11-15, 2011

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On the CoverArtist's rendition of San Vicente Dam after completion of the dam raise project to increase local storage and provide

a more flexible conveyance system for use during emergencies such as earthquakes that could curtail the regions

imported water supplies. The existing 220-foot-high dam, owned by the City of San Diego, will be raised by 117

feet to increase reservoir storage capacity by 152,000 acre-feet. The project will be the tallest dam raise in the

United States and tallest roller compacted concrete dam raise in the world.

The information contained in this publication regarding commercial projects or firms may not be used for

advertising or promotional purposes and may not be construed as an endorsement of any product or

from by the United States Society on Dams. USSD accepts no responsibility for the statements made

or the opinions expressed in this publication.

Copyright 2011 U.S. Society on Dams

Printed in the United States of America

Library of Congress Control Number: 2011924673ISBN 978-1-884575-52-5

U.S. Society on Dams

1616 Seventeenth Street, #483

Denver, CO 80202

Telephone: 303-628-5430

Fax: 303-628-5431

E-mail: [email protected]

Internet: www.ussdams.org

U.S. Society on Dams

Vision

To be the nation's leading organization of professionals dedicated to advancing the role of dams

for the benefit of society.

Mission USSD is dedicated to:

Advancing the knowledge of dam engineering, construction, planning, operation,

performance, rehabilitation, decommissioning, maintenance, security and safety;

Fostering dam technology for socially, environmentally and financially sustainable water

resources systems;

Providing public awareness of the role of dams in the management of the nation's water

resources;

Enhancing practices to meet current and future challenges on dams; and

Representing the United States as an active member of the International Commission onLarge Dams (ICOLD).

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Low Headroom Cutter 663

CONSTRUCTION OF CUT-OFF WALL BY LOW-HEADROOM-CUTTER

INSIDE DAM TUNNEL IN CHINA

Wolfgang G. Brunner1

Arthur Bi2

William Chang

3

Dung Feng Zong4

ABSTRACT

In the South of Sichuan province a 240 MW Yeleh hydroelectric plant was constructed

by Sichuan Nanya River Basin Hydraulic Power Development Company Limited, a state-owned enterprise, and China Gezhou Ba Water & Power Group Company Limited. This

project sets out to develop the mountain cascade of the Nanya River, a tributary of the

Dadu River, provide superior electricity and adjust flood peak & frequency. Thespecialist contractor Foundation Engineering Company of China Water Resource and

Hydropower (FEC) was awarded the contract for foundation treatment at the right bank,which included the construction of a 75m deep concrete Cut-Off Wall (COW) inside a

6.0m x 6.5m tunnel. The requirement for 19,317m2

COW to be constructed in permeableand very dense gravel/cobble formations and a demanding project program led FEC to

adopt Bauer Low Headroom Cutter CBC25/MBC30 in conjunction with Overlap Cutter

Joint (OCJ).

INTRODUCTION

Slurry wall technique development

The slurry wall or diaphragm wall technique was introduced in Europe in the 1950s forthe construction of underground reinforced concrete structural wall systems. The

technique initially evolved from the principal idea of stabilizing open excavations with

thixotropic fluids, a concept invented in the 1940s by Prof. Veder of Austria. During the

past 30 years it has undergone a further dramatic evolution not only in terms of analysis

and design methodologies, but also in terms of construction technology and construction

equipment.

In the early stages of development, slurry walls were constructed by cable grab mounted

on tripod rigs on rails (Figure 1). In the 1960s, a milestone in the development of slurry

wall excavation equipment was reached with the introduction, in Japan, of the reverse

circulation technique in the field of slurry wall construction. Whilst one companydeveloped a rig with blade cutters rotating around a vertical axis, another one designed a

cutter with two cutting wheels rotating around a horizontal axis. Both systems used

1 Professor, Dipl. Ing. Marketing Director, BAUER Maschinen GmbH, BAUER-Str. 1, D-86522

Schrobenhausen, Germany, [email protected] Deputy General Manager, BAUER Technologies Taiwan Ltd., Taipei, Taiwan3 Senior Engineer, BAUER Technologies Taiwan Ltd., Taipei, Taiwan4 Deputy Chief Engineer, China Water Group, Beijing, China

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664 21st Century Dam Design Advances and Adaptations

either a suction pump located outside the trench or an airlift system to remove the

excavated material (Figure 2).

Figure 1. Historical tripod rig Figure 2. Japanese rig

In subsequent years, specialist firms developed alternative excavation equipment such as

rope grabs suspended from crawler cranes (Figure 3).

The limitations of rope operated grabs, mainly due to their lack of power and speed when

excavating stiff clayey soils and rock formations, were overcome by the development of

hydraulically operated grabs capable of generating high closing forces (Figure 4).

Figure 3. Crawler crane with rope grab Figure 4. Hydraulically

operated grab

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The arrival of the reverse circulation cutter technology in Europe resulted in intense

research and development by several European geotechnical and foundation contractors.

Development of the Trench Cutter

In the early 1980s, the first BC 30 trench cutter was developed and built for theconstruction of a cut-off wall in fractured sandstone below the main dam of the

Brombach storage reservoir in Germany ( Figure 5, 6,7, 8 ).

Figure 5. BC Cutter at Brombach Reservoir Figure 6. BC Cutter wheels

Figure 7. BC Cutter Figure 8. BC Cutter set up

The introduction of the newly developed cutter system on the world market opened up

new horizons for the uses and applications of the slurry wall technology:

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Increase in depth:

Whilst the average depth of slurry walls was in the range of 20m in the 1950s, it is

now possible to construct walls to depths of more than 100m.

Increase in wall thickness:

When the slurry wall technique was first introduced, the equipment capacity waslimited to thicknesses of between 500 and 800mm. Today, projects have been

successfully executed with a wall thickness of up to 2400mm.

Wall construction in hard soil formations and rock:

Trench excavation in rock has been made possible by the introduction of trench

cutters capable of producing high torque (hydraulic power is often in excess of 600

kW) and also by the development of cutter wheels with excellent rock crushing

capabilities, such as the roller bit or round shank chisel wheels.

Wall construction in limited headroom conditions:

In addition to the fact that the capacity of trench cutters is still increasing, specialcutters have also been developed which do not require more than 6m headroom.

These 'mini' cutters were specifically requested by the Japanese, South-Korean,

Taiwanese, Singaporean and Chinese markets for the construction of diaphragm walls

in connection with mass rapid transit systems in the big cities or cut-off walls for

dams (Figure 9, 10, 11, 12).

Environmentally friendly construction process:

Trench cutters have the added qualities of noise and vibration-free excavation, which

is an important environmental factor when working in densely populated areas.

Figure 9. Low Headroom Cutter in Singapore Figure 10. Compact Cutter in Japan

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Figure 11. Mini Cutter in USA Figure 12. Mini Cutter in South-Korea

High degree of verticality:

One of the major advantages of trench cutters over other conventional cutter

equipment is their ability to construct trenches to extremely high vertical tolerances.The verticality of cutters is controlled by inclinometers, which are integrated into the

cutter system. For stringent verticality requirements a specially developed

hydraulically operated cutter steering system is deployed. Adjustments of the cutter

position in vertical direction of the trench are made via long hydraulically operated

steering plates, whilst adjustments in longitudinal direction are made by pivoting the

cutter head. Throughout the correction process, the rig operator is continuously

guided by corrective measures displayed on his on-board monitor.

Automatic quality assurance system:

The entire slurry trench construction process is monitored and controlled by a

PC-based electronic system. On completion of each diaphragm wall panel a fullcutter report can be printed out inside the operator's cab or transmitted by remote data

transfer direct to the site office, the company's head office or the consultant engineer's

office for further analysis and evaluation (Figure 13).

In conclusion, the market required slurry trench or diaphragm wall construction

equipment capable of excavating deeper and thicker trenches at greater speed through

harder soil and rock formations and sometimes with limited headroom.

In the early days, the cutter frame as well as the hydraulic hoses and slurry hoses were

simply suspended from a rope on a long crane jib and the depth of the trench was limited

to twice the length of the jib of the base carrier. As a result of the increasing demands onslurry trenches as referred to above, the base machines for cutters also increased until

gigantic cranes had to be used. But more and more frequently diaphragm walls had to be

constructed on down-town city centre areas amidst bustling traffic and on very restricted

sites even under limited headroom space or inside tunnels. This led to the development

of smaller, so called 'compact' cutters, with all hoses on hose drums, capable of con-

structing diaphragm and cut-off walls to greater depths on extremely restricted sites.

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Figure 13. Figure 14.

In addition to the rapid development of trench cutter excavation systems, it was also

necessary to improve slurry treatment, desanding plants as well as decanters, especially in

conjunction with high-performance trench cutters (Figure 14).

Key project information

In the South of Sichuan province the 240 MW Yeleh hydroelectric plant was constructedby Sichuan Nanya River Basin Hydraulic Power Development Company Ltd., a state-

owned enterprise, and China Gezhou Ba Water & Power Group Company Ltd.. This

project sets out to develop the mountain cascade of the river Nanya, a tributary of the

river Dadu, to provide superior electricity and adjust flood peak and frequency. Theproject is located in a high seismic zone with a Modified Mercalli design intensity VIII

and represents with its 125.5m high dam the highest rock-/soil-fill dam with an asphaltconcrete core in Asia. Grout curtains and cut-off walls are designed as a combined

system. The specialist contractor Foundation Engineering Company of China Water

Resource and Hydropower (FEC) was awarded the contract for foundation treatment atthe right bank, which included the construction of a 75m deep and 1m thick concrete cut-

off wall inside a 6.0m wide x 6.5m high tunnel. The requirement for the 19,317m2

cut-

off wall to be constructed in permeable and very dense gravel, cobble and boulderformations and a demanding project program led FEC to adopt the Bauer low-headroom

trench cutter CBC25/MBC30 in conjunction with the overlap cutter joint.

General

Earth dams must be safe against overtopping, their slopes must be stable under allconditions, their foundations must not be overstressed, and they must be safe againstinternal erosion, water forces and pressures. A large percentage of earth dam failures

reported by Justin (1936) and Sherard et al. (1963) likewise were seepage failures. As

such the formation of an effective water barrier beneath dams is one of the mostimportant tasks in building dams.

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The oldest method of making a barrier beneath dams still commonly and widely

used even today is the grouted cut-off technique. A complete positive grouted cut-off is often difficult and costly to attain, requiring a pattern of holes staggered in rows

with carefully planned injection sequence and pressure control. As a result of this

problem, alternative processes have been developed during which soil is excavated

and the resulting voids backfilled by a barrier material of well defined properties, e.g.concrete, soil-bentonite or cement-bentonite mix. One important method for

constructing a positive cut-off wall is the use of slurry trench, diaphragm wall

technology. A case history of a 75m deep concrete cut-off wall (Figure 15), for alarge rock/soil-fill dam in Sichuan illustrates the possibilities for constructing cut-off

walls inside an extremely limited working space, a 6.0m x 6.5m tunnel, with the latest

technology BAUER low-headroom cutter CBC25/MBC30.

Figure 15. Longitudinal section of cut-off system at Yeleh Hydropower Plant axis

1. Right bank cut-off wall upper portion with 5. River bed cut-off wall

grouting joint to tunnel 6. Left bank cut-off wall2. Tunnel for construction of right bank cut-off 7. Left bank curtain grouting

wall at lower portion 8. Tunnel for construction of left

3. Right bank cut-off wall lower portion bank curtain grouting4. Right bank curtain grouting 9. Compacted asphalt concrete

PROJECTDATA

Works for the Construction of the right Bank Cut-off wall inside a 6.0 m x 6.5 m

Tunnel

Due to high rock level encountered near the ground surface on the left bank, a

conventional grout curtain was executed. On the right bank a cut-off wall was

executed from the surface and inside a tunnel. The excavated volume for the Yeleh

1

3

4

25

6

7

8

Original Ground Line

Rock Surface

1

3

4

25

6

7

8

Total Length : 850 meters

Original Ground Line

Rock SurfaceE.L. 2440.00

E.L. 2480.00

E.L. 2520.00

E.L. 2560.00

E.L. 2600.00

E.L. 2640.00

E.L. 2680.00

E.L. 2720.00

E.L. 2760.00

E.L. 2654.50

9

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cut-off wall was 19,317m2

(whereas out of this quantity 6,284m2

were excavated by

Wu-Ka-S rigs, a local percussion rig, and 13,033m2by BAUER low-head room cutter

CBC25/MBC30). The strength of the cut-off wall concrete is 35 MPa. The wall

measures 302m in length with a width of 1000 mm, a depth of 60.95 - 77.95m and

consists of 68 panels. The verticality requirement is 1:200.

Site Geology

At the location of Yeleh hydropower station, the valley is wide and asymmetric withexposed Quartz-Feldspar rock face on the left bank and more than 200m thick alluvium

and lake deposit on the right bank (Figure 16).

Soil type estimated in % on the right bank:

5 % Quartz, Feldspar and Basalt boulders (200 mm above), 35 % Quartz, Feldspar andBasalt cobbles (60 - 200 mm), 20 % gravel (2 - 60 mm), 40 % sand and silt (< 2 mm).

The strength of boulders and cobbles was not tested but estimated to be 100 - 160 MPa.

Figure 16. Typical alluvia and lake deposit Figure 17. Local Wu-Ka-S rigs

MAIN EQUIPMENT FOR THE CONSTRUCTION OF THE CUT-OFF WALL

General

For constructing diaphragm walls, several systems and types of equipment areavailable. The selection of an appropriate equipment setup for constructing a

diaphragm wall project is a key decision in the planning phase that will significantly

govern the technical and economical success of the works. Specific characteristics of

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excavation equipment and the main reasons for selecting them for the use on Yeleh

hydropower station are described below.

Local Percussion Rig Wu-Ka-S

The locally made Wu-Ka-S rigs, due to their good mobility, were used at beginningstage for the execution of the river bed and left bank cut-off walls. But due to their

slow performance, ventilation, noise and excessive concrete over-break problems they

thoroughly moved out after the cutter started cutting work on site (Figure 17).

Custom-made BAUER Low-Headroom Cutter CBC25/MBC30

For Yeleh hydropower station, a custom made BAUER low-headroom cutter

CBC25/MBC30 was selected. It has a weight of about 25 ton, working height 5.3m

and is mounted on a specially designed hydraulic base carrier BS 100B and directlypowered from the 420 kW engine installed in the base machine (Figures 18 and 19).

Figure 18 and 19. BAUER low headroom trench cutter CBC25/MBC30 at Yeleh

The main reasons for selecting the low headroom trench cutter were that the 75m deep

cut-off wall has to be constructed inside a 6.0m wide x 6.5m high tunnel and througha dense gravel and cobble layer.

Other factors leading to its selection were that it is almost vibration free during thecutting process, the demanding project program, and the requirement of strict

verticality with water-tight wall joints.

The trench cutter is a hydraulically operated machine, which works on the principle of

reverse circulation. During the cutting process, soil and rock directly beneath the

diaphragm walling are continuously loosened by two powerful cutter wheels rotatingin opposite direction, broken down into smaller fragments, mixed with the stabilizing

slurry in the trench and moved towards the suction intake. A centrifugal pump

mounted on top of the suction box pumps the spoil enriched slurry to the regenerating

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plant (Figure 20), where the soil and rock cuttings are removed from the bentonite

slurry by a system of vibrating screens and cyclones. The regenerated clean slurry isthen returned to the trench. Built into the cutter is an electronic inclinometer, which

measures the cutter's verticality deviation in two directions. The deviation is

continuously displayed both in degrees and in centimetres on the B-Tronic monitor

inside the operator's cabin. If the cutter deviates from its vertical axis, its position canbe adjusted with the help of the hydraulically operated steering plates (Figure 21).

Figure 20. Desanding station BE 500 Figure 21. Verticality control withsteering plates

Mud plant

The bentonite slurry is continuously recycled and passed through desanding units typeBAUER BE 500, where it is cleaned, regenerated and then returned into the trench.

The units comprise coarse vibrating screens, hydro-cyclones and fine dewatering

screens. After the cutter itself the mud plant is the second most important componentin the cutter technology. Throughput and desanding capacity have to be closely

matched with the soil and the cutter performance.

CONSTRUCTION OF THE CUT-OFF WALL

Construction sequence

The cut-off wall was constructed as a series of primary and secondary panels, see

Figures (22 and 23).

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ConcretedPrimary Panel

Soil 2.20mConcretedPrimary Panel


Excavated

Secondary Panel2.80m


Concreted

Secondary Panel

Concreted

Primary Panel

Concreted

Primary Panel

2800

0.30m Overcut on each side!



Soil 2.20mConcretedPrimary Panel


Excavated

Secondary Panel2.80m


Concreted

Secondary Panel

Concreted

Primary Panel

Concreted

Primary Panel

2800



1. Primary Panel 2. Primary Panel3. Soil

2800 1200 2800

6800

Excavation Sequence

1. Primary Panel 2. Primary Panel3. Soil

2800 1200 2800

6800

Excavation Sequence

Figure 22. Typical primary panel layout Figure 23. Typical secondary panel layout

Wall joints as overlap cutter joint

The use of the cutter is also essential for the formation of the joints between panels.A secondary panel is formed between two previously completed primary panels,

usually as single or triple cuts. As the cutter descends it encroaches on the adjacentprimary panels and thereby cuts back a fillet from each end of the previously

concreted primary panels (Figure 24). The cut-back or overlap between primary and

secondary panels is 300mm required by the specifications. Joints produced in this

manner are watertight, because they consist of a serrated surface resulting from theformation of grooves cut into the concrete of the primary panels by cutter wheels.

The advantages offered by the cutter over the other available systems include a

consistently high output, an extremely high degree of verticality control as it isequipped with electronic inclinometers and its position adjusted with the help of

hydraulically operated steering plates, watertight joints and the ability to cut througheven the hardest boulders and also key into bedrock if required.

Figure 24. Overlap cutter joint

ConcretedPrimaryPanel

ConcretedPrimaryPanel

MBC 30

2.80 m

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R. C. structure

120 cm thickness

right bank tunnel(600 cm X 650 cm)

grouting joint

curtain grouting

right bank cut-off wall(lower portion)

right bank cut-off wall(upper portion)

E.L. 2572.50

E.L. 2569.20

E.L. 2561.50

E.L. 2500.00

toe of curtain

grouting

E.L. 2639.50

The arrangement of the right bank cut-off walls and the tunnel is shown in a section

(Figure 25). The dam axis view of the upper cut-off wall and the tunnel are shown in ajobsite picture (Figure 26).

Figure 25. Cross section of dam Figure 26. Dam axis view

CONCLUSION

The construction of the concrete cut-off wall at the Yeleh hydropower station is a

very recent and impressive case that proves the technical and economic feasibility ofthe diaphragm slurry wall technology for making a watertight cut-off wall more than

75m deep even in the very limited space of a 6.0m x 6.5m tunnel. It also shows quite

clearly that such a task can only be performed when utilizing not only advanced

technologies but also by planning the works on the basis of selecting feasibleexcavation equipment. This paper is presented to offer a case experience related to a

deep concrete cut-off wall in dense gravel/cobble layer using a custom made BAUERlow-headroom trench cutter CBC25/MBC30 in conjunction with cutter overlap joint and

it will serve as an important reference, and be helpful for future projects in similar

circumstances. According to Chinese information no damage occurred at the cut-off wall

during the Wenchuan earthquake with a magnitude of 7,9 (Richter scale) in Sichuan.

Dam Design 663 674

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