ijirae:: construction peculiarities of bottom outlet tunnel, its air vent and energy dissipater at...

International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163 Volume 1 Issue 9 (October 2014) www.ijirae.com

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Construction Peculiarities of Bottom Outlet Tunnel, Its Air Vent and Energy Dissipater at Upper Gotvand Dam

Ali Zabihollah zadeh* Mehdi Ghomeshi Mohammad Reza Mohseni kia Mahab Ghodss Cons.Eng.Co. Water Sciences Eng.Dep.& university of Chamran Mahab Ghodss Cons.Eng.Co.

Abstract— Located in south western area, Upper Gotvand Dam has three diversion tunnels, third of which is served as bottom outlet tunnel. The other two had already been plugged for dam watering procedure. The purpose of study is to survey construction peculiarities of seven affiliated blocks of bottom outlet. Preparations, reinforcement, formwork operation, and concrete placement of different blocks as well as casing for steel segments are explored. Main features of each individual blocks are detected as well. It was concluded that opted scheme for dowels can effectively influence precise construction, devastating of initial concrete of third diversion tunnel in certain sequences is considered fundamental for project progression. It was revealed that self-compacting concrete is highly substantial in accumulated and hard to access sections such as casing operation of steel segments.

Keywords— Bottom outlet tunnel, affiliated blocks, self-compacting concrete, construction features.

I. INTRODUCTION Modern dams must have bottom outlets or low level outlets to lowering the reservoir in an emergency. These outlets

can also be used for filling of the reservoir, drawdown of the reservoir, flushing of sediments, or discharging surplus water. Morris R. reviewed drawdown capacity at abberton reservoir and compared it to standards commonly applied elsewhere. He also explored how this capacity might be improved [1]. First filling of the reservoir must be made progressively for stability and watertightness considerations. Wieland has declared that this procedure is only safe if a bottom outlet is available, thus have to be designed so that reservoir level can be kept constantly under arbitrary levels [2]. This fundamental structure has specific peculiarities at each individual dam projects.

Gotvand dam is involved with a 12m diameter shaft downstream of plugging of second tunnel which was excavated to aerate output flow of bottom outlet gates. Up and downstream gates’ box is steel lined 76.25 and 55.5m long respectively to protect tunnel against cavitation as well as erosive flow harms. A 2.75m diameter bell mouth inlet at the elevation of about 92.4m is fabricated in reinforced steel ring using ST.37 material to protect forehead concrete of corrosion. Through this structure water is conveyed from 9.5m diameter horseshoe section and approaches gates box after 73.5m distance. Water enters 55m long U-shaped steel canal afterwards. Downstream concrete tunnel which is equipped by 5 aerator structures transfers water to stilling basin and energy dissipater afterwards. The purpose of study is to focus on construction attributes of each individual affiliated structure. As it was rather predictable relevant blocks have specific features that make some constructional schemes inevitable.

II. METHODS- CONSTRUCTION PROCEDURE

A. Access Way Route available at northern powerhouse has been extended 200m long from elevation of about 96 at 8% slope to

tunnel portal, as shown in Fig.1. Since it partially cross the river we were about to stack boulder materials to mount above the surface and then was continued in embankment. As it'll be mentioned subsequently this trend cause some inconveniences during construction of energy dissipater structure. Besides initial path from tunnel outlet another way was planned to make gate chamber accessible. Second route derives from access tunnel allotted for gallery 107(applied for grouting curtain), as shown in Fig.2. It crosses the shaft upon mid diversion tunnel in which a metal bridge makes this conjunction traversable, as shown in Fig.3. This route provides permanent access pending dam operation and serves either to transmit gate chamber segments or for reinforcement and concrete placement during construction as well.

Fig. 1 Access provided by embankment to the outlet portal of tunnel



B. Knocking Down of Initial Concrete Once access from tunnel outlet was fulfilled and transportation through which become feasible, pulling down

operation of bell mouth walls get started. Hammer machine facilitates devastating operation and continued to mid U-shaped structure. Since extant benching shear keys, 68m distant from entrance obstacle steel segments to be transmitted in, thus debris were moved out, formworks were assembled, and concrete placement was performed afterwards at this part, as shown in Fig.4,5. Structural mix design named T298 with 350 kg/m3 cement content facilitates 465m3 concrete placement.

Fig. 2 Divergence of gate chamber access of gallery 107 access way

Fig. 3 Bridge made at air vent shaft of second diversion tunnel

Fig. 4 Shear key region after initial concrete detriment

Fig. 5 Concrete placement at shear key position

C. Formwork Operation 6 to 8m long parts were assigned for concrete placement operation in which wood headstalls were applied in moulding

and adequate support props consolidate formwork panels. D. Concrete Casing for Steel Lining Zone

Once moulding at headstalls in mentioned above length was fulfilled, preparations such as cleaning job for section as well as pump line were prepared, concrete was placed in 1.5 to 2 m height lifts.

Accumulated stiffeners encompassing steel segments and rare access to most sections makes self-compacting concrete (SCC) highly fundamental for efficient casing operation of metal parts [3]. Thus multiple mix designs were tested and a well-integrated mix with satisfactory indexes on fresh as well as hardened concrete was proposed [4]. Applied SCC mix design is shown in Table 1.

TABLE I SCC MIX DESIGN, APPLIED FOR STEEL CASING

Mix Name Class Cement Content (kg/m3)

W/C Aggregate (kg/m3)

Admixture % Name

Spread (cm)

FT17 19/30 0.43 380

(0-5)mm (5-9.5) (9.5-19) 1%Gelenium51P 60 1082 176 620



In spite of satisfactory performance and filling ability provided by super plasticizer, miniature voids still remain behind steel and were detected by knocking exercises. Steel was locally perforated and voids were filled through grouting practice. E. Heading Shot Crete of U-Shaped Sections

Initial sketch of U-shaped channel was featured by reinforced concrete in between gate chamber and where shear key ends. However sufficient strength distinguished for extant concrete coating, complicated geometry of the section and intricate formwork required for construction made us to conceive of shot Crete as an alternative to protect exposed steel bars of primary concrete. F. Reinforcement, Formwork, and Concrete Placement of Air Vent Concrete Wall and gates’ threads

After encompassing concrete of metal segments up to gate chamber apron (Elev.98.55) was fulfilled, surrounding concrete of these boxes and emergency as well as service gates’ threads was enhanced against prospective vibrations during lifelong service using reinforced concrete. Thus a Φ25-made reinforcement layer was provided by taking advantage of Hilti drill and extension steel bars planted in section [5]. Metal forms satisfy concrete placement of mutual walls in 2m height lifts when reinforcement was accomplished.

Since first lift has contact surfaces with surrounding concrete of gate threads, rabitz was employed in moulding of inner walls. That way a rough surface is ensured during concrete casting.

When first and second lifts of concrete wall were casted in, 6.2 m far from gate chamber inlet a 2.6m height and 8.65m long formwork was assembled and gates surrounding concrete was executed via two lifts up to finishing surface of gate chamber apron (Elev. 101.15). Second lift is involved with ditches prepared for pertaining facilities and valves, projected for chamber obligations.

Self-compacting concrete (FT17) and structural mix design, T298 properly meet requirements for casing concrete of gates’ box and air vent walls respectively [6]. G. Block Wall of Gate Chamber Entrance

Established block wall protects inside equipment and separates inner and outer space during service, specifically considering air vacation phenomenon.

Two openings were provided in 20 cm thick wall. The bigger is planned in 4×4.5 m dimensions and less sized one in 1×2 m as passage, as shown in Fig.6.

Plates imbedded in air vent walls during concrete casting as well as steel column made of 21NP18 served establishment connections of major opening. Mentioned column was harnessed to base plates embedded in ceiling, thus become much more resistant against vacation originated air currents.

Fig. 6 Outer view of gate chamber- block wall and two openings provided in

H. Ring-Shaped Air Vent Structures (Blocks 1 to 5)

Steel lined zone is followed by concrete lined part in horseshoe section. To prevent cavitation, five ring-shaped air vent structures precede concrete tunnel which were named as first to fifth blocks. First air vent is constructed 85m downstream steel zone and the last one in 396m far from. Second to fifth one of these 5.72m blocks are located in 75m intervals.

Two alternatives had already been proposed at initial sketch. First proposal was featured by 40-80 cm thick reinforced concrete blocks which had to be constructed upon main concrete lining. Each block is followed by 2m wide gap, and then continued by secondary concrete, placed 73m long and reaches next air vent structure.

Second proposal is characterized with elimination of secondary concrete in which principal concrete lining have to be crushed 40 cm deep in air vent locations. Each air vent have to be accomplished in provided key at the same level by major lining surface and turns to a 40 cm raised step, 70 cm prior to the end of 5.72 m long block. Both schemes met technical requirements; however more easy and fast work as well as financial considerations makes second option preferable.



1) Executive Sequences: Once mentioned keys become developed, reinforcement gets started afterwards. Ring as well as semitrailer steel bars arranged in Φ14@100. Up and down networks is skein together with Φ14-made pins in arrangement of 40×20 cm.

To harness reinforcement and make it thoroughly attached with initial concrete some 160 cm long Φ20-made dowels are applied in arrangement of 1×1 m featured by 40 cm bent length [7].

Dowels were planted in 80 cm deep and 32 mm diameter holes which are drilled by Hilti utensil and filled in half by its cohesion. Every voids remaining around dowel are filled by Hilti cohesion as well using pump. In order to provide proper access and maintain quality for drilling, installation, and grouting operation of dowels this steps were accomplished prior to major reinforcement.

Bentonite water stops can satisfactorily watertight joints available between initial concrete and air vent blocks. Bentonite strips are sticked in joints by taking advantage of Hilti cohesion prior to concrete placement for blocks. These strips will expand if any moist become involved with section and easily watertight executive joints in which they are embedded. 2) Concrete Placement

An appropriate mix design named NA102 serves 1100 m3 placement of five mentioned structures. Whose details are shown in Table 2.

TABLE II III STRUCTURAL MIX DESIGN, APPLIED FOR AIR VENT STRUCTURES

Mix Name Class Cement

Content (kg/m3)

W/C Aggregate (kg/m3)

Admixture % Name

Slump (cm)

NA102 19/30 0.43 380

(0-5)mm (5-9.5) (9.5-19) 1% Rheobuild 13 928 371 557

Pump line facilitates discharge of fresh concrete into the section which was carried out in two lifts. First lift is

performed using template. Whereas curved modular forms, connected together become consolidated to established scaffolds by making use of jack heads and applied for second lift to be casted in.

Reinforcement and accomplished apron of air vent structure are shown in Fig.7, 8.

Fig. 7 Fulfilled reinforcement- ready for concrete placement

Fig. 8 Executed apron- reinforcement preparation of wall

I. Block 6

Extant situation of third diversion tunnel outlet has been changed at stilling basin initials. Stilling basin approaching slope was devastated by mechanical hammer in both ends and the remaining had been scratched. Previously used embankment as our primary access way was cut out and concrete surface become exposed. To make monolithic reinforcement with previous concrete some 32 mm diameter holes were perforated by Hilti drill in arrangement of 1×1 m.

In which 1.6 m long Φ25-made anchor bolts featured by 50 cm bents were installed and satisfied consolidating obligation for new reinforcement. Since five aerator structures were assigned as five affiliated blocks, this one has been named sixth block in which Φ25-made reinforcement of type AІІІ in 20×20 cm plan scheme was designed.

After reinforcement fulfillment, whole section was divided into four parts each one separated by rabitz. Shear key molds were implemented at rabitz headstalls as well. NA102 satisfactorily meet requirements for 700 m3 concrete placement of these four parts. Since latter part continues to the same level as the stilling basin apron, river water seeps down embankment materials and interfere with the section, thus had to be obstacle, as shown in Fig.9, 10.

A barrier made of bags filled in clay materials hinders corruption of reinforcement and concrete placement operation as well.



Fig. 9 River water seepage beneath embankment

Fig. 10 Executed parts of block 6- accumulated water at fourth block

J. Energy Dissipater Structure (Block 7)

This structure has been designed at the end of stilling basin available in third diversion tunnel outlet. It is involved with four and three 1.5m height stairs at up and downstream basin respectively. It is notable that downstream stairs were eliminated because of some restrictions outlined afterwards. Approach steps are constructed upon basin apron at elevation of about 72 and continue up to 78. To make construction possible rock fill access road had to be moved out using bulldozer and mechanical shovels. By operation progress remained dike partitioning inner and outer space lost its mass, make it more vulnerable to collapse and caused inward seepage to increase appreciably.

Accomplishment of two layers of shot Crete accompanied by wire mesh, rather enhance stability of earthwork barrier. Whereas water still leaked and corrupted executive operation. Thus a more sever preventive measure was proposed. A 1m wide and 50 cm deep ditch was filled in concrete at the top surface of dike in between outlet walls. After hardening it makes a safe platform for drill wagon to be set upon. Two lines of 10-12 m deep bore holes in 50 cm intervals were drilled. Since rock fill materials are not highly condense re-drilling was necessary because of collapse occurred in some cases. Once drilling was fulfilled and bore holes were equipped by casing, consolidating grouting was performed through which by making use of accelerant involved cement slurries. That way leakage could be controlled and residual inward water could be pumped out as well, as shown in Fig.11.

It is notable that before grouting commencement, pumping was halted to provide equal water surface at bulwark both sides. As a result existent hydraulic gradient diminished and probable escape of slurry was eliminated.

After cleaning of primary concrete surface, chipping operation was performed by pneumatic hammers. Bed surface was perforated in 1×1m scheme by Hilti drill for 1.2m long dowels to be installed in. Each Ф25 dowel is planted in 50cm deep perforation and remaining 70cm lay expose outside, 50cm of which is bent in 90°.

After dowels were planted, AІІІ-made reinforcement was accomplished in which Ф25 steel bars are applied for longitudinal as well as cross poses and Ф14 for shear rebar (pins). Then concrete placement of structure was performed by taking advantage of NA102 mix design afterwards.

When executive processes were terminated and residual earthwork is removed, pumps had been moved out the section. Completed energy dissipater structure is shown in Fig.12.

Thus downstream water surface gradually rises and reaches the same elevation as the river surface in few days.

Fig. 11 Formwork operation of final lift-pump station

Fig. 12 Completed energy dissipater -downstream stilling basin



III. CONCLUSIONS

Dowels or anchor bolts made of Φ25 steel bars which were planted in using Hilti drill and its cohesion could

satisfactorily stabilize reinforcement and make construction more precise. Accumulated stiffeners encompassing steel segments and no access to vibrate casted in concrete make self-

compacting concrete of utmost importance at concrete casing accomplishment. Turning down of initial concrete at steel lining zone, transition as well as bell mouth structure had to precede that

of gate chamber ceiling and U-shaped structure. Installation of buried parts and placement of apron concrete afterwards had been recognized as substantial to make

transportation of metal segments possible. Moulding preparation in 6m long sections and assignment of several lifts make casing placement of steel segments

much more efficient. Contact grouting was revealed as an efficient method to eliminate any porous areas behind steel coating zones by

taking advantage of knocking test, while concrete casing was fulfilled. Installation of metal segments in reduction and gates’ box whereabouts as well as their concrete placement up to

gate chamber apron was concluded as the next fundamental step before similar operation at U-shaped zone. Since rabitz is easy to install and can remain buried, it was capable of increasing progress rate as our beneficial F1

surface type. Employment of two-part semitrailers with overlap can ease reinforcement operation than that of contiguous ones.

ACKNOWLEDGMENT We acknowledge endeavors of Mr. M. Yamin pour as the head of supervision of bottom outlet project and Dr. Sohrabi

master of inspecting system at Upper Gotvand Dam. First author tenders highest respects to his passed away supervisor Dr. Mohmoud Bina. Continuous supports of Prof. A.M. Akhoond ali are greatly appreciated as well.

REFERENCES

[1] R. Morris, “Upgrading of The Bottom Outlet Facilities at Abberton Reservoir”, ICE Journal of Dams and Reservoirs,

vol.22, no.1, pp.11-18, 2012. [2] M. Wieland, Modern Dam Safety Concepts, ICOLD Committee on Seismic Aspects of Dam Design. [3] A. Zabihollah zadeh, B. Zafari, M. Yamin pour, “Multifunctional Use of Self-Compacting Concrete as a

Fundamental Material in Dam Construction: Upper Gotvand Dam”, Key Engineering Materials, vol. 629-630, pp. 391-398, doi:10.4028/www.scientific.net/KEM.629-630.391, 2015.

[4] K. Murthy, N. Rao, R. Reddy, and V. V. Sekhar, “Mix design procedure for self-compacting concrete”, IOSR Journal of Engineering (IOSRJEN), Volume2, 2012.

[5] C.K. Wang, C.G. Salmon, Reinforced Concrete Design, Book, Harper & row publishers, Incorporated, ISBN:0700225145, 1979.

[6] European guidelines for self-compacting concrete, specification, production, and use, EFNARC, 2005. [7] T.J. Freeman, “The Behavior of Fully-Bonded Rock Bolts in The Kielder Experimental Tunnel”, Tunnels and

Tunneling International, vol.10, no. 5, pp.37-40, 1978.

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