Longitudinal ts In Dams 251ul

Longitudinal ts in Dams- Some Case Studies | "MMM" Hydro Power

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Longitudinal ts in Dams- Some Case Studies

M M Madan Director (Hydro) - GVK

Ex Executive Director-NHPC Introduction Normally the provisions of longitudinal ts in Concrete Gravity Dam are made to achieve necessary temperature control and to prevent cracks parallel to the length of the dam. This is done in case of relatively high dams by subdividing the monolith into several blocks by Longitudinal Contraction ts. Subsequently these Longitudinal Contraction ts are grouted to ensure monolithic action. For consideration of Safety of the structure, the IS Code (Indian Specifications) advocates elimination of longitudinal ts. Basically IS Code recognizes the practice of dividing a monolith block into two or more blocks by introducing ts parallel to the axis of Dam as unsound. However if necessary, a high degree of perfection has to be ensured by providing suitable shear keys and then properly grouting the ts to create a monolithic block. However, the necessity of vertical or inclined discontinuity is sometimes required due to the reasons other than “thermal behaviour and crack control”. One such reason may be due to tight construction program and adoption of methodology. This paper focuses on Dams where “Longitudinal Contraction ts” have been provided successfully and the dams are functioning properly for a long time.

ts in Concrete Structures Creating a construction t in a large gravity dam is common practice and is applied on majority of dams. Where the dam is very thick, longitudinal ts are necessary to control random cracking, to accommodate volumetric changes and to facilitate construction. These should be able to transfer compressive and shear stresses and must be grouted after cooling (ICOLD bulletin 107 on concrete dams). ts are necessary in concrete structures for a variety of reasons. All the concrete may not be placed continuously in a structure under construction, therefore the construction ts are provided to allow for the work to be resumed after a certain period of time. Principally due to shrinkage and temperature changes, the concrete undergoes volume changes; therefore, it is desirable to provide ts to relieve tensile or compressive stresses that would be induced in the structure. By adopting specific construction measures, the homogeneity of the dam body shall be ensured by providing shear keys casted on the internal face of both blocks that will assure the transmission of shear stresses if any. After concreting of the area between the two blocks, the grouting will ensure that the structure is monolithic. In the second stage of concreting, internal ts are an acceptable practice as far as they allow an efficient and sound transfer of loads / stresses.

Longitudinal and Transverse Contraction ts in Dams A contraction t is formed by vertical or inclined surface between masses of concrete/ masonry placed at different times. The ts divide the dam into convenient sized monoliths to permit convenient and systematic construction and to prevent the formation of haphazard ragged cracks due to volume changes that cannot be prevented, Transverse Contraction ts

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It is good practice for normal methods of construction to provide contraction ts in gravity dams. The spacing of transverse contraction ts shall be such as to suit the methods of construction, materials of the dam, the foundation conditions and the convenience of the location of control gates outlets etc. They are usually spaced about 15m apart, experience having shown that cracks are likely to develop in monoliths much wider than this. It is however, essential to locate the ts to best advantage relative to the shape of the abutments. The general requirement is that each t extends entirely through the structure. Longitudinal Contraction ts For large structures the problems of cooling large masses of concrete are enormous. One of the measures used to control cracks parallel to the length of the dam in case of relatively high dams is to subdivide the monolith into several blocks by longitudinal contraction ts and subsequently grout these ts to ensure monolithic action. The spacing of ts is largely dictated by convenience of construction and foundation conditions. A spacing of 20-30m is generally adopted. There is also now a school of thought which believes that the longitudinal ts need to be at very close spacing of about 15m or even omit them all, since there are doubts of the final behaviour of dams built in such a manner. IS Code (Indian Specifications) advocates elimination of longitudinal ts for the consideration of safety. Basically IS Code recognises the practice of dividing a monolith block into two or more blocks by introducing ts parallel to the axis of Dam as unsound. However if necessary, a high degree of perfection has to be ensured by providing suitable shear keys and then properly grouting the ts to create a monolithic block. IS Code:6512 on “Criteria for Design of Solid Gravity Dams” vide clause 7.1.1.1 states that “It is now being increasing accepted that better alternative is to achieve necessary temperature control by pre-cooling of concrete supplemented wherever necessary by post-cooling and avoid longitudinal ts altogether, even in case of high dams.” However, the necessity of vertical or inclined discontinuity is sometimes required due to the reasons other than “thermal behaviour and crack control”. One such reason may be due to tight construction programme and adoption of methodology. The homogeneity & monolithicity of the dam body has to be ensured by provision of shear keys cast on the internal face of both the blocks and grouting performed after the concreting of the area between the two blocks. Further, a detailed stability analysis of the dam needs to be carried out considering the longitudinal ts for all relevant load cases and uplift conditions. There are examples of Bhakra Dam & Dulhasti Dam in India, Ravedis Dam in Italy, Les Toules Arch Dam in Switzerland, Bolargue Dam, Irabia Dam & Grand Dixence Dam in Switzerland where such longitudinal ts have been provided successfully.

Dulhasti HE Project The 390 MW Dulhasti Hydroelectric Project is a run of the river scheme located on river Chenab in District Doda, J&K- India. The project consists of a diversion dam at Dul with two intakes, one to provide water for the first stage and another to provide water for the proposed second stage. There are two underground desilting chambers with necessary flushing conduits and two more desilting chambers has already been constructed for the proposed second stage. The head race tunnel is 7.7 m dia and 10.6 km long. Due to changes in alignment, it doesn't have a uniform slope from upstream to downstream and hence an air-vent shaft of 400 m high has been provided to avoid airlock. An underground surge shaft of 18.5m diameter, 117m deep with other ancillary tunnels has been provided. Three penstocks of 4.8 m dia and 141 m depth take-off from surge shaft. The underground power house with installed capacity of 390 MW (3x130 MW) with all the appurtenant structures is located at Hasti. The construction work of the project was awarded to a French consortium Dumez-Sogea-Borie SAE in October 1989, but was abandoned in August 1992. The work remained standstill for a period of about three years (between 1995 to 1997). The tunnelling work was carried out departmentally. Then the work was again retendered and

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awarded to the Indo-Norwegian t venture Jaiprakash – Statkraft (JSA) during April, 1997. The project was finally commissioned in the year 2007. Geology of the Project Area Geologically, the rocks of Kishtwar area are classified into two Groups viz. Older Kishtwar Group and Younger Sinthan Group, corresponding to the metamorphic rocks and younger sedimentary and volcanic rocks respectively. The of these two groups is marked by a thrust designated as Chhattru Thrust, which is having a sinusoidal alignment. There were presence of other regional faults viz. Kishtwar Fault (runs almost N-S) and Daddhar – Buzensheru Fault within Kishtwar Group. Structurally, the area was very disturbed and subjected to intense folding and faulting. Kishtwar fault was a significant fault in the area and separating schist and gneisses of Salkhala Formation and Quartzite - Phyllite sequence of Dul Formation under the Kishtwar Group of rocks. This fault was trending in NNW – SSE direction and dipping 650 in westerly direction. The Kishtwar plateau (fossil valley), later on, is designated as “Graben” based on kinematic mechanism and sub-surface explorations carried out for HRT of the Project by French Consortium.

Dulhasti Dam The Dul dam is a mass gravity concrete dam, measuring 65 m high above foundation with a crest length of 186m. It serves the primary function of hydroelectricity. In Dul dam longitudinal and vertical ts were provided as a construction requirement. These ts were provided with shear keys casted on internal face of both the blocks for transmission of shear stresses. These ts were later on grouted to ensure monolithic structure in the dam. Necessary provisions were left during construction stage to ensure grouting after completion. The grouting was done successfully. The dam is in operation and behaving correctly.

[http://4.bp.blogspot.com/-26XrHU7k6MA/UhogIi7OaPI /AAAAAAAAINY/Je4LvggBAYU/s1600/X+section+dam.jpg]

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[http://2.bp.blogspot.com/-Jlel1cUs1lA/Uhofrfr63dI/AAAAAAAAINE/BvhveiE8dVA/s1600/Dulhasti+Dam.png]

Dul Dam Cross Section - Longitudinal t parallel to Dam Axis can be seen

[http://2.bp.blogspot.com/-pMd37LtFBZc /Uhogj6XqSMI/AAAAAAAAINg/6VIgXPGjXMc/s1600/Dul+dam+plan.jpg] Dul Dam Foundation Plan - Longitudinal t parallel to Dam Axis can be seen

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[http://3.bp.blogspot.com/-GJit1zmhY5Y/UhogDaK2vI/AAAAAAAAINQ/QIIxFedT5j0/s1600/vertical+ts-+Dulhasti.png]

[http://2.bp.blogspot.com/-wP_1FJLEK4Q/UhojzD4kF_I /AAAAAAAAINw/G4s-BLWbuFU/s1600/Dul+Grouting+arrangement.jpg]

Dul Dam – Grouting System Arrangement

Ravedis Dam The Ravedis dam in Italy is a mass gravity concrete dam, measuring 51 m high above foundation with a crest length of 173m and a reservoir storage volume of 25 million cum. It serves for Flood Control, irrigation and hydroelectricity. The construction of the Dam was completed in November, 2004. During construction of the monolithic basement of Ravedis dam, 8 (eight) different blocks were casted and connected together by ts, parallel to the dam axis. Particular attention was given to the t treatment of monolithic basement blocks in order to guarantee the monolithic characteristic requested for the foundation block.

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[http://4.bp.blogspot.com/-HCz14oXuf08/UhollWb944I/AAAAAAAAIQc/Sw6gD5idfuU/s1600/Ravedis+Foundation+Plan.png]

[http://3.bp.blogspot.com/-ssXl3NvP1WI/UhollpRO4NI/AAAAAAAAIQs /EbA0FjZB7jA/s1600/Ravedis+Dam.png]

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[http://3.bp.blogspot.com/-fU-s4D67ZPo/UhollnKmG1I /AAAAAAAAIQo/YZQDVI51Mss/s1600/Ravedis+X+section.jpg]

Cross Section of Ravedis Dam - Longitudinal ts parallel to Dam Axis can be seen

Les Toules Arch Dam Les Toules Dam in Switzerland is a 86m high double curvature Arch dam with a crest length of 460m. The first dam was built as single curvature arch dam in 1958. The heightening of this dam was carried out in 1960-1964 and converted to Double curvature Arch dam. Particular design was very slender shape without abutment thickening, high vertical curvature towards valley and no shear keys. The dam foundation lies on gneiss and mica schist rocks forming alternate subvertical strips almost parallel to the valley. In 2003, upgraded seismic regulations and recommendations of Swiss Federal Office of Energy required the dam owners to reassess the safety of their structures. In this context comprehensive studies of the existing dam structure and its behaviour were carried out, and dam strengthening was required. This arch dam was rehabilitated by adding buttresses on the downstream face of the dam, creating a longitudinal curved t between the existing structure and the new concrete structure transferring the load from the overloaded cantilevers to the thickened arches, creation of shear keys in the vertical ts, local foundation treatment and some other secondary rehabilitation works. To strengthen the dam, abutment thickening was done in both abutments, shear columns were provided, for seismic purpose downstream face was provided with 30,000cum of concrete. The bonding between old and new concrete was done with proper t treatment by hydro-demolition with high pressure rotating water jets. The between the existing dam and the new concrete was not grouted to prevent any jacking effect because of grout pressure. After rehabilitation, the final monolithic structure behaved satisfactorily along expected lines.

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[http://2.bp.blogspot.com/-TrgCHftwxKU/Uholk23tRjI/AAAAAAAAIQU/gPe-pTfDDk/s1600/Les+Vincas.png]

[http://1.bp.blogspot.com/-Av1Byk6Lr0U/Uholj9rGvHI /AAAAAAAAIQE/KX3LohV0zmw/s1600/Les+Toules.jpg]

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[http://1.bp.blogspot.com/-ckGkBD6vEOE/Uholm0Xhl7I/AAAAAAAAIRA /0liD2mkEpdA/s1600/rehabilitation+Les+Toules.jpg]

[http://3.bp.blogspot.com/-e6cqwaWbO7I/UholhN1wAiI/AAAAAAAAIPQ/QjZNrVcsUxg/s1600 /End+Block+les+toules.png]

Irabia Dam Irabia Dam was heightened three times and the fourth heightening is being planned.

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[http://3.bp.blogspot.com/-pWHgZ5YG_nw/Uholikxko0I/AAAAAAAAIPw /7w-VIc-mjEA/s1600/Irabia+Dam+stages.png]

The figure above shows the original profile over which three other profiles of the extended portion. The profile of the fourth extension being planned can be seen.

Bolarque Dam The Bolarque Dam was heightened & the base width was increased. Galleries were also provided at the foundation & at the top of the existing dam to take care of the uplift forces.

[http://4.bp.blogspot.com/-5L-77kMxhSI/UholdAFmk6I /AAAAAAAAIOg/jqLPUAHMQ0A/s1600/Bolarque+dam.png]

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[http://4.bp.blogspot.com/-XhjZ3D-ge7k/Uholj3xSIwI /AAAAAAAAIQA/bVU9mzVVm6U/s1600/Irabia+photo.jpg]

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Grand Dixence Dam 285m tall Grand Dixence Dam is located in Switzerland in Canton of Valais on the Dixence River. The river supplies water to the Rhône Valley. It is the highest Gravity Dam in the world. It is 695m long with a base width of 200m. It was carefully planned to be built in successive stages. The hydroelectric power plant has a capacity of 2000 MW. It holds back Lac des Dix which is 3.65 Sq. km (902 acres) in area. It holds 400 million cum of water and is 284m deep. It is the 5th tallest dam in the world & highest gravity dam in the world. The Grout curtain extends 200 m deep and 100 m outward in each direction. First Dixence dam was built in 1929 and construction on the new dam started in 1950. The New dam submerged the old dam.

[http://1.bp.blogspot.com/-BX8x1tX5zNk/UholhvYn1DI/AAAAAAAAIPY /527Sejv0YUE/s1600/Grand+Dixene.png]

Figure shows the profiles for each stage. Keys were provided at each stage for better bonding. Longitudinal ts were created in the process.

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[http://1.bp.blogspot.com/-MmlauEM3cNk/Uhoos3E3-CI/AAAAAAAAIRQ /6h9XrDv3POk/s1600/Grand+Diexene+stages.png]

Grand Dixence Dam Cross Section – Figure shows the years in which different blocks have been Constructed- Longitudinal ts created during successive stages can be seen

Bhakra Dam Bhakra dam in India is a mass concrete gravity dam of 225.55m height located on Satluj River corresponding to a total volume of 3.82 million cum of concrete. It is located in an area with large seasonal temperature variation. The dam construction was started in 1955 and completed in 1963. Several remedial measures were taken to avoid cracks in this large massive structure. The dam body was segmented in a series of blocks by traverse and longitudinal ts. Without longitudinal ts for such a large concrete structure, stoppages or delays in the construction during the winter time would have involved cracking of the dam body. Cross section of the dam body shows the ts provided during construction in Bhakra Dam.

[http://2.bp.blogspot.com/-psjBUnLrWzs/UhopFhvxf0I/AAAAAAAAIRY /8pjuvOPcYfs/s1600/Bhakra+Dam.jpg]

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Cross- section of Bhakra Dam

Conclusion The various examples cited above where Longitudinal Contraction ts have been provided due to varying reasons. Concrete Gravity Dams with Longitudinal Contraction ts can be planned when necessitated due to reasons such as construction programme and ultimately the viability of the project itself. However, utmost care has to be taken so that a high degree of perfection is accomplished in ensuring monolithicity by providing suitable shear keys and successfully grouting all the ts. These examples demonstrate that with proper planning, adopting strict quality control measures and maintaining all necessary design requirements, there is no reason as to why Longitudinal Contraction ts cannot be adopted in large concrete dams.

References New York, VI Congress of ICOLD 1958 Rome, VII Congress of ICOLD 1961 New Delhi, XIII Congress of ICOLD 1979 Florence, XIX Congress of ICOLD 1997 Muller, O., Wohnlich, A., Safety Enhancement and Strengthening of Les Toules arch Dam. Posted 4 weeks ago by MM Madan

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