Construction of concrete gravity dam
Regardless of the type of dam, whether concrete or embankment types, it is necessary to de-water the site for final geological inspection, for foundation improvement and for the construction of the first stage of the dam. In order to carry out the above works the river has to be diverted temporarily. The magnitude, method and cost of river diversion will depend upon the cross- section of the valley, the bed material in the river, the type of dam, the expected hydrological conditions during the time required to complete the dam construction works, and finally upon the consequences of failure of any part of the temporary works.
For concrete dams, it may be necessary to divert the river during the first phase of the construction of the dam. Once this is complete, the river may be allowed to overtop the dam and flow without causing serious damages to the structure or its foundation. For concrete dams, sluice openings are left open in the first stage of concreting and the higher stages constructed. If the second stage outlets are too small for the flood to pass, they would be submerged after the whole works. At some sites, virtually no risk can be afforded. For the Ukai dam on river, Tapi, which is 4927 m long and maximum height of 68.6 m, no risk of overtopping and possible destruction of the control section could be accepted in view of the very large resident population downstream of the dam. Hence, a diversion channel was excavated to carry 49500 m³/s passed through the blocks of the concrete gravity dam section that was intentionally left low.
The Bureau of Indian standards code IS 10084 (part 2) -1994 “Design of diversion works – criteria” describes the design criteria for diversion channel and open cut or conduit in the body of the dam.
At sites where diversion of flow through tunnels or close conduits is not possible (due to topographical considerations) or proves to be uneconomical, diversion through excavated channels called diversion channels is effected. Diversion channels are often classified according to the type of diversion namely, single stage or multiple stage diversion scheme. In the former which is more suitable for narrow valleys, the same set of diversion channel and coffer dams is utilised throughout the period of construction. In the latter, which is generally suitable for wide valleys, the channels and coffer dams are shifted from place to place in accordance with phasing of the work. A more useful classification, however, is based on the type of the dam to be constructed namely diversion channel for masonry or concrete dams and that for the earth or rockfill dams. The following paragraphs taken from Bureau of Indian Standards code IS: 10084 (Part 2) – 1994 “Design of diversion works – criteria” provides criteria for diversion channels for dam construction.
A concrete or masonry dams could be allowed to get overtopped during floods when construction activity is not in progress. The resulting damage is either negligible or could be tolerated without much concern. Therefore, it is customary to adopt diversion flood which is just adequate to be handled during non monsoon season, when construction activity of the dam is continued. Generally the largest observed non-monsoon flood or non-monsoon flood of 100 year return period is adopted as a diversion flood. This is generally a small fraction of the design flood of the spillway and, therefore, diversion channel required to handle this flood is obviously small. Advantage is also taken of passing the floods over partly completed dam or spillway blocks, thereby keeping the diversion channel of relatively smaller size. In such a case a small excavated channel either in the available width of the river or one of the banks of the river proves to be adequate.
Construction sluices are located in such excavated channels which allow passage of non-monsoon flows without hindrance to the construction activity. Such sluices are subsequently plugged when the dam has been raised to adequate height. If the pondage is not allowed even when the dam has been raised to sufficient height, the river outlets are often provided in the body of the non overflow or overflow dam to pass the non monsoon flows which later on are kept for permanent use after completion of construction. If the diversion channel is excavated on one of the river banks, it is possible to use the same for locating an irrigation outlet, a power house or a spillway depending upon the magnitude and purpose of the project. Figures 40 and 41 show typical layouts of diversion channel for masonry/concrete dams in wide and narrow rivers respectively.
Figure 40. Diversion Channel for Concrete Dam in a Wide River
Figure 41 .Diversion Channel for Concrete Dam in a Narrow River
Preparation of foundation for dam construction
A concrete gravity dam intended to be constructed across a river valley would usually be laid on the hard rock foundation below the normal river overburden which consists of sand, loose rocks and boulders. however at any foundation level the hard rock foundation, again, may not always be completely satisfactory all along the proposed foundation and abutment area, since locally there may be cracks and joints, some of these (called seams) being filed with poor quality crushed rock. Hence before the concreting takes place the entire foundation area is checked and in most cases strengthened artificially such that it is able to sustain the loads that would be imposed by the dam and the reservoir water, and the effect of water seeping into the foundations under pressure from the reservoir.
Generally the quality of foundations for a gravity dam will improve with depth of excavation. Frequently the course of the river has been determined by geological faults or weaknesses. In a foundation of igneous rock, any fault or seam should be cleaned out and backfilled with concrete. A plug of concrete of depth twice the width of the seam would usually be adequate for structural support of the dam, so that depth of excavation will, on most occasions depend upon the nature of infilling material, the shape of the excavated zone and the depth of cutoff necessary to ensure a acceptable hydraulic gradient after the reservoir is filled. An example of this type of treatment for Bhakra dam is shown in Figure 42.
Figure 42. Foundation Feature for Bhakra Dam
Improvement of the foundation for a dam may be effected by the following major ways:
Grouting of the foundation of the dam to consolidate the entire foundation rock and consequently increasing its bearing strength is done by a method that is referred to as consolidation grouting. This is a low pressure grouting for which shallow holes are drilled through the foundation rock in a grid pattern. These holes are drilled to a depth ranging from 3 to 6 m. Prior to the commencement of the grouting operation, the holes are thoroughly washed with alternate use of water and compressed air to remove all loose material and drill cuttings. The grout hole are then tested with water under pressure to obtain an idea of the tightness of the hole which is necessary to decide the consistency of the grout to be used and to locate the seams or other openings in the rock which are to be plugged .
The grout is then injected with these holes at relatively low pressure which is usually less than about 390 KN/m². Since this is a low pressure grouting it is accomplished before any concrete for the dam is laid. This grouting results in the consolidation of the foundation into more or less monolithic rock by bonding together the jointed or shattered rocks. Some of the recommendations for grouting under pressure in rock foundations have been taken from Bureau of Indian Standards code IS: 6066 – 1994 “Pressure grouting of rock foundations in river valley projects – recommendations” have been presented in the following paragraphs.
Methods of rock grouting
Rock grouting consists essentially of drilling a series of grout holes in rock and injecting grout under pressure, which eventually sets in the openings and voids in the rock. The drilling and grouting operations can be carried out either to the full depth in one operation or in successive depths either by stage grouting or packet grouting. Grouting in the valley should proceed from river bed to the abutments. There are two broad methods for grouting: Full depth grouting and stage grouting.
In the full depth method each hole is drilled to the full desired depth, washed, pressure tested and grouted in one operation. This method is usually limited to short holes 5m or less in depth or holes up to 10 m that have only small cracks and joints with no risk of surface leakage. In deep bore holes high grouting pressures have to be used for proper penetration of grout at an economical spacing of holes. As full depth grouting involves the risk of disturbance in the upper elevation it is not generally considered for grouting deep holes. For grouting in heterogeneous strata, where the nature of rock discontinuities is subject to large variations in relation to the depth, full depth grouting is not recommended and stage grouting is preferred to packer grouting in such cases.
Stage grouting is done by drilling the holes to a predetermined depth and grouting this initial depth at an appropriate pressure to its final set (within 2 to 4 hours) deepened for the next stage. Alternatively the grout is allowed to harden and re drilling is carried out through the hardened grout and the hole extension to the next stage. In another procedure called the one stage re drilled method, which is sometimes used grout is washed out within a small depth of the top of the stage being grouted and only one stage is re drilled for proceeding to the next stage. In each of procedures the cycle of grouting-drilling-washing-re drilling is repeated until the required depth of the hole is reached.
General criteria for size and depth of grout holes
The pattern and depth of holes is governed primarily by the design requirements and the nature of the rock. When the purpose is consolidation, the holes are arranged in a regular pattern over the entire surface area required to be strengthened and the depth is determined by the extent of broken rock as well as the structural requirements regarding the deformability and strength of the foundation. When the purpose is impermeablisation the grout holes are arranged in a series of lines to form a curtain approximately perpendicular to the direction of the seepage. The depth of holes is dependent on design consideration as also on the depth of pervious rock and configuration of zones of relatively impervious strata.
The size of grout holes is generally less important than the cost of drilling holes and the control of the inclination. For grouting with cement, 38 mm holes are used. The advantage gained by drilling large holes does not often justify the increase in drilling costs. In long holes the diameter at the top of the hole may have to be larger than the final diameter at the bottom of the hole to facilitate telescoping or allow for the wear of the bit.
Patterns of holes for curtain grouting
Single lines grout curtains are effective only in rocks having a fairly regular network of discontinuities with reasonably uniform size of openings. In such cases a curtain of adequate width can be achieved by grouting a single line of holes. In massive rocks, with fine fissures uplift control is primarily obtained by drainage and the grout curtain is only used as a supplementary measure to avoid concentration of seepage which may exceed the capacity of the drainage system. Single line curtain may serve this limited objective in comparatively tight rock formations.
In single line curtains (Figure 43), it is customary to drill a widely spaced system of primary holes, subsequentially followed by secondary and tertiary holes at a progressively small spacing. The usual practice is to split the spacing from primary to the secondary to the tertiary phase. One of the criteria for deciding on the primary spacing is the length of expected intercommunication of grout between holes. The initial spacing usually varies from 6m to 12 m but the choice of spacing should be based on geological conditions and on experience. At every phase of the grouting operation, the results of percolation tests and ground absorption data should be compared with the previous set of holes in order to decide whether a further splitting of the spacing of holes in worthwhile.
When no significant improvement is noticed either in terms of decrease of the grout absorption or water percolation, careful review should be made of rock features, the nature of the rock and its relation to the pattern of holes. Sometimes it may be more advantageous to drill another line of holes at a different angle and orientation than to split the spacing further. Spacing below 1 meter are rarely necessary and the requirement of a spacing closer than 1 meter often indicates an unsuitable orientation and inclination of holes. Possibly multiple line curtains may be necessary. If the area is too limited, the setting time of the grout becomes important since it is not desirable to drill close to a freshly grouted hole. Before pressure grouting is started, drilling of all the holes should be completed within a distance of 20 m of the hole to be grouted.
FIGURE 43. Single line of grout holes below a concrete gravity dam
Depending upon initial investigation and strata conditions, the spacing of primary hole treatment should be decided. If the primary holes were spaced more than 6 m apart secondary holes should be drilled and grouted. On completion of primary holes spaced closer than 6m or secondary holes (when the primary holes are spaced more than 6 m), should the percolation tests carried out in a few test holes indicate that further grouting of the area is necessary, secondary, or tertiary treatment as the case may be, should be carried out systematically thereafter in the whole area or in the particular section where the rock conditions are bad. Similarly tertiary holes should be taken over the whole area or the full length of the section which requires the treatment.
In addition to the systematic grouting of primary Secondary or tertiary and subsequent holes it may be necessary to drill and grout additional holes for treatment of peculiar geological feature such as faults, sheared zones and weathered rock seams.
Pattern of holes for consolidation grouting
The choice of pattern of holes, for consolidation grouting depends on whether it is necessary to wash and jet the hole systematically. When washing has to be carried out a hexagonal pattern (Figure 44) would be preferred as this admits for flow reversal. When systematic washing and jetting is carried out to remove all soft material in seams it is generally not necessary to use a primary and secondary system of holes.
When it is desirable to test the efficacy of consolidation grouting by comparing the grout absorption in primary and secondary holes a rectangular or square pattern (Figure 44) of holes would be preferred. This is generally the case when the joints are irregular and relatively free from in-filling or it is not necessary to remove the material filling the joints.
Figure 44.Typical Patterns for Consolidation Grouting
Rock grouting is normally performed with a mixture of cement and water with or without additives. The cement should be ordinary Portland, Portland Pozzolana, Portland slag, Supersulphated or Sulphate-resisting Portland. The solid materials which may be used as additive to the grout mixture could be Paxxolanas (such as fly ash and calcined shade), fine sand or other fine non-cementeitious materials like clay and silt. While using additives constant field checks and review should be undertaken to achieve the desired results in respect to permeability and strength. Admixtures when added in small quantities to the grout mixture impact certain desirable characteristics like delaying or hastening setting time and increasing the workability.
The entire grouting operation is carried out by first drilling and then injecting the grout under pressure. The various types of drilling equipments can be grouped as under
A. Precursive Drilling equipment
a) Standard drifter or wagon drill
b) Dom the hole drilling equipment, and
c) Overburden drilling equipment
B. Rotary drilling equipment with suitable drive, that is hydraulic, electric, diesel or compressed air Precussive drilling methods are generally more economical in all kind of rocks. For deep rocks it may be advantageous to use overburden drilling equipment. By virtue of the greater rigidity of the casing tube combined with the drill rods, better control on inclination of holes can generally be achieved in the overburden drilling equipment. Down the hole hammer is also capable of maintaining a better control on the inclination. However, the hammer may get clogged when the drill cuttings form slush in form saturated strata and cannot be removed by air flushing.
During precussive drilling in stratified rocks where the resistance of the rock is prone to variation the holes may get curved and control on inclination may be lost. In such cases guide tubes may be used for ensuring verticality of the holes or alternatively rotary drilling may be used. Irrespective of whether air or water is used for flushing the hole during drilling, thorough cleaning by water flushing is essential before starting grouting operations.
The major equipment required for carrying out grouting are Grout Mixer and Grout Pump. These are explained below.
Grout mixer: The mixer should have two tank namely mixing tank and agitating tank. Mixers are generally cylindrical in shape, with axis either horizontal or vertical or equipped with a system of power driven paddles for mixing. Grout should be mixed in a mixer operating at 1500 r.p.m. or more. The high speed of mixing serves the purpose of violently separating each cement grain from its neighbour thus permitting thorough wetting of every grain. This proves to be advantageous by chemically activating each grain to through hydration before reaching its final resting state. Further individual grains penetrate finer cracks more readily than flocs. Vertical barrel type mixers have proved satisfactory when small mixers are required for use in confined or limiting working spaces. This type of mixer consists essentially of a vertical barrel having a shaft with blades for mixing, driven by a motor mounted on top of the mixer above the barrel. Centrifugal pump mixers mix the grout by re circulating it through a high speed centrifugal pump. They are sometimes referred to as colloidal type mixers, but they don’t achieve a true colloidal grout mix. However they possess considerable merit and produce grout of excellent texture. When mixing sand-cement grouts their action tends to guard against segregation.
Grout pump: A pump suitable for grouting should permit close control of pressures, allow a flexible rate of injection, and be designed to minimize clotting of valves and ports. Grout pimps are of three types namely, piston, screw, and centrifugal.
Washing and testing of holes and surface preparation
Any grouting operation requires major washing of the holes, testing of the holes with water under pressure and surface preparation. The purpose of washing the holes is two fold. First to clean the hole to remove the material deposited on the surface during the drilling operation and second to provoke deliberate inter-connections between adjoining grout holes to remove known seams and layers of erodable material. It should be bornein mind that inter-connections between holes are effective only if he washing operations are carried out systematically to remove all the soft material. Isolated inter-connections don’t serve much useful purpose as soft material may still remain in position in a known and irregular pattern. A distinction is therefore made between washing of holes at the end of the drilling operation and systematically washing of group of holes in order to remove the erodable material in the intervening area for which the term jetting is used.
Washing of holes
On completion of a drilling of a stage and before injection, the holes should be washed by allowing drilling water to run until the return from the hoe is reasonably clean. The quantity of water flowing into the hole during the period should be adequate and generally not less than 15 l/min.
When no return of drilling or washing water occurs, the holes should be washed for a reasonable period based on site experience. This is generally for 20 minutes. If an abrupt loss of drill water occurs during drilling and similarly when a strong flow of artesian water is encountered, the drilling should be stopped and the hole grouted even if it has not reached its final depth.
For routine grouting operations, and simple water test conducted before and after grouting, the test pressure should be limited so as to avoid hydraulic fracture. The value of limiting pressure for various strata and depths should be established by preliminary investigations where cyclic tests should be conducted to evaluate pressure at which fracturing occurs. Additional tests may be carried out in trial grouting plots or in selected primary grouting holes to verify the pressure limits established during preliminary investigations.
Water percolation tests may be used to measure the effectiveness of the grouting treatment. The tests may be simple or cyclic. Cyclic testing is recommended for evaluation stage while before and during grouting operations simple tests should be carried out.
Water tests should be carried out in primary stages before injection to amplify information available from the site investigation. Tests should be carried out in secondary stages before injection to indicate the results of primary injections. Test may be carried out in individual test holes at any time to indicate the results of all treatment carried out before that time. Test holes drilled for this purpose should be sited midway between completed injection holes.
Jetting operation are carried out in order to deliberately provoke connection between bore holes and to remove known deposits of erodable material. Jetting should be carried out in group of holes arranged in a square, triangular or hexagonal pattern known as cells.
For effective treatment of the surface zones, sufficient pressure should be developed to achieve the spread required witH a convenient spacing of holes. Adequate cover should be maintained during grouting to ensure that adequate pressure is applied without causing upheaval or excessive surface leakage.
Injection of grout
As for the method of injection, grout holes should be injected by direct connection to the pump. Each pump should be provided with a packer at the surface or with a short strand pipe threaded at its outer end to accept stand or control fittings, which should be provided with a pressure gauge, bleeder valve and a valve enabling delivery from pump to be cut-off from the hole. Either single line or circulation system may be used, usually circulating system is preferred, however when adequate controls are possible to regulate the pump discharge and pressure by using pumps of suitable design, single line grouting system can be used.
Once the grouting of stage or group of holes has been commenced it should be continued without interruption up to completion. In general a stage may be considered complete when the absorption of grout at the desired limiting pressure is less than 2 l/min averaged over a period of 10 minutes.
As far as practical a continuous flow of grout should be maintained at the desired pressure and the grouting equipment should be operated to ensure continuous and efficient performance throughout the grouting operation. After grouting is completed, the grout holes should be closed by the means of a valve to maintain to grout pressure for a sufficient period to prevent escape of the grout due to back pressure and flow reversal, due to causes like artesian conditions. For this purpose a period of one or two hours is generally sufficient, however this should be verified by trial.
The grouting pressure should be adequate to achieve the desired grout and the pressure should be limited so as to avoid disturbances and upheaval of the ground and should take into account reservoir pressure.
For structures on rock foundations, it is a basic requirement that no disturbance should be caused to the surface zones of the foundation by the grouting operation. When grouting is undertaken below an existing structure no upheaval of the foundation can be allowed as it would have very harmful consequences on the structure and/or the equipment. In general the disturbance caused by the grouting is dependent more on the manner in which the pressure is developed and the nature of the rock than on the absolute magnitude of pressure. Relative higher pressures can be sustained without damage to the foundations, when pressure is built up gradually, as resistance to flow is developed by deposition of grout On the other hand when pressures are raised hastily damage can occur even at relatively low pressure. In general, horizontal stratified or low dripping rocks are more vulnerable to disturbance by grouting pressure than fractured igneous or metamorphic rocks or steeply dipped sedimentary rocks. Rocks previously
subjected to folding or fracturing or rocks in the process of adjustment after removal of overburden load are also more vulnerable to disturbances.
The most common difficulty experienced in consolidation grouting is surface leakage. It is therefore customary to pipe through the entire height of concrete or masonary and carry out the grouting after the rock has been completely covered. This not only eliminates surface leakage but permits use of higher pressure so that even the smaller seams can be grouted effectively.