Design and Construction of Concrete Gravity Dams (Part -14) - Civil Engineering (CE) PDF Download

Instrumentation 

Numerous embankment dams constructed in India and abroad and the height achieved ever increasing, like the Nurek Dam of Russia (Figure 57). From the point of view of safety as well as to garner knowledge about the physical behaviour of these dams, instrumentation has been recommended all medium and large sized dams. The data obtained from these measurements also help the commonly made either explicity or implicity in an embankment dam design. In fact, it is very important to monitor the behavior of the dams under earthquake loadings and those constructed in regions of high seismic activity need to be instrumented carefully. 

Figure 57. Elevation and cross section of Nurek embankment Dam,Russia

Even for the continued maintenance of any embankment dam, vertical and timely observations of the measurements taken will provide means of evaluating the behaviour of the structure and, if need be, allow the engineers to take appropriate remedial measures on the basis of the observed data. Hence, the importance of providing instruments in an embankment dam, or any dam for that matter, cannot be over emphasized. The Bureau of Indian Standards code IS: 7436 (part1)-1993 “Guide for types of measurements for structures in river valley projects and criteria for choice and location of measuring instruments (Earth and Rockfill dams)” provides guidelines for various types of instrumentation to be carried out in embankment dam. The following paragraphs highlights the salient features of the recommended measurements to be taken and the corresponding instruments that have to be installed. 

Pore Pressure 

The measurement of pore pressure is probably the most important and usual measurement to be made in the embankments. Their measurement enables the seepage pattern set up after impounding of reservoir to be known, the danger of erosion to be estimated, at least partially, and the danger of slides in the dam and abutments to be estimated if the reliable shear strength is known. Valuable information about behaviour during construction and drawdown is obtained. 

Movements 

Measurement of movements is as important as the measurement of pore pressures. Movements conforming to normal expectations are basic requirements of a stable dam. An accurate measurement of internal and external movements is of value in controlling construction stability. The measurement of the plastic deformation of the upstream and downstream slopes under the cycles of reservoir operation may indicate the likely development of shear failure at weak points. 

Seepage

Measurement of seepage through and past a dam, may indicate erosion or blocking of downstream drains and relief wells, by increase or decrease of seepage, respectively at constant reservoir conditions. Seepage and erosion along the lines of poor compaction and through cracks in foundations and fills may specially be indicated by such measurements. 

Strains and Stresses 

Design analysis of earth and rockfill dams is based on radical simplifications of the stress pattern and the shape of the rupture planes. Stress measurements, therefore, require considerable judgement in interpretation. Accurate measurement of stress is difficult and distribution of stress in earth and rockfill dams is complex. Strains may be calculated from displacements or measured directly. 

Dynamic Loads (Earthquakes) 

Earthquake causes sudden dynamic loading and measurement of vibrations in dams located in areas subjected to seismicity is important for evolving design criteria for such conditions. The instruments for recording the measurements can be divided into two types: Vertical Movement Gauges and Horizontal Movement Gauges. These are explained below. 

Vertical Movement Gauges 

Surface Markers: Surface marker points consist of steel bars which are driven vertically into the embankment or the ground and embedded in concrete. A reference base line is established on a firm ground outside the area of movement due to reservoir and embankment load. Position of surface stakes or markers fixed on the embankment are determined by survey with reference to this line. It measures horizontal  movements also. Surface markers may be established on lines parallel to the centre line of the dam at 50 to 100m centres. The lines may be at the edge of the top width of the dam, at the edge of berms or at suitable intervals along the slope, at the toe. of the dam and at 50 m and 100 m from toe if foundation soil is not firm. These may be provided both on upstream and downstream slopes except in locations on upstream slope which remain throughout the year below lake water.

Cross-Arm Installation: It consists of telescopic steel casing to which are attached horizontal cross-arms at predetermined vertical intervals. As the soil settles, sections of casing are dragged down and these are thus relocated in their new positions by lowering down the casing a problem fitted with retractable claws which engage the bottom of each section in turn or by using an electrical probe. Cross arms are used in order to eliminate any possibility of the casing sections not settling along with the surrounding soil. 

Hydraulic Device: It is made from two 50 mm diameter brass pipe nipples soldered to a common diaphragm. Pipe caps are secured at both ends of the assembly which is then mounted vertically on a steel base plate for anchorage in the embankment. The diaphragm separates the upper (air) chamber from the lower (overflow) chamber and encloses a plastic float valve which prevents water from entering the air chamber during flushing of the lower chamber. Three 8-mm outer diameter plastic tubes are embedded in trenches which are excavated to maintain continuous downward slopes to the instrument terminal. The instrument terminal is equipped with a pump, air compressor and high precision pressure gauges. 

Geonor Probe: It consists of a three-pronged tip connected to a double rod which is lowered down a bore hole or driven in soft ground to desired depth. When the outer rod is held and the inner rod driven with hammer, the three prongs are forced out in the surrounding soil. The outer rod is then uncovered from, the tip and withdrawn a few centimeters. The top of the inner rod, which remains in contact with the anchored tip is used as a reference point to measure the settlement of the tip. This device is particularly well suited for measuring settlements of soft foundations under-low embankments. 

Foundation Settlement Measuring Device: It is a base plate placed on the foundation line with a vertical column of steel tubings. The position of the base plate is determined by a surrounding device lowered from the top open end of the steel tubings.

Magnetic Probe Extensometer: This system consists of a magnet/lead switch probe of approximately 15mm diameter connected to an indicator with a marker connecting cable. Magnetic ring markers with stainless steel spring parts are installed over a series of PVC access pipes of 33 mm outer diameter and 27 mm inner diameter jointed together. The probes when lowered through the access pipe will give indications in the indicator where the magnet marker rings are located. When settling takes place the marker rings will move with the soil and the fresh positions of the marker rings indicate the amount of settlements with respect to earlier logged position. 

Induction Coil Type Extensometers: This induction coil type extensometers consist of an electrical probe made of PVC and having a diameter of 35mm or 43mm which houses a primary electrical exit. The probe is connected to an indicator electrical cable. Indicator has a volt/ammeter to measure the voltage/current increase when the primary coil enters a secondary coil, when there is a steel marker ring or plate, it will indicate a current/voltage which could be read through the indicator. Series of marker rings installed over a corrugated PVC pipe installed over a PVC access tubes or inclinometer tube should help monitoring the settlement. 

Horizontal movement gauges 

Cross-Arm Installation for Measurement of Horizontal Movement: This installation is similar to that described above but instead of cross-arms fixed at different sections there are two Vertical plates at the same level placed at a certain distance apart. The relative horizontal movements between the two cross-arms are measured by transmitting the same by means of a cable to a pair of counter weights, which move vertically in the tubing. A sounding probe similar to that used in measurement of vertical movement installation determines the position of the counter-weights. 

Inclinometers: Plastic or aluminium tubing is placed vertically in the dam with its bottom anchored to firm unyielding stratum. The inclination of the tubing is measured by a sensitive electrical inclinometer, step by step, starting from the bottom of the tubing. Horizontal movements are computed by integrating the movements starting from the bottom, on the basis of changes in the inclination. Vertical movements may also be measured by using telescoping couplings for connecting the sections of the tubings and noting the positions of the ends of each section by a mechanical latching device, or if metal rings are embedded in the end portions of plastic tubing, by an electro magnetic device. Each section of tubing is anchored to the surrounding soil mass by fixing flanges or collars to the tubing. Alternatively, when an electromagnetic sounding device is used, the plastic tubing passes through encircling metal discs which are free to move along with the earth mass and the position of these discs are determined by the device.

Piezometers 

Piezometers are installed in embankment dams to monitor the pressure of water within the soil or rock fragment. Typical installation locations of these devices are shown in Figure 58 and the details of some particular types are described below. 

Figure 58. Typical installation of piezometers in embankment dams

Porous Tip/ Tube Piezometer: This is a steel or PVC pipe 10 to 40 mm in diameter placed vertically during construction or in a borehole after construction. A porous element is fixed at the bottom of the pipe or alternatively, the lower portion is perforated, and soil prevented from entering the pipe by surrounding the perforated portion by brass wire mesh and a gunny bag filled with filter material. With increase or decrease of pore water pressure in the soil near the perforated portion, water level rises or drops in the pipe and this level is noted by as electrical sounding device or a bell sounder. 

Closed System Hydraulic Piezometer: It consists of a porous element which is connected by two plastic tubes to pressure gauges located in a terminal house or terminal well. The terminal house or well contains pumping and vacuum equipment, an air trap and a supply of de-aired water besides pressure gauges. Use of two plastic tubes makes possible the circulation of water through the porous element and to remove air from the system. The pore-water pressure is noted by means of gauges.

Electrical Piezometers: Electrical piezometer consists of a tip having a diaphragm which is deflected by the pore water pressure against one face. The deflection of the diaphragm is measured by a suitable strain gauge which may be suitably calibrated to read pore water pressure. The strain gauge is either electrical resistance (unbonded strain gauge) type or vibrating wire type. 

Pneumatic Piezometers: In the pneumatic piezometers, the diaphragm deflection due to pore water pressure is balanced by a known air/gas pressure and recorded at the outside indicator end using pneumatic pressure gauges or pressure transducers. 

Earth Pressure Cells: The usual instrument to measure earth pressure is the earth pressure cell. It uses a stiff diaphragm on which the earth pressure acts. The action is transmitted through an equalizing, confined, incompressible fluid (Mercury) on to a second pressure responsive element, the deflection of which is proportional to the earth pressure acting. The deflection is transformed into an electrical signal by a resistance wire (unbonded strain gauge) or vibrating wire strain gauge and transmitted through a cable embedded in the earth work to a receiver unit on the surface. The measure of the electrical signal indirectly indicates the earth pressure by appropriate calibration. 

Instruments for measuring effects of dynamic loads due to earthquake include seismographs, accelerographs, and structural response records, details of which may be had from the Bureau of Indian Standards code IS: 4967-1968 “Recommendations  for seismic instrumentation for river valley projects”.