DESIGN CONCEPTS FOR EARTH & ROCKFILL DAMS
1. INTRODUCTION
Embankment dams have been built since ancient time for conservation and utilization of waters. There is evidence of the existence of a stone faced earth dam in Jordan dating as far back as 3200 B.C. The art of construction of earth dams in the Indian subcontinent also dates back to ancient times and the 21 m high Padvil dam constructed in 504 B.C. in Sri Lanka is still functioning. In India numerous earth dams were constructed since time immemorial across streams and depressions to store rain water during monsoons and to utilize the same during dry season. Even today, the areas of central and south India are dotted with innumerable small reservoirs or tanks created by earth dams serving the needs of the people.
ALSO READ: DESIGN OF CORE AND FILTER
ALSO READ: FOUNDATION TREATMENT FOR DAMS
The embankment dams constructed in the past were mostly of heights less than 20 m and were generally constructed by simply heaping earth across an area to be blocked with negligible compacting efforts. They were built on the experiences of past performances and failures and by ‘Rule of Thumb’ with no real scientific basis, and they often resulted in failures. In recent times, particularly during the last five decades, there has been tremendous advance in the science of soil/rock leading to a better understanding of the behavior and strength properties of soil and rock. Advances in other aspects of dam building like investigation methods, testing of foundation and construction materials, design procedures, construction techniques and management have enabled us to design and construct embankment dams of heights even exceeding 300 m. These advances have given us the confidence to evolve risk free and economical design of embankment sections. The developments in computer application techniques have also greatly helped the designer to understand the behaviour of structures and to evolve optimal designs.
2. ADVANTAGES OF EMBANKMENT DAMS
Embankment dams have many advantages compared to gravity dams. The main advantages are: Embankment dams can be constructed on any given foundation condition and the excavation for foundation need not be up to rock level, where the bed rock is deep seated. Foundation excavation is negligible in most of the dams. Soil/rock materials locally available are used with negligible processing. Use of costly manufactured items like cement and steel is eliminated and there is saving on transportation costs also. Embankment dam is more resistant to seismic forces and are preferred in areas of high seismicity. Embankment dam can be constructed in stages and the dam height can be increased later on easily, if needed. With modern earth moving machineries, the dam can be completed in less time compared to a rigid dam. Embankment dams are generally much cheaper. The dam sites which have sound geo-technical set-up having been almost, exhausted, the embankment dam with its flexible requirements remains almost the only choice.
It may be noted that more than 90 per cent of high dams (15m and above) planned or constructed in the country since 1950 are embankment dams.
3. TYPES OF DAMS
Embankment dams are classified as earth fill dams or rock
fill dams based on the use of construction material. An earth dam relies mainly
on earth/soil and a rock fill dam relies mainly on rock with impervious
membrane either an upstream facing or an interior impervious core. While an earth dam generally fits into any
ground situation, rock fill type is adopted in locations where supply of
rock/boulders is ample, foundation rock is at or near the ground surface and
suitable soil for earth fill is not readily available.
Earth dam classification based on method of construction includes rolled fill, semi-hydraulic fill and hydraulic fill dams. Rolled fill dams are generally selected now a day with the availability of efficient earth moving and compacting equipments making the construction both quicker and economical. The rolled fill dams are also known to be more stable than hydraulic fill dams against seismic forces.
Earth dams are further classified as ‘homogeneous’ or ‘zoned’ type. In ‘zoned dams’ a central or an inclined impervious core is introduced to ensure control of seepage and cracking. Several zones can also be included in a dam section to get the most economical section using the different types of soils available at site and their physical and strength properties. Fig. 1 to 3 shows some typical sections of different types of earth dams.
Rock fill dams have an impervious interior core or an upstream impervious membrane. The upstream membrane can be cement concrete or asphalt concrete. In smaller dams wooden, geomembrane or steel facings can also be used. The rock fill can be from quarrying or naturally occurring river bed material with a fair degree of grading of fragments. Figs. 4 and 5 show details of the two types of rock fill dams.
ALSO READ: DESIGN OF CORE AND FILTER
4. BASIC DESIGN NEEDS
A competent design of an embankment dam should be both safe and economical. The safety shall be adequate in respect of failure or damage resulting in danger to life or property. The structure designed shall also be durable and as maintenance free as practicable. The structure shall also serve the purpose for which it is designed.
The design of embankment dam is to be based both on precedent and on analytical studies. At a given site, a number of design types can be evolved and hence the personal experience of the designer plays a more important role in the final embankment section selection compared to design of other structures.
A satisfactory design must ensure the following basic criteria:-
1. The design of the dam is adjusted to the ground conditions so that the foundation is adequately water tight. Necessary treatment for foundation to achieve desired water tightness is also to be provided. In case where foundation susceptible to build up of pore pressures resulting in stability necessary treatment by way of proper drainages is also to be provided. Interfaces of the dam with concrete structures and abutments should be treated with caution
2. The embankment, foundation, abutments and reservoir rim must be controlled to prevent excessive uplift pressures, piping, instability, sloughing and removal of material by solution or erosion of material. Concepts of design for stability will be discussed later.
3. Camber or additional height of dam should be sufficient for allowance for settlement of foundation and embankment. Generally upto one per cent of height of embankment is taken for settlement allowance.
4. Surplussing arrangement viz. spillway and outlet should have sufficient capacity consistent with the designed flood and its routing to prevent overtopping.
5. Freeboard must be sufficient to prevent overtopping through a combination of wave run up, wind set-up and standing waves. The I.S. Codes provides a minimum value of 2 m for normal freeboard as well as for minimum freeboard. I.S. Codes 11223-1985, 6512-1984, 10635-1983 may be referred to in this regard. 6. Finished upstream dam surface shall be designed to withstand effects from waves and erosion to prevent breaching. The protection can be by rip rap, soil cement, concrete slabs, turfing etc. The downstream slope shall withstand effects of surface water due to rains and wind action. Turfing with a series of collecting drains on the downstream slope with a well designed toe protection measure with rip rap is generally adopted.
5. SEEPAGE CONTROL MEASURES
The design of a dam should consider the seepage control measures both through the foundation and embankments, such as:-
1. Foundation grouting – both consolidation and curtain routing.
2. Foundation cut
off – impervious fill, diaphragm etc.
3. Upstream impervious blanket.
4. Special foundation treatment of geological discontinuities in foundation.
5. An impervious core of sufficient width in zoned
section.
6. Transition zones and filters.
7. Internal drainages like inclined, vertical or horizontal drains, drainage blankets etc.
8. Toe drain and relief wells.
9. Strict quality control during foundation treatment, use of fill material, compaction control etc.
ALSO READ: DESIGN OF CORE AND FILTER
ALSO READ: FOUNDATION TREATMENT FOR DAMS
6. DESIGN CONCEPTS AND STABILITY ANALYSIS
The most important cause of failure of an embankment dam is sliding. A portion of the Earth or Rockfill will slide downwards and outwards with respect to remaining part, generally along a well defined slice surface. The failure is caused when the average shearing stress exceeds the average shearing resistance along the sliding surface due to various loading conditions.
Slope stability is generally analyzed by two methods depending upon the profile of failure surface viz.
(a) Circular arc method and
(b) Sliding Wedge method.
In the ‘Circular arc’ method or ‘Sweedish Slip Circle’ method, the rupture surface is assumed cylindrical or in the cross-section by an arc of a circle.
The sliding wedge method assumes that the failure surface is approximated by a series of planes.
6.1 STATIC STABILITY ANALYSIS
The design studies for slope stability include consideration of : (i) Loading conditions, (ii) Material properties, (iii) Pore pressures and (iv) Factors of safety requirement under various loading conditions.
6.1.1 LOADING CONDITIONS:
A dam is required to be safe and stable during all phases of construction and operation. Analysis is done for the most critical combination of forces likely to occur. The loading conditions considered generally include:-
1. Construction condition ( u/s and d/s slopes).
2. Reservoir partially full (u/s slope).
3. Sudden Drawdown (u/s slope).
4. Steady seepage (d/s slope).
5. Steady seepage with sustained rainfall (d/s slope).
6. Earthquake condition ( u/s and d/s slopes).
Allowance for pore pressures in the dam are varyingly determined depending upon the loading condition, fill material properties, stress due to fill, hydrostatic head at locations considered, and reservoir water level variations. Effective shear strengths obtained by consolidated undrained tests are used for all loading conditions except in construction condition where unconsolidated undrained test results may also be used. Consolidated drained test results may be used only in cases where the material is cohesion less and free draining.
6.1.2 SELECTION OF DESIGN PARAMETERS
The embankment material shear strength is obtained by performing triaxial tests of borrow area materials compacted to densities aimed at during construction. The foundation material strength is obtained by tests with undisturbed samples from triaxial shear testing. Testing in each case shall be from zero to maximum normal stress expected in the dam. The design shear parameters for fill material is fixed at 75 per cent availability from an adequate number of samples, and for foundation soils minimum shear strength values along foundation obtained are adopted after rejecting extreme or freak values.
6.1.3 ANALYSIS PROCEDURE
The procedure of arriving at driving and resisting forces involves assumption of a tentative cross section of the dam, a possible circular failure surface, division of the slip circle mass into a number of slices, calculation of forces on each slice and summation of the forces
The factor of safety against sliding for assumed failure surface is obtained by the equation:-
Until the advent of computers, the stability analysis was being done by arithmetical or graphical method, which is both laborious and time consuming considering a large number of assumed slip circles to be analyzed to evolve minimum factor of safety under each loading condition. Now-a-days, the stability analysis for dam slopes without or with earthquake forces (pseudo static method) are carried out using computer progammes developed by various persons or organizations. The notable methods and programmes in vogue include spencer method, Sarma’s method, Morgenstern method, Bishop’s method, etc.
ALSO READ: FOUNDATION TREATMENT FOR DAMS
6.1.4 MINIMUM FACTORS OF SAFETY
I.S. Code IS 7894-1975 prescribes the minimum desired values of factors of safety for various loading conditions as under:
|
Loading Condition
|
Minimum factor of safety |
Slope |
|
1. Construction condition |
1.0 |
u/s & d/s |
|
2. Partial Pool
|
1.3 |
Upstream |
|
3. Sudden drawdown
|
1.3 |
Upstream |
|
4. Steady seepage
|
1.5 |
Downstream |
|
5. Steady seepage with
sustained rainfall |
1.3 |
Downstream |
|
6. Earthquake Condition: |
|
|
|
(a) Steady Seepage
|
1.0 |
Downstream |
|
(b) Reservoir full
|
1.0 |
Upstream |
7. EARTHQUAKE RESISTANT DESIGNS
India has been divided into five seismic zones taking into account past earthquake occurrences, and basic horizontal seismic co-efficient is given for each of the zones.
The forces induced on a structure during an earthquake is dynamic in nature and is a function of ground motion and properties of the structure itself. The dominant effect is equivalent to a horizontal force varying over the height of a structure. The assumption of a uniform force along one axis at a time is an over simplification, but this practice still prevails for saving efforts in dynamic analysis.
Two pseudo static methods viz. seismic co-efficient method and Response Spectra method are in vogue:
7.1 SEISMIC CO-EFFICIENT METHOD
The mass of structure is multiplied by design seismic coefficient and it is assumed to act statically in any one direction. The magnitude of the co-efficient is considered uniform for the entire height of the structure. The design seismic co-efficient is computed from:
αh = β. I. αo
where,
αh = Horizontal design seismic co-efficient
β = A co-efficient depending on soil foundation system
I = Importance factor (depends on importance of structure)
αo = Basic horizontal seismic co-efficient
7.2 RESPONSE SPECTRUM METHOD
In this method, the response acceleration coefficient is first obtained for natural period and damping of the structure, and the value of design horizontal seismic coefficient is obtained by:-
αh = β.I.Fo Sa/g α h where
αh = Horizontal design seismic coefficient.
β = A coefficient based on foundation – soil system
I = Importance factor
Fo = Seismic zone factor for average acceleration spectra.
Sa/g = Average acceleration coefficient for appropriate natural period and damping of the structure.
7.3 STABILITY COMPUTATION
The slope stability analysis is carried out to get the minimum factor of safety for a tested section under steady seepage and reservoir full conditions for downstream and upstream slopes respectively. The computer programmes cited earlier and used for static analysis are used for the computations.
For the analysis, dynamic properties of soil/rock fill are determined using block vibration test, cyclic plate load test and wave propagation test. These tests give in-situ dynamic and damping properties. Triaxial tests are carried out using repeated static loading for shear parameters.
8. DYNAMIC ANALYSIS AND DEFORMATION STUDIES
While the pseudo static methods are considered enough for project report designs, dynamic analysis and deformation studies are necessary for detailed designs of all important dams. The methods include both simplified and rigorous methods of analysis.
8.1 SEQUENCE OF STUDIES
Deformation and/or liquefaction studies are performed to predict embankment and foundation response to specific earthquakes. It is considered that for a dam not subject to liquefaction, deformation is not a problem if the following conditions are satisfied:-
The dam is well built (densely compacted) and the peak accelerations are 0.2 g or less, or the dam is on clay or weak rock and peak accelerations are 0.35 g or less.
Slopes of the dam are 3H: 1V or flatter.
The static factors of safety of the critical failure surfaces involving the crest are greater than 1.5 under loading conditions expected prior to earthquake
The freeboard is minimum 2 to 3 per cent of dam height. If these conditions are not satisfied a deformation analysis should be made.
8.2 DYNAMIC ANALYSIS STUDIES
Inputs for deformation analysis include seism tectonic exposure assessment of dam site, selection of ground motion parameters, assessment of dynamic properties of embankment and foundation materials, static stress analysis and one, two or three dimensional dynamic analysis.
The site-specific seism tectonic studies are done to identify earthquake source areas, maximum credible earthquakes and estimates of the magnitude – recurrence interval relationships. Potential for fault rupture in foundation and in the reservoir will also be assessed.
Appropriate earthquake ground motion parameters for the earthquakes are provided by the seismic exposure study (peak acceleration, peak velocity, response spectrum shape and an accelerogram). Most of dynamic analysis procedures require an accelerogram, i.e. a record of acceleration versus time during earthquake. Accelerograms are obtained from existing records or from synthetic records that produce a specific response spectrum. Computer programmes are used for arriving at response spectra.
The dynamic properties of construction material are classified into mainly two groups viz. stress – deformation parameters (shear modulus and damping ratio) and the strength parameter given by the liquefaction curve. Other significant parameters needed are degree of saturation, in-situ dry density, static strength and stress deformation parameters. A number of field and laboratory tests for determining the relevant parameters are required to be performed.
Using the static stress analysis, the initial stress conditions required for the dynamic stress conditions for dynamic stress analysis are established. For this purpose two dimensional finite element analyses is generally used, though 3-D FEM studies are restarted to when situation warrants, like for a dam situated in a deep and narrow valley. In the F.E.M. analysis, the structure is idealized as an assemblage of a number of discrete elements which are connected to one another at nodal points. The stresses in each element obtained from the static analysis will be useful in selecting the dynamic properties for use in the dynamic analysis. Several programmes are available for use on computer for these studies. The dynamic analysis studies are done by one dimensional or two dimensional F.E.M. studies. One dimensional study is best suited for analysis of a level ground, and the 2-D studies are widely used for analysis of embankment dams. 3-D dynamic F.E.M. analysis is very costly, and is not generally used for embankment dams.
ALSO READ: DESIGN OF CORE AND FILTER
ALSO READ: FOUNDATION TREATMENT FOR DAMS
8.3 DEFORMATION ANALYSIS
The deformation analysis is made using `Newmark` approach. Several procedures are available for the analysis which includes both simplified and rigorous procedures. The steps involved are:
To obtain response of the structure to earthquake ground motion, and To calculate displacements on one or more potential sliding masses. Deformation analysis using F.E. approach can also be used.
8.4 LIQUEFACTION ANALYSIS Embankment dams are designed on the basis of maximum earthquake and the dam should be capable of retaining the reservoir under the loading imposed by maximum earthquake. If the soils of either the foundation or the embankment suffer serious loss of strength under cyclic loading, then an evaluation of liquefaction potential and post-earthquake stability must be done. The liquefaction analysis can be done by simplified methods (using semi-imperial methods) or by rigorous analytical procedures using F.E.M. analysis.
8.5 COMPUTER PROGRAMMES AND METHODS
Some of methods of dynamic analyses of embankment dams in vogue are given below :-
Dynamic Analysis of Embankment Dams
Given: Earthquake magnitude and distance, Ground Motion parameters etc.
Aim : 1. To
evaluate liquefaction potential of dam or foundation.
2. Permanent displacement of dam.
Steps
1. Dynamic Response evaluation.
2. Deformation Response Computation.
Computation Methods and Programmes Some known programmes and methods for liquefaction studies Empherical, simplified analytical (seed), SHAKE (one-D), FLUSH, TLUSH (2-D), MASH (3-D), QUAD (2-D) etc. Some known programmes & methods for deformation analysis Empherical, Makdisi & Seed (simplified analytical), SHAKE (1-D), FLUSH, SLUSH (2-D), MASH (3-D F.E.M.) etc. Pore pressure and deformation Response (methods & programmes) Empherical, Makdisi & Seed (simplified), Newmark - DYNDSP (Analytical Math Model), Seed-Idris, F.E.M. studies, SEDIA (simplified seed).
9. FINAL SELECTION OF DAM SECTION
Based on the results of studies for slope stability by static, psudeo static and dynamic response analyses for liquefaction potential and final displacement potential, the designer will select the final section of the dam. In this selection, great emphasis is put on the experience of the designer and the data of behavior of dams constructed in almost identical situations. The data obtained from instruments installed in performine dams are also used in the process.
10 DEFENSIVE DESIGN MEASURES
While dynamic analysis of important dams is essential to avoid catastrophic failures, the design details should also include defensive measures to enhance their performance.The measures may include:-
· Provision of adequate freeboard to allow for settlement, slumping and fault movement.
· Use of wide transition zones of materials not vulnerable to cracking. Use of drains near critical zones and central portion of dam.
· Use of wide core zones.
· Use of adequate well-graded filter zone upstream of core to serve as a crack stopper.
· Controlled compaction of dam zones.
· Removal or treatment of foundation materials that are of low strength or density
· Widening of core at abutment interfaces.
· Special treatment of foundations at faults including provision of transition dam sections.
· Stabilization of hill slopes susceptible to sliding around reservoir rim.
11 INSTRUMENTATION IN EARTH DAM
At present the purpose of instrumenting dams in general are:
· To compare actual observations with assumed conditions which provided an opportunity to adjust design criteria based on reliable data.
· To monitor the structural behavior of a dam during construction, initial reservoir filling and routine project operation.
· Forewarning an impending distress.
Because of the uncertainties inherent in the design of a dam it is often necessary to conduct observations by either mounting or embedding instruments within the dam and its foundation to ascertain the performance of the structure commencing from the construction stage. This is done specifically to determine whether the expected behavior of the structure assumed during design are reflected in the actual behavior of the structure right from the construction stage to the end of its operating life time.
The code states that the objectives of instrumentation are two fold. The instruments embedded in, or installed on the surface of dams keep a constant watch on their performance in service and indicate distress spots which call for remedial measures. These instruments thus play an important role in checking the safety of the structures. In addition to these observation from instruments from a cumulative record of structural behavior which when analyzed can be used to modify purely theoretical assumptions in the design and place future designs on finer footings. The future design criteria for dams can therefore be made more realistic.
For earth and Rock fill dam the following types of measurements are recommended.
· Pore Pressure
· Movements
· Seepage
· Stresses & Strains
· Dynamic loads (Earthquakes)
INSTRUMENTS FOR PORE PRESSURE MEASUREMENT
There are many types of Piezometers which are normally used in earth / rockfill dams such as: Stand-pipe Piezometers – located at downstream of dam for monitoring piping. Hydraulic Type – USBR type used the world over – Economical, easy availability, easy installation. Electrical Type – Not widely used in India. Pneumatic Type – These are used where hydraulic type can’t be used due to freezing of water.
EARTH PRESSURE CELLS
The measurements of earth pressure is generally aimed at
investigating the conditions associated with differential settlement and
cracking of the core. Earth pressure cells are being installed in all modern
high dams in the country.
ALSO READ: DESIGN OF CORE AND FILTER
ALSO READ: FOUNDATION TREATMENT FOR DAMS
INSTRUMENTS FOR DEFORMATION MEASUREMENT
· USBR Type Movement Device A variety of devices are available for measurement of movements in the dam. However, most extensively used is USBR type cross-arm device and surface settlement points.
·
Slope Indicator This instrument is a widely used device for
measuring horizontal and vertical deformations accurately.
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ALSO READ: FOUNDATION TREATMENT FOR DAMS
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