Geological Investigations for Dams Imran Sayeed Chief (Geology) NHPC, Sector 33, Faridabad, Haryana, India
[email protected]
1. Definition and Early Structures:
A dam is a barrier across flowing water that obstructs, directs or slows down the flow, often creating a reservoir, lake or impoundments. Most of the first Dams were built in Mesopotamia up to 7,000 years ago. These were used to control the water level, of Tigris and Euphratus rivers. The earliest recorded dam is believed to have been on the Sadd Al-Kafara at Wadi Al-Garawi, which is located about 25 kilometers south of Cairo, and built around 2600 B.C. It was destroyed by heavy rain shortly afterwards. The oldest surviving and standing dam in the world is believed to be the Grand Anicut, also known as the Kallania, an ancient dam built on the Kaveri river in the state of Tamil Nadu located in southern India. It was built by the Chola King, Karikalan, and dates back to the 2nd century AD. Before embarking upon the path of geological investigations it is imperative to know the types of dams and their fundamental requirement regarding foundation and abutments. 2. Types of dams
Man-made dams are typically classified according to their size (height), intended purpose or structure. International standards define large dams as higher than 15 meters and major dams as over 150 meters in height. 2.1 Classification by Purpose:
Dams are built to fulfill different objectives such as power generation (hydroelectricity), irrigation, drinking water supply, flood control, navigation, fisheries, and tourism. There are very few dams that serve all the above purposes at one go but otherwise there are many dams that are built for more than one purpose. They are known as multipurpose projects.
2.2 Classification by Structure:
Based on structure and material used, dams are classified as Masonry dams, concrete dams, embankment dams, timber dams etc with several subtypes.
2.2.1 Masonry and Concrete dams Gravity dams In a gravity dam, stability is secured by making it of such a size and shape that it will resist overturning, sliding and crushing at the toe. For this type of dam, impervious foundations with strong bed rock of high bearing strength are essential. The Gilboa dam in the Catskill Mountains of New York State, U.S.A. is an example of a "solid" gravity dam. Similarly, the examples of concrete gravity dam are Bhakra and Chamera dams in India. Strictly speaking however, Chamera is an arch gravity structure. Gravity dams inspire more confidence than any other type of dam in layman sense. However, when built on strong and stable foundations with carefully designed loads they indeed represent one of the most stable structures in dam engineering. This is significant because the fear of dam failure and resultant flood is a strong factor. This has resulted in gravity dams being built in some instances where an arch dam would have been more economical
A dam site in Garhwal Himalaya, India Masonry dams
There are gravity structures making use of local construction material available. Mainly it is the dressed stone masonry structure made from strong rock types like
Granite, basalt etc. One of the highest masonry dams in the world is Nargarjunasagar in Nalgonda district of Andhra Pradesh.
Arch dams In the arch dam, stability is obtained by a combination of arch and gravity action. The distribution of the loads is more complicated and is taken both by the foundation as well as the abutments. For this type of dam, firm reliable supports at the abutments are more important. The most desirable place for an arch dam is a narrow gorge with side walls composed of sound rock. The safety of an arch dam is dependent on the strength of the side wall abutments, hence not only should the arch be well seated on the side walls but also the strength and character of the rocks in the abutments should be carefully inspected. Hoover dam on the Colorado River is one of the famous arch dams in the world. In India, Idduki dam in Kerala is an example of arch dam.
2.2.2 Embankment dams
Embankment dams are made from earth or rock fill, and have two main types, rock-fill and earth-fill dams. Embankment dams rely on their weight to hold back the force of water, like the gravity dams made from concrete. Rock-fill dams
Rock -fill dams are essentially embankments of compacted free-draining granular earth or rock fill material with an impervious zone. The impervious zone may also be within the embankment in which case it is referred to as a core. In the instances where clay is material the dam is also referred to as a composite dam. Examples of rock-fill dams are Salal in Jammu and Kashmir and Baira – Siul Siul in Chamba district of Himachal Pradesh. Earth-fill dams
Earth-fill dams, also called earthen, rolled-earth or simply earth dams, are constructed as a simple embankment made out of well compacted earth. There may be homogenous or zoned earth fill dams. dams. Hirakud dam in Orrissa is a composite structure of earth dam and concrete and masonry. Garrison dam in North Dakota in U.S.A. is one of the biggest earthen dam in world. Concrete face rock-fill dam
In this type of dam the upstream side is made impervious by a concrete face. The first such dam in India has been recently built across Dhauliganga river in Pithoragarh district of Uttarakhand.
There are other dams of timber and steel also but they are not popular as permanent structures. 3. Geological investigations:
The most important failure in the history of dams was the St. Francis dam in California in 1928 which gave birth to new kind of technical development. Engineering geological investigations gained importance after this event. The failure of St. Francis dam was attributed to a fault zone and conglomerates which got dissolved after saturation and lead to leakages in foundation. Consequently, failure of the structure had occurred. The dam was built on foundation of mica schist, red sandstone and conglomerates. Nearly 450 people lost their lives in the disaster. Similarly Vajont dam in Italy and Malpasset in France were large disasters due to natural causes and technological failures. Therefore the importance of geological investigations cannot be undermined simple because nobody can afford to have a disaster. For meaningful geological investigations, due cognizance is to be given to the type of dam, size, overflow structure and other ancillary structures also. The geological study for dam sites mainly involves recognition of various formations and geological structures at the dam site and neighborhood. 3.1. Gravity dams
The geological investigation for concrete dams are well documented in the Indian Standard (IS 15662) “Geological exploration for gravity dams and overflow structures-Code structures- Code of practice” 2006. The investigations are to be rationally planned so that the relevant information is gained in minimum possible time and funds. A complete program of geological investigations should be able to bring out the following: (i) Thickness and type of overburden at the dam site. (ii) Types and extent of underlying bed rock or lithological units present at the foundation and abutments. (iii) Structural features such as bedding planes, foliation, joints, shear zones, fracture zones and folds, faults etc. (iv) Open joints, weak zones, hidden layers like inter-trappean beds in the deccan traps, susceptible soluble formations. (v) Extent of Weathering and slumping particularly on the river banks. (vi) Joint orientations and stability of the abutments. (vii) Any weakness running parallel to the river. (viii) Engineering properties of overburden or rock mass. (ix) Landslides, avalanche zones and related hazards in the area. (x) Slumping in rock mass in the abutments. (xi) Glacial, fluvial materials, buried channels, effect of geology on landforms and geomorphology. (xii) Tectonic features and active faults, seismic status.
(xiii) (xiv)
Geo-hydrology of the area. Availability of suitable construction materials.
Entire gamut of investigations must necessarily culminate in assessment of the feasibility, reliable design and cost estimate of the project so that a bankable Detailed Project report is prepared. 3.1.1 Stages of Explorations:
Geological investigations for large engineering projects such as dams are divided into stages (i) (ii) (iii) (iv)
I.
……….. PFR Stage Reconnaissance or Pre-Feasibility Pre-Feasibili ty Stage Preliminary Investigation or Feasibility Feasibilit y Stage ...……… FR Stage Detailed investigation investigati on or DPR Stage ………...Bankable DPR Stage …………During Const. Const. Construction Stage
Reconnaissance or Pre-Feasibility Stage :
This stage involves Selection of suitable sites on the basis of regional geology, topography and surface features. Many alternatives are identified. (i) Data Collection and Desk study: All the available geologic information reports etc are gathered. Broad assessment of depth to bed rock, hazard identifications, rock types their strength and weaknesses. Thrusts, Folds and faults in the area are broadly identified and recorded for the area of interest before the site visit is undertaken. (ii) Reconnaissance: Alternatives sites are examined on ground and geological assessment is made. General geological maps and sections are prepared and given in the geological report. Tentative lay-out of the dam is formulated in this stage. Further explorations for FR Stage are identified and estimates are prepared for quantities etc.
II. Preliminary Investigation or Feasibility Stage
Engineering geological data is collected for the most preferred dam site. Geological explorations should cover for establishing techno-economic feasibility of the dam. Investigations should be done to minimize the geological uncertainties. A.
Methods: (i) Surface geotechnical mapping on 1:1000 scale for the dam site and ancillary structures with sufficient coverage as given in IS code. (ii) Remote sensing studies
(iii) 1. 2. 3.
4. 5.
Sub-Surface geological explorations: Geophysical surveys for thickness of overburden, ground water and characteristics of rockmass. For Dams upto 30m height, trial pits, trenches and drill holes are undertaken. For Dams more than 100m height, trial pits, trenches, drill holes, drifts and shafts may be required depending on geological complexity. Rock mechanic tests, in-situ and laboratory Construction material investigations: Assessment and related testing.
The feasibility report is prepared at the end of feasibility stage investigations. It should contain the following: geological plan and sections of the dam site, pit, trench and drift logs as required with rock mass classifications. Drill hole logs with photographs of cores and permeability test analysis and results. Field and laboratory test reports and geological evaluation of the dam site. Availability of construction materials particularly aggregates for concrete dam is to be discussed. Preliminary layout of the dam superimposed on geologic and exploration plans. Further detailed investigations during the DPR stage are to be outlined as well. III.
Detailed Investigation or DPR Stage :
(i) Surface geotechnical mapping on 1:1000 scale for the dam site and ancillary structures with sufficient coverage as given in IS code. Updating as required. (ii) Remote sensing studies: Use of satellite imageries and aerial photographs is made right from the PFR or FR stage. (iii)
Sub-Surface geological explorations:
Geophysical surveys for thickness of overburden, ground water and 1. characteristics of rock mass.
2. Drill holes: Three to five drill holes in river bed/channel portion, two hole on each abutment are to be completed. Two holes are suggested in toe portion also. Core samples for laboratory tests are to be obtained. Permeability tests are to be conducted in all the drill holes. For diversion tunnel three holes are required. Inclined holes in river bed or drift below the river bed may be required. Geological logging of the core samples and presentation of data on specified formats shall be done. 3. Drifts: For dams upto 50m height one drift at mid height is recommended for ascertaining the abutment conditions. In case of 50-100m high dams, two drifts are necessary and for over 100m high dams, three or more drifts are to done
according to Indian Standard (IS:15662) . For diversion tunnel one or two drifts may be made. Geological logging and rock mass classification is to be carried out for all investigation tunnels. 4.
Groutability tests are required.
5.
Rock mechanic tests are to be carried out as follows:
a) 1) 2) 3) 4) 5) 6) 7) 8)
Laboratory Tests: Unconfined compressive strength Modulus of elasticity Sp. Gravity and water absorption Slake durability tests. Shear and compressional ultrasonic velocities. Shear strength of infilling material. Swelling. Joint stiffness test.
b) 1)
In-situ Tests: Shear strength parameters; interfaces. Modulus of deformation. Seismic wave velocity. Bearing capacity test.
2) 3) 4)
rockmass,
discontinuities
and
Special tests such as blast monitoring etc be conducted as required. Geohydrology should be well defined. 6. Construction Material Surveys:
Suitable sites for obtaining the construction materials shall be identified. Mainly the requirement is for: a) Coarse and fine aggregates b) Rock fill material for coffer dams c) Impervious material for clay core. For coarse and fine aggregates the sources could be river shoals, terrace deposits or rock quarries. Similarly other materials are also located and the quantities assessed with respect to the requirement. Then the process of testing for suitability is further taken forward which was initiated in the feasibility stage itself. There is also a chance for review the work completed in the FR stage and go for additional sources if required. In entire process of the construction materials, environmental considerations are to be given priority and maximum use of materials from the excavations in the project shall be made. Different tests for the construction materials are
a) b) c) d) e) f)
Physical properties tests Rapid chemical tests Petroraphic study. Mortor bar tests for alkali aggregate reactivity. Suitability tests for clay core materials Suitability tests for rock fill materials.
Considering all the above explorations and the finalized layout, the geotechnical assessment should be given. The bankable DPR may thus contain fairly detailed description of geological conditions, geological maps and sections, geological logs of drill holes and drifts, geophysical survey reports and their correlation with geology, rock mechanic tests reports and discussion of results and their implications on the structures, detailed account of geological structure, geohydrology, foundation and abutment conditions. A separate volume is devoted to the construction materials as it is the availability of the same which sometimes governs the choice of dam. The engineering remedial measures are also to be included in the DPR so that deviations are avoided.
IV
Construction Stage
Foundation mapping is done in the construction stage as the bed rock/founding levels are exposed. Treatment of shear zones etc is to be given as the work progresses. Foundation grade maps are prepared on 1:100 scale after proper cleaning of the foundation by water jet etc.
4.1. Earth and Rockfill Dams:
A separate Indian Standard (IS:6955-2008) is available for subsurface explorations for Earth and Rock fill dams. The entire gamut of investigations for Earth and Rockfill dams is similar to concrete gravity dams except for some specialized tests and evaluation part. Moreover, the characteristics of overburden are very important in case of earth and rockfill dams whereas for gravity dams the properties of bed rock are more significant. It is very important to ascertain the bed rock configuration in case of earth and rockfill dams also. Due precautions are required against buried channels, slumped rockmass, open joints, landslides, variations in rock types etc. Permeability of overburden and bed rock are important parameters that are to be estimated. In case of overflow structures or spillway tunnels separate investigations are necessary. Availability of suitable rockfill or earth material is a big consideration in feasibility of such dams.
Exploration may be done by both non-coring and coring methods in the feasibility and DPR Stages. In the FR Stage field penetration and field density tests are recommended in addition to other laboratory tests. In the DPR Stage, the requirement of drilling is one additional row of holes in case the height of dam is less than 30m and two additional rows of holes in case the dam height is 30-100m. Similarly for dams higher than 100m three lines of holes are required. In the DPR stage, confirmation of construction material both in terms of availability and suitability shall be ensured. Large quantities of rockfill and earth material preferably from project excavations or from reservoir area are identified. The soil and rock mechanic tests required for earth and rockfill dams are different from concrete gravity dams. The list is given below: a)
Laboratory Tests
For Soils
1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)
Natural Moisture content Liquid and plastic limits Sp. Gravity Particle size distribution Bulk density Permeability Consolidation characteristics Swelling Tests Strength Characteristics Compaction Tests Density index Mineralogical composition. Chemical analysis
For rock
1) 2) 3) 4) 5) 6)
Petrographic study Shear Strength Sp. Gravity and porosity Water absorption Chemical analysis Water Analysis
b) Insitu Tests
1) 2)
Deep Penetration tests Shear tests
3) 4) 5)
Density of foundation material Permeability tests Blasting tests.
Bankable DPR is prepared in the same manner as concrete and masonry dams. However, the geotechnical assessment is framed keeping in view the earth /rockfill dams dams together together with ancillary ancillary structures. structures.
IV
Construction Stage
Foundation mapping is done in the construction stage as the overburden or bed rocks at founding levels are exposed. Foundation grade maps are prepared on 1:100 scale after proper cleaning of the foundation is done and before placement of the embankment material. The mapping of cut-off trench is also carried out.
