Unit 5
Role of Engineering Geology in reservoir, dam and tunnel
(a)Storage Dam
(b) Detention Dam
(c) Diversion Dam
(d) Cofferdam.
Classification on basis of structural behaviour:
(a) Gravity dam
(b) Arch Dam
(c)Buttress Dam
(d) Embankment Dam
Following points are required that-
They are generally built of clay, sand, gravel, etc.
They are relatively cheaper as compared to masonry dams.
(a)Gravity dam- The resultant force acting on dam wall is transmitted downward and hence due to this entire load, weight is transmitted on small area of foundation.
Table rocks are required for the foundation of gravity dam.
(b)Buttress dam: The dams in which there deck sloping upstream. The entire load is supported from behind by walls called as buttress, which are further supported by strut.
Hence weaker rock can also be used for foundation of buttress dam.
(c)Arch Dam- The wall is in arch shape, convex upstream.
It is suitable for narrow deep water bodies.
Rock below arch dam should be stable i.e. modulus of elasticity of rocks should be high enough.
Geology of dam which are investigated is:-
Stages of Investigation in selection of dam site
In this investigation, we can determine the main advantage and disadvantage of area suitable for dam or not.
(a) Lithology: ‘Lith’ means Rock and ‘logy’ means study. Hence, study of rock is termed as lithology. This study provides details of Rock type, occurrence of Rock debris and soil, weathering action of rocks in that area. This study provide the presence or absence of rocks which are suitable for which type of dam.
(b) Structure and altitude: This study provides information on the dip and strike of beds, folds, faults, joints, foliation, cleavage, etc. These studies focus on bearing of rocks for dams.
(c) Topography: This study give details of surface features present on the site of dams as valley ,hill, stability of slope is of great importance because it control landslides and it also gives study of depth of bedrock.
(d) Groundwater condition: This study provide information on sea pages, wellspring etc. The data effect the ground water on rock, if any shallow water table does not make leakage problem whereas deep water table allow or make leakage problem. Hence, it needs to be very carefully observed.
(a) Surface investigation: When all the data is approved in preliminary investigation, then it is gone for detailed investigation. Preparation of map which shows geological features of area, engineering properties of rock such as types, strength, durability, porosity, etc. The details on occurrence of bed plains, depth of solid rock, etc.
(b)Subsurface investigation: It is expensive and laborious venture, and more care is required. Drilling is done which gives detailed information on cavities and fractures if present.
1) Piping failure: Water which percolates through the bed of rocks at foundation carry soil particles that are also lifted from downstream and leads to pipe like structure.
Precaution: The foundation should be highly strong and well compacted.
2) Dams on suitable soluble Rock: Soluble rocks mean dolomites, limestone and marble are soluble rock. These rocks are strong enough to take the weight of dam but if they contain underground solution channel and caverns. If it is present, then leakage should be at greater extent.
Prevention: At preliminary and detailed investigation should be taken carefully to avoid soluble rocks.
3) Careful attention should be given to the orientation of joints, bedding plains, fault, folds, foliations at foundation of dam.
4) The Rocks that are present in abutment of an arch dam should be strong enough to resist the pressure without being crushed.
5) Permeable rocks: Growing is the prevention for the permeable rocks present at bottom of dam.
6) Dams on Shales: The compaction shales are soft and change their property by alternate wetting and drying and hence bearing strength become slow.
Prevention: Heavier structure like gravity dam is avoided.
7) If dams are constructed at strata dipping downstream, then it is very dangerous and not safe. Hence, the ideal condition is stratadipping upstream which is safe for construction of dam and reservoir.
8) Dams are constructed in synclinal fold the downstream limb provides better location.
9) The crust Rock may be removed and replaced by concrete.
Strength, stability and water tightness of foundation rocks
Geological age Mineral composition Density Specimen prep. Moisture content
1) Stability within gravity dams
2) Stability within arch dam
3) The foundation stability, avoiding the risk of superposition of potential unstable wedges.
4) Foundation should be free from holes and faults.
Water tightness
When water flows over loose rocks or soil, it is naturally some water of river percolates underground. Before construction of dam or reservoir, this problem shall be solved first. If sufficient compaction of gravel is therefore foundation rock, then water percolation can be avoided.
Factors affecting Dam type selection
Tunnel is defined as underground routes. Generally, they are constructed in hilly areas, to save time for the traffic and also to save distance.
Types of tunnel:
In an area which apparently appears to be suitable for construction of tunnel, preliminary investigation determines the merit and demerit. And also without preliminary investigation detailed investigation cannot be started. And which is more expensive and hence carelessness is avoided. It includes the following points:
a) Physiography-It is the first thing to be investigated in the site area. It gives information about hills, slopes, valleys and other environmental features related to tunnel area. It also gives alignment information of the tunnel and possible route or location.
b) Lithology- This gives knowledge of rock present in the area. It includes various rock types available, there weathering action and formation of rock and soil. It helps in calculating the stability for cutting and filling through tunnel.
c) Structures-This study includes observation on various weak zones are present, and proper reinforcement can be provided. These pictures are of extreme importance as they have great bearing on suitability of tunnel. It also gives knowledge of beds along the tunnel and stability of site.
d) Groundwater conditions-It is very important and critical feature to be measured because groundwater conditions play important role in tunnelling. It includes sea page of water, water table and water bearing beds. It also gives to use necessary dewatering equipment and other precautionary measures if use.
For alignment of tunnel, detailed survey is necessary before aligning the tunnel on ground. Accurate survey is carried to ensure the centreline of alignment and level are drop properly to their position. A small mistake can cause bigger problem in construction.
Following points should be kept in mind before alignment of a tunnel:
a- Minimum gradient is provided if possible for tunnel.
b- The alignment should be straight as far as possible because such roots will be shortest.
c- Proper ventilation and adequate lightning is provided inside tunnel.
d- Drains in tunnel is also a major problem hence, 1: 500 (1 in 500) of gradient is provided for effective drainage.
1- Rock type- Strong Rock and massive rock such as granite, quartzite. Granite- genes. Basalt are highly durable and stable.
2- Folded formations-The folded formations tunnel is aligned along or across fold axis. Tension effect in the rock impart highly fractured along the crest and trough. In case of tunnels along fold axis anticline provides suitable location and better suitability, because fractures along crest diverge upward, such that fractured blocks are held together, decreasing the rock falls. A syncline does not provide suitable location because the fractures along trough diverge downward with result that fractured blocks are left loose to fall out. Leakage problem is also a severe problem in tunnel. Any aquifer if touch upon tunnel can cause flooding. Therefore, anti-clinical fold provides better location than synclinal fold. Advantage include greater stability and lesser problem of infiltration.
3- Faulted formation-It is serious problem if exist and any connection to this problem leads to complete collapse of whole tunnel. Care should be taken to avoid this structure. Small and inactive fault are not cause failure of tunnel. If weak or crushed rock is present in fault zone then it should be excavated and filled back with reinforced concrete. Proper arrangement for interlocking is necessary. The movement along fault crossing tunnel is dislocated and causes failure.
4-Jointed formation-Joint if present make the rock weak and they also release the stored strain. Extensive joint only can cause stability problem as well as sea page. For this problem growing is very important as well as lining is provided.
5-Attitude of bed-Horizontal beds are ideal condition for tunnel if soft bed underlain and covered by hard rock. Tunnel should be constructed along soft bed and hard bed is used as floor and ceiling. If steeply dipping beds is present, then tunnel should be aligned along strike and cutting should be done at song and impervious bed. If tunnel across strike, then leakage problem along bedding planes and permeable beds then choice of Rock fall increases.
The groundwater presence in the Tunnel construction leads to a major hazard or sometimes leads to collapse. Special precautions should be taken such as dewatering or injection programs need consideration. The study of water inflow qualities is however difficult and special measures are to be taken. Rocks which are impervious are to be selected for tunnelling. If however sea page is there, then growing and lining are to be done. Solution effect of groundwater is also very important. It may make some rocks highly cavernous. Such case is to be avoided. At the process of excavation, water table play a vital role. If large amount of water in loose ground is present, then it lead to formation of cavities around the tunnel excavation and produce wet and lose ground to flow the tunnel. Therefore, groundwater occurrence should be assessed during the site investigation stage. It is very difficult to tell the water inflow rate of any ground. The ground water problem becomes more critical as thickness of overburden increases.
Suitability of common Rock types for excavation and tunnelling
1) Suitability of igneous rock- Rock Sachin plutonic and hypabyssal varieties are competent but difficult to work. Lining is not important or any special maintenance because they are strong, hard, impervious and rigid and after telling do not have chance to collapse.
2) Suitability of Metamorphic rock: Rock such as gncioses, granite in terms of durability and workability. Rock such as phyllites which are highly foliated and generally soft are easily excavated but lining is important or necessary in this rock. Slates, marbles are also soft and weak but require lining.
3) Suitability of sedimentary rock: Rock such as siliceous or ferruginous sandstones are more competent and better for tunnelling. These rocks are strong i.e. do not require any linings. These rocks are not affected by geological structures and groundwater conditions whereas poorly argiliacioces sandstones are weak and does not suitable for tunnelling. Dolomitic limestone are harder and durable therefore better than other varieties whereas calcareous limestone are naturally soft and weak and not suited for tunnelling.
Important case studies in Kasara and BorGhat sections of central railway in Maharashtra
Himalayan
Main challenges and obstacles
The international link between China, Nepal and India require the construction of more than 500km of new rail .The scariest of many challenges include the following:
• Large height drop within a short distance
The most difficult section is the Chinese side of the alignment where the elevation drops by about 2,000m over a distance of 20km between the towns of Nyalam and Zhangmu.
• Long tunnels under deep overburden
Deep tunnels on the routes pass through tectonic faults and shear zones which is usually characterised by high rock stresses and high groundwater pressures, frequently at high temperatures.
• Lack of detailed geological data
Lack of data and experience is available ahead of excavating long tunnels in the region. Very few tunnels excavated in the Himalayan, and current projects are the conditions for the first time.
If the railway was to be built, it would boost the socio-economic development of two of the most developed countries in the world today and improve substantially underdeveloped situation of landlocked country Nepal, benefiting roughly about 2.5 billion people and providing a modern 'silk route'
Reference:
1. Physical Geology by P. K. Mukarjee, World Press, 2013.
2. Physical Geology by Arthur Holmes, ELBS Publication.
3. Principles of Engineering Geology and Geotechniques by D. P. Krynine & W. R. Judd. CBS Publishers, New Delhi.
4. Engineering Geology by F. G. H Blyth and De Frietus,2006, Reed Elsevier India Ltd.
