Embankment lecture 2
- 1. EMBANKMENT DAM DESIGN OF HYDRAULIC STRUCTURE MODULE 1 LECTURE 2 ON 10 45 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 1
- 2. COMPONENTS OF EARTHEN DAM The main components of an earth dam are as follows : 1. Impervious core 2. Pervious shell 3. Filter 4. Rock toe 5. U/S slope protection 6. D/S slope protection 7. Cutoff 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 2
- 3. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 3
- 4. Impervious core : The impervious core at the centre checks seepage through the dam. It is constructed of clay, silt, silty clay or clayey silt. At the top of top width of impervious core should not be less than 3 m and its height should be 1 m more than the maximum water level in the reservoir. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 4
- 5. Pervious shell : The pervious shell gives stability to the dam. It consists of sand, gravel or a mixture of these materials. The upstream pervious zone provides free drainage during sudden drawdown. The down stream pervious zone acts as a drain to control the phreatic line. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 5
- 6. FILTER Filter zones are invariably provided in all earth dams. They are constructed of pervious materials. Provision of filter zones at d/s side serves the 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 6
- 7. following functions : to drain off the water seeping through earth dam safely. To reduce the pore water pressure in the downstream portion of the dam. To keep the phreatic line within the body of the dam. To check the piping by checking migration of the particles. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 7
- 8. Usually following types of filters are provided : (1) Toe filter (2) Horizontal drainage filter (blanket) (3) Chimney drains 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 8
- 9. Filter criteria : Generally, a multilayered filter is provided in which each Subsequent layer becomes increasingly coarser than the previous one. Such a filter is sometimes known as inverted filter or reverse filter. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 9
- 10. The soil to be protected is known as base material. Basic requirements of filter material are : (i) The filter material should be sufficiently fine and properly graded so that the voids of the filter are small enough to prevent base material particles from penetrating and clogging the filter. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 10
- 11. • (ii) The filter material should be sufficiently coarse and pervious compared to the base material so that the incoming water is rapidly removed without any appreciable build up of seepage forces within the filter. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 11
- 12. (iii) The filter material should be coarse enough not to be carried away through the drainage pipe openings. (iv) The filter layer should be sufficiently thick to provide a good distribution of all particle size throughout the filter and to be able to carry the seepage discharge. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 12
- 13. The Filter material should satisfy the following two criteria. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 13
- 14. Rock toe : At the d/s end of an earth dam a rock toe is provided by arranging 15 to 20 cm size stones. The height of rock toe is usually kept 30 to 40% of the reservoir head.“ Rock toe keeps the phreatic line well within the section and also facilitates drainage. It also prevent failure of d/s of the dam due to piping or sloughing. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 14
- 15. u/s slope protection : The u/s slope should be protected against the wave action and the d/s slope should be protected against rain. The following measures are taken to protect the slope. (1) Rock riprap (2) Concrete pavement (3) Steel facing (4) Bituminous pavement (5) Precast concrete blocks 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 15
- 16. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 16
- 17. Cut off : Cut off is required to (1) reduce loss of stored water through foundation and abutments (2) Prevent sub surface erosion by piping. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 17
- 18. • As per IS : 8826-1978 recommendations : (1) The alignment of the cut off trench should be fixed in such a way that its center line should be within the u/s base of the impervious core and it should be keyed into rock. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 18
- 19. (2) The bottom width of cutoff trench may be fixed taking following factors into considerations : • Provide sufficient working space to carry curtain grouting. • Provide sufficient working space for compaction equipment. • Provide safety against piping. 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 19
- 20. Cutoff may be provided in the following ways : • by providing concrete cutoff wall • by providing cutoff trench filled with impervious material • by driving sheet pile • by curtain grouting 1/8/2014 prepared by V.H.KHOKHANI, ASSISTANT PROFESSOR, DIET. 20