SEEPAGE PROBLEMS IN DAM; TERM PAPER

A TERM PAPER ON PRESENTED BY (CVE/10/2833) IN PARTIAL FULFILMENT OF THE REQUIREMENT OF THE COURSE DAM & WATER RETAINING STRUCTURES (CVE 825) DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING, SCHOOL OF ENGINEERING AND ENGINEERING TECHNOLOGY, FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE, ONDO STATE, NIGERIA COURSE LECTURER: FNSE, P.Eng (Ont. Canada) , Ph.D., (Tor.), APRIL, 2011. TABLE OF CONTENTS Cover Page Table of Contents i List of Tables ii List of Plates ii List of Figures ii 1.0 Introduction 1 1.1 Causes of Dam Failure 1 1.2 Seepage 2 1.3 Seepage forces 3 1.4 Typical failure modes that leads to seepage in earth dams 4 1.4.1 Flow erosion 4 1.4.2 Embankment leakage 4 1.4.3 Outlet conduit leakage 4 1.4.4 Tree Growth 5 1.5 Seepage failure 5 2.0 Effects of seepage 6 3.0 Detection 6 4.0 Control 7 5.0 Monitoring 8 6.0 Conclusion and Recommendations 9 References 11 i LIST OF TABLES Table 1: Causes of failure 2 LIST OF PLATES Plate 1: Land slide (slope failure) near the right abutment 13 Plate 2: Uncompleted spillway 13 Plate 3: Complete failure of the dam 14 Plate 4: Exit of the main canal from the release facility 14 LIST OF FIGURES Figure 1: Embankment with seepage 15 Figure 2: Problem of Dam Break 16 Figure 3: Seepage Force Components 16 Figure 4: Use of horizontal and inclined drainage layer to control seepage through an embankment 17 Introduction Failure of earth dams can be caused by seepage, piping, foundation instability, deformation and deterioration, and from earthquakes. However, most of the recorded failures around the world are related to seepage problems. All dams have some seepage as the impounded water seeks paths of least resistance through the dam and its foundation. Seepage becomes a concern if it is carrying material with it, and should be controlled to prevent erosion of the embankment, or foundation, or damage to concrete structures. http://www.dev. ny.gov/lands/4991.html#Owners, New York State, Department of environmental conservation, 22nd March, 2011 Causes of Dam Failures The incident of failures demonstrate that depending on the type of dam, the cause of failure may be classified as: a. hydraulic failures; (for all types of dams) b. failures due to seepage. (i) through foundation, (all except arch dams) (ii) through body of dam (embankment dam) ii c. failures due to stresses developed within structure. A study of dam failures in the world has revealed the percentage distribution of dam breaks and its attributes causes of failure is shown below in Table 1 ii ii Table 1: Causes of failures Cause of failure Cause of failure Foundation Problems 40% Inadequate spillway 23% Poor construction 12% Uneven settlement 10% High Pore Pressure 5% Acts of war 3% Embankment slips 2% Defective materials 2% Incorrect Operations 2% Earthquakes 1% Source: Prof. B.S. Thandaveswara, hydraulics-Indian Institute of Technology The causes of failures may be classified as (i) foundation deterioration (ii) foundation instability (iii) defective spillway (iv) defective outlets (v) defects in embankments (vi) concrete deterioration and defects (vii) overtopping (viii) inadequacy of spillway and (ix) sudden filling of reservoirs. Typical Failure Modes that leads to seepage in Earth dams Http://www.dev. ny.gov/Dam/, New Hampshire, Department of Environmental Services 22nd March, 2011 Dams are susceptible to several forces that can ultimately lead to their deterioration and failure. The forces that may contribute to deterioration on earth embankment dams which eventually lead to seepage are water flowing over the dam embankment (flow erosion), leakage, and trees on the embankment. Flow Erosion A high percentage of the earth dams have inadequately sized spillways to allow for the passage of any abnormal size storm event. As a result, these dams are frequently overtopped for short periods of time. Studies have indicated that this type of dam can sustain limited overtopping without major structural damage. However, any degree of overtopping will accelerate deterioration and should be guarded against. Embankment Leakage Most dams in active use today exhibit seepage of one form or another. The location, rate of flow, and turbidity (clear or murky) are the critical factors when evaluating the seriousness of seepage from a dam. Water pressure cannot build up against this face because the voids allow seepage to drain freely. Also, a large number of these dams have been built with sufficient amounts of gravelly material, which acts to plug seepage paths over time. Outlet Conduit Leakage Breaks, separation of joints, or loss of conduit material within the dam structure itself could lead to leakage of water under pressure into the interior of the dam. This action could cause the washing out of material from within the dam embankment, creating the possibility for structural failure of the dam. Probably the most potentially serious situation is when a rupture occurs in the conduit on the upstream side of the gate. Because high water pressures are maintained on the upstream side of the control mechanism, a leak which develops can cause greater internal erosion and at a faster rate. The simple fact that high pressures exist in the conduit makes the development of leaks and seepage more likely. For this reason new dams are constructed with their low level outlet controls located at the upstream side of the dam. Tree Growth Tree growth on stone faced earth dams can lead to failure in a number of ways. The most sudden of these is when trees growing along or near the crest of the dam are blown over. This reduces the available freeboard of the dam and can lead to overtopping, or the amount of dam embankment removed could lead to structural failure because of the reduced cross section of the dam. The root systems of these trees could extend from the upstream side all the way through the embankment at the same time providing a convenient path for seepage to develop and progress along. Seepage Failure Sherard et al, (1963) carried out an extensive survey on dam failures and he reported that failure in earth dams could be as a result of overtopping, embankment and foundation piping, differential settlement and cracks, embankment and foundation slides, slides during construction, earthquake damage, reservoir wave action, damage due to borrowing animals, damage caused by water soluble material, flow slides due to spontaneous liquefaction, and damage due to surface drying. In the early times Terzaghi in his experience in geotechnical engineering encountered many cases of failures - significantly due to lack of ability to predict and control ground water. Piping failures were abundant and also slope failures, bearing capacity failures and excessive settlements. (Burland, 2006) Anonymous, (2003) pointed out that earth dam failures can be grouped into three general categories: overtopping failures, seepage failures, and structural failures. The three types of failure are often interrelated in a complex manner. On the basis of investigation reports on most past failures by Punmia and Lal, (1992), they were able to categorize the types of failures into three main classes: (1) Hydraulic: 40% (2) Seepage: 30% (3) Structural failures: 30% Investigations carried out by Arora, (2001) also showed that about 35% of failures of earth dams are due to hydraulic failures, about 30% are attributed to seepage failures and about 20% are as a result of structural failure. The remaining 7% of the failure are due to other miscellaneous causes such as accidents and natural disasters. Effects Seepage can cause slope failure by creating high pressures in the soil pores or by saturating the slope. The pressure of seepage within an embankment is difficult to determine without proper instrumentation. A slope which becomes saturated and develops slides may be showing signs of excessive seepage pressure. Uncontrolled seepage may weaken the soil and lead to a structural failure. A structural failure may shorten the seepage path and lead to a piping failure. Surface erosion may result in structural failure http://www.dnr.state.oh.us/water, Ohio Department of Natural Resources, Division of Water. Dam Safety Engineering Program, Detection http://www.dev.ny.gov/lands/4991.html#Owners, New York State, Department of environmental conservation, 22nd March, 2011 Seepage can emerge anywhere on the downstream face, beyond the toe, or on the downstream abutments 1t elevations below normal pool. Seepage may vary in appearance from a "soft," wet area to a flowing "spring." It may show up first as an area where the vegetation is lush and darker green. Cattails, reeds, mosses, and other marsh vegetation often become established in a seepage area. Another indication of seepage is the presence of rust-colored iron bacteria. Due to their nature, the bacteria are found more often where water is discharging from the ground than in surface water. Seepage can make inspection and maintenance difficult. It can also saturate and weaken portions of the embankment and foundation, making the embankment susceptible to earth slides. If the seepage forces are large enough, soil will be eroded from the foundation and be deposited in the shape of a cone around the outlet. If these "boils" appear, professional advice should be sought immediately. Seepage flow which is muddy and carrying sediment (soil particles) is evidence of "piping," and will cause failure of the dam. Piping can occur along a spillway and other conduits through the embankment, and these areas should be closely inspected. Sinkholes may develop on the surface of the embankment as internal erosion takes place. A whirlpool in the lake surface may follow and then likely a rapid and complete failure of the dam. Emergency procedures, including downstream evacuation, should be implemented if this condition is noted. Seepage can also develop behind or beneath concrete structures such as chute spillways or headwalls. If the concrete structure does not have a means such as weep holes or relief drains to relieve the water pressure, the concrete structure may heave, rotate, or crack. The effects of the freezing and thawing can amplify these problems. It should be noted that the water pressure behind or beneath structures may also be due to infiltration of surface water or spillway discharge. A continuous or sudden drop in the normal lake level is another indication that seepage is occurring. In this case, one or more locations of flowing water are usually noted downstream from the dam. This condition, in itself, may not be a serious problem, but will require frequent and close monitoring and professional assistance. Control Recently, great efforts have been paid to develop effective techniques for detecting, positioning, and mapping of seepage under and through earth dams. These efforts will help to find ways and means to minimize and control seepage and increase safety of earth dams. Li and Ming (2004) studied the driving seepage force and its effect on earth dams through a set of fully coupled finite element analysis. Xu et al., (2003) formulated an optimum hydraulic design regarding an earth dam cross section and the design depends mainly on reducing the saturated zone and minimizing material cost. Li et al., ( 2003) proposed element free method for seepage analysis with free surface and the method was applied to steady seepage and transient seepage in uniform earth dams and the application showed satisfactory results. Panthulu et al., (2001) utilized an electronic method for delineation of seepage zones. Leontiev and Huacasi (2001) used mathematical programming technique to conduct numerical simulation for unconfined flow through porous media. They perform boundary element discretization and applied interior point algorithm to solve it. They propose to use the method of solution for 2D real size problems and extended to 3D problems. Zhang et al., (2001) proposed a simplified approach based on finite element technique to predict the seepage line (phreatic line) through non-homogenous rock fill dam with toe drain or core wall. Kalkani (1997) presented the case of Bakoyianni earth dam in Greece in which the dam abutment experienced seepage problem and he evaluated the dam safety and remedial measure to control seepage. Huang (1996) described and applied a numerical method using finite element technique to check the stability of earth dams after filling of their reservoirs. The need for seepage control will depend on the quantity, content, and location of the seepage. Reducing the quantity of seepage that occurs after construction is difficult and expensive. It is not usually attempted unless the seepage has lowered the pool level or is endangering the dam or appurtenant structures. Typical methods used to control the quantity of seepage are grouting or installation of an upstream blanket. Of these methods, grouting is probably the least effective and is most applicable to leakage zones in bedrock, abutments, and foundations. These methods must be designed and constructed under the supervision of a professional engineer experienced with dams. Controlling the content of the seepage or preventing seepage flow from removing soil particles is extremely important. Modern design practice incorporates this control into the dam design through the use of cutoffs, internal filters, and adequate drainage provisions. Control at points of seepage exit can be accomplished after construction by installation of toe drains, relief wells, or inverted filters. Weep holes and relief drains can be installed to relieve water pressure or drain seepage from behind or beneath concrete structures. These systems must be designed to prevent migration of soil particles but still allow the seepage to drain freely. The owner must retain a professional engineer to design toe drains, relief wells, inverted filters, weep holes, or relief holes, and regular monitoring of these features is critical. Monitoring Regular monitoring is essential to detect seepage and prevent dam failure. Knowledge of the dam's history is important to determine whether the seepage condition is in a steady or changing state. It is important to keep written records of points of seepage exit, quantity and content of flow, size of wet area, and type of vegetation for later comparison. Photographs provide invaluable records of seepage. All records should be kept with the Inspection and Maintenance Plan for the dam. Every inspector should always look for increases in flow and evidence of flow carrying soil particles, which would indicate that a more serious problem is developing. Instrumentation can also be used to monitor seepage. V-notch weirs can be used to measure flow rates easily and inexpensively, and piezometers may be used to determine the saturation level (phreatic surface) within the embankment. Regular surveillance and maintenance of the internal embankment and foundation drainage outlets is also required. The rate and content of flow from each pipe outlet for toe drains, relief wells, weep holes, and relief drains should be monitored and documented regularly. Normal maintenance consists of removing all obstructions from the pipe to allow for free drainage of water from the pipe. Typical obstructions include debris, gravel, sediment, mineral deposits, calcification of concrete, and rodent nests. Water should not be permitted to submerge the pipe outlets for extended periods of time. This will inhibit inspection and maintenance of the drains and may cause them to clog. Rodent guards are readily available and should be installed where needed. Conclusion and Recommendations All earth and rock-fill dams are subject to seepage through the embankment, foundation, and abutments. Seepage control is necessary to prevent excessive uplift pressures, instability of the downstream slope, piping through the embankment and/or foundation, and erosion of material by migration into open joints in the foundation and abutments. The following recommendations are thus given with a view to reducing earth dam failures to the barest minimum: a) Adequate study should be carried out on the project area to include, hydro-meteorology, geology and soil among others; b) Design should be based on the results of the feasibility study carried out; c) Projects should not be commissioned before they are fully completed; d) Experts from all the relevant areas must be involved in the planning and development of the project; e) Engineering procedure of project conception, implementation operation and maintenance should be strictly adhered to; f) There should be a well designed and constructed spillway; g) Construction should be strictly based on the design specifications and standards; h) Side slopes in the upstream and downstream side of the dam should be about 3: 1 as this provides a very reliable stability; i) Allowance of 60cm freeboard after settlement above maximum height of water if the length of the dam does not exceed 300m and if larger than this, more than 60cm; j) The downstream slope should be protected against rainfall erosion by heavy gravel or rock riprap. Sod may also be provided to guard against erosion if the rainfall is sufficient to grow and maintain grasses; k) If highly permeable material would be used at all in constructing the dam, it will be found least objectionable if applied at the outer parts of the dam to aid drainage as a fill. Particularly attention must be given to the use of impervious materials in the core; l) There should be no danger of over-topping by water; m) The seepage line should be well within the downstream face the dam. This is to prevent sloughing and possible failure; n) Water passing through or under the dam should be unable to remove materials of the dam or the foundation; o) There should be no opportunity for free flow of water from upstream to downstream face; p) The foundation shear stress should be smaller than the shear strength to provide a suitable margin of safety; q) Well equipped and adequate dam safety monitoring team should be on site all the time; r) The operation and maintenance should be based on a standard manual; s) Log books should be provided to enhance accurately in record taking as well as record keeping; t) The site monitoring team should be well trained and they should be sent to refresher courses from time to time; and u) All the instrumentation facilities should be well maintained to avoid malfunctioning. References Anonymous. 2003. Dam Safety: Earth Dam Failures, Fact Sheet 03-03. Indiana Department of Natural Resources, Water Division. http://www.in.gov/dnr/water. Arora K.R. 2001. Irrigation Water Power and Water Resources Engineering. Naisarak India: Standard Publishers. Burland J. 2006. Terzaghi: back to the future. Journal/Bulleting of Engineering Geology and Environment. http://www.springerlink.com. 66: 29-33. Http//www.arpnjournals.com, Asian Research Publishing Network(ARPN), Journal of Engineering and Applied Science ,19th March, 2011. Http://www.dev.ny.gov/Dam/, New Hampshire, Department of Environmental Services, 22nd March, 2011 Http://www.dev.ny.gov/lands/4991.html#Owners, environmental conservation, 22nd March, 2011 New York State, Department of Http://www.dnr.state.oh.us/water, Ohio Department of Natural Resources, Division of Water Dam Safety Engineering Program. Http://www.ejge.com, 22nd March, 2011 Http://tatanggustawan.blogspot.com/2009/04/seepage homogenous earth dams.html, 11th March, 2011 through homogenous and non- Huang, T. (1996) “Stability analysis of an earth dam under steady state seepage,” Journal of Computer and Structure 58 (6), 1075-1082. Kalkani, E.C. (1997) “Geological conditions, seepage grouting, and evaluation of piezometer measurements in the abutments of an earth dam”, Journal of Engineering Geology, 46, 93-104. Leontiev, A., and W. Huacasi (2001) “Mathematical programming approach for unconfined seepage flow problem,” Journal of Analysis With Boundary Elements, 25, 49-56. Li, G., J. Ge, and Y. Jie (2003) “Free surface seepage analysis based on the element-free method,” Journal of Mechanics research Communications, 30, 9-19. Li, X. S., and H. Ming (2004) “Seepage driving effect on deformations of San Fernando Dams,” Journal of Soil Dynamics and Earthquake Engineering. 24, 979-992. Panthulu, T. V., C. Krishnaiah, and J. M. Shirke (2001) “Detection of seepage paths in earth dams using self-potential and electrical resistively methods,” Journal of Engineering Geology, 59, No. 3 and 4, 281-295. Punmia B. C. and Lal P. B. B. 1992. Irrigation and Water Power Engineering. 12th Edition. J. udpur India. Laxmi Pupblications (P) Ltd. Sherard J.L., Richard S.D, Woodward J, Stanley N.S., Gizenski F., Willaim M.S. and B.S. Clevenger. 1963. Earth and Earth Rock Dams. John Willey and Sons Inc. Xu, Y., K. Unami, and T. Kawachi (2003) “Optimal hydraulic design of earth dam cross section using saturated-unsaturated seepage flow model,” Journal of Advances in Water Resources, 26, 1-7. Zhang, J., Q. Xu, and Z. Chen (2001) “Seepage analysis based on the unified unsaturated soil theory,” Journal of Mechanics Research Communications, 28(1), 107-112. Plate 1: Land slide (Slope failure) near the right abutment Source: www.arpnjournals.com(Asian Research Publishing Network) Plate 2: Uncompleted Spill-way. Source: www.arpnjournals.com(Asian Research Publishing Network) Plate 3: Complete failure of the dam. Source: www.arpnjournals.com(Asian Research Publishing Network) Plate 4: Exit of the Main Canal from the release facility Source: www.arpnjournals.com(Asian Research Publishing Network)