Earth Embankment Dam Filters - Essay Example

? Embankment dam of Introduction An embankment dam can be defined as a gigantic but artificial water barrier. It is classically created by the emplacement and compaction of a multifaceted semi-plastic mound of diverse compositions of sand, rock or and soil. It is characterized by a semi-permanent waterproof natural covering for its surface and very dense waterproof core. This kind of protection makes the dam impervious to seepage erosion (INDRARATNA, & ASHOK, 2006). Two kinds of embankment dams exist. They include the rock and earth filled dams. Some of the causes of embankment include embankment deformations under static loading, which occur due to volumetric changes, tangential and shear displacements within the embankment and groundwork materials. This study establishes that the volumetric changes are due to either a raise in the normal stresses on a soil constituent, which causes a decrease in void volume, or dilation of soil rudiments undergoing shear (INDRARATNA, & ASHOK, 2006). The riskiest event of an embankment dam is when it overtops beyond its spillways thus causing the ultimate failure. What is the purpose of upstream and downstream filters? Upstream can be defined a course away from the supply of power in a fluid system under the pretext of an embankment dam. In other words, downstream in a hydraulic structure is in the same bearing as the fluid is moving. To start with, the purpose of the downstream filters involves upgrading the dams intended to meet the safety standards. It is imperative to note that retrofitting stepped up spillways to the dam is the most recent and accepted method, where when the waters flows down a stepped spillways, the water surface changes from a smooth surface to a rough surface with visible white water (INTERNATIONAL COMMISSION ON LARGE DAMS, 1994). Designing of the stepped spillways, and stilling basins finds the knowledge of the beginning of the white water fundamental. This paper indicates that stepped spillways used to embankment dams particularly on the downward filtering have become a common phenomenon with the rehabilitation of aging watershed dams more particularly those experiencing a hazard classification change from low to high hazard (PERRY, 2007). This study finds that the initiation point is significant region for a spillway design factor applied in energy debauchery and entrainment forecast associations. For instance, Chanson developed an inception point correlation for gravity stepped; spillways with an ogee crest control section. However, Chanson’s correlation tends to overvalue the distance from the downstream periphery of a broad-crested barrier to the inception point for stepped spillways with is presented by () when the Froude surface coarseness is less than 10 (PERRY, 2007). Meireles and Matos maximized Chanson's association for broad-crested weir stepped spillways retrofitted for embankment dams (PERRY, 2007). There are numerous new associations for projecting the original point location for broad-crested step spillways classically designed for embankment dams for a extensive range of flow conditions such that the F*? 100 and Froude surface roughness ?100. Upstream can be defined as a course towards the spring of power in a fluid structure in the context of an embankment dam. Upstream in a hydraulic system is in the direction from which the fluid is coming (PERRY, 2007). It is imperative to note that, both the downward and upward streams filters are fundamental in numerous functions, which include acting as cut offs, casing, slope protection, surface drainage and as impervious blanket. How are such filters designed? This study intends to consider the design of these filters that makes them perform the listed functions in the current dynamic environmental changes. Cut off The cut off is significant in plummeting the loss of stored water through foundations and abutments. In addition, it prevents sub-surface erosion by piping (PERRY, 2007). The design that fits makes this filters to function efficiently depends on the latest design models, which include Location of the cut off (for filtering) should be in such way that its centreline is within the base of the impervious core and upstream of the dam’s centre line. The positive cut off should be pegged 40 cm deep into the non-erodible rock A 4-metre bottom width is recommendable 1:1 ration of side slopes should be provided to educe burden in case of soft rocks. The flank’s cut off should be extended to the impervious rock top (INTERNATIONAL COMMISSION ON LARGE DAMS, 1994). Internal drainage system The filters remain fundamental in ensuring the safety of the dam. The filters handle the seepage of water in the dam consequently maintaining the original particles of soils in their place. The main filters involved in the safety of the dam include vertical, horizontal, rock toe and toe drain. The design of the vertical filter should be fitting to the dynamic environmental changes and challenges. It is normally situated at the downstream slope of the core. Its design should ensure a minimum thickness of 1 meter (ENGINEERING AND RESEARCH CENTER (U.S.) 1984). The horizontal filter functions in collecting the seepage from the foundation and chimney filter. It carries it to the toe rock and rock toe filters. Its design should ensure a minimum of 1-meter thickness. In addition, the typical filter measure amid the filter and the foundation should be fulfilled. On a similar note, the occurrence of seepage under filters is due to the water level difference at both the upstream and downstream sides of the structure. It is imperative to note that, poor or lack of consideration of this significant parameter would lead to numerous problems, which include undermining and piping immediately downstream the filter. Moreover, appreciation of perforated drainage blanket just below the downstream filters is fundamental in attaining security against piping. In addition, the optimum length of both the downstream and upstream filters remains critical in designing the filters (UNITED STATES, 2007). This study reveals that, granular materials should be used in the build up of filters and similarly, permeable materials of 10^-6 m/s which consists of crusher-run rocks with an optimum size of 150mm, should be placed just under the concrete to provide barriers to both downstream and upstream halves. In addition, other effective factors in designing the filters include distribution of the pores in the filter, which is the main feature of a properly functioning filter. This feature influences the permeability and retention mechanism (INTERNATIONAL COMMISSION ON LARGE DAMS, 1994). The following parameters form few of the numerous effective factors to be considered when designing of filters. Fine content of the filter, filter particle shape, hydraulic gradient, relative density, filter thickness, physic-chemical aspects and void distribution. Are there systems in place to monitor the performance of the filters? This study finds out that fundamental filter monitoring is one of the principal segments in an embankment dam. This process is done by installing a simple but most effective tool called a critical filter. This instrument ensures that the dam is immune from erosion seal unfavourable cracks and prevents water escape. Some of the systems in place to monitor the performance of the filters include carrying out the No Erosion Filter (NEF) test (FOSTER, FELL, & SPANNAGLE, 2000). This apparatus is made of a plastic cylinder with both the upper and lower caps. Each contains holes for inlet and outlet water. It contains base soil, filter and Gravel drain in the cylinder gravel drain from the top to the bottom in that order. Then a 1mm hole is created in the pedestal soil that represents the replicated cracks in the core. A 4.2 Kpa pressure is provided by water should be applied to the system after replication. The essence of this high pressure is to cause erosion in the foundation soil. This performance practice, which is commonly used due to the geographical, physical and chemical changes attributed to the processes involving assessment of filter performance (INDRARATNA, & ASHOK, 2006). Outputs such results like water rate, the magnitude of darkness of the water and the diameter of the hole. Then what follows this out put is the ascertaining that the filter is successful to control the erosion or not. This study finds out that, decrease in water rate output and darkness reducing from dark to bright are the first hand evidences to indicate or prove that the filter is properly controlling the erosion and the trend of erosion will come to an end as soon as possible. More over, the diameter of the hole crowns the evidence of proper control of the filter on erosion. Another, performance check on the filter is the Continuing Erosion Filter (CEF), which scholars and researchers tend to incline their critical analysis to a modification of the NEF test. The CEF test performs almost a similar performance filter monitoring by searching and checking the continuing erosion boundary and this aid in ascertaining the ability of the filter to control erosion (INDRARATNA, & ASHOK, 2006). Besides the two tests most performed in ascertaining the performance level, other systems that should be put in place to monitor or ensure effective functioning of the filters include, ensuring that the poles amid the filter particles are sufficiently small to sustain the retention criterion by holding some of the relatively larger particles. The filters should be in such way that they sustain the coarse nature, which is fundamental in allowing seepage stream to bypass through the filters (CHAHAR, & KALSI, 2004). In other words, the coarse nature is intended in sustenance of the permeability criterion by the filters, which contributes immensely in preventing upsurge of high pore pressure and hydraulic gradient. What is the relevance of the critical hydraulic gradient in the context of dam filters? The relevance of the critical hydraulic gradient in the context of dam filters surrounds the determination of the critical hydraulic gradient needed for moving a base of a particle within a pore cannel. Essentially, this can be demonstrated though calculation, however, in this study the general overview indicates that, the particle is presumed to displace when applied hydrodynamic forces exceed this critical hydraulic gradient to include the effect of the drag which aid in determination of the aspects of permeability and retention in the filters (INDRARATNA, & ASHOK, 2006). References CHAHAR, B. R. M., & KALSI, A. P. (2004). DESIGN OF HORIZONTAL FILTER LENGTH IN HOMOGENEOUS EARTH DAMS. ISH Journal of Hydraulic Engineering. 10, 8-18. ENGINEERING AND RESEARCH CENTER (U.S.). (1984). Design standards no. 13. Embankment dams. Denver, Colo, United States Dept. of the Interior, Bureau of Reclamation, Engineering and Research Centre. FOSTER, M., FELL, R., & SPANNAGLE, M. (2000). A method for assessing the relative likelihood of failure of embankment dams by piping. Canadian Geotechnical Journal. 37, 1025-1061. FOSTER, M., FELL, R., & SPANNAGLE, M. (2002). A method for assessing the relative likelihood of failure of embankment dams by piping: Reply. Canadian Geotechnical Journal. 39, 497-500. INDRARATNA, B. R., ASHOK K. (2006). Enhanced Criterion for Base Soil Retention in Embankment Dam Filters. Journal of Geotechnical and Geoenvironmental Engineering. 132, 1621. INTERNATIONAL COMMISSION ON LARGE DAMS. (1994). Embankment dams granular filters and drains: review and recommendations = Barrages en remblai filtres et drains granulaires. Paris, Commission Internationale des Grands Barrages. PERRY, E. B. (1987). Laboratory tests on granular filters for embankment dams. Vicksburg, Miss, U.S. Army Engineer Waterways Experiment Station. SCHMERTMANN, J. (2002). A method for assessing the relative likelihood of failure of embankment dams by piping: Discussion. Canadian Geotechnical Journal. 39, 495-496 UNITED STATES. (2007). Design standards no. 13: Embankment dams. Chapter 5, protective filters. Denver, Colo, U.S. Dept. of the Interior, Bureau of Reclamation. Read More