Design And Construction Fundamentals Of Earth Dams Environmental Sciences Essay

This paper focuses on earth dams, one of the oldest earth dams. Forming service reservoir for domestic water supply and irrigation purpose, earth dams comprise the most numerous class. The design and construction of Earth dams are complex because of the nature of the varying foundation conditions and the different properties of the materials available for the construction of the embankment. Last but not least, an understanding of the potential failure of Earth dams is important in the design and construction process of the Earth dams.

Earth dams, also called earthen or earth-filled dams are generally built in or near drainage line which has the variety of purposes including domestic water supplies, agricultural irrigation, water for landscape improvement and others. One advantage of the Earth dams is because it can adapt to a weak foundation provided proper consideration is given to the foundation and design of the Earth dams. However, failure of any important part of the Earth dams such as seepage, structural and foundation will lead to the collapse of the whole dam.

Types of Earth Dams

Constructed as a simple homogenous embankment of well compacted earth, earth dams are classified as Homogenous Earth Dams, Zoned Earth Dams and Diaphragm Dams. A brief discussion of the types of dams is as follow:

Homogenous Earth Dam is a kind of common earth dams which consist of two main components; the impervious to semi-pervious structure and the toe filter and the drain. The purpose of the filter and the drain is to provide a through way for seepage to exit the dam structure without causing erosion. This type of dam is usually built on an impervious foundation such as solid rock or clay.

The next type of Earth dam is Diaphragm Dam. A diaphragm dam, can be used when there is no impervious layer below the dam and modification of the dam can be made to suit at an impervious layer. Besides, this type of dam is limited to a height of 8.0m in order to keep seepage forces at a safe level.

Last type of Earth Dam is Zoned Dam. A Zoned Dam is known to be the most efficient use of soil as each material is used to its greatest potential. The slope protection protects the shoulder from erosion and wave action, the core retains the water and the shoulders stabilize the core. Lastly, figure 1 shows the 3 types of earth dams.

Figure 1. Cross section of Homogenous Earth dam, Zoned earth Dam and Diaphragm Dams

2. DESIGN OF EARTH DAM

Earth dam is built for the purpose of storing water and is made of compacted earth. It is usually more convenient when concrete is expensive and timber is scarce.

2.1 Foundation

A site investigation should be conducted prior to construction so as to investigate the nature of the foundation. By knowing the actual foundation condition at site, the earth dam can then be designed according to it.

A dam foundation is said to be adequate if it is capable of providing a stable support for the embankment under all condition of saturation and loading and that it provide enough resistance to seepage to avoid excessive loss of water (Bureau of reclamation 1987).

According to Bureau of reclamation (1987), dam foundation can be generally classified into three types namely foundations of rock, foundations of coarse-grained material and foundation of fine-grained material.

These foundations may need to be treated to stabilize any weakness as well as to reduce seepage. For instance, coarse-grained, pervious foundations present no difficulties in the matter of settlement or stability whereas a fine-grained, weak foundation is subject to settlement or displacement usually present no problem in seepage. Rock foundation on the other hand must be checked for erosive leakage and excessive uplift pressure. If such conditions exist, grouting to the foundation must be considered.

2.2 Soil

Earth dam is usually built with clay, sand and gravel and hence is also known as earth fill dam. The type of soil used in the construction of an earth dam must be suitable to allow for optimum compaction as well to reduce seepage.

Soil permeability is one of important criteria to be considered in dam design, seepage-confinement units, drainage and other structural elements (Goldin, AL & Rasskazov, LN 1992). Seepage usually occurs in fine-grained soils such, with greater permeability, as well as in some coarse-grained soils (Goldin, AL & Rasskazov, LN 1992).

Higher frictional resistance and greater permeability in granular soils is the reason for its greater stability as compared to cohesive soils. Greater permeability permits rapid dissipation of pore water pressures resulting from compressive forces.

2.3 Embankment

In determining the height of the embankment, it is necessary to allocate for a settlement between 5 to 10 percent. This is to prevent its height becoming lower than the spillway height (Figure 1) since settlement is unavoidable despite having a good compaction since air and water out are being forced out of the voids by the weight of the dam, hence causing consolidation (Shaw, R & Smout, I 2009). During compaction, equal effort should be applied throughout the dam to prevent differential settlement. The minimum compaction effort should not be less than 95% of the Standard Maximum Dry Density (Department of Primary Industries and Water 2008).

The stability of the embankment depends largely on its ability to resist shear stress which comes from internally applied loads such as the weight of the soil and the embankment slopes, and externally applied loads such as reservoir and earthquake loads.

As mentioned earlier, steeper slopes can be adopted for granular soils since they are more stable. In contrast, a gentler slope needs to be applied for homogenous material with low permeability.

Figure 1. Cross section of an earth dam

2.4 Spillway

Spillway plays an important role in earth dam since it allows excessive water to flow without overtopping the dam wall. There are two types of spillway namely the cut spillway (Figure 2) and natural spillway.

Figure 2. The position of a cut spillway

In order to reduce water velocity and subsequently minimize erosion, a gentle slope for the spillway must be provided. However, for steep slope, loose stones or geotextile can be provided to reduce erosion.

Apart from that, adequate capacity of the spillway must be provided to prevent overtopping and to enable the spillway to effectively handle run-off.

3. CONSTRUCTION ON EARTH DAMS

Stripping

Organic soil and vegetation must be removed from the area that will covered by the dam base. The removed organic soil are stockpiled and used on the downstream slope of the fill.

Key trench

Key trench which is also known as cutoff trench is excavated below the base of the fill to a minimum of three feet deep for dam with 10 to 12 feet height. The key trench is included in the dam design to secure the dam to the base material and to prevent seepage under the fill.

Diversion tunnel

Diversion channels are excavated before the dam can be constructed. This tunnel often lined with concrete. The tunnel is built around the dam construction area at one side of the valley. In the drill holes, explosive materials are placed and blasting will takes place. The soil or broken rock is then removed. The steps are repeated until the tunnel is completed.

Figure : Building a diversion channel

In Figure, after the dam construction is completed, the diversion channel is closed o allow the lake to begin filling.

Figure : Closure of diversion tunnel

Cofferdam

In summer, the construction on diverting the river starts when the river levels are low. At the upstream of the upstream of the main dam construction area, small dam which is also called as cofferdam are built. It is built using earth-moving equipment. The cofferdam will acts as a barrier to the river and the water will flow to the diversion tunnels.

The second cofferdam will be built downstream of the main dam construction area. At this area, the cofferdam will prevent the river flow to flow back into the construction area.

Figure : Cofferdams act as barriers to the river

Foundation Grouting

Grouting consists of lines or a line of holes that are drilled into the dam foundation. The types of foundation grouting are curtain and consolidation grouting.

Curtain grouting

Curtain grouting consists of a single row of gout holes that are drilled and grouted in sequence by packer testing to the base of the permeable rock or to a depth that acceptable hydraulic gradients are achieved (Fell et al 2005).

Based on Figure, primary holes are drilled first. The last hole spacing will normally be 1.5m or 4m, but also may be as close as 0.5m (Fell et al 2005). This approach allows control over the efficiency of the curtain grouting.

Figure : Curtain and consolidation grouting (Fell et al, 2005)

Figure :Grouting downstage with packer (WRC, 1981)

Fill construction

Earth dam is usually constructed from impervious material which is clay or clay-based material. The construction materials, taken from the surrounding area have to be placed and compacted horizontally in the fill. Dry soil are usually added with moisture and compaction equipment such as a sheepsfoot packer used to obtain the proper compaction.

The construction started by filling the cutoff trench with well-compacted material and six inch layers is added until the maximum height is obtained. The top of the dam at the center of the draw is usually built 10 per cent higher than the design to allow settlement of the fill.

Based on Figure, a riparian pipe is placed through the bottom of the fill and a frost-free valve is placed on the riparian pipe. This pipe and valve system function is to allow water to be released downstream to other water storage facilities during water shortages.

Figure : Elevation view of dam

4. POTENTIAL FAILURES OF EARTH DAMS

Potential failures of dams have always been great importance and much attention has been given to safety evaluation and research due to their disastrous effects. According to Fell, MacGregor, Stapledon and Bell (2005), 79% of Earth dams with less than 30m high suffer more failures than higher dams. This may reflect better design, monitoring and surveillance of larger dams. Potential failures of earth dam can be grouped into three categories: Hydraulics failure, seepage failure and structural failures. A detail discussion of the types of potential failure of earth dam is as follow:

4.1 Hydraulics Failure

Hydraulics failure occurs by the surface erosion of the dam by water. Hydraulics failure is due to several reasons. One of hydraulics failure is overtopping of dams. Overtopping failure occurs when the level of the reservoir exceeds the capacity of the dam. According to Fell, MacGregor, Stapledon and Bell (2005), over-topping is one of the main causes of failure in Earth dams. Overtopping may also be caused by insufficient freeboard provided. Next, erosion of upstream force due to continuous wave action caused erosion of the dam. However, this can be avoided if the surface is protected by stone rip-rap and filter. The next reason is due to erosion of downstream slope by rain water. Although the downstream face of the embankment is not affected by reservoir water, it may get eroded by heavy rain flowing down the face of the dam, causing the formation of gullies and finally collapse of the whole dam.

4.2 Seepage failure

Seepage failure is also known as piping failure. Seepage failures are generally caused by pervious foundation, leakage through embankments, conduit leakage and sloughing. All dams have seepage as the impounded water seeks path of least resistance through the dam structure and its foundation. If the surface seepage intersects the upstream face of the dam, erosion may occur which will lead to possible failure of the dam. Typical method used to control the quantity of seepage is rock fills installed at the downstream toe or gravel blankets to intersect the line of seepage before it reaches downstream toe as shown in Figure 1.

Figure 1. Seepage through a dam or embankment with rock toe or gravel blanket.

4.3 Structural Failure

Structural failure includes failure of the upstream, downstream slopes of the earth dam, foundation as well as cracking deformation and settlement of the dam structure that may lead to overtopping or seepage failure. Slides in Embankment occur when the slopes are too steep for the shear strength of the embankment material to resist the stresses imposed. Failure of this type usually cause by faulty design and construction. As for foundation failure, it occurs because of excessive pore water pressure which reduces the strength of the soil which it may not be able to resist the shear stresses induced by the embankment. Furthermore, damage caused by burrowing animals lead to structural failure. This causes seepage water to flow out quickly, carrying fine materials along which consequently leads to piping failure within the structure of the dam and finally lead to a complete collapse of the dam.

5. CONCLUSION