Design and Construction Fundamentals for Earth Dams

Abstract:

The following articles produce an introduction to Earth-fil dams, various design elements as to how they are build and maintained and construction procedures needed to have adhered.

Furthermore, it is explored the benefits and downfalls of these designs elements, what factors may affect and precautions needed to be taken to full-fill the dam’s purpose.

1.0 Introduction

An Earth Dam, or an Earth-fill dam is type of dam built up of mainly earth-based materials. These materials are compacted successively layer upon layer in order to build the dam structure. Impervious materials are used for the construction of the core and more pervious materials to be used on the upper stream and downstream sections of the dam. Man-made materials such a concrete is used to re-ensure there is no washout if so the damn over fills. The use of an earth-fill dam may vary accordingly with size and shape. From the production of hydroelectricity, creation of inlet and outlet channels for water irrigation purposes to acting as spillways. (Encyclopedia Brittanica,2019)

1.1 Components of an Earth-Fill Dam

Earth-fill dams consist of many components, but can be categorized into six main sections. (Civilblog.org, 2015)

Foundation is the area where all vertical and horizontal loads are resisted. The foundation section is made up of earth or rocks and has the ability to resist underwater seepages beneath the dam

Core or the membrane is the section where the water of the dam is held back. Most commonly the core is situated at the center or the upstream section of the dam. The core has impervious materials as its main construction material source.

The transition filter is situated between the core and shell of the dam. The job of a transition filter is to stop movement of fine grained core materials into pores of the course grained materials

Internal drain is typically placed at the downstream section of the dam; the water seepage is carried away from the core and the cut-off. Another purpose of the internal drain is to stop or minimize the saturation of upper stream shell by rain on the dam. The Toe drain on the other hand is used to cover the upstream face which helps to stop erosion in the upstream face. (Civilseek,2013)

Figure 1: Different Components of an Earth-Fill dam.

1.2 Pre-required Knowledge

Before understanding the concepts of building an earth-fill dam, it is highly recommended to know what factors might affect the failure of one(Harradine,2008). Undertaken research says 1 out of 60 earth-fill dams are experiencing a fault.

Poor planning, poor investigation of site and soil type, poor designing and poor construction practices could directly affect the longevity and the safety of the dam.

The first filling of the earth layer of the damn is the reason behind majority of failures, this allows leakages to seep in between layers and cause internal stresses and bond breakages of layers.

2.0 Legalization and Site Clearance

2.1 Contract

The permit holder of the dam is recommended to hire a contractor with dam construction. The ‘Water Management (Safety of dams) Regulations 2003’ (link) states that the contractor has the above specifications and experience with height, build quality and what type of Earth-fill dam is to be constructed accordingly.

2.2. Easements and boundaries

It is essential to identify that the building sight is clear of any pipelines, easements, right of way and any other reserves or boundaries. It is common that everyday utilities such as Electricity, natural gas, water supplies are damaged. It is the sole responsibility of the permit holder to bare all expenses and inconvenience that may have been caused.

2.3 Site Investigations

In this stage, the contractor needs to establish the type of earth-fill dam and the required material for construction. Soil tests are needed to be carried out to ensure that its able to withstand pressures and carry its purpose.

3.0 Classification of Earth-fill Dams

The classification of earth-fill dams are broken into different factors. The most common few being(link);

3.1Rolled filled earth dams

As the name suggests, the rolled filled earth damns a made of layer upon layer of moistened soils not exceeding over 20cm in thickness. The moisture content of the dampened soil should be in its optimum moisture for the next layer to be laid upon.

3.2Hydraulic Fill Dams

This type of earth-fill dams are created by excavation and transportation of the earth. The outer embankments are needed to be higher than of the middle portion of each layer.

4.0 Designs of Earth-fill damns

Though Earth-fill dams follow the same concept of earth layers being laid over one another, it is critical to understand the what type or design structure is needed.

4.1 Homogenous Earthen Dams

Homogenous dams are built with homogenous materials. Meaning the materials are of uniform composition. It could range from different types of soils to rocks. Commonly a homogenous earth dam should contain 20%,30% of clay with the balance made up of sand,clay, gravel and rock,(Nelson,1986). This design is mostly used for low height dams (10m). Mainly consisting of soils and grits in measured ratios, homogenous dams are not approving for seepage action. Due to this, in case if a drawdown the upstream slope is kept proportionally flat (3:1) ratio.

4.2 Zoned Earth dams

With central portions called the ‘core,’ is a collection of impervious materials. The sole purpose of impervious materials is to minimize of ideally stop leakage of water through the body or flow channel of the dam. Zoned dams are usually made for irrigation of water for crops by farmers.

4.3 Dam with a Diaphragm

The Diaphragm dam is introduced when the availability of clay is limited. he majority being pervious materials and a thin lining of impervious materials such as clayey soil, cement concrete is used to construct the diaphragm in the central to stop water seepage, these dams require more labor to build. The diaphragm is placed in the upper stream of the dam. The main difference Diaphragm compared to a Zoned dam is the thickness of the impervious membrane. Usually the diaphragm membrane is below the 10m mark.

5.0 Construction

As proclaimed by Gavin Hunter and Robin Fell, two renowned researches in the University of New South Wales Once all the necessity pre-requirements are ticked off, the actual building process can be initialized. But there are five main preliminary design assumptions to be made. (Hunter, Fell,2003). The mass of the damn that will make it stable (link).(i)Design elements of cores to prevent water seepage through the dam.(ii)The cut-off design or seepage prevention.(iii)Stopping erosion on the upstream face. (iv)Economics

Once these above factors are considered the building process is started. Usually earth-fill dams are built during the dry season, where the water levels in streams and rivers are much lower. This is since use of machinery and equipment’s are easy to use and the rain or high water levels wouldn’t affect the précised mixture ratios of materials. Before actual excavating is done, the site is measured for dam alignment on the ground, areas needed to be excavated, borrow areas for the soil and rocks used for construction. A field engineer will measure groundwater levels and monitor the affects for adjacent hillsides. The water stream must be stopped by diverting the stream or temporally stopping it by a cofferdam.

The foundation area must be prepared before excavation; this is because in most sites the ground is compressible (resulting in the dam to be irregular) and permeable (allowing water to seep through). So initially, soils, and rocks that are soft or hard are excavated and piled up in different areas for later use. The surface of the foundation is thoroughly cleaned, it is boomed and hosed to expose any irregular forms or materials that would affect the construction process.

After the above processes, a trench (Keyway or cutoff)(LINK) that is of full width of the damn is cut into rock. The use of the trench is to allow a longer pathway for seepages that might try to flow below the layer and to stop the dam from sliding along the foundation.

The soil in the keyway are raised to the same level simultaneously. Ramps are being cut to access and move equipment. Now, the initial layer is placed with the use of earthmovers. A layer of about 6-8 inches is placed and rolled and hosed with water for the right water content ratio. If gravel is used in the process, a vibrating roller is used as it enables the gravel to fit with one another and leave no spaces.

Throughout the compacting process, the site is thoroughly investigated for quality control purposes. Unwanted debris such as grass, irregular shaped stones and rocks are removed. A test called “Nuclear density gauge,” is carried out during this process. A controlled and safe radioactive material is pulsed into the soil by a certified technician, these radioactive particles bounce back into a detector which then indicates the moisture content and density of the soil layers. This enables the engineers to inspect and analyze the soil if its needed to be re-compacted until the right requirements are met. The advantage of this process is that no excavation is needed to carry the test.

The construction of the earth-fill dam proceeds until the crest height and zoned heights are reached the specific and required levels.

Once the dam is complete, the cutoff water is re-diverted back into the dam, as the dam fills up, it is closely monitored seepages or quality and functional breaches..

5.1 Soil types and level of sustainability for core construction

Soil Type

Sustainability

Well graded mixture of sand, gravel and fine clay. 15%

coarser than 50mm,50%

coarser than 6mm

Excellent sustainability

Well graded mixture of sand, gravel and clayey fine, to be coarser than 25mm. Inorganic clay to be present in clayey or high plastic tough clay with a plasticity index larger than 12.

Good Sustainability

A low plasticity clay with little or no coarse fractions. The plasticity index lying between 5-8 and has a liquid limit that exceeds 25.

Silts containing little or no coarse fraction with a plasticity index exceeding 10.

Very Poor Sustainability

Table 1: Differentiating sustainability of types of soils for core construction (The constructor Civil Engineering Home,2016)

5.2 Side Slopes of earth-fill dam for different types of soil

The outer shell of an earth-fill dam is to protect the core. As mentioned earlier on the article, the outer core consists of pervious materials. But what type of pervious materials are to be used solely depends on the soil used in the core.

Type of Soil

Upstream slope

Downstream slope

Homogenous well graded

2.5:1

2:1

Homogenous course silt

3:1

2.5:1

Homogenous silty clay (H>15)

2.5:1

2:1

Sand with clay core

3:1

2.5:1

Sand with RC diaphragm

3:1

2:1

Table 2: Ratio of slope in upstream and downstream shells according to soil type of core. (The constructor Civil Engineering Home,2016)

6.0 Maintenance

It is vital a maintenance inspection is carried out during the first filling of the dam. As mentioned by author K.D Nelson the first filling the dam can be controlled, the filling should be done gradually not exceeding over 300mm of water height per day(Nelson, 1986).

6.1 Spillways

Spillways are to be cleared of any debris as it could directly affect the water flow capacity. Any cumulated debris could overtop the dam. Such situations are caused by actions taken by dam owners itself, for example, some owners fences around the reservoir to stop fish leaving from leaving or sandbags being placed over the spillway to provide more storage. It is crucial all these supports are to be removed under safe conditions before a flood occurs. Not doing so, could result in Sevier consequences to the person and the dam withhold alike.

Having a well grassed and dry spillway could reduce the chances of erosion taking place when high water pressures are faced, and a trickle tube can be further added to act as a supportive element.

6.2 Earth Embankments and Foundations

Cracks and erosions are initial signs of poor construction and cheap materials. Examining these flaws would generally mean that the water levels

6.2.11Cracks

Cracks are needed to be filled tightly with fine clay. There are three main type of cracks (Nelson,1986) (i)transverse, (ii) longitudinal and (iii) Shrinkage. Transverse cracks run straight through the stomach of the dame, starting from the upstream and all the way to the downstream section of the dam, they are to be known as the most consequential and worse type of cracks. Transverse cracks are commonly found in dams that are too dry, as the water fills, it gets saturated and slumps, this results in the formation of the cracks. Longitudinal cracks are present parallel to the crest length and are considered to be not too serious. Fine clay is filled to prevent water collection from rain. If such water is collected, it could substantially act as lubrication to aid slides. Shrinkage Cracks develop as both longitudinal and transverse and does not have a specific area of occurrence.

6.2.2 Piping

Piping is referred to when a water flow is creating a tunnel or water flow through the weaker parts of soil. A serious effect of piping is internal soil erosion. In-order to stop piping, firstly water levels are reduced, and a sandbag is place around the inlet.

6.3Quality Control

Quality engineering is a vital part of the dam building process. Since most of the materials used are raw and much weaker in strength compared to steel or concrete the placement of the layers and particles determine the longevity and strength. Geotechnical engineers and geologists alike continue assess the safety and quality of the dam.

6.4Byproducts/Waste

The majority of being earth based materials in these types of dams, there is almost no byproducts formed in the process, the excess fill may be used to build access roads and support structures. But it is important to take note on toxic blue-green algae been grown in earth fill dams. As K.D Nelson concluded that these algae are the main reasons for stock deaths (Nelson,1986). They tend to cover the surface of the reservoir within weeks and block out all sunrays penetrating the water surface, simultaneously excreting toxic byproducts to the water during respiration. Though this could be a handful of a problem, a clever method is to use ferric alum. Ferric alum is known to be a less pure version of aluminium sulphate, (Oriental packaging,2010). A 100 milligrams per liter is added to suppress the growth of this algae.

Figure : Harmful algae growth in a reservoir (Mudgee Gurdian,2017)

7.0The Future

Due to environmental concerns, the construction of any type of dam will be a controversial argument. However, earth-fill dams are more onto the environmentally friendly side due it is earth based materials rather than concrete and steel structures. Since it much less expensive than other types of dams, they will always be favoured when it comes to irrigation of water, flood control and water consumption.