Dam Construction Machinery Manuals

DESIGN OF EARTHFILL EMBANKMENT DAMS This section deals with the design aspects of earthfill embankments. Where the site conditions and soil test results are favourable a safe and economical design for structures within the feasibility limitations set out in Chapter 10 can be achieved through careful application of the requirements specified below. In situations where the standard design requirements cannot be met or where the suitability of site or material conditions is uncertain, then specialised investigations and analysis and the services of a government approved dam design engineer, hydrologist and geologist/geotechnical expert will be required. Any embankment dam must meet design requirements for stability under all conditions of construction and operation, and imperviousness, both through and beneath the embankment. This chapter examines the design required for earth-fill embankment dams which are widely used in Kenya. Rockfill dams are not discussed due to their limited utilisation in Kenya, primarily due to the difficulty in providing a robust impermeable membrane over a rockfill embankment. 12.1 Types of Embankment Dams Earthfill embankment dams in Kenya are generally homogeneous or zoned embankments with a drainage blanket for internal seepage control for structures greater than 5 metres in height as shown in Figure 12-1 and Figure 12-2.

The choice of whether to use a homogeneous or zoned embankment will be a function of the availability of suitable materials. Where there are limited quantities of impervious material, more pervious material can be placed on either side of the core creating a shell.

Construction standards, etc., are purposely omitted from this manual. Ated due to faulty gates, valves, or operating equipment, the dam could be in danger of. The editors of this Technical Manual for Dam Owners. A construction operation that is typically done following the. Mulching – the.

The most economical type of dam will usually be the one for which materials can be found within the site or a reasonable haul distance. Figure 12-1: Homogenous Earthfill Dam with Drainage Blanket Figure 12-2: Zoned Earthfill Dam. 12.2 General Guidelines for the Design of Embankments 12.2.1 Design Criteria The basic requirements for the design of an embankment dam are to ensure:. Safety against overtopping.

This is a function of the spillway capacity and freeboard;. Stability.

The stability of the slopes should be considered for the case of construction, steady state and rapid drawdown. Acceptable values for upstream and downstream slopes are provided in Table 12-3;. Safety against internal erosion. The selection of material for the downstream shell and the design of internal drainage blanket and toe drain address this aspect;.

Functional performance in terms of excessive seepage. The design of the cut-off and impervious core address this aspect. 12.2.2 Dam Axis The location of the dam axis should be chosen in such a way that the amount of fill required for the embankment is minimal. Usually the most appropriate location will be indicated by a narrowing of the contour intervals on the topographical map (see also Section 10.7). The dam axis should normally be designed straight, unless special topographical features impose a curved axis. Consideration should be given to the stability of the abutments and to avoid abrupt topographic discontinuities which can lead to differential settlement and shear cracks in the embankment.

12.2.3 Height of Embankment The height of the embankment should be determined in accordance with the water depth calculated in Section 8.12 (Determination of the Required Storage Capacity) and then increased by the required gross freeboard (GF) which is a function of the width of the spillway; the wider the spillway, the lower the gross freeboard. This means that the final embankment height should be established through an iterative process which considers the cost of spillway excavation and the cost of embankment construction as the cost of the intake and other structures is constant irrespective of the height of the embankment. An extra allowance or camber should be provided along the crest of earthfill dams, to ensure that the freeboard will not be diminished by post-construction settlement of the dam and the foundation. For small earthfill dams on relatively non-compressible foundations, a camber of about 2% of the embankment height (with a minimum of 0.20m) should be provided.

Linear equations should be used to vary the amount of camber, and make it roughly proportional to the height of the embankment. Figure 12-3 and Figure 12-4 show a diagrammatic cross-section and lay-out plan respectively of a small earth embankment. Figure 12-3: Cross Section of a Small Earth Dam It should be noted that dam heights less than five metres be carefully considered as the freeboard is usually 1.00 – 1.50 metres and evaporation in arid areas is above 2.00 metres with the result that the effective storage available for use is less than is justified by the cost of the project. Figure 12-4: Layout Plan of a Small Earth Dam 12.2.4 Dam Freeboard The embankment crest must be sufficiently higher than the maximum design water level in the dam to prevent any overtopping, including the possibility of waves washing up the embankment.

The critical condition is when the inflow design flood (IDF) (for the design return period) is passing through the spillway. The net freeboard (NF) is defined as the minimum freeboard that occurs when the spillway is flowing at its maximum design flood capacity.

The gross freeboard (GF) is therefore the minimum freeboard (NF) plus the water depth in the reservoir ($hA$) above the spillway crest when the IDF is passing. Equation 12-1 GF = h A + NF Where GF = Gross Freeboard (m) h A = water level in reservoir when spillway is passing inflow design flood (m) NF = net or minimum freeboard (Table 12-1) The WRM Rules (2007) specify a minimum freeboard of 0.6m for Class A dams and 1.0m for Class B and C dams, unless otherwise specified by the WRMA. MoWI (2005) provides a relationship between fetch and minimum freeboard which has been summarised in Table 12-1. The more conservative value should be used.

The water depth (hA) is established from the spillway design described in Section 12.3. Table 12-1: Fetch and Minimum Freeboard. Fetch (Km) Minimum Freeboard (m) 0 – 0.10 0.80 0.10 – 0.50 1.00 0.50 – 1.0 1.10 1.0 – 3.0 1.30 3.0 – 5.0 1.60 5.0 Reference should be made to publications for the required minimum freeboard 12.2.5 Crest Width The main criteria for crest width is related to construction and post-construction use of the crest, rather than slope stability. The crest width (CW) should therefore comply with Table 12-2. Consideration should be given to the camber and surface dressing of the crest. The crest should be sloped at 1% to shed rainwater.

The crest should be dressed with a minimum of 200 mm of compacted murram or gravel. This provides a hardwearing surface that can handle periodic light traffic and is less likely to erode. If the crest will not be used by traffic, it can be grassed which requires at least 200mm of top soil, lightly compacted, into which grass splits are planted.

The grass species that are suitable are creeping (stoloniferous) grasses which cover the ground closely. These species include Kikuyu, Signal ( Brachiaria humidicola), Bahia ( Paspalum notatum), and Star ( Cynodon spp) grass. Grass species that form tuffs should be avoided. Table 12-2: Crest Widths.

Depth of Water (m) Minimum Crest Width (m) Comments 0 – 3.0 3.00 Note: minimum width for machinery access is 4.00 metres. A comfortable roadway width is 6 metres 3.1 – 5.0 4.00 Greater than 5.0 5.00 12.2.6 Impervious Core for Zone Embankments For dams of 5 - 15 metres high, where suitable soil is not available in sufficient quantities for constructing a homogeneous embankment, the construction of a 'zoned' embankment can present a solution. In such cases a core of impervious material (generally clay) is incorporated in the embankment (See Figure 12-2), while more pervious fill material (a soil containing more sand than would normally be admissible) can be utilised for backfilling the shoulders of the embankment. The more pervious material on the downstream shoulder serves to lower the phreatic line to keep it within the embankment.

A more granular material on the upstream also helps to reduce the uplift pressure under the embankment. In the case of a zoned embankment, the impervious core should by designed with upstream and downstream slopes of 1.5:1 and should constitute at least 30% of the cross sectional area. The impervious core should penetrate through the cutoff trench to the impervious foundation layer. The top of the impervious core should exceed the flood water level. 12.2.7 Embankment Slopes Embankment slope stability usually considers three critical conditions, namely:.

Sudden drawdown. This is a post-construction condition that assumes that the reservoir water level has dropped but the upstream face remains saturated;.

Sudden post-construction drawdown. Steady state. This assumes that the water level is at full supply level; Embankment slope stability depends on the type of fill material used and on the height of the embankment. Analysis of the slope stability for different embankment heights and fill material has informed the recommended steepest slopes given in Table 12-3 for well compacted material. Table 12-3: Recommended Slopes for Earth Embankments. Embankment Height Fill Material Type Casing Slopes (h: v) Upstream Downstream 500m) a protective layer of hand placed rip-rap (rubble stone/hardcore) should be placed on the areas of the upstream embankment slope which are likely to be affected by the wave action. This zone is usually 0.6 metres above the normal water level to 2/3 of the water height.

The thickness of the rip-rap layer should not be less than 0.30 metres. A gravel blanket (min 150mm) will normally be provided under the rip-rap layer. The bottom toe of the rip-rap layer needs to be keyed into the embankment face to prevent gradual movement of the rip-rap down the slope. This can be achieved by the construction of a step or inset at the appropriate height along the embankment face.

The space between the top line of the rip-rap and the crest can be grassed to reduce erosion. Figure 12-6: Upstream Slope Protection 12.2.14 Downstream Slope Protection The best form of erosion protection for the downstream face is a good cover of creeping grass (e.g. Kikuyu, Signal ( Brachiaria humidicola), Bahia ( Paspalum notatum), and Star ( Cynodon spp) grass). Grass species that form tuffs should be avoided. In order to obtain a good grass cover, a layer of top soil (200 – 400 mm thick) is placed on the downstream face. This requires particular attention during construction to schedule stockpiling of top soil and inclusion of the top soil along the downstream face during the construction process.

12.3 Design of Spillway Structures The function of the spillway is to discharge the normal and flood flows safely around the embankment and back to the water course without compromising the long term functionality and integrity of the dam. 12.3.1 Location and Type of Spillways The common type of spillway used with earth embankments is a side channel spillway, excavated in earth or rock next to the embankment. The incorporation of relatively large concrete structures as spillways for small earth dams is difficult to justify on economical grounds.

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The basic factors to be taken into account when choosing a spillway location are:. The spillway should be kept away from the embankment in order to avoid the need for concrete protection structures, and. Excessively steep valleys should also be avoided, in order to prevent erosion problems in the spillway channel and to reduce excavation volumes. Consequently spillways are usually located on the side of the embankment where the valley slopes are flattest. In the case of large discharges to be catered for, the possibility of constructing two spillways -one on either side of the embankment- can be considered; the quantity of excavation required usually being the decisive factor. In cases where the topography of the site favours such a solution the possibility of discharging the flood waters into a valley other than the original river valley can also be considered. This could however have adverse effects on eventual water users downstream of the dam and on the flow regime of the other river.

Construction

Because of the cost of rock blasting, extensive excavation in rock should be avoided, but the location of the spillway channel on a relatively horizontal layer of bedrock is wherever possible a handsome solution to all erosion problems in the spillway channel. Problems with spillway channel erosion prohibit the construction of spillways on backfilled soil. Spillways should always be excavated in original material. It is always preferable to let spillway channels discharge on bedrock. Where this is not possible, it is advisable to protect the river-bed from scouring at the location of the spillway discharge.

Lining with reno-mattresses, gabions or pitched stone is usually appropriate. Only side channel spillways excavated in earth or rock will be considered. For all other types of spillways, reference is made to the United States Department of the Interior - Bureau of Reclamation, 1987. A site may require a side spillway on both sides of the embankment.

The side spillway normally consists of three parts: Inflow Section, Control and Outflow Channel (see Figure 12-7) and Drawing Type III in Appendix B. Figure 12-7: Spillway Design 12.3.2 Control Section The normal water level in the reservoir is controlled by the height, length (i.e. Width of spillway channel) and geometry of the spillway sill. The sill level is controlled by a reinforced concrete sill (minimum width 300 mm), thus preventing lowering of the crest level by erosion. This sill is usually aligned with the dam axis. The depth of the sill (minimum 1.00 m) below ground level should be determined by the engineer to minimise seepage underneath the sill.

Where the sill is proud of the spillway bed and there is a risk of erosion and undercutting of the sill, a 150 mm thick reinforced concrete apron should be placed downstream of the sill. The width of the control section should be a minimum of 10 m unless a detailed analysis justifies otherwise. Consideration should be given to the likelihood of erosion along the spillway floor and side slopes, particularly in the control section. Grouted masonry can be laid along the floor and side slopes to protect against erosion where the spillway is cut into soil and where a good grass cover cannot be guaranteed. 12.3.3 Inflow Section The inflow section leads the flood water to the control section.

Usually, it slopes moderately (maximum 1%) upwards to the sill. The cross-section is usually narrowed down gradually towards the sill. Care should be taken that the water flowing to the control section remains far enough from the earth embankment to minimise the risk of erosion of the embankment face.

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12.3.4 Outflow Channel The outflow channel discharges the flood water back into the riverbed at acceptable velocities that do not cause erosion. For spillways excavated in undisturbed earth, a maximum velocity of 2.5 m/s is usually acceptable under Kenyan conditions. Control of the outflow channel water velocity is usually achieved through adequate slope selection. Otherwise lining of the channel (or parts thereof) with rip-rap will be required.

In such cases velocities up to 6-7 m/s can be accepted. In case of unacceptably long outflow channels, the possibility of incorporating a gabion or concrete drop structure can offer a solution. The Manning formula (Equation 12-5) can be used to establish the velocity in the outflow channel for different gradients, widths and channel roughness. Recommended values for outflow channel slopes are presented in Table 12-4. Table 12-4: Recommended Values for Outflow Channel Slopes. Type of Soil Slope (%) Earth.

Q m 3/s/m h A m h C m 0.25 0.28 0.19 0.50 0.44 0.29 1.00 0.70 0.47 1.50 0.92 0.61 2.00 1.12 0.74 2.50 1.29 0.86 3.00 1.46 0.97 3.50 1.62 1.08 4.00 1.77 1.18 4.50 1.91 1.27 5.00 2.05 1.37 5.50 2.18 1.46 6.00 2.31 1.54 6.50 2.44 1.63 7.00 2.56 1.71 7.50 2.68 1.79 8.00 2.80 1.87 8.50 2.92 1.95 9.00 3.03 2.02 9.50 3.14 2.10 10.00 3.25 2.17 For supercritical flow conditions, the normal depth of flow in the outflow channel ($hN$) is smaller than the critical depth ($hC$). If $hN$ is greater than $hC$ the flow in the outflow channel is subcritical, and the depth of approach $hA$ will depend of the water velocity and height in the outflow as in Equation 12-8.

Equation 12-8 h A = h 1 + 5 4.

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