What Is an Earth Dam?

A dam constructed using local soil and sand, gravel, gravel, ballast, stone, etc. It is an ancient and yet widely used water-retaining building, sometimes called earth-rock dam.

A dam constructed using local soil and sand, gravel, gravel, ballast, stone, etc. It is an ancient and yet widely used water-retaining building, sometimes called earth-rock dam.

Chinese name

Earth dam

Foreign name

earth dam

Definition

Dam built using local soil and stone

Main materials

Near the dam site

Features

Flexible, adapt to deformation of foundation, etc.

Earth Dam Type

Homogeneous earth dam, clay inclined wall dam, etc.

Dam structure

Dam slope, dam surface drainage, dam roof structure

Earth Dam Introduction

A dam that consists of soil, sand, or stone as the main part and an impervious material (such as clay or concrete) that forms the core of the dam. Earth dams are mainly used near the dam site.

Earth dam

Soil materials, water-retaining buildings constructed by rolling, filling and other methods. The dam building materials of this type of dam can be taken in situ, and the dam body is flexible and can adapt to the deformation of the foundation. The geological conditions of the foundation are lower than that of rigid dams such as concrete dams and masonry dams. The earth dam has a relatively simple structure, reliable work, easy maintenance, heightening and expansion, and easy construction techniques to facilitate rapid mechanized construction. Therefore, earth dams are widely used at home and abroad.

Construction technology and equipment have been greatly developed, and soil mechanics theory and experimental technology have made great progress in aspects of seepage, stability, and calendaring. These are all favorable conditions for building high earth dams.

Types of earth dams : (1) According to the dam material and anti-seepage body settings, there are homogeneous earth dams, clay inclined wall dams, clay core wall dams and various earthy dams. (2) According to the different construction methods, earth dams can be divided into compacted earth dams, hydraulic flushing and filling, water fall dams, water dump earth dams and soil irrigation dams.

The choice of earth dam form should be determined through comprehensive research based on factors such as the topographical and geological conditions of the dam site, the quantity, nature and distance of construction materials, climatic conditions, and construction conditions.

Earth Dam Benefits

A dam constructed using local soil and sand, gravel, gravel, ballast, stone, etc. It is an ancient and yet widely used water-retaining building, sometimes called earth-rock dam. The advantages of earth dams are: the dam materials are taken from the local area, which can save cement, steel and wood; the engineering geological conditions of the dam foundation are lower than other dam types; the seismic performance is better. Its shortcomings are: It is generally necessary to construct expensive drainage structures outside the dam, such as spillways, tunnels, etc .; If the reservoir is flooded, the dam will collapse and it will have poor ability to withstand over-standard floods.

Earth Dam History

Over 2200 BC, Babylonians have built earth dams on the Euphrates. Earth dams were also built in India and Egypt BC. In order to prevent the flood of the Yellow River, the Chinese people built earth embankments along the banks of the Yellow River as early as in the spring and autumn (770-476 BC BC). In terms of structure, earth embankments are also earth dams. An embankment was built in Shouxian County, Anhui Province from 598 to 591 BC to form the Anfengtang Reservoir. In the 17th and 18th centuries, Russia built more than 200 earth dams in Ural and other places. At the end of the 19th century, some hydraulic flush dams were built in the United States. By 1900, there were more than 100 earth dams in the United States.

Soil mechanics has become an independent discipline in the 20th century, and its theoretical practice and testing methods have become increasingly perfect. The emergence of large-scale earthmoving machinery with high power and high work efficiency, coupled with the above-mentioned advantages of the earth dam itself, has led to the rapid development of earth dams, and its proportion in the world's dam work has increased. Earth dams accounted for only 38% of the 100m-high dams built in the world from 1961 to 1968, and increased rapidly to 62% from 1975 to 1977. The Soviet Union's Rogon Dam is 335m high and is the highest dam in the world. From the end of 1949 to 1980, about 90% of the 2,600 large and medium-sized reservoir dams in China have been built.

Earth Dam Dam Materials

There are three types of functions in the dam.

Earth dam impervious material

It is used to fill the anti-seepage body of earth dam and reduce reservoir water leakage. The impervious earth material should have a small permeability coefficient, good permeability stability and a certain degree of plasticity. Cohesive soil can generally be used as anti-seepage material, but marsh soil, bentonite, surface soil and soil materials containing incompletely decomposed organic matter should not be used. The salt content of the impervious material must not exceed the allowable value of the specification. The water content (weight ratio of water and dry soil) of the dam soil should be controlled near the optimal water content so that the maximum dry bulk weight (dry soil weight per unit volume) can be obtained by applying the same compaction function. If the natural moisture content of the material yard is too dry or too wet, it should be treated. Gravel soil is widely used in the world as an anti-seepage material for high earth dams. This kind of soil contains both coarse particles with a diameter larger than 5mm and fine particles with a diameter smaller than 5mm, but the content of coarse particles should generally not exceed 50% in order to meet the anti-seepage requirements. Gravel soil has the characteristics of both coarse-grained soil and fine-grained soil, with high strength, low compressibility, and low water permeability, making it an ideal anti-seepage material. The disadvantage is that the crack resistance is poor. Due to the use of gravel soil, the range of gradation of impervious soil is greatly expanded (Figure 1). In addition to earth, concrete, reinforced concrete, and asphalt concrete can also be used as impervious materials.

Earth dam

Earth Dam Dam Shell Material

It is used to fill the dam shell, support the impervious body, keep the dam slope stable, and drain the seepage through the impervious body and the dam foundation to the downstream. Sand, gravel, pebbles, crushed stones, stones, and ballasts excavated from foundation pits can be used as dam shell materials. Even fine sand is not easy to compact, and it is easy to liquefy in the event of earthquake, so it should be used with caution. After the 1960s, large-scale vibration flat milling and thin laminated solid dam shell materials were used to make excavated weathered and soft rocks accessible to the dam, which expanded the use of dam shell materials.

Earth dam reverse filter

Set between the coarse and fine materials in the dam, the texture is required to be dense and hard, and the mud content does not exceed 5%. The anti-filter material should be able to drain water without blocking the protected material. Material gradation and number of layers are determined by calculation or experiment. The anti-filter material can come from screening natural sand gravel or artificial crushed aggregate, or directly use natural sand gravel. The transition material installed in the dam to prevent sudden changes in stiffness has both anti-filtration effect and is generally thicker, which is good for construction.

Earth Dam Classification

Earth dams can be divided into: Homogeneous earth dams: The dam body is mainly made of a kind of dam-building material. A variety of soil dams: The dam body is made of several dam building materials; the anti-seepage material is located in the middle or upstream of the dam body. Core wall earth dam: anti-seepage material is located in the middle of the dam body, and the upper and lower dam shells are a single permeable material. Inclined wall earth dam: anti-seepage material is located upstream of the dam body and downstream is a single permeable dam shell. In addition, earth dams can be divided into the following three types according to the construction method.

Earth Dam Roller Dam

The roller compaction equipment is used to separate the dam building materials. The roller compacted dam is relatively compact, and the amount of subsidence after completion is small, generally not more than 1% of the height of the dam. The shear strength is high, and the slope of the dam is steep, saving engineering costs. This dam has a long history and is the most widely used. Most earth dams in the world are roller dams. Taking dry bulk density as the standard for controlling rolling compaction, the water content of the upper dam soil should be controlled near the optimal water content. There are a variety of roller compactors used in roller compaction dams. From manual compaction to mechanical compaction with various powers, they can be selected according to dam building materials and meteorological conditions. The optimal roller compaction parameters are determined through field compaction tests, such as Layer thickness and number of rolling passes.

Earth fill dam

The soil is filled into the still water in layers, and the granular structure of the soil is disintegrated by water, and compacted under the action of soil transport and the weight of the soil. The soil used requires easy disintegration and wetting in the presence of water. Water-filled dams have large plasticity and strong ability to adapt to deformation. Basically, no rolling equipment is needed. The filling is less affected by the climate and the water content requirement of the material yard is not strict. However, the construction process is complicated, the dry bulk density of the fill is low, and the water content is high. The strength is low and the compressibility is high. The slope of the dam is slower than that of the rolled dam, and the amount of subsidence after completion is large. The stability of the dam slope during construction is the key to controlling the safety of the dam. The water content of the filling soil and the rising speed of the dam body should be strictly controlled, and drainage inside the dam should be provided. Water-filled dams were first developed in the Soviet Union, and China has built more than 700. The Shanxi Fenhe Reservoir water-filling dam with a dam height of 61.4m was built in 1960 and was the highest water-filling dam in the world at that time.

Earth dam hydraulic flushing

Use water guns, dredgers, and other hydraulic machinery to dig the soil and mix it with water. Use a mud pump to transport the soil to the block surrounded by soil through the mud pipe. The water is discharged to the outside of the dam through the drainage pipe. Settle down and get compacted under the action of self weight and osmotic pressure generated by drainage. If the terrain is suitable, the mud can flow freely into the dam surface through channels, which is called a water drop dam in China. Hydraulic flushing dam is suitable for sandy soil with strong water permeability, which can be continuously operated and has high work efficiency, without the need for transportation and rolling equipment. During the construction of this kind of dam, the filling soil is completely saturated with water, the dry bulk density and strength are low, the compressibility is high, and a "fluid zone" is formed on the upper part of the dam body. The application of muddy water thrust to the upstream and downstream dam slopes can easily lead to landslides and cracks. It is necessary to slow down the slope of the dam, set drainage inside the dam, and limit the dam ascent speed. After the Second World War, the Soviet Union built large-scale hydraulic flushing dams on some large water conservancy hubs such as Zimlyan and Gubyshev. China has built many water dams, of which the highest one is Guangdong Gaoping Dam, 68m in height.

Earth Dam Dam Structure

Decided according to natural conditions, application requirements and engineering design standards.

Earth dam slope protection

In order to prevent the harmful effects of wind and waves, rain erosion, and frost heaving and cracking, upstream and downstream slope protection should be provided. Upstream slope protection materials are available: throwing stone, dry stone, mortar stone, concrete, reinforced concrete, asphalt concrete, etc. Downstream slope protection materials are available: dry masonry, gravel, pebble, turf, etc.

Earth Dam Drainage

In order to eliminate rainwater on the dam surface, vertical and horizontal drainage ditches should be set on the downstream dam slope to intercept the collected rainwater and discharge it downstream of the dam.

Earth dam dam structure

Earth dams are generally not allowed to pass through the dam top, so the dam top should exceed the reservoir water level by a certain height, and the specific superelevation is determined by calculation. Generally, the impervious wave-proof wall is built on the upstream side of the dam top with mortar stone, concrete or reinforced concrete. The width of the dam top is determined by the requirements of construction, operation and traffic. Crushed rock, single-layer masonry or precast concrete blocks are commonly used as materials for the dam roof.

Connections to bank slopes and concrete buildings The bank slopes connected to earth dams should be treated to be even and smooth from top to bottom, not excessively steep, without sudden changes, steps or slopes, so as to avoid cracks due to uneven subsidence of the earth dam. At the joint of the bank slope, the core wall or inclined wall is often partially enlarged to extend the seepage diameter of the contact surface. For rocky bank slopes, concrete tooth walls are usually set along the contact surface between the anti-seepage body of the earth dam and the bedrock, and grout curtains are set at the bottom of the wall to extend a certain distance inward to increase the seepage diameter around the dam. The nearby bank slope where the earth dam is connected, should be kept stable after water storage, otherwise it should be excavated into a stable slope or filled with a slope prism.

Earth dams and concrete buildings are connected by gravity walls. According to its planar shape, it is usually divided into L-shape, side wall type and plug-in type (Figure 2). The plug-in type is mostly used for high dams, and the rest is mostly used for low and medium dams. For barbed walls of side wall type and plug-in type standing deep into the anti-seepage body of earth dam. Within a certain width of the contact surface between the gravity wall and the anti-seepage body of the earth dam, it is best to fill the soil with high viscosity to improve the ability of contact erosion resistance. In order to avoid uneven subsidence, vertical slopes are not adopted when the gravity wall contacts the impervious body.

Earth dam foundation seepage prevention and drainage

The purpose is to ensure the stability of dam foundation seepage and control the seepage flow after water storage.

Seepage prevention of earth dam foundation

Soil foundation: Remove the humus soil layer, then compact the topsoil, and set several small alveolar backfill impervious materials along the contact surface between the anti-seepage body of the earth dam and the soil foundation.

Earth dam

Extend the penetration to strengthen the connection. Rock foundation: remove loose rocks on the surface, and seal surface cracks with cement mortar or shotcrete; perform curtain grouting on the rocks below the impervious body. Gravel foundation: If the gravel layer is not thick, excavate the interception trough (Figure 3a <;), backfill and compact with anti-seepage material, and cut off the gravel layer. The upper and lower sides of the interception trough are respectively connected with the impervious body and the bedrock. If necessary, grouting curtains are set in the bedrock. If the gravel layer is relatively thick, it can be filled with anti-seepage material to connect the upstream horizontal pavement and the dam anti-seepage body (Figure 3b) to extend the seepage diameter of the dam foundation and ensure stable seepage. .

Earth dam

Deep thick gravel dam foundations often adopt vertical anti-seepage, and a concrete impervious wall is constructed to cut off the gravel layer (Figure 3c). The anti-seepage body of the earth dam is inserted into the upper part of the anti-seepage wall, and the lower part is connected to the bedrock. If necessary, a grouting curtain is set in the bedrock. The impervious wall of Canada's Marnick III dam reaches 131m in depth, making it the deepest in the world. If the gravel grading is suitable and the suction capacity is suitable, the gravel layer can be grouted to form an anti-seepage grouting curtain to intercept the gravel layer (Figure 3d).

Earth dam foundation drainage

The dam foundation and dam body seepage water is discharged outside the dam. The water permeability of drainage facilities should be much larger than the surrounding materials and meet the transition requirements. Common drainage forms

Earth dam

There are: prism drainage: located at the downstream dam site, reducing the dam body infiltration line, eliminating water seepage from the dam body and dam foundation, and increasing downstream dam slope stability (Figure 4a). Slope drainage: laying along the downstream dam slope (Figure 4b), construction and maintenance are convenient but the dam body infiltration line cannot be reduced. Drainage inside the dam: commonly used pad drainage, which extends from the downstream dam site into the dam a certain distance (Figure 4c), is often used for uniform soil dams on weakly permeable foundations, which significantly reduces the dam body infiltration line. If the drainage material is insufficient, a mesh drainage can be used instead, consisting of a vertical continuous drainage belt and a horizontal interval drainage (Figure 4d). The longitudinal drainage zone reduces the infiltration line of the dam body and collects the seepage water, and drains out of the dam through the horizontal interval drainage. Decompression wells: Permanently located in the pervious foundation with upper and lower layers being a weakly permeable layer and a strong permeable layer, respectively. The weak permeable layer is thick, or the strong and weak permeable layers are interlayers. Decompression wells generally extend into a strong permeable layer to a certain depth or penetrate it directly to the bedrock surface to decompress the dam foundation drainage. The seepage water is led to the surface through the filter and outlet pipe in the well. If there are several strong permeable layers in the bedrock, decompression wells should be set up separately. Drainage ditch: If the weakly permeable layer on the surface is thin, it is often dug through to reach the strong permeable layer below to form a drainage ditch to eliminate water seepage in the strong permeable layer of the dam foundation.

Earth dam

Earth Dam Earth Dam Calculation

Generally includes the following four aspects.

Earth dam seepage calculation

Determine the dam body infiltration line, the dam body and the dam foundation flow network, the seepage flow and the escape ratio drop, and the reservoir water level and the pore pressure (relative pressure values in the pores of the soil exceeding the atmospheric pressure) for the dam when the water level of the reservoir decreases Slope stability analysis is used to understand the leakage and ensure stable infiltration. According to the dam body and dam foundation's permeability coefficient, boundary conditions and various upstream and downstream water level combinations, it is solved through hand-drawn drift nets, numerical calculations and simulation experiments. When solving, it is generally simplified as a two-way problem. For three-dimensional seepage fields such as the flow around the shore, numerical analysis or simulation experiments can be used to solve it.

Earth Dam Stability Calculation

Earth dams are large in volume, and the possibility of horizontal sliding of the entire dam body under water pressure usually does not exist, so only the anti-sliding stability of the upstream and downstream dam slopes need to be calculated. Generally divided into three types of construction period, stable seepage period and reservoir water level fall period. The shear strength of the soil is determined by the following formula:

Earth dam

Where is the shear strength of the soil; is the normal normal stress perpendicular to the sliding surface; is the pore water pressure; is the normal effective stress; and are indicators of the effective shear strength of the soil material. Representing cohesion and internal friction angle, respectively, determined by experiments.

The homogeneous earth dam, thick core wall and thick inclined wall dam are usually calculated by sliding arc method (see the following formula). Assume that the sliding surface is an arc, divided into several soil strips, irrespective of the force between the strips, the calculation formula:

Earth dam

In the formula, K is the anti-sliding safety factor, not lower than the specified value; N and T are the normal and tangential stresses acting on the bottom of the soil strip; W is the weight of any soil strip; U is the pore acting on the bottom of the soil strip Pressure; is between the gravity line of the soil strip and the radius of the arc passing through the midpoint of the bottom of the soil strip

Earth dam

Angle; L is the arc length of the cohesive part through which the sliding arc passes. Calculate several sliding arcs to find the minimum safety factor K. In the 1950s, AW Bi Xiaofu and others also proposed a calculation method that takes into account the force between bars.

For dams with weak interlayers or thin core walls or thin inclined walls, the sliding wedge method should be used (see left picture). The sliding surface is assumed to be a polyline, and the force between sliding wedges is assumed to be parallel to the slope or horizontal. After dividing the shear strength of various materials along the fold line by K, the sliding wedge is in the limit equilibrium state, and this K value is the required safety factor. The minimum safety factor is calculated by assuming different sliding interviews. If in an earthquake zone, the seismic force should be added as an external force to the calculation.

Earth Dam Settlement Calculation

Determine the total settlement of the dam body and dam foundation under its own weight, the relationship between the settlement and time, and the settlement after completion. Based on this calculation, the overfill of the dam roof reserved to offset the settlement after completion is calculated, the amount of uneven settlement is predicted, the possibility of cracks in the dam body and the preventive measures are judged. The calculation method is based on the compression curve of the dam body and dam foundation soil, and the vertical total stress and pore pressure distribution of the dam body and dam foundation at time t. That is, the dam foundation is divided into several layers, and the vertical effective stress (equal to the vertical total stress minus the pore pressure) and the corresponding settlement amount at the center of each layer at time t is calculated. The settlement amount of each layer is added to obtain the time t and completion Settlement of rear dam body and dam foundation.

Earth Dam Stress and Strain Calculation

The finite element method is used to calculate the stress and strain of the fill under the load of the fill and other loads in the dam foundation and bank slope joints to determine whether shear failure occurs, whether there is excessive deformation, whether there are tension zones and cracks, and impervious soil. Whether hydraulic cracking occurs in the body, and provide a basis for the dam body stability analysis and the design of the buildings connected with the earth dam.