3 Refer: Chinese National Committee on Large Dams (edt.), 1979. Dam construction by the Chinese people.
History and development of sluicing-siltation dams
Advantages of sluicing-siltation dams
Construction technique
Layout of the borrow areas
Sluicing-siltation operation and division into mud ponds
Construction of levees
Soil loosening and mud mixing
Control of mud concentration
Draining embankment water
Embankment construction by sluicing-siltation method was developed after Chinese liberation in 1949 by the Chinese laboring people for building dams. This is for storing water, for warping, and for soil and water conservation in the basin of middle reaches of the Yellow River. These are severely eroded areas. Since the founding of New China, soil conservation has been carried out by the masses on a large scale. Thousands of gullies and ravines were checked by building check dams by people's labor on farm lands for stable yields. As a consequence of this, both the flood water and silt pouring into the Yellow River have reduced. In the process of check dam construction, the people developed a new method of constructing dams, i.e. sluicing-siltation method, which makes full use of local favorable topographic conditions and natural materials such as sand and loess. In fact, it is a further development of the ancient Chinese experience of diverting water to sluice soil for the purpose of forming farmland and building levees. Such type of dams has the advantages of simple construction, less labor, high efficiency, low cost and good quality.
According to statistical data, since 1970 a total of more than 6, 000 check dams and storage dams higher than 15 m have been built by sluicing-siltation method in the northern parts of both Shaanxi and Shanci provinces. The soil materials suitable for applying this method can vary from sand to light, medium and heavy silt loam as well as gravelly clay.
The popularization of building dams by sluicing-siltation method has promoted the development of agricultural production and water conservation in the region. It has also played as active role in the control of soil erosion and harnessing the Yellow River.
The method of building earth dams by sluicing-siltation method has higher labor efficiency as compared with the rolled-fill method. It has greatly reduced the amount of work in loading, hauling, unloading and compaction of soil materials. This it leads to 3-6 fold increase in the efficiency of labor. In the sand blown areas of northern Shaanxi, still higher efficiency was achieved as the water for sluicing the soil materials could be diverted by gravity flow. In Zizhou County, a 36 m high Dianshi Earth Dam of 49 ha-m volume was completed in 5 months by a small number of local laborers.
The quality of this type of dam is good. Sluicing-siltation earth dam is built of high concentration slurry, which consolidates to become a homogeneous dam with the escape of excess water from the embankment. After completion, the dry density of the embankment can quickly attain the value as required in the design, and is generally over 1.5 t/m3 as per a field study.
The cost is low which includes the cost of labor, pumping water, power consumption, soil loosening by blasting, management and so on. The costs are 60 % cheaper than that of a rolled-fill dam.
Preparation works
Acquisition of water source and storing of water by building cofferdam
Selection of the pumps
Treatment of foundation and abutments
The location of the dam is decided based on near to possible water source, topography and geological conditions. The preparation of the construction work mainly involves the acquisition of water source, storing of water by building cofferdam, selection of pump units, stripping of foundation and abutments etc.
To guarantee the supply of water for construction use source of water with adequate supply is needed. If it is possible to get water from a reservoir, or a canal, or a spring upstream of the dam site and also at a high location, diversion of water by gravity flow is preferable to the use of pumps. If water diversion by gravity flow is not possible or the base flow in the river does not provide the necessary quantity, it would be necessary to build a cofferdam for storing water. The amount of water required may be taken as the volume of the embankment. Based on the needed storage, the height of the cofferdam can be fixed. In general, the cofferdam for storing water may be incorporated with the upstream levee of the embankment used as a check dam or a storage dam.
This involves mainly the selection of the suction head and the capacity of the pumps as well as their corresponding power equipment. The suction head of pump depends on the anticipated height of the dam, the elevation of the borrow area and its distance from the dam, and should generally exceed the design dam height by 20-30 m so as to ensure a sufficient gradient of the mud-mixing ditches and mud-conveyance canals. In case that the pumps available do not satisfy the requirement of the suction head, several lift pump stages are needed. The pump capacity is decided upon in accordance with the construction schedule and the daily volume of earthwork. However, a discharge of at least 30 t/hour has to be provided for efficient work. Separate units are to be installed on both banks if the slurry is sluiced from both banks
The treatment requirements of foundation and abutments of a sluicing-siltation earth dam, apart from those in common with the rolled-fill earth dam, differ accordingly to the construction features, purposes of the dam, and height of the dam. The popular practice for check dams below 20 m height is to clear only the top soil from the foundation and abutments to a depth of 0.1-0.3 m. If the abutment is too - steep, it should also be flattened suitably. For storage dams and higher check dams, be it a rock foundation or a pervious over burden layer of sand and gravel, one or two cutoff trenches should be excavated in the foundation upstream of the dam axis, and then backfilled by the sluicing-siltation method. This is to prevent erosion at the interface or to prevent serious leakage or piping.
In northern Shaanxi and western Shanci, most of the sluicing-siltation earth dams were built on thin alluvia of pervious sand and gravel, which serve as natural drainage blankets and facilitate consolidation of the embankments to achieve stability quickly. The rivers there carry large amount of sediments, which are mainly composed of fine silt loam. After one or two floods, the sediments deposited behind the dam would naturally form a thick blanket for controlling seepage flow. Therefore, check dams for warping or small storage dams in the area of serious soil erosion do not need any treatment of the foundation so long as the foundation is stable and pervious.
The borrow areas are classified according to the purposes i.e. whether the soils serve for sluicing-siltation of the embankment or for placing the levees. When supplying the soils for placing the levees, the borrow areas are to be located on places with gentle slope, and preferably on both upstream and downstream of the dam for shortening the hauling distances.
The borrow areas used for sluicing-siltation should be as near as possible to the dam, generally over 20 m higher than the design height of the dam so that the natural gradient might be used to mix the slurry uniformly. When the condition permits, the borrow area should be chosen in combination with the spillway excavation to reduce the volume of earthwork.
The functions of the mud ditches are to first mix the slurry and then convey it to the embankment where it is to be deposited. Therefore, the ditches are divided into two sections: those for preparing the slurry at the borrow area are referred to as mud-mixing ditches and those for conveyance ditches are called canals.
Popular experience shows that the mud-conveyance canals should be narrow, deep, steep and long so as to increase the sluicing effect of the water and also the capacity of carrying the soils. When water is delivered from a 3" - 4" pump, the ditch may have a depth of 1 m and a bottom width of 0.5 m. The flowing slurry may easily be stumbled in a shallow ditch and may spill over it. The mud-mixing ditches should also be deep and narrow, with a length of 70-130 m and a gradient of 15-20 %. The alignment of the mud ditches may be changed with the locations of the borrow areas and the siltation ponds as well as with the topography and gradient of the ground.
The exit section of a mud-conveyance ditch should be kept parallel to the side levees and at a certain distance from them, lest the side levee might be damaged by the slurry flow. If the slurry pond is wide, two or more exits from the mud-conveyance ditch are used alternately to prevent the concentration of thin slurry at one place.
Sluicing-siltation operation
Sluicing-siltation from one bank
Sluicing-siltation from two banks
Intermittent sluicing-siltation
Continuous sluicing-siltation
Division of construction surface into mud siltation ponds
One mud pond on the entire construction surface
Two or more mud ponds
Sluicing-siltation operation may proceed from one bank or from both banks of a gully (or rivulet or river) according to the locations of the borrow areas. The operation may also be intermittent or continuous according to the sluicing requirement.
Sluicing-siltation from one bank: this applies in the case where the borrow areas are all located on one side of the river/gully on which dam is to be built. With this type of operation, the mud-conveyance ditches usually extend a long length and the gradient of the mud surface on the embankment will be great, causing the water to accumulate at the farther end of the embankment. The water should be drained away on time for facilitating the consolidation and stability of the dam.
Sluicing-siltation from two banks: in this case borrowing, mixing and conveying of soil materials proceeds at both banks, enabling the slurry to flow into the embankment from two opposite directions. The two fan-shaped slurry surfaces will overlap each other. This may avoid the escaped water to accumulate at one place for a long time to the advantage of the embankment stability. Such kind of sluicing operation should be used wherever the locations of borrow areas and the pumping equipment permit. If the mud siltation pond (a section of the dam dyke) is too wide, the exit section of mud-conveyance ditch should be shifted from place to place to prevent the escaped water from collecting at one place and to prevent the gradient of the mud surface from varying.
Intermittent sluicing-siltation: This means that the sluicing-siltation operation will stop when the mud deposited on the embankment has reached a certain thickness (say 0.5-1.0 m). The work of sluicing-siltation for the next layer will not resume until the moisture content in the mud is reduced through infiltration and evaporation and the mud surface is hard enough to be trodden upon by men (in other words, the moisture content has been reduced to liquid limit or below it). The time interval between the successive sluicing-siltation operation is called the intermittent period. It depends on the soil properties, air temperature, layer thickness for one sluicing-siltation operation, drainage condition and others, and may take at least 1-3 days or at most 10 days. This kind of operation is usually applied for low dams or for the dams being constructed to a height for flood retention. Successive roving operations of sluicing-siltation at multiple mud ponds also fall within the category of intermittent sluicing-siltation (sketch below).
Continuous sluicing-siltation: This refers to the method of sluicing-siltation at daytime but suspended at night. The method is often used to ensure the dam safety during flood season in those cases where there is a need to hastily raise the dam height for flood retention due to both a large volume of embankment and watershed area.. For the purpose of ensuring the safe construction of the dam, the daily rate of heightening the dam by sluicing-siltation should be uniform over the construction surface of the embankment and not too fast. According to the statistical data of completed dams in northern Shaanxi and western Shanci, the rate is controlled within a certain limit depending on the soil types as shown in Table 1. If the rate of construction is high, to ensure the dam stability a wide side levee is necessary which may render a large amount of labor to be uneconomical. The 34 m, high Moshigou Dam with a 47 ha m silt volume was completed in only 5 months with a daily rate of sluicing-siltation height of 0.23 m/day averaged over the entire construction period. Soil testing of samples from deep bore holes one year after the end of construction revealed that the embankment water content had decrease from 39% at the initial stage to 22-26%, and the dry density increased from 1.35 t/m3 to 1.53-1.66 t/m3. The embankment is good in quality and has been in performing normally under reservoir operation.
Table 1. Permissible rate of construction (height/day)
|
Soil type |
Permissible rate of sluicing-siltation (m/day) | |
|
Daily rate |
Averaged over the construction period |
Maximum daily rate averaged over 10 days |
|
Sandy Loam |
0.25-0.3 |
0.35 |
|
Light silt loam |
0.25-0.25 |
0.30 |
|
Medium silt loam |
0.15-0.20 |
0.25 |
The construction conditions, construction schedule, the size of construction surface and the intermittent period determine the number of mud siltation ponds.
One mud pond on the entire construction surface: This is usually applied to low dams. Only two side levees are needed to be built, one on upstream and the other on downstream The sluicing-siltation operation is simple and labor saving.
Two or more mud ponds: When the construction surface of the dam is wide, it is often necessary to arrange two or more mud ponds by building one intermediate levee or more parallel to the dam axis. The sluicing-siltation operation is carried out in turn on these mud ponds. If the water level behind the dam rises rapidly or it is necessary to retard the flood temporarily, the upstream mud pond can be raised by continual sluicing-siltation operation so as to first form a small cross section. At the same time, the sluicing-siltation is done at the other mud ponds in order to raise the embankment surface like a ladder. Near the top of the dam, the embankment surface is changed to having only one mud pond. It is necessary to reduce the mud ponds where ever possible to reduce costs and increase efficiency.
There are two kinds of levees. The function of the upstream and downstream side levees is to retain the slurry and help in the stability of the embankment slope during the construction stage. Therefore, the width, and construction quality, and the stability of the dam.
By roller-fill method
By compacting with spades
By filling loose soil
By sluicing siltation
Levees are of trapezoidal cross section and are placed layer by layer. The outer slope of the side levees is combined with the embankment outer slope. The inner slope is 1:
1 or at an angle of repose. The intermediate levees generally have a slope at an angle of repose. During construction the outer slope of the side levee can be steeper than the embankment outer slope with the toe of the side levee placed on the embankment slope line. It is then trimmed after the end of sluicing-siltation operation. The building of the levee should be kept higher than the slurry surface preferably not greater than 1 m. The upper side levee should overlie the lower one by a width greater than two-thirds of its bottom width.
By roller-fill method: This method can ensure good construction quality of the levee It is mainly used in the building of a storage dam or a large check dam by continual sluicing-siltation operation. During compaction, the layer thickness is about 0.3 m and the dry density is controlled at greater than 1.5 t/m3
The top width of the levee should be kept more or less constant, being neither too great at one time nor too small at the other so as to prevent the occurrence of defects.
By compacting with spades: This method avoids the hauling and compaction of soil materials borrowed from the bank. The deposited soil in the mud pond after consolidation is directly dug and placed on the levee and then compacted by beating with the spades. The levee build by this way has a steep slope and small width. This method has ease of digging in pervious soils such as sand and sandy loam. The mud should not be dug close to the levee but 1 to 2 m away only.
By filling loose soil: The levee is built of loose soil hauled onto the dam by machine or by manual labor. The loose soil is compacted by the hauling vehicle movement. Such method is simple and needs less labor, but the levee is of poor quality. The intermediate levees or the side levees of small check dams are built in this way.
By sluicing siltation: The side levees can be built by sluicing-siltation method. The mud pond is arranged on the side levee and generally sandy soils are used. This method has the advantage of decreasing the labor cost of hauling soils and omitting the compaction procedure. However it is not applicable in cohesive soils.
The upstream and downstream side levees are the component parts of the embankment slopes. In the course of building a dam of light to medium silt loam by continual sluicing siltation, the embankment with a certain depth and range of 'fluid-like' slurry zone needs the support of side levees for maintaining the its stability. Hence, the side levee is required to have a suitable width and good quality construction. The statistical data show that the earthwork of the levees constitutes generally 20 percent of the total volume of the entire dam. The width of the side levee depends on the dam height, soil type, concentration of the slurry, rate of sluicing-siltation operation, the volume of 'fluid-like' slurry, and topographical and geological conditions. Generally speaking, the side levee can be narrower in case of gentle embankment slope, pervious sandy soils, high concentration slurry, narrow valley and slow rate of construction, and vice versa. Higher dams of different soils, the top width of a side levee can be calculated by the following empirical formula:
b = 0.005 (n+ a) (1+ v) Hp
in which
b = top width of side levee (m);
n = gradient contained in 1:n;
v = the rate of construction (m/day);
H = the design height of the dam (m);
a = empirical coefficient;
p = clay content of the soil material (%).
In the course of mud mixing, it is necessary to loosen the soil in time and to dump it into the mud-mixing ditches with spades so as to ensure the slurry to have an adequate concentration. Soil loosening may be done by manual excavation, blasting, bulldozers, water jets and other methods. The number of laborers needed for taking care of soil loosening and mud mixing depends on the condition of borrow areas, usually 1 person for every 2,000 liters of water pumped are needed along the mud mixing ditches. Sufficient loose soils should be dumped continually into the ditches as water flows along. The ditches are cut deep into which the soil is dumped. At the outlet of the mud-conveyance ditch, special persons are needed to measure and control the concentration of the slurry.
The mud concentration will affect the labor efficiency and construction quality of a sluicing-siltation earth dam. A high concentration of slurry means a high labor efficiency and low cost. Slurry of low concentration will not only result in high cost and low labor efficiency but also cause a high pore water pressure and a large zone of 'fluid-like' slurry in the embankment slope. On the contrary, if the slurry is too high in consistency its flowability is poor, and the gradient of the mud surface on the embankment may be so great as to affect the construction of the levee. A suitable concentration of slurry can be identified as in table 2.
Table 2. Identifying the mud concentration of light to medium silt loam
|
Color of slurry |
Corresponding moisture content % |
Corresponding soil volume transported by per unit volume of water (M3/m3=T) |
Concentration of slurry |
|
Light Yellow |
>42 |
<2.2 |
Low |
|
Dark Yellow |
37-42 |
2.2-2.6 |
Suitable |
|
Blackish |
<37 |
>2.6 |
High |
where
T = soil volume transported by per unit volume of water (m3/m3);
rn = density of slurry (t/m3)
G = specific gravity of the soil particles,G=2.71 for medium silt loam;rd = Average dry density of the soils in the borrow area (t/m3);
G=2.7 for light silt loam;
G=2.69 for sandy loam.
w = Average moisture content of the soils in the borrow area (in decimal figures);
rw = Density of water (t/m3)
According to measured resulted of some sluicing- siltation dams made by the testing and researching group of sluicing-siltation earth dam in Shaanxi and Shanci, a suitable concentration of slurry prepared from sandy loam, and light to medium silt loam should have a moisture content of 37-41%, and a corresponding soil discharge T of 2.32-2.6 as shown in Table 3.
Table 3. measured mud concentration of some sluicing-siltation dams
|
Name of dams |
Soil material classification |
Clay |
Mud concentration (average value) |
Water content |
Soil volume transported by per unit volume of water T m3/m3 |
|
Taoerzui |
Light silt loam |
12 |
1.83 |
39 |
2.46 |
|
Shangliu-jiachuan |
Ditto |
13 |
1.81 |
41 |
2.32 |
|
Dianshi |
Ditto |
11 |
1.81 |
41 |
2.32 |
|
Dongfeng |
Medium silt loam |
16 |
1.8 |
40.8 |
2.32 |
In order to ensure a suitable concentration of the slurry during construction,
it is necessary to take regular samples for measurement from the exit section
of the mud conveyance ditch. If undesirable value is detected, the persons in
charge of the mud mixing must be notified immediately and must increase the
concentration of the slurry.
Dewatering embankment surfaces
Laying dry soils on the slurry surface
Using filters
Special internal drainage facilities
After the slurry flows onto the construction surface of the embankment, the water moisture will escape through natural evaporation and seepage. In addition to this, some effective measures are necessary to accelerate the drainage and consolidation of the embankment slurry with an objective of promoting the stability of the embankment.
After the slurry enters the mud ponds on the embankment, a part of the free water will be squeezed out by the weight of the soil grains and will accumulate at the low places in the slurry surface. It can be dewatered away from the dam by using siphon or small pump.
Buried pipes may also be adopted i.e. steel or rubber pipes are embedded in the levees with the pipe mouths at the same level as the slurry surface so that the water collected on it may be drained immediately by gravity. With the raising of the slurry surface, these pipes should also be raised to guarantee the drainage uninterrupted.
A layer of dry soil is placed on the slurry surface just after sluicing siltation to absorb the water in the slurry. This will expedite drainage and consolidate the slurry. Such a method is suitable to the slurry prepared from cohesive soil.
Apart from constructing diversion tunnel for releasing the base flow to reduce the storage of water in the reservoir during construction stage, it is advisable to build the downstream toe prism in advance or to build it to a greater height for accelerating the drainage and consolidation of the embankment. This is useful when the soil materials used for sluicing siltation are not very pervious but sand and gravel are available at site.
The internal drainage facilities include sand ditches, sand drains, drainage blankets and so on. A number of sand ditches (1×1 m cross section) or sand drains (1 m diameter) should be distributed in the embankment under sluicing-siltation. Sand ditches of trapezoidal cross section are filled with such filter materials as sand and gravel, or porous concrete pipes are used instead. The main trunk sand ditch leads to the toe prism. Vertical sand drains are of circular section and are connected to the trunk sand ditch.
The drainage blanket consisting of sand and gravel arranged as a filter is used on impervious foundation. It is directly connected to the toe prism. If the drainage blanket is located in the middle portion of the dam, it can be connected to the toe prism through sand drains or porous pipes. Whether the internal drainage facilities are necessary or not will depend on the specific conditions. Practical experiences show that sluicing siltation earth dams built of light to medium silt loam do not require the provision of internal drainage facilities if the foundation is relatively pervious.
