Discussion on Related Problems in The Testing Process of GCL Flexible Wall Permeameter
Abstract: Geosynthetic clay liner (GCL geosynthetic clay liner factory price) is a new type of geosynthetic anti-seepage material with good water impermeability. In engineering applications, its hydraulic seepage performance is a vital reference. The high quality GCL geosynthetic clay liner flexible wall permeation device and its testing principle are mainly introduced, and the related problems encountered in the development of the device are discussed.
Keywords: geosynthetics, clay liner (GCL geosynthetic clay liner for sale); flexible wall permeameter; permeability coefficient; back pressure
Geosynthetic clay (high quality GCL geosynthetic clay liner) is a new type of geosynthetic anti-seepage material. It is a composite material formed by pinching, stitching or bonding between two layers of geosynthetic materials, which may be a geotextile fabric or a non-woven fabric. Spinning fabric. According to the processing technology, it can be divided into bonded GCL geosynthetic clay liner manufacturers, needled GCL geosynthetic clay liner manufacturers and sutured GCL geosynthetic clay liner factory price. Currently, the main producers of GCL geosynthetic clay liner for sale are the United States and Germany. GCL geosynthetic clay liner manufacturers product characteristics mainly include physical properties, hydraulic properties, mechanical properties, and durability. Among them, hydraulic characteristics include hydration liquid, free expansion, hygroscopicity, fluid loss, and water permeability.
As a kind of anti-seepage material, high quality GCL geosynthetic clay liner has a permeability coefficient which is one of the most concerned indicators in engineering applications. The permeability coefficient of GCL geosynthetic clay liner factory price is small, generally between 1 x 10-10 cm/s and 1 x 10-7 cm/s. In foreign countries, the flexible wall triaxial permeameter is mainly used to test the permeability of GCL geosynthetic clay liner manufacturers in the triaxial apparatus by the backpressure method. Most of the domestic GCL geosynthetic clay liner for sale penetration performance tests use the soil permeability test method in the geotechnical test. Most of the instruments used for the GCL geosynthetic clay liner manufacturers permeability test refer to the soil permeation instrument in the geotechnical test. Different types of instruments are selected according to different soil qualities, such as 70. Type permeation instrument, soil sample tube, negative pressure type permeation instrument, South 55 type permeation instrument and test pit seepage method. Among them, the South 55 type permeation instrument is characterized by simple structure and strict water stop, suitable for constant head and deformation. The two methods of water head belong to the basic stereotyped permeameter, which is widely used in various laboratories in China. There are also some spontaneously developed instruments, such as Tsinghua University and Hohai University, which use a self-developed permeameter to test the permeability of GCL geosynthetic clay liner factory price.
In order to more accurately measure the anti-seepage performance of high quality GCL geosynthetic clay liner, the Chinese Academy of Water Sciences introduced a flexible wall triaxial permeameter from the United States and made relevant research and development on the testing principle and usage of the device.
1. Composition, characteristics and working principle of flexible wall permeameter
1.1 Introduction to flexible wall permeameter
The flexible wall permeameter consists of three parts: the water tank, the control panel, and the permeator chamber.
The water tank is a solution reservoir that provides the required permeate for the hydraulic system. The osmotic liquid may use deionized water, distilled water or other degassed water, or corrosive permeate or other chemical solution may be used according to the test requirements, as long as the permeate does not affect the performance of the instrument itself. Connected to the water tank are water, air compressors, vacuum pumps, and control panels. When air is present in the osmotic liquid in the tank, the liquid is vented by a vacuum pump. At the same time, a small pressure is applied to the permeated liquid in the water tank by the air compressor, so that the liquid enters the three glass measuring tubes on the control panel, respectively. When the test is finished, apply less pressure to the tank to remove any remaining liquid from the tank. The maximum working pressure of the water tank is 138 kPa.
The control panel is the operational control interface for the sample hydraulic performance test. It is connected to a water tank, a permeameter, an air compressor, a vacuum pump, and the like. Pressure digital display, pressure gauge, vacuum gauge, pressure regulator, system inlet and outlet control valve, glass tube and corresponding operation control valve are installed on the control panel. Through the control valve on the control panel, the injection, discharge and degassing of the permeated liquid in the water tank, the glass tube, and the permeation chamber can be completed. There are three glass tubes on the control panel, which correspond to the sample confining head, the sample inlet pressure head, and the sample outlet pressure head. The glass tube is divided into an inner tube and an outer tube, and a flow rate value is marked on the inner tube wall. During the test of the sample, the inner and outer tubes can be separately or simultaneously injected into the permeate according to the test requirements. At the end of the test, the change in the water level in the glass tube of the control sample results in a change in the flow rate of the permeate.
The triaxial permeator chamber is a device for mounting a sample, which is a cylindrical cavity having a diameter of 15 cm and a height of 32cm and is supported by the top plate and the chassis. A 10.2 cm diameter base was mounted on the permeator chassis to support the sample. The sample is placed with filter paper of the same size as the base, porous permeate, and finally placed on the top cover. The sample is surrounded by a flexible rubber film, and the upper and lower ends of the rubber film are respectively sheathed with an O-ring, and the flexible film is pressed to press the sample. A drain line is provided at both the upper and lower ends of the sample, and the water is allowed to permeate the sample under pressure, and the amount of penetration through the sample is determined by the change in the water level in the glass tube on the control panel.
There are 4 drainage pipes on the bottom of the permeator, and each end of the drainage pipe is connected with a constant volume of on-off valves. Two of the drainage pipes have one connection port outside the permeation chamber, and the connection can exclude four The gas in the pipeline can also be connected to a pressure sensor. The pressure sensor is mainly used to verify whether the sample is saturated. There is also a quick connector on the chassis to inject and discharge the permeate through the quick connector. There is also a quick connector on the top plate that is vented through the quick connector before the penetrator is filled or drained.
1.2 Characteristics of flexible wall permeameter
Compared with other domestic GCL geosynthetic clay liner for sale test devices, the flexible wall permeameter can better meet the high quality GCL geosynthetic clay liner test requirements and has the following unique features:
(1) Avoid leakage of the sidewall of the sample. Most of the instruments used in the domestic high quality GCL geosynthetic clay liner penetration performance test are based on the permeameter of the soil in the geotechnical test, while the penetration test of the soil sample generally uses a hard wall permeameter, and sidewall leakage is difficult to avoid. The flexible wall triaxial permeameter allows the flexible film to be pressed against the sample by the pressure difference between the confining pressure and the back pressure, thereby effectively avoiding leakage of the sample sidewall.
(2) Backpressure saturation. The so-called back pressure is to artificially increase the pore water pressure and the surrounding pressure of the sample at the same time so that the pore air in the sample is completely dissolved in water under pressure. Compared with the saturator hydrostatic pressure saturation method and the suction saturation method, this method can quickly saturate the sample in a short time and has high saturation.
(3) Automatic pressure valve. The features of the automatic pressurization valve allow the operator to easily control and maintain a constant pressure differential between the sample peripheral pressure and the sample osmotic pressure during the test, making the test operation simple and accurate.
(4) Bridge valve. The function of the bridge valve makes it easy for the operator to apply the same pressure at both ends of the sample, which facilitates simultaneous wetting of the test sample from both ends of the permeator so that the sample reaches saturation as soon as possible.
(5) Check the convenience of leaking and checking the saturation of the sample. By observing the variation of the pressure sensor readings, it is convenient and quick to determine whether the sample has been saturated and leaked.
(6) Accurate measurement of flow changes. When the sample begins to perform the permeability test, the precise change in the flow rate of the inflow and outflow can be obtained by observing the change in the water level in the glass tube that controls the inlet and outlet of the sample on the control panel.
1.3 Flexible wall permeameter test principle
The penetration performance test of high quality GCL geosynthetic clay liner is based on the American standards ASTM D5887 and ASTM D5084. ASTMD5887 is a test standard for standardized tests. The osmotic liquid is distilled water, deionized water or other degassed water. The permeation pressure is 550 kPa and the back pressure is 515 kPa. After the sample is saturated under backpressure for 48 h, the bottom of the sample is raised. The pressure was 530 kPa (the sample outlet pressure was kept unchanged at 515 kPa), and at osmotic pressure of 15 kPa, the permeated liquid was subjected to a permeation performance test from the bottom up through the sample. See Figure 1 for the principle of the penetration test.
Considering that the thickness of GCL geosynthetic clay liner for sale is small, the permeation channel and the percolation velocity are also small, and the water flow should be laminar, so it is considered that the Darcy law commonly used in the soil can also be used for the penetration of high quality GCL geosynthetic clay liner.
The permeation area of the sample can be calculated from the size of the sample (diameter 10.2 cm). The thickness of the sample can be determined according to the method recommended in ASTM D5887, or other feasible measurement methods can be adopted. During the permeation test, the flow change value of the glass tube of the control sample is recorded separately, and the seepage flow of the sample can be obtained. According to the calculation formula of Darcy’s law, the permeability coefficient KT of the GCL geosynthetic clay liner factory price product at the test temperature can be calculated. Then, by multiplying the ratio RT of the viscosity of the water at the test water temperature to the viscosity of the water at the water temperature of 20 ° C, the permeability coefficient K20 at 20 ° C is corrected as a reference index in engineering applications.
2. Discussion on related issues in the experiment
During the development of the method of use of the instrument and the infiltration performance test of high quality GCL geosynthetic clay liner using a flexible wall triaxial permeameter, the following problems worthy of discussion by peer experts and scholars were encountered.
2.1 The value of the thickness
In the penetration test, if the permeability coefficient of GCL geosynthetic clay liner factory price is to be obtained, the thickness of the GCL geosynthetic clay liner product is an important parameter. The thickness of the GCL geosynthetic clay liner manufacturers is affected by the compressive load, and the load acting on the GCL geosynthetic clay liner for sale is different, and the thickness thereof is also different. Since GCL geosynthetic clay liner manufacturers are thinned under pressure and expands after being exposed to water, it is difficult to accurately measure the thickness of GCL geosynthetic clay liner. In addition, there are some differences in the method of determining the thickness of GCL geosynthetic clay liner manufacturers. Some people think that the thickness of GCL geosynthetic clay liner manufacturers generally refers to the thickness of the entire GCL geosynthetic clay liner manufacturers composite, while in ASTM D5887 Appendix 2, GCL geosynthetic clay liner is proposed as a barrier material, and bentonite is its For the body that acts as a barrier, it is recommended that the thickness of the GCL geosynthetic clay liner factory price only take the thickness of the clay portion.
In combination with the characteristics of the test instrument, a dial gauge is installed at the stainless steel piston at the top of the permeameter to measure the thickness of the GCL geosynthetic clay liner factory price sample. First, in the case where the GCL geosynthetic clay liner for sale sample is not installed, the filter paper and the porous seepage stone are placed in order, the stainless steel piston is loosened and gently inserted into the top cover, and then the piston is fixed by the side fixing screws. The initial reading of the sub-meter is zero, and then the sample is loaded into the permeameter. The finished (dry) thickness of the sample before the penetration test and the thickness of the sample after the penetration test can be separately measured according to the reading of the dial gauge.
2.2 Verify the accuracy of sample saturation
The saturation of the sample is verified by the pressure sensor connecting the two drain lines of the sample, that is, the pore water pressure value. Therefore, accurate measurement of the pore water pressure is necessary for verifying the saturation of the sample. When the high quality GCL geosynthetic clay liner sample is saturated, the pore water pressure in the entire sample is equal under confining pressure. The resulting pore water pressure is transmitted to the zero indicators faster through the porous seepage stone, causing a slight displacement of the sensing film of the pressure sensor. At this time, a small amount of pore water flows from the sample to the pore water pressure measurement system. If the pipe volume of the measurement system and the volume of the bubbles mixed in the pipe change, there will be more pore water flowing out of the sample. The flow of pore water in the sample must be generated under a certain hydraulic ratio. Therefore, the pore water pressure inside the sample is greater than the pore water pressure at the contact between the sample and the measurement system, and the larger pore inside the sample. The water pressure must pass through the measurement system after a period of time. This is the time lag of the pore water pressure measurement caused by the volume change of the pore water pressure measurement system. For this problem, Bishop has conducted a large number of pore water measurement tests on representative clays and gave the time required for the clay to reach 98% saturation. From the results of Bishop’s test, it can be concluded that the time lag caused by the volume change of the measurement system is negligible under normal circumstances. A severe time lag can occur only when the compressibility of the sample is low and air bubbles are present in the measurement system. In order to eliminate various errors caused by time lag, the four drain lines and pressure sensors must be completely vented before the penetration test to ensure accurate verification of sample saturation and accurate measurement of sample inlet and outlet pressure values.
2.3 The way pressure is applied in the hydraulic system
The permeator pressure is 550 kPa and the back pressure is 515 kPa. The application of this pressure will have an effect on the test. Under the confining pressure, the shape of the sample remains basically the same, so the stress and pore water pressure of each part in the sample are relatively consistent. However, since the upper and lower ends of the sample are restrained by the top cover and the base, the shear stress between the sample and the top cover and the base limits the lateral deformation of the upper and lower ends of the sample, resulting in the uneven pore water pressure inside the sample. Therefore, when the pore water pressure is measured at the top and bottom of the sample, the true result can be measured only when the pore water pressure in the sample is completely equal. For this reason, during the test, the backpressure of the sample must be applied slowly and stepwise to ensure that the pore water pressure in the sample has sufficient time to be equalized. Lundgren recommends using a stress-controlled pressurization method at the beginning of the test to wait for the pore water pressure to stabilize and equalize after each stage of pressure before applying the next stage pressure.
During the application of the back pressure in this test, after applying each stage of back pressure to the sample, observe the reading of the pore water pressure sensor. When the measured pore water pressure is the same as the bottom pressure of the sample, and the inside of the glass tube The reading is stable, the reading is recorded and the next stage bottom pressure is applied until the preset pressure (ie 515 kPa).
2.4 Leakage of sidewalls of flexible films
In the permeation test, the pore water pressure in the sample may sometimes be unstable, and it will change after a long time. There are many reasons for this phenomenon, such as the volumetric creep of the pore pressure measurement system under pressure or Leakage, the pore air in the unsaturated soil slowly dissolves in water and temperature changes. The more common cause is caused by leakage of the rubber film. There are three ways to leak:
(1) Leakage through the needle eye. This leakage can be carefully checked for cracks or pinholes in the flexible film before the test, or it can be tested.
(2) Water leakage through the joint between the flexible film and the sample top cover and the base. In the triaxial penetration test, this osmotic route is considered to be a leak that occurs frequently and has a large influence. Therefore, the method of tying the flexible film to the top cover and the base of the sample can be improved, and when the sample is installed, it is ensured that there is no fiber or the surrounding sidewall of the base, the top cover, the flexible film and the porous seepage stone of the sample or Other sundries.
(3) Leakage of the flexible film itself. A flexible film is not absolutely impervious to water, even without a pinhole. The permeability coefficient of the flexible film caused by the hydraulic ratio drop measured by Poulos is 4.8 x 10-16 cm/s.
2.5 Effect of sample saturation on infiltration samples
GCL geosynthetic clay liner for sale is a porous material, so air is inevitably present. When the permeation test is carried out, the permeability coefficient tends to be lower than the actual value due to the gas in the sample. Therefore, before the permeation test of the sample, the GCL geosynthetic clay liner for sale is firstly subjected to a confining pressure of 550 kPa and backpressure of 515 kPa. Under the action of 48h infiltration saturation, the saturation of the GCL geosynthetic clay liner factory price sample is smaller, and the more residual gas in the pores of the bentonite, the effective permeation area of the bentonite is reduced. At the same time, since the gas shrinks due to the change of pore water pressure, the influence of saturation becomes an indefinite factor. In order to maintain the test accuracy, the sample must be sufficiently saturated to exhaust the gas in the GCL geosynthetic clay liner factory price pores.
2.6 Influence of indoor temperature
ASTM D5887 requires that the temperature of the permeator, sample and permeate tank should not exceed ±3 °C and that the test equipment must be placed in a relatively constant temperature room. If such a room is not found, the test equipment should be placed in a constant temperature water tank or insulated container, or other instruments that can maintain the specified accuracy. Because in the infiltration stage, the amount of water permeation is not only generated under the osmotic pressure but also changes in the volume of the instrument caused by the temperature change to cause a change in the amount of water permeation. In order to accurately test the permeability coefficient of GCL geosynthetic clay liner for sale, it is necessary to ensure that the temperature variation range during the test is minimized. In addition, the temperature must be measured and recorded at regular intervals, and the temperature at the beginning and end of the infiltration phase must be determined at a minimum.