Permeability coefficient is also called hydraulic conductivity.In isotropic media, it is defined as unit flow under unit hydraulic gradient, representingfluidThrough the difficulty of pore skeleton, the expression is: κ=k ρ g/η, where k is thepermeability , it is only related to the nature of the solid skeleton, κ is the permeability coefficient;η is powerViscosityCoefficient;ρ is the fluid density;G is the acceleration of gravity.In anisotropic media, the permeability coefficient is expressed in tensor form.The greater the permeability coefficient, the stronger the permeability of rock.Permeability coefficient of strongly permeable coarse sand gravel layer>10m/day and night;The permeability coefficient of weakly permeable sandy soil is 1~0.01 m/day and night;The permeability coefficient of impervious clay is less than 0.001 m/day and night.It can be seen that the soil permeability coefficient depends on the soil texture.
Chinese name
Permeability coefficient
expression
κ=kρg/η
Decided on
soil texture
influence factor
It mainly depends on the shape and size of soil particles
Permeability coefficient k is a quantitative index representing the permeability of soil, and it is also a basic parameter that must be used in seepage calculation.The k value of different kinds of soil varies greatly.Therefore, it is very important to accurately measure the permeability coefficient of soil.
computing method
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The permeability coefficient K is an index that comprehensively reflects the permeability of soil mass. The correct determination of its value is of great significance to the seepage calculation.There are many factors that affect the permeability coefficient, mainly depending on the shape and size of soil particles, uneven coefficient and viscosity of water. It is difficult to establish an accurate theoretical formula to calculate the permeability coefficient k, which can usually be determined by test methods, including laboratory measurement, field measurement or empirical estimation.
Determination method
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Permeability coefficient k measured by constant head method
The determination methods of permeability coefficient are mainly divided into "laboratory measurement" and "field field measurement".
1. Laboratory determination method
At present, there are many kinds of instruments and test methods for determining the permeability coefficient k in the laboratory, but from the test principle, they can be roughly divided into "constant head method" and "variable head method".
Constant head test methodThat is to keep the head constant throughout the test, so the head difference is also constant.As shown in the figure:
During the test, fill the transparent plastic cylinder with a saturated sample of section A and length L, open the water valve, make the water flow through the sample from top to bottom, and discharge it from the water outlet.After the head difference △ h and seepage flow Q are stable, measure the water volume V flowing through the sample within a certain time t, then
Constant head test is suitable for measuring the permeability parameters of sandy soil with high permeability.Since the permeability coefficient of cohesive soil is very small and the amount of seepage water is very small, it is difficult to measure accurately with this test, so the variable head test must be used instead.
Measurement of permeability coefficient by variable head method
Variable head test methodThe water head difference is always changing with time during the test. The device is shown as follows: water flows through the soil sample from bottom to top from a vertical glass tube with a scale and a U-shaped tube.During the test, after filling the glass tube with water to the required height, start the stopwatch to measure the initial head difference △ h1, and after the time t, measure the final head difference △ h2. By establishing the instantaneous Darcy's law, the expression of the permeability coefficient k can be derived.
Assume that the head difference of t acting on the two sections at any time during the test is △ h, and the water level in the pipe drops dh after the time dt, then the amount of water flowing into the sample during the time dt is
dVe = －a dh
Where, a is the sectional area of glass tube;The negative sign at the right end indicates that the water volume increases with the decrease of △ h.
According to Darcy's law, the seepage flow of the sample flowing out in dt time is:
dVo = k*i*A*dt = k*（△h/L）*A*dt
Where, A - sectional area of sample;L -- sample length.
According to the principle of water flow continuity, dVe=dVo, that is
k = （a*L/A*t）㏑（△h1/△h2）
Or withCommon logarithmIndicates, the above formula can be written as
k = 2.3*（a*L/A*t）lg（△h1/△h2）
2. Field measurement
Infiltration test generally adopts test pit seepage test, which is a simple method for measuring permeability coefficient of loose layer and rock stratum in aeration zone in the field.The test pit method, single ring method and double ring method are often used in the test pit seepage test.
Test pit method
It is to dig a square or round test pit with a certain depth (30-50cm) in the surface dry soil. The bottom of the pit should be 3-5 meters away from the groundwater level. The bottom of the pit should be paved with 2-3 cm thick inverted coarse sand, and water should be injected into the test pit. The water level in the test pit must always be about 10 cm higher than the bottom of the pit.In order to observe the water level in the pit, a ruler shall be set at the bottom of the pit.Calculate the water quantity Q infiltrated from the pit bottom in unit time, divide it by the pit bottom area F, and then obtain the average seepage velocity v=Q/F.When the height of water column in the pit is not large (equal to 10 cm), it can be considered that the head gradient is close to 1, so K (permeability coefficient)=V.This method is suitable for measuring sandy soil with little influence of capillary pressure. If it is used in cohesive soil, the measured permeability coefficient is high.
Single loop method
An iron ring with a height of 20 cm and a diameter of 35.75 cm is embedded at the bottom of the test pit, and the area enclosed by the iron ring is 1000 square centimeters.The iron ring is pressed 10 cm deep into the bottom of the pit. The ring wall should be in close contact with the soil layer, and 2-3 cm thick inverted filter sand should be laid inside the ring.At the beginning of the test, use a Mariotte bottle to control the water column in the ring and keep it at a height of 10 cm.When the test is carried out until the infiltration water Q is fixed, the seepage velocity can be calculated as follows: v=Q/F, and the seepage velocity obtained is the permeability coefficient value of the loose layer and rock stratum.
Double loop method
Two iron rings are embedded at the bottom of the test pit, and an inner ring is added to form a concentric ring. The diameter of the outer ring can be 0.5m, and the diameter of the inner ring can be 0.25m.During the test, inject water into the iron ring, and use a Mariotte bottle to control that the water column of the outer ring and the inner ring is kept at the same height (for example, 10 cm).The permeability coefficient value of loose layer and rock stratum shall be determined according to the above method according to the data obtained from internal ring.Since the water in the inner ring only penetrates in the vertical direction, which eliminates the error of the lateral seepage zone, it is more accurate than the test pit method and the single ring method.The water seepage between the inner and outer rings is mainly lateral diffusion and capillary absorption, while the inner ring is the actual seepage of loose layers and rock layers in the vertical direction.
When the water seepage test is conducted and the water seepage volume tends to be stable, the permeability coefficient can be accurately calculated as follows (taking into account the additional influence of capillary pressure): K (permeability coefficient)=QL/F (H+Z+L).
Where:
Q ---- Stable infiltration water volume (cubic centimeter/minute);
F ----- seepage area of the inner ring of the test pit (square centimeter);
Z ----- water thickness in the inner ring of the test pit (cm);
H ---- Capillary pressure (generally equal to half of the capillary rise height of rock and soil) (cm);
L ---- Water penetration depth at the end of the test (determined by excavation after the test) (cm).[1]
Main applications
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Determination of groundwater flow rate: the groundwater flow on the groundwater contour map is upward, and the adjacent two classes are calculatedWaterlineAnd then calculate the groundwater flow velocity V=kI using the formula
