The term “permeability of soil” refers to the capacity of soil to allow the passage of liquids such as water. For buildings that will be in contact with water, this is a crucial consideration. A soil’s empty areas are linked together, allowing water to move through them.
Water flows in a meandering course rather than a direct one. But in soil mechanics, the assumed flow direction and effective velocity are both straight lines. When pores are larger, flow rates are higher.
What does soil permeability mean?
Soil permeability refers to the property of soil to transmit water and air. The more permeable the soil, greater the seepage. Generally, soils comprise of layers and their quality varies considerably from one layer to another. Several factors such as soil texture, structure, cracks, and holes, layering, visible pores, etc. must be considered when determining permeability.
Soil permeability significance
- The permeability of soil affects how quickly it settles when loaded on top of a saturated subgrade.
- Permeability can have a major role in determining the stability of slopes and retaining structures.
- Soil permeability plays a crucial role in earth dam design.
- Soil filtration systems are optimised for permeability.
Soil permeability characteristics
- Managing issues with seepage water pumping from a construction excavation.
- Attempting to gauge the amount of groundwater seepage.
- Analysing the stability of earthen buildings and earthen walls against seepage.
all about: degree of saturation formula
Darcy’s law
All soil permeability test findings must be interpreted in light of Darcy’s law, an equation describing fluids flow through a porous media. As a ratio of how fast water moves through the soil to how steep the slope is, the coefficient of permeability or hydraulic conductivity is defined by this equation:
- V = discharge velocity or superficial velocity
- k = coefficient of permeability or hydraulic conductivity
- i = hydraulic gradient
- = fall in total head
- L = length of soil specimen
Permeability coefficient
The permeability coefficient (k) measures how quickly water may move through soils in metres or centimetres per second. A sand and gravel formation may have values of 104 metres per second or higher, while fine-grained soils like clays may have values of 10-8 metres per second or lower.
Empirical techniques such as soil survey mapping, soil texture, and particle size distribution can be used to make estimates of soil permeability. On the other hand, direct measurements of these characteristics are equally straightforward, thanks to a number of various lab and field test methods. The choice of test technique is based on the type of soil being tested, the reason for the test, the level of precision necessary, and the type of specimen to be tested.
Soil permeability: How is it measured?
Permeability tests of soil are conducted with either a constant or a decreasing head.
The term “constant head test” is used to describe a piece of equipment that maintains a constant head pressure over a sample by keeping the top of the water column at a constant height above the sample. High-flow soils, such as sands, gravels, and even some clay soils, pass the test.
The pressure is gradually reduced during a falling head test because the head is allowed to fall while water seeps into the sample. As a rule, falling head techniques can only be used on very fine-grained soils.
Soil permeability measuring instruments
Flexible-wall permeability cells
Flexible-wall permeability cells measure the hydraulic conductivity of soils through a variety of techniques. This standard’s procedures permit numerous variations on the constant head and falling head approaches, such as testing for a constant flow rate and constant volume tests with regulated pressures.
The test sample can be made from undisturbed samples from boreholes (Shelby tubes) or by packing soil in a mould to a certain density. The sample is wrapped in a latex membrane and placed within a fluid-filled, pressured test chamber.
With a system of valves and burettes installed on a logic panel, the confining pressures on the sample and the permeant (usually water) can be controlled in three dimensions. Throughout the entire procedure, sample deformation and volume change are monitored. Although this is a basic and commonly described test, it requires a lot of preparation of samples and can take several days to finish.
Features
- Caps and pedestals for specimens, porous stones, latex membranes, and end caps.
- Individual components or entire kits for various specimen sizes are available. Other equipment is utilised for sample preparation and handling.
Constant head permeameter
Constant head permeameters evaluate the permeability coefficient of non-plastic soils in which no more than 10% of particles pass a 75m (No. 200) test sieve.The test is done in a rigid-walled sand and gravel permeameter with a diameter 8–12 times the largest particle size and porous stones to keep the sample from leaking out when the head pressure stays the same.
Two manometer ports are connected to a double-tube manometer in order to measure variations in head pressure during the test. A continuous head tank provides the specimen with de-aerated water. As required, permeability tests can be conducted with samples ranging from 0% to 100% relative density.
After packing thin layers of the prepared granular soil sample in the permeameter, a special sliding-weight compaction hammer or vibrating tamper is used, if needed, to get a higher relative density. After saturating the sample with degassed water under a vacuum, the test is initiated.
Readings of time, head (the level of water in the manometer tubes), and flow rate during periods of rising head pressures are used to figure out the final results.
Features
- Conform to ASTM and AASHTO specifications for constant head permeability measurements of granular soils up to 0.75 in (19.0 mm).
- No. 100 mesh put over manometer port covers inhibits dirt leakage.
- Different diameters for a wide variety of particle size distribution.
- Robust acrylic and anodised aluminium framework.
Falling head permeameter
In the falling head permeability test, water is allowed to flow through a soil sample that is relatively short and is linked to a standpipe. The standpipe provides the water head and also enables the volume of water that passes through the sample to be measured.
The permeability of the soil being examined will determine the diameter of the standpipe. A falling head permeability cell is all that is required to carry out the test.
Features
- Permeability tests can be performed at a constant head pressure or by dropping the head.
- Strong, transparent acrylic testing chamber.
- Durable valves and fittings that won’t break the bank.
Compaction permeameters
Compaction permeameters conform to the standards of standard 4in (102mm) or 6in (152mm) proctor soil compaction moulds through the use of density moulds and collars. You can test the permeability of the soil with a falling head or a constant head using one of these flexible permeameters.
The mould and collar are held in place by the upper and lower plates of the permeameter, which are made of anodised aluminium. Connections and inlet/outlet valves are included. The compressed sample can be drained with water thanks to a built-in air bleed valve. The set comes with the tubing, two porous stones, and sealing rings required to use the device.
One must acquire, independently, a single-tube manometer. The reservoirs and other extras are sold individually. extra soil density moulds can be used to rapidly create representative samples.
Features
- You can compact dirt using standard collars and moulds of 4 or 6 inches in diameter.
- Soil permeability testing under both constant and declining head conditions.
- Only high-quality fittings and valves are used.
Shelby tube permeameters
By conducting the test in the undisturbed soil sample tube, Shelby tube permeameters are able to save time and money while providing a near-in-situ environment for soil permeability measurement. Non-cohesive materials and sands, which require particular handling when transferred to a typical permeability cell, are ideal candidates for these devices.
The permeameters are made up of two anodised aluminium end caps with O-rings, which are then fastened with threaded tie rods over a standard 2in (50.8mm), 2.5in (63.5mm), or 3in (76.2mm) diameter Shelby tube segment that is up to 6in (152mm) in length (not included). Tilting clamping knobs make it easy to get the clamps over the tie rods quickly.
A porous stone and a valve for regulating permanent flow are built into both ends of the caps. The internal top cap’s concave profile helps remove excess air.
Each permeameter comes with ten feet (three metres) of 1/4-inch (0.635 millimetres) ID tubing, three threaded tie rods, clamp knobs, a stainless steel valve opening, two porous stones, two valve openings, a stainless steel valve, corrosion-resistant top and bottom end caps; and two stainless steel valve openings.
Features
- Prevents delicate samples from being disturbed or damaged.
- Cost and effort for set up are reduced.
- Try either the constant head or falling head approach.
Double-ring infiltrometers
In the field, double-ring infiltrometers measure soil infiltration rates for geotechnical and environmental applications like designing dams and reservoirs or looking into liquid waste and leaching. Two concentric metal rings are positioned and pushed into the ground at the testing site.
After the test rings are filled with water, two Mariotte tube devices maintain a steady level of the liquid. The infiltration rate is determined by volume changes observed in the test tubes.
This rate is a way to measure how fast water moves through the soil, but it can only be directly linked to the coefficient of permeability or hydraulic conductivity if the hydraulic properties are well understood. Nonetheless, the test gives useful information, and its application is well-established.
Features
- The preferred method for the ASTM D5856 test.
- Compatible with both constant head and falling head methods.
- Available both with and without a soil compaction mould.
Soil permeability: Factors that affect permeability
Grain size or particle size
K=CD210
Alan Hazen gave the above equation. The shape and size of soil particles affect how well they let water through. The size of the particles affects how well they pass through.
Void ratio
If there are more holes, then there is also more permeability.
Composition
Mica can make it harder for water to pass through gravel, sand, and silt. When water gets between clay particles, it makes the clay less permeable.
Structural arrangement
Reworking natural soil makes it less permeable. If the soil has more round particles, it is more permeable.
Stratification
When water flows parallel to the strata, the permeability is greater than when water flows across the strata.
Foreign particles and air trapped inside
This changes the permeability because there is less empty space, and the pores can’t connect with each other.
Degree of saturation
If the soil is dry or only partly wet, it will always let less water through.
FAQs
What makes the soil more permeable?
If the volume of soil voids rises, the flow path becomes wider, and the interconnectivity of the voids increases.
Why is clay impermeable?
Clay-textured soils have limited pore spaces, which results in poor water drainage.
