Pore water pressure and the importance of drainage

Pore water pressure and drainage are key considerations when designing earthworks, including reinforced soil structures, using compacted clay fill.

Without appropriate drainage, rainfall can saturate the clay and increase its pore water pressure. This, in turn, reduces the soil’s effective stress, results in a loss of strength and can potentially lead to failure.

This guide explains what pore water pressure is, the formula used to calculate pore pressure, and its role in determining the structural integrity of earthworks. It also outlines how drainage helps to maintain pore water pressure and effective stress, preventing structures from collapsing.

Read on to learn more about:

• What is pore water pressure?
• Effective stress, negative pore pressure and suction
• Pore water pressure in saturated soils
• The importance of drainage in reinforced clay fill
• Designing drainage in geosynthetic-reinforced soil slopes
• Pore water pressure formula

What is pore water pressure?

So, what is pore water pressure? Essentially, pore water pressure is the pressure of groundwater held between soil or rocks in the gaps (or ‘pores’) between particles. It is affected by the soil type, water flow conditions and level of the water table. Pore pressure is important in geotechnical engineering as it influences soil behaviour, including its shear strength and bearing capacity.

Factors that contribute to pore water pressure development

Hydrostatic water pressure

Hydrostatic water pressure occurs due to the weight of the water and any soil or sediment above a given point. The deeper you go into the fluid or saturated soil, the greater the pressure transmitted through the water in the pores. 

Hydrostatic water pressure can be calculated using the formula P=ρgh, where P is the hydrostatic pressure, ρ\rhoρ is the density of the fluid, g is the acceleration due to gravity, and h is the depth below the fluid surface.

Changes in the water table

Fluctuations in the water table can lead to variations in pore water pressure. When the water table rises, pore water pressure increases, and when it falls, it decreases. When water moves from higher to lower elevations, it creates dynamic pressure changes, which can lead to fluctuations in pore water pressure, especially in saturated soils.

Osmotic pressure

Osmotic pressure occurs when there are different amounts of dissolved particles (like salt) in water. Water naturally moves toward areas with more particles because of attraction at the molecular level. As a result, this movement creates pressure that can affect how water behaves in soil.

Absorption pressure

Soil particles can stick together because of thin films of water around them. These water films help to bind the soil particles together through a cohesive force which influences the stability and structure of the soil. 

Matric suction

Matric suction is the phenomenon that describes the pressure difference between pore air pressure and pore water pressure. As water moves from areas of high moisture to areas of lower moisture, it influences the water retention and flow in soils. Matric suction plays a crucial role in the behaviour of unsaturated soils, affecting their strength and stability.

Effective stress, negative pore pressure, and suction

Ground movement and soil slope instability can be caused by changes in total stress and pore pressure changes, such as after rainfall when pore pressure can increase.

The difference between total stress and pore pressure is the ‘effective stress’, which controls soil behaviour, such as shear strength, compression, and deformation. Effective stress is a measure of how much load a soil can carry.

Below the water table, pore pressure is positive; at the level of the water table, pore pressure is zero. Above the water table, soil can remain saturated due to capillary rise, with negative pore pressure (suction). As the soil becomes partially saturated, suction increases due to water molecules being attracted to soil particles and generating surface tension between them. This draws the particles together, giving the soil strength.

A good example of this is wet sand in a sandcastle. The sand will remain damp in the short term due to suction, which increases effective stress and holds the sand grains together. However, over time, as the sand dries, suction is lost and the sandcastle will collapse. It will also collapse if more water is added, as pore pressure will increase and effective stress will reduce, causing a loss of strength.

Ground Coffee – Episode 17. Andrew is in the sand pit this week, explaining the importance of negative pore pressure in sandcastles.

Pore water pressure in saturated soils

Loose, dry sand compresses if we apply a load to its surface. However, say that we were instead working with saturated soil and the gaps between particles were filled with water, preventing compaction. In this scenario, an applied load would increase pore water pressure – and the pore water would try to flow to an area of low pressure such as the ground surface.

But if a large load is applied to saturated soil over and over, such as during an earthquake, the water doesn’t have sufficient time to flow away toward lower pressure zones between loading cycles. Pore water pressure builds, and can eventually reach a point where the soil particles are forced apart. At this stage, liquefaction can occur: the strength of the soil is lost, and it starts to flow like a liquid, with potentially catastrophic consequences for any structures built on the affected ground (see Soil Liquefaction: Causes, Effects, and Solutions to discover more).

The importance of drainage in reinforced clay fill

It is very important to consider negative pore pressure when designing earthworks, including reinforced soil structures, particularly when using well-compacted clay fill. In particular, expansive clay soils are sensitive to moisture changes, swelling when saturated, and shrinking when dry.

Clay fill compacted to a normal earthworks specification is likely to have negative pore pressure up to considerable heights. Pore pressures are only likely to become positive at the base of very high structures (more than 10m to 15m high), or at lower heights if the clay was wet and soft during placement.

Suction is typically ignored in design, which provides an additional margin against failure or poor performance of the structure. As a result, it is a good idea to maintain suction in the long term.

This means incorporating drainage to prevent the clay fill from coming into prolonged contact with free water, which would otherwise saturate it. If the clay fill were to become saturated with water, its pore water pressure would increase, reducing the effective stress and resulting in a loss of strength (potentially leading to failure).

Internal slope drainage must let water drain out and must not let water in. It should be designed to intercept groundwater flow and be free-draining, allowing water to drain out easily and stay ‘dry’ most of the time. It should also not be ‘daylight’ at the upper surface of the structure, preventing run-off from flowing into the fill (run-off should be handled by surface water drains, to avoid ponding).

Any movement of a soil structure has the potential to disrupt, or even reverse, the fall of a drain, which could result in water flowing back in – something worth considering when specifying maintenance regimes.

Designing drainage in geosynthetic-reinforced soil slopes

Geosynthetics are used in reinforced soil slopes using clay fill and, in some cases, reinforcement layers incorporate a geotextile drainage fabric, designed to reduce excess pore water pressure.

Eastwood health centre - Clay slope being built

However, while this may be useful in very wet fill, in the more common situation where clay is compacted to normal earthworks specification, any regularly spaced fine drainage layers could allow water to penetrate well into the fill. This could increase pore pressure, causing swelling and softening, so it’s best to avoid it.

It is far better to incorporate internal drainage behind geogrid reinforcement. This will intercept groundwater flow and, when combined with adequate surface water drains to collect run-off, can reduce the risk of slope failure.

Diagram -Reinforced earth ground water flow

 Pore water pressure formula

The formula for pore water pressure (u) is:

u=γw⋅hu = \gamma_w \cdot hu=γw⋅h

Where:

• uuu is the pore water pressure (in Pascals or kPa),
• γw\gamma_wγw is the unit weight of water (approximately 9.81 kN/m³),
• hhh is the height of the water column or the depth below the groundwater table (in meters).

To calculate pore water pressure, measure the depth below the groundwater table. Then multiply this by the unit weight of water. This gives the pressure applied by the water in the soil pores.

How Tensar can help

Tensar can assist you with accounting for drainage and pore water pressure on your next project through specially designed wall and slope systems and geotechnical engineering software, Tensar+. With 30 years of construction knowledge, design experience, and innovative geogrid products, the TensarTech permanent and temporary retaining wall and slope systems provide several facing types and construction options to suit the structure’s end use, location, and required design life.

Our retaining wall systems include precast concrete, dry-laid modular block systems (with the option of adding architectural, masonry, or brick finishes), precast concrete panel systems, gabion and crib walls, and robust units suitable for aggressive marine environments. Our TensarTech® GreenSlope System can create vegetated slopes with angles of up to 70˚.

Contact us for any questions or enquiries.

Next steps

If you enjoyed our article about pore water pressure and the importance of drainage, you may be interested in these other guides:

• Retaining walls: types designs and functions
• Bearing capacity of soil
• What is the shear strength of soil?
• What are expansive soils?