Almost everywhere on earth, we can find water below the land surface, which is called groundwater. For example, approximately 50% of the drinking water in the US comes from groundwater per American Institute of Geosciences. We also use groundwater in agriculture, and therefore it is a very essential resource.
Although it obvious benefits for our survival, from civil engineering point of view, groundwater is usually something to be remedied when designing foundations. As we have seen before, presence of more water (which means more water pressure) weakens the soil by reducing the strength of the soil skeleton, which is the real source of soil strength. It also makes the soil heavier, which becomes an issue for retaining structures. The increasing amount of water can actually be fine up to a certain point, as far as the angle of internal friction, but after reaching a certain limit, the increasing water content will decrease angle of internal friction (to visualize the relation, view the curve for the proctor test, that we showed in previous pages. The angle of internal friction follows that curve directly and becomes optimum at optimum dry density, so for saturated soils , the angle of internal friction is never optimum and always less than the maximum it could potentially be.
We can work with water in different ways. For some structures we do not want to allow water inside the foundation system at all. For others, we can allow it but in that case we want to drain it as fast as possible, as efficiently as possible, during the life of the structure.
Groundwater is sometimes not static but it flows. This is called seepage. Seepage and the force it exerts comes into play in a variety of situations during design.
Groundwater also causes the soil to freeze. This can create problem for foundations.
Now let’s discuss the concepts we have just mentioned about groundwater, in a little more detail…
Dewatering is the control of surface water or ground water (but in most cases groundwater) within acceptable limits during construction, so that the work can proceed. This control is done by pumping the water away to another location, which can receive that water. The water may be just sucked out from an excavated area or be pumped from wells that are penetrated underground, which can be just one or as required. The number, size and depth of the wells are based on factors such as the required water table depth for construction to proceed effectively versus the existing water table depth, amount of water seepage into the area (which directly depends on soil permeability), and of course as always, economical and environmental concerns. Dewatering can be performed by a specialized contractor or in many cases it can be performed by the contractor whose work will be affected. Dewatering is one of the easier items to miss during budgeting of a construction project.
So basically, by pumping the water away, ground water table is lowered at the construction area, so that we will not have to work underwater. At the locations where the soil grain size is large, sump pumps and well points can be suitable, but for soils with small grain size and low permeability such as clay, sand filters should be used around the pump and pipes or other methods may also be more suitable, such as accelerating water flow in the soil thourgh electroosmosis process.
The amount of lowering of water table is called drawdown. If it is too rapid, it can cause risk for slopes, as was mentioned under slope stability section. For example, when the water stored in an earth embankment dam is suddenly lowered, there will still be water within the dam volume, contained in soil pores. But now the surrounding water will be gone, so the sloped and saturated earth, which also includes the water weight, will need to stand on its own, which can cause that slope to fail. Therefore rapid drawdown is a situation to be avoided at slopes.
Drainage is removing surface water or underground water either by natural or artificial means. The difference between dewatering and drainage is that dewatering is done only during construction so that work can proceed (unless there is a need for a permanent dewatering operation for some special cases but needless to say it will be costly to operate and maintain pumps constantly). Drainage however is a permanent system that is designed to keep the structure and its foundations safe and operational from negative effects of water.
Surface water removal is covered in other places in this book under chapters for roads, stormwater systems and airports.
In this section we will focus on underground drainage. They include the types below:
- Consolidation Drains: These are vertically installed drains, to allow water to escape easier from impermeable, cohesive, compressible soils such as clay, during their consolidation, by providing a shorter path for water to escape. In other words, they are installed to make consolidation process (which can take even years) much faster. The drains are made of a permeable inner core wrapped in geotextile drain filter. Sometimes the holes can just be filled with clean sand, as they are permeable and will serve the same purpose. The drains are installed vertically in the ground by drilling narrow deep holes in the ground and then inserting the drains or filling the holes with sand. If drains are inserted, they will be cut at ground level. As seen in the figure below, the drainage path is much shorter with these drains in place, which makes the process much faster. To drill holes to the gorund, relatively heavy equipment is used, but the sites requiring these drains are made of soft soils, therefore it may be necessary to first cover the site with sand, before drilling of holes can begin. After installing vertical drains, at the ground surface, they are connected by horizontal drains, so that the water coming out from the vertical drains can be transmitted outside of the area. After the drains are installed, load can be applied on the area, to start consolidation process and literally squeeze the water out.
- Earthquake Drains: These are also drains made of similar materials as above, but their purpose is different. They are installed in sand soils, which carry risk of liquefaction during earthquakes. As we talked about before, liquefaction happens when water in loose sand does not have time to escape from pores suddenly, and the pore pressures increase, turning the soil into a liquid. Earthquake drains help to prevent this situation by providing easy paths for water to escape during an earthquake, therefore preventing development of excess pore pressures. One more benefit of installing earthquake drains is that the holes drilled to install them also causes some densification for the soil around these holes, which also helps to increase resistance to liquefaction in the first place. So in a way earthquake drains can also be considered a ground improvement method in that aspect, by creating a denser, stronger soil.
Coarse grained soils can be drained by gravity, but for fine grained soils this is not possible. Well points are more suitable for fine grained soils. For very fine grained soils, procedures such as electro osmosis can be more suitable.
Seepage is the process of water flow inside the soil. It mostly depends on the permeability of the soil, which is directly related to soil grain size. We had defined permeability before, under soil testing section. The larger the average grain size of soil, the greater permeability through greater connection of pores. Fine grained soils such as clays have very low permeability, while coarse grained soils such as sands have very large permeability. Gravels for instance, which are even larger than sand particles, are between hundreds of thousands to few million times more permeable than clays.
The term “hydraulic conductivity” is often confused with permeability, due to the two terms’s similarity. Permeability is the property of the porous medium, soil in our case, but hydraulic conductivity is considering the whole system of soil and fluid, and how easily fluid flows through it. So, for finding hydraulic conductvity, density and viscosity of the fluid is also taken into account, in addition to soil permeability. So permeability of a soil is the same, no matter what fluid at what temperature flows through it. But hydraulic conductivity will change with changing fluid properties, even the permeability remains the same. Permeability is largely dependent on the porosity of the soil, because it directly depends on how well pores within the soil are connected. So for example low permeability materials have isolated pores and water moves through them with great difficulty or practically not at all.
To see how we can measure seepage, let’s introduce “water head” and “hydraulic gradient” concepts first.
In the next post of this series, we will discuss “Water Head”