Geotechnical engineering for stable sewer systems

Concepts from soil mechanics provide clarity.

A below-ground sewer system, or parts of it, will unavoidably be exposed to a range of pressures in varying directions. What these pressures are, and how large they are, is entirely determined by the local situation. At what depth is the system? In what type of soil? How high is the groundwater table? Is there a road on top? All of these geotechnical variables play a role. Basic concepts from soil mechanics make it possible to perform the right geotechnical engineering calculations where needed.

The soil makes the difference

Clay, earth, loam and sand: under the microscope they all consist of separate particles, but their behaviour as soil types is totally different. Figure 1 gives an impression of the multitude of minute forces that are at work at the grain level, demonstrating that it is impossible to capture them in precise calculations. That is why in geotechnical engineering we use two characteristics of the soil material to calculate the effective pressure in the soil: the soil density and the internal shear angle.
 

Figure 1. An impression of the multitude of forces impacting every single particle in the soil.

The density or specific mass varies between 1500 kg/m3 for dry loam and 2260 kg/m3 for wet sandy loam; the internal shear angle is 23° for wet clay, for example, and 45° for humid earth. We will see later what role these two variables play.

Contributions to the earth pressure

The vertical stress on an underground element may consist of one or more components. The first is, of course, the weight of the total soil mass straight above it: with a completely dry soil, this is the product of the density and depth. This weight increases in direct proportion to the depth (see figure 2). If at the surface a pressure is applied to the soil, this is passed on downwards in its entirety and consequently has to be added to the weight of the soil (see figure 3).
 

Figure 2. The weight of the body of soil above an underground surface is directly proportional to the depth.

Figure 3. The weight of a load at the surface is added to the weight of the soil.

Figure 4. Saturated soils have higher densities than dry ones, so at any given depth the earth pressure is also higher.

If a component is in the groundwater, this results in an upward pressure, as described by Archimedes' principle. If that were the only force in effect, a sewer pipe would float up in the groundwater. This never happens, however, because the design of a sewer system is based on geotechnical engineering principles, ensuring that the total vertical downward pressure is more than sufficient to compensate for the theoretical propensity to float. 

In addition to vertical stress, there may also be lateral forces in the soil, in situations where the volume of soil has room to move outwards. Think for example of a mound of sand: when sand is added at the top, the mound maintains a cone shape because of material sliding to the side. According to soil mechanics, the shear angle mentioned earlier determines under which slope such a cone forms. In our case the shear angle affects the horizontal pressure that the soil exerts on a sewer element. Like before, this horizontal pressure increases with 

 

Traffic also plays a role

If a sewer is under a road, as is often the case, it is obvious that the weight of traffic can be an important factor in the pressure exerted on the system. Under its wheels, a fully loaded truck causes a considerable 'point pressure', which is absorbed and distributed by the soil. The formula of Boussinesq is an important tool for calculating what is the effective pressure of a certain traffic load at a certain depth.

 

Thorough calculations guarantee a stable position of each sewer system

With the available geotechnical engineering knowledge, it is possible to calculate for each situation which precautions must be taken to ensure that a sewer stays in place permanently or does not move beyond the tolerances of the system. The above overview shows only very superficially what is involved. For a somewhat more in-depth discussion of this subject, a download is available in which the underlying mathematics are also discussed by means of formulas.