Geotechnical engineering for stable sewer systems
Concepts from soil mechanics provide clarity.
The soil makes the differenceClay, 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.
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 pressureThe 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).
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.