Substation Sub-Surface Engineering: An Overview of What a Geotech Survey Reveals

Constructing a substation on an empty piece of land is not as simple as laying foundation and erecting steel structures. You have to consider the soil condition at the site before building anything. Here is what you need to know:

 Soil Analysis

Some soils perform better in providing structural support than others. They accept more structural load without settling due to compressive forces or change shape based on its plasticity. To better understand various qualities of soil, the United Soil Classification System (USCS), among other standards, has classified soils under three broad categories with consideration to their percentage of gravel/sand/fines in the mixture, shape of grain, and plasticity/compressibility characteristics.

So, in the order of good to bad soil, here’s the list:

  1. Coarse Graded Soils: mostly gravel with little sand
  2. Fine Grained Soils: silts and clays
  3. Highly Organic Soils: peat and other decaying matter

For structures with significant load inside the substation, coarse grained soil provides best support when properly drained and well confined. For silty locations, the soil needs to be tamped and compacted using heavy machinery. When organic soil is found, the only remediation is to replace it using imported soil. Therefore, depending on the location, earth work can add significant cost to the substation design.

Water Retention Capability of Soil

Storm water is another issue requiring attention while grading. Standing water inside the substation can disintegrate foundations. Therefore, it is necessary to grade soil such that the storm water can run-off to detention ponds or culverts outside the substation. Grading is necessary if the station has cable trenches, manholes, or other sub-surface recessed areas. Installing coarse grained soil around these structures will allow the water to drain away from them.

Soil Erosion Issues

Preventing soil erosion during storm-water run-off is another challenge. The loose soil revealed during excavation can get easily washed to the nearby surface waters and contaminate it or they can settle in residential areas and ruin the landscape. Fortunately, there are many measures one can take to prevent soil erosion. One of the most common method during construction is to maintain a little bit of vegetation (if not plant them) at the site or install silt fences at locations involving steep terrain.

Other Issues Explored in a Geotech Survey

The subsurface work discussed in this article represents some of the important items to consider while scoping a project. Typically a geotechnical study by an engineer determines the soil conditions by analyzing the cross section of the soil bored out of earth. It is not uncommon to see a geotech report highlighting the following issues:

  1. Site accessibility.
  2. Identifying underground utility lines, fiber optic cables, pipes, etc. that can halt the grading work. Permits need to be obtained to work around these.
  3. Seismic conditions at the location.
  4. Topography.
  5. Climate conditions.
  6. Ground resistivity readings (before and after grading for ground grid design).

So the next time you plan on building a new substation, make sure you get a geotech survey done early and proceed with design work accordingly. If you are expanding an existing station then take a peek at the proposed location. Does it have a steep terrain? Is the soil soaked? Are there boulders and big rocks that won’t budge? All these factors will affect your decision on choosing that location.

3 thoughts on “Substation Sub-Surface Engineering: An Overview of What a Geotech Survey Reveals”

  1. Taymor,
    Consider this scenario:
    A generator produces power at 13.8kV. The transformer connected to this generator has 15kV/138kV voltage rating. So in the equation (3), the voltage ratio to calculate %Z using the base values of S and V on the primary side of the transformer will be:

    All I am emphasizing is, you do NOT calculate the %Z by assuming that the V_old=15kV and V_new=138kv, both of which are transformer ratings.


  2. Dear Aleen,
    Question refers to your one-line power diagram posted on your web. Could you please explain the (V-old/V-new)^2 and the system nominal voltage in calculating the Xt1, Xt2, Xt3 and Xt4 of the example?

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