As part of the RICS construction technology we (land surveyors and quantity surveyors) need to know a number of basic construction technology. In this series of blog posts we’ll be covering the substructures.
Buildings are separated into two parts: the part generally below the ground floor (substructure) and which extends down into the ground, and the part above the ground floor. This blog post covers the substructure, from ground conditions to building construction from the foundation below.
The classification of ground conditions
When we think of ground conditions, they’re usually classified as either topsoil or subsoil. Organic matter is a big part of topsoil, which also has a high concentration of insects and worms. Vegetation grows in topsoil, and that’s what people usually refer to when they say “topsoil.”
What is Topsoil and how do you deal with it
Below the layer of topsoil there is a thin layer where both topsoil and subsoil mix. It’s considered “topsoil” for our purposes in building construction. This transitional area marks the meeting between the two types of soil.
The layer beneath the topsoil is without any organic constituents, although this layer might be home to living creatures that burrow below the soil.
Building Regulations require that soil beneath the building be completely excavated. The consequences of not doing so vary depending on a range of factors, but are mainly based on three reasons:
- Topsoil isn’t strong enough to hold up buildings, and it can’t keep them anchored in place.
- Topsoil contains organic matter which if left under the building will rot and cause a health hazard. Rotting vegetation also attracts vermin, especially insects, and this can be a source of disease, among other things.
- Surfacing the area will remove all roots, bulbs, corms, seeds or tubers that are left behind. Roots can penetrate drainage and ducting systems and block supplies or discharges.
The thickness of topsoil in the UK
With any building, the thickness of the topsoil will always affect the cost. Remember, you have to remove thicknesses, which makes things more expensive. Around the UK, there’s a general thickness of 200mm at ground level where farmers have cultivated it and a depth of 100-150mm in untouched areas with good vegetable growth. Obviously, there are exceptions—areas where soil heath is prevalent and large areas with peat or little growing on them. But as well as these less-than-ideal conditions, you’ll find demolition sites (where typically there is little or no topsoil) and dumping sites that result in depressions (where there can be less than usual). Still, we won’t go into foundations for these extreme conditions but will stick to discussing simple topsoils over stable subsoils.
One of the difficulties with drawn presentations is that many of them are designed without thinking about what site or situation they will be used for. It then falls to the designer to make assumptions about a ‘standard’ which can be seen by everyone wishing to interpret the drawings. This idea of having a standard will come up again as we progress through the posts on this blog. The ‘standard’ thickness of topsoil is typically 150 mm, so this will be how we represent measurements on drawns in our presentation.
When the majority of any structure is being built, it’s typical to remove the topsoil from the immediate area of the building. Excavators (360) are so much cheaper and easier to work with now. So instead of removing the soil in a small space, it’s more common to remove the topsoil across a larger site and stored aside or taken offsite.
The topsoil is often needed for garden ground around the building, so you must find a way to preserve it. (And there is a special way of preserving and storing them on site which is detailed in the British standard (3882:2015)).
Spoil and taking it offsite
Any excavated material is called spoil and can be disposed of offsite or placed onsite.
Topsoil can be removed from the site as long as it’s disposed of, stored correctly.
Topsoil should be immediately spread around the site being worked on when performing this process. Bringing the surface to a desired level, this step is typically called “spreading and levelling”.
The spoil heaps for topsoil shouldn’t be higher than two and a half metres. Storage in higher heaps can cause a problem for the soil in the long term and kill the bio-organisms living in there for the plant.
Three types of subsoil
There are three types of subsoils:
- These soils are able to carry the weight of a low-rise building without any special construction techniques or precautions.
- Those who are able to manage the structural load of a much larger building with efficient foundation techniques.
- Unique circumstances require special techniques or precautions for even the most basic of structures.
We will focus on the first type. But first, let’s look at the soils. The soil we’re talking about is called subsoil. It doesn’t start at a dividing line separating it from the topsoil rather, there is a gradual transition between one and the other and it generally occupies a layer 50-75 mm thick. This transitional layer isn’t an ideal place to build on, so foundations are placed in the subsoil rather than above it
So, is there a standard thickness of subsoil? Again, no. There’s no end to the subsoil because it should theoretically be from the surface all the way to the earth’s core. But for practical purposes, it usually only goes so deep into the crust. Is there a standardized type of subsoil? No. Subsoil varies all over the world according to what soil that can be found at different depths with various thicknesses.
Regardless of the thickness or density of the soil, it is important to keep loadbearing capacity in mind for individuals placing foundation. Loadbearing capacity can be defined as the force that acts on a unit area which will cause a foundation to fail. To be safe, we need a margin of safety and arrive at what we assess to be the safe loadbearing capacity. There are many factors that go into determining loadbearing capacity, not all of them applicable to every kind of soil:
- Kind of soil
- The thickness of the layer of subsoil.
- Kind and thickness of layers underlying the subsoil
- The depth and thickness of soil layers
- Moisture content and water level
- Degree of containment of the layers
- The presence or absence of underground flowing water.
In order to determine the loadbearing capacity of a foundation when constructing walls on a building site, the general approach is to classify the subsoil into various types. Designs have been greatly improved since the early 20th century, and new books generally contain an approximation of this capacity in kilonewtons (kN) or newtons (N). This capacity is adequate for common work that includes foundations as long as it’s adjusted to safe loadbearing capacity. Thinner soils or layers with lower capacities may require professional intervention from soil mechanics specialists. These specialists are available to inspect the subsoil at your building site and analyze its strengths and weaknesses by testing for loadbearing capabilities.
The categorization of subsoils and their loadbearing capacity
The Building Regulations generally have a lot to say about loadbearing capacities. In this case, the Regulation takes the form of reference tables for foundations that identify a range of widths for strip foundations in various mixtures.
Soil type can be determined on the basis of whether it is gravel, sand, silty sand, clayey sand, silt or rock. The condition of each soil type can assessed with regard to its compactness (compact or stiff), firmness (firm or loose), and softness (soft or very soft).
Only rock foundation strips are allowed to be as wide as the wall itself. Other foundation services must meet specific minimum widths depending on the subsoil and its condition. The widths allowable are noted underneath our table of readings.
Here are some tables that have been used in building research to develop tables for optimum foundation widths. Although they vary slightly, they’re all based on Building Research Station Digests 64 and 67 published in the 1970s. For example, one table quoted in some Regulations gives a width of 400 with a wall load of 40 kN/m^2. If you take the time now, you’ll see that this equals 100 kN/m^2. This is despite the fact that maximum loadbearing capacity is up to 600 or even 1000 kN/m^2 in many textbooks! Approved by rule of thumb cited above, calculated safe bearing capacities would be 150 and 300 kN/m^2 respectively, which indicates that the figures quoted in the Regulations for foundation widths and safe bearing capacities have a much higher margin of safety.
Now that we’ve looked at the properties of different soil types, it’s time to look at foundations. There are a wide variety of loadbearing characteristics in the soil tables mentioned earlier, and you’ll need a few techniques to build on different ground conditions. First, you should know that most foundations are made by pouring wet concrete into holes drilled in the ground. The shape can be made as simple or complicated as you require, and everything in between. You’re only concerned with straight pieces of concrete cast into the ground that present a flat surface where walls can be built upon. Concrete is discussed more thoroughly in Appendix D, which should be read if this chapter gets too technical for you.
If we were building with a rock that has simple foundations, the foundation would be rock solid. No elaborate foundation is needed. All you need to do is level out the ground and build your frame or walls straight off of the ground. Strip foundations are often used for medium-sized buildings that stand on stable ground and can withstand natural earth movements. In contrast, deep strip foundations are used for larger buildings that need more support against forces that could move the base of the building away from its walls
In strip foundations, a wide trench is dug with walls built off of the layer of concrete. In deep strip foundations, a narrow trench that’s not much wider than the wall is dug, and then filled up with the concrete almost to ground level. We’ll examine these two types of foundation in greater detail later in this chapter. The other extreme would be soft ground overlying a firm strata with good loadbearing characteristics.
When it comes to building a structure, an engineer must be aware of the type of ground (e.g. crystalline, sedimentary) that is supporting the building and choose appropriate construction methods and materials accordingly. In cases where there are large piles present in solid soil strata beneath the building site, they can be driven until they reach the bearing layer. Once this has occurred, beams can be cast into these piles to form the foundation for desired buildings. However, if there are high concentrations of waves or slides present beneath the site, this process would most likely prove ineffective. For this case we could use the raft type of foundation instead — which would involve placing a thin layer of concrete over areas that requires support for the building and then hoisting them up higher so that water filters through and carries away any rough spots below it. If this is not a feasible option or something you feel like doing yourself, then piling might be another great alternative. This involves driving long columns of materials – usually concrete – into the earth to ensure a stable foundation can be built on top of it afterwards; just make sure to keep in mind that you need to know what type of ground (e.g. crystalline, sedimentary) you’re working with before building the foundation.
I will cover the different types of foundations in another post.
The principal considerations of installing foundations
Building foundations are designed to provide a base on which the building can be:
- built so that it won’t sink or cave in.
- Stop a building from being lifted off of the ground
- Both mentioned above
The regulations state that any movement in the soil cannot compromise foundations on this type of land. The most common causes of this form of foundation instability are groundwater seepage and shifting soil layers.
(1) it swells or shrinks, causing the building to be displaced. Plants take up a great deal of moisture from the ground and in transpiration this passes into the atmosphere. Trees are major water users and many species such as poplars and willows use more than most. This allows them to extract moisture from clay during drought conditions, which can cause clay to shrink. Trees should be kept away from buildings or vice versa when an encounter with shrinkable clay is present.
(2) The subsoil is moist; because of this, it freezes and causes your building to be unstable.
We’ve now learnt about the two different types of soil and covered the three types of subsoil. If there are any issues or have a question please leave your response in the comments below.