When constructing any size or style of building, a suitable foundation must be established at the very beginning of the project. Many people are unaware that the ground that we walk on is made up of different layers, with the specific make up in the materials and arrangement of these layers varying based on location. When establishing the foundations of a building, the structural specifications are calculated by an engineer based on the load distribution requirements in combination with the conditions and make-up of the ground. In fact, there are wide range of foundation styles that are each suited to applications pertaining to the various types of ground conditions.

There are also many unknown factors below the surface that make calculating an accurate cost for this type of work difficult. Once the foundations are poured and the oversight is established, the approximate cost of the remaining works can be estimated through a square metre formula. For large scale construction, extensive surveying is undertaken before work begins to ascertain the conditions of the ground and criteria for the groundworks. For small scale residential construction, there is often no allowance in the budget for extensive surveys to take place, with the majority of the investigations taking place as the groundworks are carried out. As such, unfavourable ground conditions, as well as unforeseen issues under the ground can become apparent during the digging process, quickly raising the cost of carrying out building works. Underground cables, water supplies, drainage, tree roots, and debris are just a handful of the considerations that affect where and how we can carry out groundworks.

Existing utilities to a property or site are typically mapped on the drawings that are held by local councils. These drawings are referenced by architects and engineers when plans for new builds and extensions are drafted, providing the approximate location of existing utilities for the contractors. However, this only applies to known utilities. Properties that have undergone DIY or unplanned works may contain unmapped underground utilities; a cause for concern when undertaking groundworks.

Areas that consist of “built up ground” can also cause issues when carrying out groundworks, a scenario that is relatively common in the UK and other dense urban areas. Especially in the context of renovation and extension works, it's not uncommon for existing ground to be built up with manmade materials. If an area of ground has been excavated in the past for building works, with the building being subsequently demolished, it's likely that the ground is going to contain rubble and rubbish. It's also not uncommon for materials leftover from previous developments to be buried intentionally to either build up height as part of a landscaping endeavour, or to dispose of the waste. This scenario can often be found when excavating the ground around large developments constructed in the last century or so, with gardens and oversights being full of brick rubble. For footings that are already laid in ground like this, the repercussions are typically minimal, with little to no cause for concern provided that the original ground before excavation was of good quality. An issue arises when excavating built up ground to carry out new construction works.

In good ground, when a trench is excavated for a footing to be poured, the sides of the trench stay intact up to a certain depth without the need for support. Even poor-quality original ground will remain in place provided adequate supports are installed. When it comes to digging in built up ground, maintaining the correct dimensions of a trench is tremendously difficult. As the trench is excavated, the loosely compacted rubble and soil continuously collapse inwards, as there is no body to the ground. Due to how loosely formed this ground is, even the installation of supports to define the edges of the trench can be futile. In addition to this, the required depth of the trench is increased, due to the fact that the ground has been previously disturbed. For an effective footing to be poured in conjunction with the structural requirements of the building, the bottom of the trench must reach undisturbed ground of some variety.

With the above information highlighting the impact that the ground conditions have on the foundations of a building, we will take a quick look at the different types of soil and their properties, as well as a look at the common soil layers. 

Starting at the top of the diagram, the first layer of soil that we can see is the organic layer. This layer is typically only a few inches deep and provides nutrients to the shallow roots of grasses and small plants that grow out of the ground, as well as providing a loose and suitable habitat for burrowing insects.  

Below the organic layers is the topsoil - more nutrient dense and well-draining soil. This soil layer may also contain insects and roots from small plants. The soil itself is often easy to dig by hand and is not stable enough for structures to be erected on top.

Below this topsoil is the subsoil, a more compact and dense soil layer. Larger roots from trees are likely to be found in this more stable soil, providing adequate stability to the parent vegetation. This soil layer is often relatively deep. Despite its more stable characteristics, residential buildings are still unable to be erected atop it, though the preparation of the foundations for lightweight structures such as sheds or summer houses can begin at this layer.

Below the subsoil is an earthen layer known as the parent material. This layer is far more compacted, typically consisting of compacted soil and stones, and is much more difficult to dig by hand. In most instances for small scale residential construction, this layer is suitable for pouring foundations. More often than not, the top of this layer is found at around 1m or more below the common ground level, which meets the foundation depth regulations. This stoney compacted layer features the “parent” material of the less compacted soil layers above, whether it be soil, sand, gravel, or clay.

Below this hard compacted layer is the bedrock - a solid rock layer. For large scale construction projects that require immense load bearing capacities, this layer is often used for the pouring of foundations. Alternatively in other poor ground buildup scenarios, large piles are driven down through the less stable layers until they hit the bedrock layer.

The specific arrangements, thicknesses, depths, and material buildups of these ground layers are very dependent on the geographical location. Some areas feature incredibly dense rocky ground a short distance below the surface, whilst others feature very deep layers of soft subsoil. Regardless of the exact ground makeup, suitably stable ground must be found for regular strip foundations to be poured, else other options such as concrete rafts or piles must be implemented.