Best way to dig foundations
A Guide to Foundations for New Builds and Renovation Projects
Whether you are planning to construct a new build home or adding additional building projects to your existing property, effective preparation is the key to success. Although many new home builders are heavily focused on the design, getting planning permission, or finalising your choice of contractors, the foundation for your building project is an important aspect of the planning process that should not be ignored or taken lightly. Getting it wrong is all too easy when it comes to foundations, and this could mean additional costs for you right from the start.
Getting the Foundation Right:
Getting the first steps correct not only matters a lot to you, but also to the local council or planning authority, who need to ensure that your property is built on the position that is shown on the approved plans. You and your contractors are the only people who are going to check the setting out at the beginning of the project, so it’s important to take the time to ensure that it is correct. Once you have done this, driven pegs into the corners, and spray chalk lines on the ground between them, it’s time to start digging the foundation trenches.
There are various different types of foundation that are used for different projects and with different types of soil. These include:
Trench Fill Foundation – Best Foundation for Clay Soil:
This type is popular with most self-builders and large-scale construction companies. They avoid the need for laying bricks below the ground. Concrete is poured to up to 150mm of the surface ground level, which makes it a quick and convenient option to get your project off the ground quickly. With this type of foundation, both the sides and the bottom of the trench play an important part in supporting the load. Because of this, this type of foundation is only suitable for use in stable ground where the sides of the trench are firm and able to bear the building load. Chalk or clay soils are just some examples of this type of ground, making them a suitable option for this foundation type.
Strip Foundations – Best for Softer Soils:
Wider strip foundations will typically use less concrete compared to trench fill foundations since the layer of concrete required is thinner. Strip foundations are typically around 300mm thick, but the exact thickness will be determined by a range of factors including the building design, the soil conditions, and the number of masonry courses in the walls. If your project has a sloping site, the foundations may need to be stepped in order to keep them level. When concreted, steps should overlap at least the width of the trench, which usually means shuttering across them. This type of foundation is often needed for softer soils such as sand since they are able to spread the building load out over a larger area.
The Depth for Clay Foundation and Other Types:
The ground type will impact the necessary depth for your foundation. Foundations that are formed in rock and other types of stony ground will often be shallower compared to foundations that are dug in soils such as clay. At least 1m deep is typical for clay foundations, and nearby shrubs or trees may mean that the depth will need to be extended to even more.
The Best Way to Dig Foundations:
In order to dig the foundation for your project, you have the option of either using a hydraulic digger or doing the job manually with several people who can use digging shovels. In some cases, the two might need to be used together, particularly if your foundation runs over narrow or isolated sites that the digger cannot reach. It is important to consider the safety aspects and the risk of trenches collapsing while using machinery. A digger is usually the best choice in the case of large, open foundations that are easy to access.
Digging a Clay Foundation:
It’s important to take extra care when digging foundations in clay soils, particularly if there are nearby trees. Clay soils are made up of around 40% water, however, trees can impact this amount and lead to significant changes throughout the year, which can cause the soil to swell or shrink enough to affect the foundation of the building. Because of this, it’s important to ensure that the foundation is the correct depth to avoid movement.
You can tell if the soil is clay by simply squeezing it. If it does not crumble and stays in the shape that you have made by squeezing it, it is probably clay. However, bear in mind that even clay soil will crumble in very dry conditions. There are three types of clay in the UK, which include:
High Plasticity Clay:
Clays with the highest plasticity and therefore the highest risk are generally found more often in the South East of England, up to the East Midlands, to the northern area of the Humber, and down to Bath.
Medium Plasticity Clay:
Medium plasticity clays are typically found in the South East, the Midlands, and towards the North East of England beyond the Humber Estuary. It can also be found in certain isolated areas in North West England close to the coastline.
Low Plasticity Clay:
The rest of England and Wales generally has low plasticity clays, however, they still carry some level of risk, particularly where trees are present.
Minimum Depths for Clay Foundations:
Seasonal changes in the environment will have an impact on clay soils, causing them to shrink during the summer and swell during the winter. As a result, there are minimum foundation depths required for each type of clay soil in order to ensure that there is room for this expected movement, which will not cause damage to the building. Strip and trench fill foundations in clay soil must be dug at least 750mm in low plasticity clays, 900mm for medium plasticity clays, and at least 1000mm for the highest plasticity clay soil.
How Trees Impact the Clay Foundation:
The minimum depths for clay foundation digging are not usually enough when there are trees present in the area. During warm weather, trees derive moisture from the clay, which causes the soil to shrink and leads to settlement and cracking issues in properties that have a narrow foundation. In normal weather conditions, the clay will adapt to the way that the trees nearby are using water and will maintain a steady amount as the trees draw out the amount of moisture that they need.
Cutting down the trees is unhelpful since removing a mature tree leaves the clay unable to contain the excess water, causing it to swell. In some cases, this could be with enough force to lift concrete ground floors and significantly move foundations, which can cause serious structural damage to the property. As a result, the only solution is to dig a deeper foundation to below the area that is affected by trees.
Can Bamboo Damage House Foundations?
Homeowners are receiving warnings about planting certain types of bamboo in their garden. Even if a foundation is dug to below the area affected by trees, certain types of bamboo can spread rapidly and become a threat to the foundation of your home. Bamboo that is linked to the notorious Japanese knotweed can be so destructive that some mortgage lenders are refusing to lend against homes where the plant is present unless there is a plan in place to remove it. The plant has a very invasive nature, shooting up quickly and encroaching on the garden towards the house, which can cause damage to patios and foundations.
What is the Typical Victorian House Foundation Structure?
If you live in a Victorian home and are planning to extend it with building, it’s worth understanding the foundations used for properties in the Victorian era. During this period, the use of foundations for property stability was not fully underwood. As a result, the vast majority of homes were built on a foundation that was simply laid in a shallow soil trench.
Stepped foundations were the most commonly used method for Victorian homes. Solid brick walls, and later on early cavity walls were built on top of this. Foundations, as they are understood today, were not present during the Victorian era, which is why older houses tend to move more throughout the year and homeowners in these Victorian properties will often deal with issues such as mortar bed widening or slight cracking.
Despite foundations that have their faults, Victorian houses are still standing after being built between 1837-1901 and many still have their original features, making them a popular choice for home buyers. A Victorian property that has been well-maintained will be worth the money, however, it is a wise idea to have a full structural survey carried out before buying or extending a Victorian home to discover any foundation issues that have the potential to cause damage.
The foundation is the most important part of the building process. Whether you’re looking into building your own new build property or renovating an existing one, understanding the best foundation type and depth is crucial for preventing future damage.