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Constructing Foundation on Sloping Grounds

Foundation is an important part of the structure as it transfers all the loads from the structure to the soil and provides stability to the structure. A foundation provided as a greater depth is able to give lateral support, resist forces and carry a huge load. However, there are some soil factors and ground conditions that affect the depth of the foundation. They include:

  • Bearing capacity of soil
  • The allowable bearing capacity of the soil affects the depth of the foundation. It means there is need for soil improvement or increase in depth of foundation.

  • Change in clay materials
  • For a site that is more of clay, changes in clay materials such as shrinkage and swelling of particles occur due to seasonal weather changes. It is very important to identify the zones where these happen and perform penetration of the foundation below it.

  • Frost action
  • This happens on sites with fine sand and silts. In this case, foundation must be constructed to penetrate the zone.

  • Maximum depth of scouring
  • In construction of foundation for bridges and piers, the depth of scouring must be taken into consideration. The foundation depth must be below the scouring depth.

General Foundation

Usually when the foundation is considered below the natural ground level, it is penetrated down to a minimum depth of 0.5m. So the penetration must go beyond the depth of fill if the site is a filled-up type.

Foundation for Sloping Grounds

Building on a sloping ground comes with its peculiarities and challenges even though it offers enviable benefits when it is completed. The owner will enjoy the views, the landscaping, natural lighting, the seclusion and the space. This explains why some of the most coveted homes in the world are built on slopes. However, it requires a complex foundation systems which eat deep into construction time and lead to additional cost. The cost is often more than that of an entire house on flat ground. It requires more concrete, deeper excavation by specialized excavation or blasting, retaining walls or terraces and specialized solutions for drainage and septic systems to ensure that the house is up to code and safe to inhabit.

[READ THIS: General Safety Measures on Construction Sites]

How to Construct Foundation on Sloping Grounds

The two method used in construction of foundation on a sloping site are the cut and fill method or the use of stilts.

  • Cut and fill method

    This is the process of carving out a level plinth on a sloping site, in order to build a home that is essentially designed for use on a level site. The spoil that is cut from the bank is reserved in order for it to be brought back to make up the levels on the lower edge. 

    As noted earlier, the slope of the land and the requirement that the foundations should find original subsoil bearing increase the foundation costs.

    Provision will have to be made for the oversite level within the building to be brought up to within 600mm of the proposed external soil level, in order to equalise the pressure on the walling if the spoil is piled up against the lower or built up section of the new home.

  • Use of Silts
  • The use of stilts is an alternative to cutting into the slope that involves using a crane to lift the home onto supporting wood or steel columns. This can be a much more cost-effective method than cut and fill. It can open up the realm of possibilities for home placement, such as building out over trees or water.

Recommendation for Construction of Foundation on Sloping Grounds

While constructing the foundation on a ground with slope, the distance from the edge of the footing to the ground surface is taken as:

  • The distance between the lower edge of the footing to the sloping surface should not be less than 1 m for soils and 60 cm for rocks.
  • 90 cm for soil
  • A line drawn at an angle of 30 degrees with the footing base must be parallel to the slope of the ground. This means that these lines must not intersect with each other.

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