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Soil Testing

Soil Tests, Ground Strength and Soil Types

When constructing a building it is very improtant to check the strength of the ground before construction starts, this is particularly important when building on clay, sand or alluvial soils. The leaning tower of Piza is an example of what can happen if the ground is not strong enough to support the building. Here we explain what soil tests are, how they are carried out and the different types of ground that might be found.

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soil test bore sample

Investigating the ground before you build

Soil tests are very important for anyone wishing to build a building. You see the most important part of any building is, in fact, the part you cannot see, the bit under the ground, the foundations. The larger the building the more important this is.

The first thing required before designing any building is an investigation of the ground to determine what sort of foundations are required and indeed what sort of building can be built. The Leaning Tower of Pisa is a case in point. Giovanni Di Simone was only going to build a bit of a bandstand, they thought it would keep him out of trouble for a while, no one ever expected him to go up 8 floors (that was a highrise back in 1178!) checking the ground first might have been useful. It has only a 3 metre foundation in a weak unstable subsoil. Mind you nearly 900 years isn't too bad is it?

What are soil tests?

So what are soil tests and why are they carried out? A soil test is a scientific examination of a piece of ground to determine how stable it is and what sort of weight it can support. Samples are taken and tested to determine the weight, density, water content, shear strength, elasticity, the nature of the particles and of course, the load bearing capacity of the earth. Bores are sunk to extract cores so that the soil can be examined and tested at different depths and in different key locations on the site.

A comprehensive plan of the nature of the ground across a building site is compiled. From this information structural engineers can calculate the forces involved and design suitable foundations for the building that is to be built.

Soil tests are becoming more important as the nature of buildings change. Traditional Balinese houses tend to be relatively lightweight single storey structures. These days more and more people are building heavy multistorey buildings and even apartment blocks that are far more susceptible to damage or collapse from unstable land or earthquakes. (We must always keep earthquakes in the back of our minds when designing buildings in Bali – it is only a matter of time).

The nature of the ground anywhere varies enormously but let us consider five different types of ground that may be encountered in Bali.

Volcanic Rock

Volcanic rock varies from very light crumbly pumice to very hard basalts and granite. Basalt and granite are probably the strongest and most stable foundations you could find anywhere. Of course many volcanoes here are live and the best idea is to have the basalt under your house not on top of it. Perhaps volcanoes provide a new marketing angle for real estate salesmen - “Adventure Living”.

Sedimentary Rock

The southern hills of Bali (the Bukit) are made up of limestone which tends to be very stable and provides a good foundation. It is very soft and crumbly as limestones go and is in fact more of a chalk. True limestones tend to be hard (though nowhere near as hard as basalt). Limestone tends to be porous so, without a layer of topsoil it is usually very dry.


In the coastal areas such as Sanur, Nusa Benoa and Kuta you may find the ground near the beach is sand. Sand is good for building on and can be very stable, as long as it is contained. If it is not contained it can, of course, be easily washed away by heavy rains or high tides.

Alluvial Deposits

Look on a map and see if the area is a flood plain. Note that flood plains do, from time to time, have a tendency to flood. If the area is not very high above sea level and may be susceptible to Tsunamis. Land of this type varies considerably, some is fairly stable and is relatively straight forward to build on provided that foundations are properly designed. Other areas are clay which can be challenging.


Former Rice fields need special care. Rice fields, of course, are required to hold water and so are made of clay which is impermeable – it holds water. The clay is formed into rectangular basins which have a layer of mud in the bottom in which the rice is grown. Many, many buildings here are being built on the clay of former rice fields. Clay is a tricky material to build on because it changes it's nature between the wet season and the dry season. When it absorbs water it expands as the water content increases, when it is very wet it may be the consistency of toothpaste, a heavy building can squeeze it out. When it dries out it shrinks and large cracks can form in it. This is known as expansive clay.

This expanding and contracting of clay becomes a particular problem under the floor of a building. As the wet seasons starts the ground around the edge of the floor start to become saturated and expands while the ground under the centre of the floor is still dry and contracted. Gradually the water permeates into the area under the centre of the floor. Then as the dry seasons starts the effect is reversed with the ground around the edge starting to dry out and contract while the ground under the centre is wet and expanded.

This process puts enormous stresses on the structure of the floor slab and is why a common building foundation method used in Australia, a raft slab, is not suitable for the expansive clay of former rice fields. A raft slab, as the name implies, is a concrete slab that “floats” as a self contained building platform on the surface of the ground.

Building on former rice fields

With former rice fields it is particularly important to carry out soil tests to determine how far down stable ground can be found and the nature of the clay in between.

If stable is a long way down reinforced concrete piles may be used. if stable gound cannot be found special foundations may have to be built or lighweight buildings used.

Building on filled land

A final but important consideration is whether land has been filled. Most sites are not level and in the course of levelling some earth is cut out from one place and used to fill another. Unless the earth has been fully compacted or left to settle for years the filled areas will be softer and weaker than the cut areas. This is a very important consideration in designing foundations and the structure of the building.

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Copyright © Phil Wilson June 2009
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