Site investigation and soils
The construction of a building is a very expensive undertaking. It is, therefore, essential that the ground on which the structure will rest is thoroughly investigated.
INVESTIGATION saves on money and time which would be required to repair the cracked building. Most defects and structural failures in buildings are as a result of not properly carrying out preliminaries and site investigations. This article shall seek to explain what is contained in the Model Building By-laws Chapter 4 Section 2 and Schedule.
Investigating a site
In some cases due to knowledge of sub soil conditions in the locality of the proposed building, the local authority may permit the erection of a building without a site investigation.
When required by the local authority, the site investigation should be done from the point of view of soil conditions which will affect the foundations. To sum up, site investigation is a thorough study of the nature and type of the soil
◆ Position and existence of underground and overhead installations e.g. electrical cables, drains etc.
◆ Slope of the ground
◆ Existence of obstructions e.g. boulders, big
trees existing over site, etc.
◆ Position and existence of adjacent buildings
i.e. if it is a built up area
◆ Position of local authority building line and benchmarks and many other site conditions which affect the proposed building The investigation must be done by someone qualified and experienced to do so. Before preparing working drawings, an architect carries out a site survey. Also before preparing a tender, a contractor visits the site.
The local authority may require that a copy of the site investigation report be submitted to them and should include one or more soil profiles.
Knowledge of the nature and type of the soil enables both the architect and the contractor to determine the type of foundation to be used; the methods of digging and supporting the trenches and the materials to be
used to erect the building. In addition site investigation by the contractor enables him/ her to:
◆ Plan the operations properly and correctly ◆ Identify site problems and solve them
before work commences on the site ◆ Price the bills of quantities more accurately ◆ Identify building bye laws affecting the site and avert locking horns with the local authority during the construction stage.
Soil
All building structures rest on soil. It’s therefore important to determine the nature and type of soil which helps establish the soil bearing capacity. Soil bearing capacity is the soil’s ability to sustain and distribute loads without any appreciable settlement. Settlement is simply the sinking of the building i.e. the difference in the level of a building at the beginning of the works and it’s level at the completion of the works. Soils have different bearing capacities and this depends on ◆ Type of the parent rock
◆ Arrangement of particles
◆ Particles composition
◆ Moisture content
◆ Amount of organic material e.g. humus and vegetable matter which they may contain
The more compressible a soil is, the lower it’s bearing capacity and the less suitable for construction work it is.
Soil profile
This refers to the vertical section of soil from the ground surface to the parent rock. On the building site the soil profile should be ascertained and recorded. Every soil profile should be recorded from data obtained from a freshly excavated trial hole done to a depth sufficient to ensure that the whole depth of subsoil which will be affected by the building has been adequately examined.
Each layer/stratum in any profile is described in terms of colour, consistency, soil structure and soil type. The level of the permanent water table and any perched water table which may be present can also be recorded on each profile.
Top soil — which usually occupies a stratum 150mm in depth from the top - is excavated from a building site before any construction work can commence. This is known as a reduced level. The soil below the
topsoil, known as the subsoil is the fairly firm layer where excavations for foundations to carry a building are done. This soil should be tested for compressibility and shear failure. Shear failure is the sliding of the soil under a load, owing to a weakness or failure. To investigate different soil strata, trial holes or pits should be dug
◆ Trial pits: these permit in-situ visual inspection of the soil strata. The number of pits, sizes and depth depends on the type of structure to be erected and the site condition e.g. 600mm wide by 1200 long and 900mm to 1200mm deep ◆ Handboring: this is usually ideal where completion of a trial pit is difficult owing to presence of water. Handboring allows for the inspection of the soil strata and the results are the same as that for trial pits. In soft soils, handboring can be done to a depth of even 9 metres.
Soil consistency
This is a measure of the hardness of the soil in it’s natural state. Soils fall into two main categories
◆ Cohesive soils: generally slow draining soils such as silts and clays; combinations of silts and clays with sand and gravel
◆ Non cohesive (granular) soils: generally free draining e.g. gravels and clean sands In putting the grades in the groups, moulding means the moulding of a fresh sample taken from a trial hole and the freshly exposed surface is the surface within the trial hole. The table of grades can be found on page 7 of Chapter 4 in the Model Building By-laws.
Various tests can be carried out to determine the shear strength of cohesive soils e.g.. Vane shear or Dutch test; penetration tests; compression tests; and geophysical tests.
The results of the tests help in the designing of sub structures for such soils. Failure to carry out the tests during site investigations may result in great financial losses because of repairs which would be required after the structure has developed some cracksman.
Soil structure
Soil structure refers to the description of the jointing condition in the natural soil. The structure may exist in different structural forms e.g.
◆ Intact — an absence of fissures or joints ◆ Fissured — presence of random closed
joints. When cut with a pick, the soil tends to break along these joints
◆ Shattered — indicates fissures in which joints have opened up and are filled with air. The soil fragments are usually stiff or very stiff, and break out Ina cubical or granular fashion when the soil is cut with a pick. The fragments break down with difficulty when wetted and worked in the hands
◆ Micro shattered — shattered extensively , with the shattered fragments of the size of coarse sand grains
◆ Laminated or foliated — indicates that the soils show the laminated or foliated structure of the parent rock from which they are derived.
Knowledge of soil structure helps to determine what type of foundation to use for maximum stability of the building
Soil types
Soil type is a description mainly according to the grain size of the soil. Most natural soils are a combination of one or more of the soil types e.g. silty-clay is a clay with some silt.
The basic classification of soils into gravels, sands, silts and clays relate to the drainage characteristics of the soils.
All clays and most silts are slow draining soils. On a building site, the contractor is likely to encounter any of the following types of soil ◆ Fresh rock or bed rock: means a natural geological material which has not been weathered and normally requires blasting for breaking in excavation. This forms one of the finest natural foundation for heavy buildings provided that it covers the entire area on which the building will rest. However, where one part of the building is on bedrock and the other on softer material, differential settlement will occur causing the walls of the building to crack ◆ Decomposed rock: means natural geological material which can be excavated without blasting. Such decomposed rock should be classified and described as a soil ◆ Boulders: fragments of rock larger than 150mm in cross section. Fragments can be rounded, subangular or angular and may have a range of sizes
◆ Gravel: fragments of rock measuring between 3mm to 150mm. Gravel is a mixture of stones and sand. It gives the second best foundation from bedrock. Gravel is considered the most hygienic soil to build on because it’s easily drained
◆ Loam: made up of a mixture of clay and sand with other organic materials. It provides a good foundation for building both small and heavy buildings
◆ Sand: means discrete particles which are clearly visible to the naked eye. Sand is usually classified as river and pit sand, and both provide a fairly stable foundation for buildings except when they contain excessive water.
◆ Silt: means soil having particles smaller than 60 micrometers but larger than two micrometers. In general, silts are very fine discrete particles which may be felt when rubbed with water on the palm of the hand ◆ Clay: means soils having particles smaller than two micrometers. It’s composed of very fine particles of sand and organic matter. When rubbed on the palm with water, the soil has a soapy or greasy feel. It has the tendency to be firm in dry weather and elastic in wet weather. This causes soil movement which is a danger to the life of the building. To prevent such movements, clay is made to remain dry and firm by the installation of subsoil drainage
◆ Marshy soils: sometimes called swamp or bog. They are formed by the decay of plants in the presence of water over a long period of time. Marshy soils are not recommended for building purposes because their bearing properties are uncertain
◆ Made ground or artificial soil: made up of many different materials dumped from town refuse, mining residue and other wastes from factories. These soils are not recommended for heavy structures, but with suitable foundations small buildings eg houses can be built on it.
◆ Innocent Chatikobo is an engineer by profession with AtroServe Engineering Zimbabwe. He has extensive knowledge and experience in structural engineering and construction. For your comments, views, questions and feedback he can be contacted on the following platforms Cell: +263 777 950 224; +263 712 376 037; +263 782 502 732, Email: eng.chatyk@gmail.com; Facebook: AtroServe Engineering Zimbabwe, Follow the Whatsapp Channel: AtroServe Engineering Zimbabwe