Importance of Site Investigation

Some types of rock/ sand

Importance of soil investigation in civil engineering

Civil engineering works have some form of foundation which is supported by the ground. The interaction between a civil engineering structure and the ground that holds it is complex. Hence it is appropriate that proper soil study and investigation is carried out before structure is built in it.

It is standard practice to examine any soil on which a structure is intended to be erected in order to determine the following:

  • The suitability of the soil site for the proposed work.
  • Adequate and economic foundation design.
  • Difficulties that may arise during construction.
  • Situations that may occur after construction.

Rock types

Rock types can be classified by their method of excavation. That is, whether blasting is required, by load bearing or other physical properties.

From a civil engineer’s point of view, A rock is a solid mass composed of soil material in a very stable position of which it’s removal is only possible after blasting, or breaking by compressed air and hydraulic tools or by breaking tools such a wedges or sledge hammer.

Rocks can be placed into 3 basic categories, the are:

Igneous, sedimentary and metamorphic.

Igneous rocks include basalt and granite. They are formed by the solidification of molten material from the hot lower levels of the earth crust which have ascended towards the surface. They have very high bearing capacities about 3 times that of sedimentary rocks and 40 times that of alluvial clays and sand.

Sedimentary rocks includes, Limestone and sandstone.They were formed by stratification and cementation of earth materials over time.Their strength depends on properties such as angle of stratification and cementation as well as behaviour under wet conditions.

Metamorphic rocks are any sedimentary or igneous rock deposits which after consolidation have become changed by heat and pressure. Examples of metamorphic rocks are gneisses, slates, schists.

The most suitable type of  rock for civil engineering  foundation work is the igneous rock. Sedimentary rocks have lower  load bearing capacity due to the presence of soft clay material in their deposits. However, the load bearing capacity of all types of rock is greatly reduced as a result of weathering and earth movements.

Soil types

Types of soil includes;

  • Residual soil ( too soil)
  • Detrital sediments ( sands, gravel, slits)
  • Organic deposits (peat)
  • Calcareous deposits ( shell, coral)
  • Uncemented volcanic dust.

Soil types are identified by;

  • Size and nature of soil particles
  • Density and structural properties

You can download the table of Field Identification and Description of Soils. (BS 5930:2015) HERE

The table can be found in page 110 of BS 5930: 2015.

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