Geotechnical Engineering is a branch of Civil Engineering that is concerned with exploring  the  ground  properties  on which a construction project is to be made. Checking the soil quality, the quality of the stones and if it will be able to hold the project safely.

It is the job of a Geo-technical engineer to detect any vulnerabilities. After understanding all these aspects, a Geo-technical Engineer will assess the feasibility of a project.

Geotechnical Surveys

Geotechnical surveys are essential parts of the preliminary design works on any construction project to obtain information regarding the engineering properties of sub surface strata and the ground water properties, so that suitable foundations can be designed. In addition, a Geotechnical investigation is often necessary to assess the safety of an existing structure or to investigate cases where structural failure has occurred.

Why is Geotechnical survey important?

• Helps to locate any soft pockets in the supporting soil which would adversely affect the safety and performance of the proposed design.
• Inform and pprovide details and information relevant for foundation design
• Inform the structural engineer on the permissible loads the subsoil can safely carry
• Guide on earthwork repairs
• To fore see possible setbacks that may arise during design or construction

Stages Involved in Geotechnical Surveys

Test Pit Excavation

Trial pits are sunk at approximate nominal distribution, each pit attaining a specified target depth usually of 1.5m for basic storeyed structures and 3m + for structures with basement floors. The test pitting operations can be carried out manually with aid of hand tools such as spades and pick axes or by means of mechanical augers. It is best practice to have as many test pits as possible and these should be spread out on the site under survey to ensure that the samples picked are a true representation of the nature of the site in entirety.  When the soil is homogeneous over the whole area, the number of boreholes could be limited. When the soil condition is erratic, limiting the number would be counterproductive.

Test pit logging

Test pit layer details are also examined recorded as profiles. The stratigraphy revealed by each pit is carefully logged with special note taken to the thicknesses and conditions of the various layers. The soil descriptions and classifications discussed on test pit logs are generally based on visual observation and should be considered approximate.

A detailed record of boring operations and other tests carried out in the field is an essential part of the field work.

Sampling

Bulk samples are taken from each of the test pits at target depths for laboratory testing. Both Disturbed and Undisturbed samples are picked. Core cutters are driven into the ground for undisturbed samples.

Representative soil samples obtained throughout the exploration program are placed in airtight containers and transported to the laboratory for testing. 

Laboratory Work

Laboratory tests are carried out on samples obtained from the excavated test pits to identify the physical properties of the soils and obtain parameters for determining their strength and compressibility characteristics. Several different standards is prescribed for different tests.

Report

A report is the final document of the whole exercise of soil exploration. A report should be comprehensive, clear and to the point. Many can write reports, but only a very few can produce a good report. A report writer should be knowledgeable, practical, and pragmatic.

A good report will clearly present the subsoil and Geo-technical characteristics of the area under survey. And this will inform on the type and size of the foundation to be adopted for a client’s structure

Common unsafe practices!!!

Many pseudo practitioners, whom clients usually want to engage on the pretext of “professionals are expensive” don’t perform these geotechnical surveys and they just assume soil parameters (especially bearing capacity) and use it to design foundations. What you get with this is either an expensive over design or an unsafe inadequate design!!