Understand The Difference Between Conhesive And Non Cohesive Subsoils

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It is vital to understand the difference between cohesive and non-cohesive subsoils as described in the assignment scenario.

Non-Cohesive
Non-cohesive subsoils are sandy in nature and they present less of a problem for building on. They are composed mainly of sand and gravel, and they are closely compacted together which means that they are not affected by water. Therefore, their load bearing capacity does not change with the various seasons, summer or winter, drought or flood, it makes no difference. This means that they are unchanging.

Cohesive
Cohesive subsoils are predominantly clay they rely on water for their strength and to bond the very fine particles together. They will be particularly affected by drought and the clay is the most
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Loads
There are different types of load and they have to be taken into account when the construction of a building is going to take place. The subsoil has to be able to carry these loads. The primary loads are natural loads, forces due to gravity. That are as follows;

Dead load
This is the permanent elements of the structure, a consistent load as the materials of the house will always be there. The architect can calculate this load. The objects involved are;
• Floors
• Walls
• Ceilings
• Slabs
• Columns etc

Imposed or live load
This is the load of all the people in the building, for example in a school during the day there might be a load of 60 tones added with the students in the school. There is also tables, chairs, cabinets
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This means that if there was a building constructed on this ground then there would be a possibility that it would sink because the load is too much for the subsoil. How do you carry out a subsoil investigation?
Where foundation loads are small, for example a bungalow which is approximately 35kNm2 and the nature of the subsoil is well known already then I generally sufficient to dig out the foundation trench and examine the exposed subsoil in the trench. The Building Control Officer will visit the site and confirm that the subsoil is capable of supporting the loads of the building.
However when the building loads are greater and there is no available evidence or information about the subsoil then it will be necessary to dig a trial pit or drill a borehole. The depth to which you will have to go will be determined by the loads of the building and until you hit sufficiently strong subsoil.
Trial Pits
Trial pits would be excavated by a mechanicals digger to a depth of 3-4m and 1.2m2 wide. This is a more economical method than boreholes for depths of up to 3m. When the pit is dug you should simply examine the sides of it and then follow this by drawing up a trial pit

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