The very first decision one has to take regarding the type of foundation is :
Whether to go for an Open foundation or resort to a Pile foundation ?
Quick reference : An open foundation is where a open pit is done for laying the footing whereas in the pile foundation , there is no need of any open pit .
The piling is a relatively clean process in which a cylindrical hole is bored in the soil up to desired depth, reinforcement cage is lowered and concreting done.
For this selection ,we need to ask the first question :
How is the sub strata ?
whether hard strata is available at a reasonable depth from the ground level ?
Before going further ,let us see the typical composition of the substrata :
As we go from top to bottom:
Black cotton soil (SBC: 10 Ton/SqM)
Yellow soil (SBC: 15 Ton/SqM)
Mixed soil (SBC: 10 Ton/SqM)
Soft murrum (SBC: 20 Ton/SqM)
Hard murrum (SBC: 30 Ton/SqM)
Soft rock (SBC: 80 Ton/SqM)
Hard rock (SBC: 100 Ton/SqM)
1.SBC: Safe bearing capacity of the soil.
2.These values are indicative and are provided to highlight the relative difference between various layers.
Normal question then comes to the mind is how does one find the SBC at a given location ?
It is suggested to have a survey done of the various construction sites in the vicinity. This gives an overall view of the type of strata available & types of foundations being used.
It is also useful to have a trial pit excavated at the site. This gives a correct picture of what’s exactly lying below the surface.
However, to arrive at the exact Safe bearing capacity ( SBC ) more accurate methods need to be used such as :
1.Triaxial compression test.
1.1 Undisturbed samples
1.2 Disturbed sample test
2.Plate load test
Test no 1 is carried out in the laboratory based on the samples collected from site . This is quite fast & quite economical.
Test 02 is conducted on site at the specific level where we need to know the SBC. This takes time , quite expensive but gives comprehensive insight about the probable behaviour of the soil.
For an open foundation hard murrum with a SBC of 30Ton/SqM. can be considered as a good hard strata.
soft rock & hard rock are considered.
Now in this context let us examine a few cases :
Case 1. The cross section of the substrata indicates that black cotton soil exists upto 4′ . Yellow soil is between 4 to 6 ‘. Hard murrum with a safe bearing capacity (SBC) of 30 T/SqM is available from 6 ‘down wards upto 10′ .The fragmented or soft rock formation is between 11′ to 16′ and the hard rock is beyond 17′.
Now looking at this strata ,it is clear that it is possible to select Open foundation.
The excavation will have to be done only for a depth of 6′ to 7’.