The purpose of site exploration is to provide reliable, specific and detailed information about the soil and groundwater conditions of the site that may be required for a safe and economic design and execution of engineering work.

Dr. BC Punima

Generally soil excavation should be carried out to such a depth that the increase in pressure due to structural load has no detrimental effect on the structure (such as settlement and shear failure). In other words, the depth at which the soil does not contribute to the settlement of the foundation. this depth is called critical depth,

The various factors affecting critical depth are as follows.

Structure type
weight of structure
Structure Dimensions
load area settlement
Soil Profile and Layer Properties

The following 3 rules of thumb can be used to find critical depth.

This can be the depth where the net increase in vertical pressure is . falls short of 10% of the initial overburden pressure.
maximum depth achieved by pressure bulb or isobar diagram drawn with a pressure intensity different from 1/5^th or 1/10^th Surface loading intensity (ie.) 0.2Q To 0.1Q) (where Q = initial loading intensity).
it can be equal to one and a half to two times the width or the smaller lateral dimension of the loaded area.

Rules of Thumb for Determining Soil Exploration Depth

The following rules (Table-I) can be used as a guide to decide the depth of exploration of soil for undertaking exploration work.

Table-1 (Rules of Thumb for Estimating Depth of Exploration)
No. No.	Foundation Type	depth of exploration
1	isolated protruding base or fleet	one and a half times Width
2	with adjacent footing clear spacing. less than twice the width	one and a half times, foot length
3	Pile and good foundation	10 to 30 meters or more, or one and a half . to the depth of times the width of the structure by bearing level (pile toe or bottom toe) properly)
4	retaining wall base	One and a half times the width of the base or one and a half times the exposed height of the wall face, whichever is greater
5	floating basement	depth of construction
6	dams	1. Half of the bottom width of earthen dams 2. Twice the height from stream level to crest for concrete dams less than 30 meters in height 3. Bed rock up to, or even though all soft, unstable and permeable levels of overburden
7	road cut	1. A meter where a little cut or fill is required 2. In cut sections, one meter below the formation level 3. In deep cuts, equal to the width or depth of the bottom cut off
8	road fill	two meters below the ground level or height whatever the filling Greater
9	lending area	Excavation facility and thickness of material available

pay attention: The above values may be modified depending on the type of soil found at the site.

Reference

IS: 1892 – 1979 – Subsurface probe for foundation

Soil Mechanics and Foundations By Dr. BC Punima, Ashok Kumar Jain, Arun Kumar Jain