Week 2 Challenge

The depth of boreholes is controlled by the depth of the soil that is under the influence of bearing pressure of the foundation. The recommended borehole depth is equal to one to three times the width of the

loaded area. This is because it is suggested that bearing pressure below this depth would cause detrimental effect on the structure. If the shallow strip foundation is considered with considerable spacing, then

deep boreholes would not be required.

However , it will be required to dig deep Boreholes in the case of large raft foundations except in the case of encountering rock within the specified depth.

If strip or pad foundation effects are overlapping, then their behavior would be similar to that of raft foundation and eventually deep boreholes are needed to be sunk.

Following are the objectives of site investigation or subsurface exploration.

• To know about the order of occurrence of soil and rock strata.
• To know about the location of the groundwater table level and its variations.
• To determine engineering properties of soil.
• To select a suitable type of foundation.
• To estimate the probable and maximum differential settlements.
• To find the bearing capacity of the soil.
• To predict the lateral earth pressure against retaining walls and abutments.
• To select suitable soil improvement techniques.
• To select suitable construction equipment.
• To forecast problems occurring in foundations and their solutions.
• 

Stages in Site Investigation

1. Site Reconnaissance
2. Preliminary site exploration
3. Detailed exploration
4. Preparation of soil investigation report

Site Reconnaissance:-

Site reconnaissance is the first stage of site investigation. In this stage, visual inspection of the site is done and information about topographical and geological features of the site are collected. The general

observations made in site reconnaissance are as follows :

1. Presence of drainage ditches and dumping yards etc.
2. Location of groundwater table by observing well in that site.
3. Presence of springs, swamps, etc.
4. High flood level marks on the bridges, high rise buildings, etc. are observed.
5. Presence of vegetation and nature of the soil.
6. Past records of landslides, floods, shrinkage cracks, etc. of that region.
7. Study of aerial photographs of the site, blueprints of present buildings, geological maps, etc.
8. Observation of deep cuts to know about the stratification of soils.
9. Observation of Settlement cracks of existing structures.

Preliminary site exploration:-

Preliminary site exploration is carried out for small projects, light structures, highways, airfields, etc. The main objective of preliminary exploration is to obtain an approximate picture of sub-soil conditions at low

cost. It is also called general site exploration.

The soil sample is collected from experimental borings and shallow test pits and simple laboratory tests such as moisture content test, density, unconfined compressive strength test, etc. are conducted. Simple

field tests such as penetration methods, sounding methods, geophysical methods are performed to get the relative density of soils, strength properties, etc.

Detailed Exploration:-

Detailed exploration is preferred for complex projects, major engineering works, heavy structures like dams, bridges, high rise buildings, etc. A huge amount of capital is required for a detailed site exploration

hence, it is not recommended for minor engineering works where the budget is limited. For such type of works, data collected through preliminary site exploration is enough.

In this stage, numerous field tests such as in-situ vane shear test, plate load test, etc. and laboratory tests such as permeability tests, compressive strength test on undistracted soil samples are conducted to get

exact values of soil properties.

Preparation of Report of Sub-Soil Exploration:-

After performing preliminary or detailed site exploration methods a report should be prepared. A sub-soil investigation or exploration report generally has the following sections :

1. Introduction
2. Scope of site investigation
3. Description of the proposed structure, purpose of site investigation
4. Site reconnaissance details
5. Site exploration details such as number, location and depth of boreholes, sampling details etc.
6. Methods performed in site exploration and their results.
7. Laboratory tests performed and their results.
8. Details of Groundwater table level and position.
9. Recommended improvement methods if needed.
10. Recommended types of foundations, structural details, etc.
11. Conclusion.

Standard Penetration Test:-

Parameters of site/geotechnical investigation for design of the foundation:-

1.Loads of Structure

Loading conditions coming from the structure is one of the factors that govern foundation selection. It is influenced by type of building, type of construction material, environmental factors, earthquake

susceptibility.

Selection of material such as bricks, stone, steel, and concrete for construction affect the selection of foundation. Settlement calculation of foundation is another factor that associated with loads of the structure

and affects foundation choice.

In case of low-rise building, shallow foundation is preferred. However, for high-rise building, deep foundation is required. Deep foundation is provided because ground at greater depth are highly compacted.

2.Soil Bearing Capacity

Soil bearing capacity is one of the most crucial criteria that affect the selection of the appropriate type of foundation. Decision can be made to choose shallow or deep foundation based on the soil bearing

pressure.

An allowable bearing pressure of at least 100kN/m^2 or higher is effective for shallow foundations up to 4 stories. However, higher structures can consider a raft foundation provided that the modulus of subgrade

reaction shall not be exceeded when calculated.

3. Soil Types

There are different types of soil like sandy soil, loose soil, clayey soil, and expansive soils. Form the surface to a depth of 3m is termed as top soil, and subsoil begins after 3m depth. Characteristics specifically

bearing capacity of both topsoil and subsoil layers would to a great extent controls the foundation selection.

For low bearing capacity soil, stronger and suitable foundation shall be selected compare to with case where the soil has strength bearing capacity. The bearing capacity is the ability of soil to safely support

structural loads without suffering shear failure and intolerable settlement.

4. Ease of Construction

The choice of foundation should be made keeping in mind the ease with which it can be executed/constructed at the site. The construction of different types of foundation requires labor of different skillset and

different level of skills.

5. Water Table Level

Water table is another significant criterion that affect the foundation selection. foundation should not be placed on soil that undergo expansion and contraction due to water table fluctuation. So, it should be

construction on fully dry soil or on fully wet soil.

If the groundwater table is located below the formation level of the foundation, then a shallow footing like isolated or combined footing should be chosen. Moreover, for higher water table, raft/mat footing should

be proposed.

This is to counteract uplift pressure and counter the effect caused by water to avoid any overturning moments at the early stage of construction. If this option cannot be executed, then deep foundations like micro

piles or bored piles should be considered to provide the necessary uplift resistance.

6. Adjoining Structure/Water Bodies/Slopes

Adjoining structures, water bodies, slopes are parameters that need to be considered for selection of foundation types. When foundation of the neighboring structure is very close the foundation to be

construction, it may influence the choice and the safety of the adjoining structure.

If the foundations of an adjoining structure are very near to the target foundation, it may affect the choice as the safety of adjoining structure is of paramount importance. the selection of proper foundation type

would be more challenging it the neighboring structure is high rise building or an old property.

Factors such as closeness to a river, lake, natural drain, or vicinity to a slope should be taken in counting. Isolated RCC pad foundations under stilts are usually preferred for buildings on sloping grounds. No

residential buildings should be constructed on slopes steeper than 25 degrees.

7. Natural Disaster and Extreme Weather

If the area has a history of severe natural events or extreme weather, then these parameters should be selected taken these parameters into considerations. The past record or data of natural disasters and

extreme weather should be employed as a base for foundation selection.

8. Economic Design

When there are number of feasible foundation choices for the given project site, economical factor may influence the selection of the foundation. Nonetheless, choosing an economical foundation should not

compromise the safety, workmanship, strength, and durability of the foundation.

Type of soil/rock :-

1.Clay Soil

clay soil has great capacity for water retention that is why great expansion and shrinkage are expected in this type of soil. As a result, foundation structure can suffer from great settlement and uplift pressure that

is why clay soil is not desirable. Applicable codes such as British standard recommends minimum depth of 1m for foundation and 3m if there are trees around.

Raft/Mat foundation is the best foundation type to be built on clay soil, and ribs and beams can be incorporated into it to increase its stiffness. If raft foundation is costly, imposed loads are large, or strong soil

layer is not available at shallow depth, then under reamed pile should be selected.

In clay soil, it is recommended to collect and drain rainwater, extend foundation to a depth where moisture fluctuation does not occur, remove weak and shallow soil layer such as black cotton soil, execute

construction during dry season if possible, and distribute structural loads as uniform as possible.

In the case where a shallow firm soil layer cover soft clay soil layer, it is advisable to use wide reinforced strip foundation. In this manner, the affect of loads on weak soil layer is reduced. Pile foundations are

recommended for high rise buildings and whenever uplift is expected.

2 Peat Soil

It is considerably porous, easily compressible, and dark brown or black color soil which is commonly present near wetlands. It undergoes expansion and shrinkage due to moisture fluctuation, extremely weak in

terms or load carrying capacity.

So, it should be either removed strong strata and strip foundation is good option in this case. If the thickness of peat soil is great and its removal is not economical, then other foundation types should be selected.

For instance, concrete piles extended to the firm soil layer below, pad and beam foundation took to firm strata blow for small projects, or raft foundation for the case where firm strata are not available at

reasonable depth but there is hard surface crust with 3-4m thick of suitable bearing capacity.

3 Silt

Silty soil, which is smooth to touch, is generally not suitable for foundation structure because of its expansion which exert pressure against foundation and damage it.

The silt retain moisture and does not drain water easily. Reinforced concrete spread and isolated pad footings are appropriate if silt or silty clay is stiff and extends to a great depth. The depth of the foundation

should be greater than the zone of erosion and the zone of swelling and shrinkage.

4 Sand and Gravel

Sand and gravel allow water drainage that is why do not cause structural movement. Moist compaction of soil and sand make good support for foundation structure.

Dry compact gravel, or gravel and sand subsoils are adequate for spread and strip foundations. Generally, a depth of 700mm is acceptable, as long as the ground has adequate bearing capacity.