Foundation Design And Analysis

Foundation design is a detailed analysis of substructural members of a building or structure to determine the foundation’s depth, size, and capability. The load transfer from the superstructure to the soil is obtained through an appropriate foundation design. Foundations are horizontal or vertical members supporting the superstructure of the entire structure and transmitting the loads to the soil below. They are substructures supporting the superstructure such as columns, beams, walls, slabs, and roofs.

Classes of Foundation

Foundations generally can be classified as deep foundations or shallow foundations. The choice between the deep or shallow foundation can be determined through foundation design and analysis after a thorough examination of the following elements:

• The magnitude of the transmitted loads from the superstructure
• The nature of the soil
• The economic benefit of the elements of the foundation works
• Problems associated with foundation construction

Shallow Foundation

A shallow foundation is a class of foundations that transfers the structural load of a building to the surface layer of the earth. Usually, a  foundation is considered a shallow foundation when the width of the entire foundation is greater than the depth of the foundation. When compared to deep foundations, shallow foundations are commonly less technical in terms of design and construction, which makes them more economical and the most widely used class of foundations for relatively light buildings or structures. Examples of shallow foundations include:

1. Strip Foundation
2. Pad Foundation
3. Raft Foundation
4. Wide Strip Foundation
5. Strap Foundation

Deep Foundation

Unlike the shallow foundation, a deep foundation on the other hand is a type of foundation that transfers structural loads to the soil farther below the surface of the earth compared to a shallow foundation.

Examples of deep foundations are:

1. Pile Foundation
2. Diaphragm Foundation
3. Displacement Foundation

The foundation type to choose for a particular structure or building depends largely on the loads to be transmitted and the receiving soil bearing capacity must satisfy the following fundamental requirements obtainable in the foundation design and analysis:

1.  The factor of safety against shear failure of the supporting soil must be adequate
2. The settlement should neither cause any unaccepted damage nor interferes with the function of the structure.

Therefore the bearing capacity of the soil must be determined through soil investigation before the foundation design and analysis.

Soil Investigation For Foundation Design

Most clients tend to neglect or reject the idea of soil investigation based on cost when though in most cases the cost of soil investigation is less than 1% of the project total cost. It is therefore advisable for the engineer to convince his/her client of the need for soil investigation. Soil investigation is intended to achieve the following results:

• Confirmation of soil-bearing strata
• Confirmation of soil bearing capacity and,
• Confirmation of soil settlement rate.

Soil investigation can be limited to penetrometer tests only depending on the load type usually for relatively good soil and light loads. For heavier loads, a combination of both penetrometer test and borehole tests are required. The soils samples recovered are taken to the laboratory and the following tests are carried out:

1. Soil classification tests
2. Atterberg (liquid and plastic) limits tests
3. Natural moisture content tests
4. Hydrometer analysis
5. Particle size distribution tests
6. Triaxial tests

Soil Classification Test

This soil investigation is carried out to determine the different percentages of soil grain sizes contained in a given soil sample of interest. This is done through mechanical or sieve analysis to determine the distribution of the larger-sized and coarser particles.

There are several soil classification systems which include:

1. Unified soil classification system (USCS)
2. The American Association of State Highway and Transport Officials (AASHTO) system of soil classification.
3. U.S Department of Agriculture (USDA) classification system
4. Massachusetts Institute of Technology classification system (MIT)

Atterberg Limits Test

The Atterberg limits test was first named after  Albert Atterberg the Swedish chemist who was the first to develop a classification system of soil to determine the different states and limits of soil consistency.  Arthur Casagrande and Karl Terzhagi later standardized and refined the tests which are commonly used today in determining the LL, PL, and SL of soils.

The Atterberg limits test is simply the soil classification test used to determine the moisture content of which fine-grained clay and silt soils change between the different phases.  Atterberg limits tests are usually carried out on the fraction of soil that will pass through sieve No. 40 or 425µm or 0.425mm as per ASTM D 4318-00.

Natural Moisture Content Test

The natural water content is also known as the natural moisture content which is the ratio of the weight of water contained in the soil to the weight of the solids in a given mass of soil. The ratio is usually represented as a percentage. The natural moisture content tests are used to determine the state and the bearing capacity of soil under investigation.

Hydrometer Analysis

Hydrometer analysis is a soil analysis method used to determine soil particle size in a given soil sample. Hydrometer analysis is specifically carried out to determine the soil particle sizes that are less than or approximately 0.75 mm in diameter.

Particle Size Distribution Test

Determination of particle size is very important in foundation design and analysis, as the particle size determines the suitability of the final product. The characteristics of particles such as physical stability, permeability, bulk density, and rest are determined by their size. The sieve analysis test is an effective method of determining the size distribution of particles. Usually, in sieve analysis, the particle size distribution is measured by either mass or volume. Sieve analysis is a laboratory test method of soil investigation in which particles allow to move vertically or horizontally through sieve mesh. Depending on the particle material and the needs, different sieving methods are available for particle size distribution tests:

• The manual sieving method
• Mechanical sieving method
• Dry sieving method and
• Wet sieving method

Triaxial Test

A triaxial test is a soil investigation done on a cylindrical core soil or rock sample to determine the shear strength of the soil.

This test attempts to replicate the in-situ stresses (stresses that existed in the original place where the soil sample was taken) on the core soil or rock sample.

Determining the shear strength of soil and rock samples enables engineers to ascertain both the long-term and short-term effects of the core soil in large-scale engineering applications such as earth dam construction, slope failure, tunnel linings,  embankments subsoil structures, and foundation design and analysis.

Design Loading

Likewise, loads for foundation design must be expressed both in the serviceability and ultimate limit states.  The ground bearing capacity is generally expressed at the serviceability limit state, hence, the area of foundation required to sustain the load must be determined based on the serviceability or working loads. Once the area is obtained, the net pressure is then expressed in the ultimate limit state. This is used for the design of the foundation base.

Foundation Design Procedures

1. Chose the location of the columns and decide the type of load that will be applied to them such as live load, dead load, or wind load.
2. Estimate allowable soil bearing capacity pressure using the soil investigation report.
3. Determine the depth of the foundation not less than 450mm depending on the site condition the depth can exceed 1.0m.
4. Calculate the foundation area considering the affordability, construction speed, and buildability
5. Calculate the rate of settlement.
6. Calculate the Cost
7. Consider the construction duration

Soil type	Bungalow	2- Storey	3-5 Storey	Medium Rise	High Rise
Good soil >1ooKN/m2	Strip	Strip	Pad	Pad	Pile
Average soil 75 – 100KN/m2	Strip	Wide strip	Pad	Pile	Pile
Poor soil  45 – 75KN/m2	wide strip	Wide strip	Raft	Pile	Pile
Bad soil < 40KN/m2	Slab raft	Beam & Slab raft	Beam & Slab raft	Pile	Pile

Table 1.0: Suggested appropriate Foundation type for different types of buildings