BUILDING FOUNDATION INVESTIGATION

BUILDING FOUNDATION INVESTIGATION

By Falae Philips Omowumi

The foundation is the building system component that interfaces the building structure with the ground. It provides the proper support to keep the structure from settling differentially (which would tear the building apart) and hopefully from settling at all. Foundations are designed with the soil conditions in mind.

Definition “The foundation is the lowest part of a building. It transmits the load of the structure to the soil below”.

Soil mechanics site investigations are conducted to discover the characteristics of the soil at the particular location. In the recent time, many methods have been employed to investigate subsoil on which foundation is to be laid. These methods include geotechnical methods (SPT, CPT, DCPT etc) trier pits for sample analysis, Geophysical method (Electrical method, magnetic, seismic etc). The design of foundation is basically based on the result of the report from the investigation carried out.

Individual footings connected by a plinth beam. Note that the footings have been cast on top of beds of plain cement concrete (PCC), which has been done to create a level, firm base for the footing. Photo credit (UnderstandConstruction.com)

The Purposes of Building Foundations

·        To distribute the load from the building over a large area

·        To load the substratum evenly so as to prevent unequal settlement

·        To take the structure sufficiently deep into the ground to prevent overturning

Before a foundation is decided, it is necessary to determine the characteristics of the soil at the site of construction. This is done by conducting soil investigations.

Soil Investigations for Building Foundations

Soil investigations involve the following steps:

1.     Planning the details and sequence of operations

2.     Collection of soil samples from the field

3.     Conducting all field tests for determining the strength and compressibility characteristics of the soil

4.     Study of groundwater level conditions and collection of water samples for chemical analysis

5.     Geophysical exploration if necessary

6.     Testing in the laboratory of all samples of soil, rock, and water

7.     Preparation of drawings and charts

8.     Analysis of the results of the tests

9.     Preparation of report

Methods of Soil Investigations

Soil investigation methods are:

1. Reconnaissance survey
2. Test pits
3. Probing
4. Boring

Reconnaissance survey

The first step in foundation investigation is the inspection of the site and its vicinity to get a preliminary idea of the site conditions. This includes the study of the existing buildings in the neighborhood and if possible the type of their foundations. The cuts made in the nearby areas should also be looked into. The purpose of inspection can be discussed with those persons who were associated with constructing buildings in the surroundings with regard to their experiences and difficulties encountered by them when building in the area.

Test Pits

Test pits are dug by hand or by excavating machines. The size of the pit should be such that a person can easily enter the pit and have a visual inspection. Both disturbed and undisturbed soil samples are collected from the pit at a regular interval for detailed Laboratory analysis. Sample are well preserved to enable it to remain in in situ conditions.

Probing

This will give a rough idea of the underlying soil. In this, a steel bar of 25 to 40 mm (1 inch to 1.5 inches) in diameter is driven into the ground until a hard stratum is met with. The bar is driven by a hammer. The bar is then drawn out at intervals and the soil sticking to the bar is examined to get an idea of the type of the soil. An experienced workman can assess the nature of the soil by observing the way the rod is penetrated into the soil.

Boring

In this process, bore holes are made on the ground and the soil samples collected. Boring helps in obtaining the following:

Ø Extent of each stratum of soil/rock

Ø  Nature of each stratum and the engineering properties of the soils

Ø Location of ground water table.

The depth and number of boreholes will depend upon the type of the structure and nature of the soil as obtained from the preliminary examination. The depth of boreholes is governed by the depth of the soil affected by the loading. As a rough estimate, it is advisable to investigate the subsoil to a depth of at least twice the width of the anticipated largest size of the foundation. In the case of a pile foundation, the depth of boring should extend into the bearing stratum.

Methods of Boring for Soil Investigations

The different methods of boring are:

Auger Boring

The examination of the soil for ordinary buildings can be done by a post hole auger. The auger is held vertically and is driven into the ground by rotating its handle. At every 30 cm of depth, the auger is taken out and the soil samples collected.

Wash Boring

Wash boring is commonly used for boring in difficult soil. The hole is advanced by an auger and then a casing pipe is pushed to prevent the sides from caving in. A stream of water under pressure is forced through the rod into the hole. The loosened soil in suspension in water is collected in a tub.

·        Percussion Boring

In this method, the substrata are broken by repeated blows by a bit or chisel. Water is circulated in the hole and then the slurry is bailed out of the hole.

Core Drilling

When rocks are to be penetrated for examination, core drilling is resorted to. In this process, a hole is made by rotating a hollow steel tube having a cutting bit at its end.

·        Bearing Capacity

The bearing capacity of a soil is defined as the capacity of the subsoil to support the load of the structure without yielding. The bearing capacity of the soil depends upon the characteristics such as cohesion, friction, and unit weight. The bearing capacity can be determined in the field and also from the results of tests conducted in the laboratory on the soil samples.

·        Field and Laboratory Testing

Field Tests

The commonly adopted field tests are:

Standard penetration test, Dynamic cone penetration test, Static cone penetration test, Plate load test, Vane shear test, and Pressuremeter test.

Laboratory Tests

A set of laboratory tests is required to be done to obtain the soil parameters for the design of foundation. These tests are Shear Strength, Compressibility, Permeability, Chemical and Mineralogical Composition, and Soil Classification.

Type of Foundation

Shallow foundations

Shallow foundations are those founded near to the finished ground surface; generally where the founding depth (Df) is less than the width of the footing and less than 3m. These are not strict rules, but merely guidelines: basically, if surface loading or other surface conditions will affect the bearing capacity of a foundation it is 'shallow'. Shallow foundations (sometimes called 'spread footings') include pads ('isolated footings'), strip footings and rafts. 

Shallows foundations are used when surface soils are sufficiently strong and stiff to support the imposed loads; they are generally unsuitable in weak or highly compressible soils, such as poorly-compacted fill, peat, recent lacustrine and alluvial deposits, etc.

Pad foundations

Pad foundations are used to support an individual point load such as that due to a structural column. They may be circular, square or rectangular. They usually consist of a block or slab of uniform thickness, but they may be stepped or haunched if they are required to spread the load from a heavy column. Pad foundations are usually shallow, but deep pad foundations can also be used.

Strip foundations

Strip foundations are used to support a line of loads, either due to a load-bearing wall, or if a line of columns need supporting where column positions are so close that individual pad foundations would be inappropriate.

Raft foundations

Raft foundations are used to spread the load from a structure over a large area, normally the entire area of the structure. They are used when column loads or other structural loads are close together and individual pad foundations would interact.

A raft foundation normally consists of a concrete slab which extends over the entire loaded area. It may be stiffened by ribs or beams incorporated into the foundation.

Raft foundations have the advantage of reducing differential settlements as the concrete slab resists differential movements between loading positions. They are often needed on soft or loose soils with low bearing capacity as they can spread the loads over a larger area.

Deep foundations

• Deep foundations are those funding too deeply below the finished ground surface for their base bearing capacity to be affected by surface conditions, this is usually at depths >3 m below finished the ground level. They include piles, piers, and caissons or compensated foundations using deep basements and also deep pad or strip foundations. Deep foundations can be used to transfer the loading to a deeper, more competent strata at depth if unsuitable soils are present near the surface.

Piles are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. They are used when for economic, constructional or soil condition considerations it is desirable to transmit loads to strata beyond the practical reach of shallow foundations. In addition to supporting structures, piles are also used to anchor structures against uplift forces and to assist structures in resisting lateral and overturning forces.

Piers are foundations for carrying a heavy structural load which is constructed in situ in a deep excavation.

Caissons are a form of deep foundation which is constructed above ground level, then sunk to the required level by excavating or dredging material from within the caisson.

Compensated foundations are deep foundations in which the relief of stress due to excavation is approximately balanced by the applied stress due to the foundation. The net stress applied is therefore very small. A compensated foundation normally comprises a deep basement.