PILE FOUNDATIONS CLASSIFICATIONS BASIC INFORMATION

A pile is a slender column made of wood, concrete or steel. A pile is either driven into the soil or formed in situ by excavating a hole and then filling it with concrete. A group of piles are driven to the required depth and are capped with R.C.C. slab, over which super structure is built.

The pile transfer the load to soil by friction or by direct bearing, in the latter case, piles being taken up to hard strata. This type of foundations is used when top soil is not capable of taking the load of the structure even at 3–4 m depth. Pile foundations are classified according to the materials used and also on the nature of load transfer.

Classification According to Materials Used:

Piles may be classified as:

(a) Timber piles

(b) Concrete piles

(c) Steel piles and

(d) Composite piles.

(a) Timber piles: Circular seasoned wood can be used as piles. Their diameter may vary from 200 mm to 400 mm. Similarly square piles of sizes 200 mm to 400 mm are also used. The length of timber pile should not be more than 20 times its lateral dimension.

The bottom of the pile is sharpened and is provided with iron shoe, so that it can be driven in the ground easily by hammering. These piles should be always kept below water table; otherwise alternating wet and dry condition cause the decay.

These piles are cheap and can be easily driven rapidly. The main disadvantage is their load carrying capacity is low and are likely to be damaged during driving in the soil.

(b) Concrete piles: These piles may be further classified as precast piles and cast in situ piles. Precast piles are reinforced with steel and are manufactured in factories. The cross-section diameter/dimension varies from 200 mm to 500 mm.

Square, circular and octagonal sections are commonly used. The length of piles may be up to 20 m. They are provided with steel shoe at the lowest end. These piles can carry fairly large loads. These piles are highly resistant to biological and chemical actions of the soil. The disadvantage of these piles is they need more time to manufacture and are heavy to handle.

Cast in situ concrete piles are formed first by boring the holes in the soil and then concreting them. Concreting is usually made using casing tubes. If the hole is filled with only plain concrete it is pressure pile.

The load carrying capacity of the piles may be increased by providing enlarged base. The reinforcement caging may be inserted in the bored holes and to increase load carrying capacity one or two under reams may be formed. After that concreting may be carried out.

Such piles are known as under reamed piles. These piles are provided at regular interval of 2 to 4 m and capping beam is provided over them.

(c) Steel Piles: A steel pile may be a rolled steel I sections, tubes or fabricated in the form of box. These piles are mostly used as bearing piles since surface available for friction is less and also the coefficient of friction is less. If tubes are used the soil inside the tube is driven out by compressed air and concrete is filled. These piles are very useful for driving close to existing structures since they disturb the soil least.

(d) Composite Piles: Composite piles may be of concrete and timber or of concrete and steel. Wooden piles should not be subjected to alternating wet and dry conditions. Hence they are preferred for the portion below water table.

The portion above water table are built with cast in situ concrete piles. If the required length of steel piles is less than the depth of pile, many times upper portions are built with concrete. Thus steel and concrete composite piles are sometimes used.

Classification of Piles According to Load Transfer:

According to the load transfer to the soil piles may be classified as

(a) Bearing piles and

(b) Friction piles.

Bearing piles rest on hard strata and transfer the load by bearing. Such piles are preferred. These piles are used if the hard strata is available at reasonable depth.

Friction piles transfer the load to the soil by the friction between soil and the pile. Such piles are used if hard strata is not available to a considerable depth. The friction developed is to be properly assessed before deciding the length of the pile. The surface of such piles is made rough to increase the skin friction so that required length of pile is reduced.

THE USE OF PILES FOR LATERAL LOAD BEARING BASICS AND TUTORIALS

In practice one has to make a choice between the use of vertical piles used singly or in groups to carry such loads or of groups incorporating at least some piles installed to an angle of rake. The capacity of a pile as a structural unit to carry shear loads at its head depends on the strength of the section, and when the forces become high, one is impelled to find some structurally acceptable solution which keeps stresses within reasonable limits.

However, in choosing the possible option of raking piles one should be aware of the problems and limitations that may be involved. Some of the factors involved are as follows:

1 Raking piles are usually more expensive than vertical piles. This is partly involved with extra time taken to set up and maintain the equipment in position, the less efficient use of hammers in the case of driven piles, and the difficulties of concrete placing in bored piles.

2 The standards of tolerance that can be maintained in the installation of raking piles are not as good as for vertical piles. Most analyses of pile groups of this kind ignore the effect of tolerances, but if tolerances are properly taken into account they can have a significant effect on calculated pile loads, depending on pile grouping and numbers, with small groups being usually most sensitive.

3 Where the upper part of a raking pile is embedded in a soil that is likely to suffer time-dependent settlement, the pile will in due course be subject to bending stresses unrelated to the structural design load conditions. This may require increase of strength of the section, which is in turn reflected in costs.

4 Many machines used for pile installation carry the pile driving or forming equipment on a long mast, so that they become intrinsically less stable, particularly as the line of the pile gets further from the vertical position. In certain cases, when working close to river banks or railway lines, for example, there is a major limitation on how machinery can be positioned to produce the desired end result.

5 Design of groups involving raking and vertical piles and with loads that are both vertical and horizontal should have regard to the constancy of the relationship between these. If, for example, the vertical load is near constant, but the horizontal force varies greatly, then it is better to employ groupings with rakers balanced in two opposed directions rather than to have an arrangement of vertical piles plus piles raking in one direction only. This is simply to minimize the shears in the

heads of the piles when horizontal load falls to a minimum value.

6 The use of raking piles to ‘spread’ load under vertically loaded foundations, where the piles are fully embedded in the soil mass and where the whole foundation is expected to undergo significant consolidation/creep settlement, must lead to large bending stresses being developed in the piles. In certain cases this can lead to such stress levels in the piles that the section will suffer damage, which may in turn lead to severe problems in the supported structure.

It should, however, be said that where groups of raking piles derive their axial capacity from strata that are hard and relatively non-deformable, they provide a stiffness in terms of laterally applied forces which can be very desirable. The main issue in design is to avoid large and unquantifiable secondary stresses, and provided this can be achieved all will be well.

Where there are very heavy lateral loads to be carried and neither raking piles nor single piles other than perhaps those of very large diameter are suitable, then diaphragm piers or ‘barrettes’ have a useful potential application. They can be given high stiffness in the direction of applied horizontal loading without fear of the problem of major secondary stresses.

PILE DRIVING PRE-BORING ACTIVITIES BASIC INFORMATION AND TUTORIALS

Pre-boring is a commonly referenced method for easing the passage of some driven piles into the ground. However, its use can also be misunderstood or misguided. It is not a satisfactory way of overcoming significant obstructions to enable piles to be driven because that which impedes the driven pile will also in general impede progress of the pre-boring tool.

Pre-boring in sand and gravel presents a problem because of the inherent instability of the soil through which the pre-bore passes. When such soil is dense, pre-bores may stand open temporarily because of arching and the influence of temporary pore water suction.

However, as soon as a piling tube or pile enters the bore and the hammer begins striking, the upper granular soil collapses into the lower part of the bore. The lower section of the bore will possibly not collapse in this circumstance at the initial driving strokes because the soil is relatively more dense and the hammer influence more remote.

The result is frequently that because of re-compacted debris in the lower bore, piles will not drive back to the same depth as originally bored. Only if the bore is temporarily cased to prevent collapse, and if the casing is of large enough diameter to allow access for the final pile, can a satisfactory load bearing unit be inserted, albeit with loss of potential friction resulting from loss of displacement effects and the need for in-filling around the pile.

As an alternative to trying to form an open hole in sand soils, the pre-boring tool is sometimes used simply to stir up the ground, leaving disturbed soil in position. This may be sufficient to deal with dense soil near ground level.

However, if deep bores are attempted after this manner, again when a piling tube or pile is entered and driving begins, the loosened material is compacted down into the lower part of the bore and becomes virtually indistinguishable from the original natural soil. Piles will frequently not drive back to the depth of the pre-bore or may behave inconsistently under applied load.

It is therefore not generally satisfactory to use deep pre-bore methods in sands, for example, for the purpose of ensuring that piles reach a deeper stratum such as rock unless special temporary casing methods are adopted.

Pre-boring sockets into rock or very hard soils for the supposed purpose of enhancing end bearing or reaching strong soil, where there are overlying fill, sand or clay layers, is also generally futile. For the same reasons as stated above, it will be found that without guaranteed bore stability and measures to prevent soil from collapsing into the socket, a satisfactory load bearing and consistent unit cannot be formed because of debris falling before the pile arrives.

Pre-bores are satisfactory only under specific circumstances:

1 To loosen dense upper crust soils and enable long piles to be driven without breakage. Long piles struck at the head are really slender columns and so the possibilities of buckling failure can be very real.

2 To make an open hole in stiff clays or similar cohesive soils into which a pile is pre-entered. The purpose in this instance is to avoid or diminish soil heave. If using the method for the purpose of eliminating ground heave, it is generally legitimate to choose the area of the bore so that the pile cross-sectional area is just slightly larger.

Jobs with pre-boring are frequently associated with claims and cost overruns, partly because it is difficult to synchronize the activities of boring and driving machines with consequent delay, and partly because, where the motivation is to achieve stringent ‘sets’ this may be a major source of damage to equipment.  

PILE FOUNDATION DESIGN NATURE OF LOADING BASIC INFORMATION

Usual. Usual loads refer to conditions which are related to the primary function of a structure and can be reasonably expected to occur during the economic service life. The loading effects may be of either a long term, constant or an intermittent, repetitive nature.

Pile allowable loads and stresses should include a conservative safety factor for such conditions. The pile foundation layout should be designed to be most efficient for these loads.

Unusual. Unusual loads refer to construction, operation or maintenance conditions which are of relatively short duration or infrequent occurrence. Risks associated with injuries or property losses can be reliably controlled by specifying the sequence or duration of activities, and/or by monitoring performance.

Only minor cosmetic damage to the structure may occur during these conditions. Lower factors of safety may be used for such loadings, or overstress factors may be applied to the allowables for these loads. A less efficient pile layout is acceptable for these conditions.

Extreme. Extreme loads refer to events which are highly improbable and can be regarded as emergency conditions. Such events may be associated with major accidents involving impacts or explosions and natural disasters due to earthquakes or hurricanes which have a frequency of occurrence that greatly exceeds the economic service life of the structure.

Extreme loadings may also result from a combination of unusual loading effects. The basic design concept for normal loading conditions should be efficiently adapted to accommodate extreme loading effects without experiencing a catastrophic failure.

Extreme loadings may cause significant structural damage which partially impairs the operational functions and requires major rehabilitation or replacement of the structure. The behavior of pile foundations during extreme seismic events is a phenomenon which is not fully understood at present.

The existing general approach is to investigate the effects of earthquake loading at sites in seismic Zones 1 or 2 by applying psuedostatic forces to the structure and using appropriate subgrade parameters.

In Zones 3 or 4 a dynamic analysis of the pile group is appropriate. Selection of minimum safety factors for extreme seismic events must be consistent with the seismologic technique used to estimate the earthquake magnitude. Designing for pile ductility in high risk seismic regions is very important because it is very difficult to assess pile damage after earthquakes and the potential repair costs are very large.

PILE TYPE SELECTION FOR CIVIL ENGINEERING FOUNDATION BASIC AND TUTORIALS

CIVIL CONSTRUCTION PILE TYPE SELECTION GUIDE
How To Select The Pile Type On Civil Engineering Construction?


Selection of pile type
The selection of the appropriate type of pile from any of the above categories depends on the following three principal factors.

The location and type of structure.
The ground conditions.
Durability.

Considering the first factor, some form of displacement pile is the first choice for a marine structure. A solid precast or prestressed concrete pile can be used in fairly shallow water, but in deep water a solid pile becomes too heavy to handle and either a steel tubular pile or a tubular precast concrete pile is used.

Steel tubular piles are preferred to H-sections for exposed marine conditions because of the smaller drag forces from waves and currents. Large-diameter steel tubes are also an economical solution to the problem of dealing with impact forces from waves and berthing ships. Timber piles are used for temporary works in fairly shallow water.

Bored and cast-in-place piles would not be considered for any marine or river structure unless used in a composite form of construction, say as a means of extending the penetration depth of a tubular pile driven through water and soft soil to a firm stratum.

Piling for a structure on land is open to a wide choice in any of the three categories. Bored and cast-in-place piles are the cheapest type where unlined or only partly-lined holes can be drilled by rotary auger. These piles can be drilled in very large diameters and provided with enlarged or grout-injected bases, and thus are suitable to withstand high working loads.

Augered piles are also suitable where it is desired to avoid ground heave, noise and vibration, i.e. for piling in urban areas, particularly where stringent noise regulations are enforced. Driven and cast-in-place piles are economical for land structures where light or moderate loads are to be carried, but the ground heave, noise and vibration associated with these types may make them unsuitable for some environments.

Timber piles are suitable for light to moderate loadings in countries where timber is easily obtainable. Steel or precast concrete driven piles are not as economical as driven or bored and cast-in-place piles for land structures.

Jacked-down steel tubes or concrete units are used for underpinning work. The second factor, ground conditions, influences both the material forming the pile and the method of installation.

Firm to stiff cohesive soils favour the augered bored pile, but augering without support of the borehole by a bentonite slurry, cannot be performed in very soft clays, or in loose or water-bearing granular soils, for which driven or driven-and-cast-in-place piles would be suitable.

Piles with enlarged bases formed by auger drilling can be installed only in firm to stiff or hard cohesive soils or in weak rocks. Driven and driven-and-cast-in-place piles cannot be used in ground containing boulders or other massive obstructions, nor can they be used in soils subject to ground heave, in situations where this phenomenon must be prevented.

Driven-and-cast-in-place piles which employ a withdrawable tube cannot be used for very deep penetrations because of the limitations of jointing and pulling out the driving tube. For such conditions either a driven pile or a mandrel-driven thinwalled shell pile would be suitable.

For hard driving conditions, e.g., boulder clays or gravelly soils, a thick-walled steel tubular pile or a steel H section can withstand heavier driving than a precast concrete pile of solid or tubular section. Thin steel shell piles are liable to tearing when being driven through soils containing boulders or similar obstructions.

Some form of drilled pile, such as a drilled-in steel tube, would be used for piles taken down into a rock for the purpose of mobilizing resistance to uplift or lateral loads. The factor of durability affects the choice of material for a pile.

Although timber piles are cheap in some countries they are liable to decay above ground-water level, and in marine structures they suffer damage by destructive mollusc-type organisms. Precast concrete piles do not suffer corrosion in saline water below the ‘splash zone’, and rich well-compacted concrete can withstand attack from quite high concentrations of sulphates in soils and ground waters.

Cast-in-place concrete piles are not so resistant to aggressive substances because of difficulties in ensuring complete compaction of the concrete, but protection can be provided against attack by placing the concrete in permanent linings of coated light-gauge metal or plastics.

Steel piles can have a long life in ordinary soil conditions if they are completely embedded in undisturbed soil but the portions of a pile exposed to sea water or to disturbed soil must be protected against corrosion by cathodic means if a long life is required.

Other factors influence the choice of one or another type of pile in each main classification, and these are discussed in the following pages, in which the various types of pile are described in detail. In UK practice specifications for pile materials, manufacturing requirements (including dimensional tolerances) and workmanship are given in a publication of the Institution of Civil Engineers(2.1).

Having selected a certain type or types of pile as being suitable for the location and type of structure, for the ground conditions at the site, and for the requirements of durability, the final choice is then made on the basis of cost. However, the total cost of a piled foundation is not simply the quoted price per metre run of piling or even the more accurate comparison of cost per pile per kN of working load carried.

The most important consideration is the overall cost of the foundation work including the main contractor’s costs and overheads. It has been noted  that a piling contractor is unlikely to quote a fixed price based on a predetermined length of pile.

Extra payment will be sought if the piles are required to depths greater than those predicted at the tendering stage. Thus a contractor’s previous experience of the ground conditions in a particular locality is important in assessing the likely pile length on which to base his tender.

Experience is also an important factor in determining the extent and cost of a preliminary test piling programme. This preliminary work can be omitted if a piling contractor can give an assurance from his knowledge of the site conditions that he can comply with the engineer’s requirements for load-settlement criteria.

The cost of test piling can then be limited to that of proof-loading selected working piles. If this experience is not available, preliminary test piling may be necessary to prove the feasibility of the contractor’s installation method and to determine the load-settlement relationship for a given pile diameter and penetration depth.

If a particular piling system is shown to be impracticable, or if the settlements are shown by the test loading to be excessive, then considerable time and money can be expended in changing to another piling system or adopting larger-diameter or longer piles.

During the period of this preliminary work the main contractor continues to incur the overhead costs of his site organization and he may well claim reimbursement of these costs if the test-piling work extends beyond the time allowed in his constructional programme. To avoid such claims it is often advantageous to conduct the preliminary test piling before the main contractor commences work on the site.

Finally, a piling contractor’s resources for supplying additional rigs and skilled operatives to make up time lost due to unforeseen difficulties, and his technical ability in overcoming these difficulties, are factors which may influence the choice of a particular piling system.

SCREW CAST IN DISPLACEMENT PILES BASIC AND TUTORIALS

SCREW CAST IN DISPLACEMENT PILES BASIC INFORMATION
What Are And How To Install Screw Cast In Displacement Piles?

Whilst the installation of this type of pile is effected by means of a type of auger, the process involves compaction rather than removal of the soil and, in this respect; the piles are of a displacement type. In forming the pile, a heavy-duty single-start auger head with a short flight is screwed into the ground to the required depth.

The auger head is carried on a hollow stem which transmits the considerable torque and compressive forces required, and through which the reinforcement cage is inserted after completion of the installation process. The end of the hollow stem is sealed with a disposable tip.

Following placement of the reinforcement, concrete is placed through this tube from a hopper at its head. As concrete filling takes place, the auger is unscrewed and removed, leaving behind a screw-threaded cast-in-place pile.

By virtue of the combined rotation and controlled lifting applied at the extraction stage the ‘threads’ are of robust dimensions. The sequence of pile construction is shown in Figure 3.6.

This method of forming a pile is known as the Atlas Piling System, and is marketed by Cementation Foundations: Skanska Limited in the United Kingdom, in association with N.V. Franki S.A of Belgium. A purpose-designed, track-mounted rig provides hydraulic power for auger rotation and the application of downward force and is fitted with a crane boom for handling reinforcement and concrete skips.

For a given pile size and volume of concrete, pile capacities are greater than for traditionally constructed bored piles, although the restricted diameter of the reinforcement cage may be a disadvantage if the pile is required to resist high bending stresses. The system does however combine many of the advantages of a displacement pile with the low noise and vibration characteristics of a bored pile.

It will operate in most cohesive and granular strata to a maximum depth of 22 m, providing piles ranging in diameter from 360 to 560 mm. To achieve the torque of perhaps 250 to 350kNm required at the auger, power requirements are relatively high.

DAMAGE TO ADJACENT PILES DURING DRIVING BASICS AND TUTORIALS

DAMAGE TO ADJACENT PILES DURING DRIVING BASIC INFORMATION
What Are The Damage To Adjacent Piles During Driving?

Driven cast-in-place piles can be damaged by driving adjacent piles too close or before the concrete has reached a suitable strength. The piles may be damaged by lateral forces or by tensile forces, as the ground heaves.

When it is suspected that pile damage of this type has occurred it may be decided to carry out a pile load test or integrity test as a check.

On a pile which is cracked by this means, a load test may yield an apparently satisfactory result but the long-term performance of the pile may be impaired if the steel reinforcement is exposed via the cracks.

To lessen the risk of cracking caused by soil movements, a minimum spacing of 5D, centre to centre is often employed when driving adjacent piles when the concrete is less than 7 days old. The use of integrity tests may be considered to provide sufficient information to modify this rule if necessary.

During installation of cast-in-place piles with relatively thin bottom-driven permanent steel casing, collapse of the tube can occur from lateral soil displacement if the piles are driven at centres that are too close.

This has sometimes resulted in the loss of the hammer at the base of the pile, when the collapse occurs above the hammer as the pile is driven. The occurrence is more likely, however, when driving piles inside a coffer-dam.

Where this problem is encountered, and there is no way to reduce the piling density, pre-boring may be considered as a method of reducing the effect over the upper part of the pile.

At the design stage, if high-density piling is unavoidable in soils prone to heave such as stiff clays, a low displacement ‘H’ section pile may be selected as more suitable. Alternatively, the multi-tube technique described by Cole (1972) can be employed.

All piles within 12 diameters of each other are considered to form a part of a group, and are driven (and if necessary, re-driven) to final level before basing out and concreting.

PRE CAST CONCRETE PILES BASICS AND TUTORIALS

PRE CAST CONCRETE PILES BASIC INFORMATION
What Are Pre Cast Concrete Piles?


Pre-cast concrete piles are now usually of the jointed type, unless a large contract with a more or less constant depth of piling makes it economical to pre-cast the piles on site, thus overcoming a potential difficulty in transport.

Pre-cast non-jointed piles are generally of square section and may be up to 600×600mm to work at loads up to approximetely 3000kN in suitable ground. Typical sizes and capacities are given in
Table 3.1.

Extending pre-cast piles that do not have pre-formed joints is a lengthy process, involving breaking down the projecting pile head to provide a suitable lap for the steel and casting concrete to form a joining surface.

The pile sections are then butted together in a steel sleeve using an epoxy cement, or joined by inserting steel dowel bars into drilled holes and using an epoxy cement to fix them in place.

Good alignment of the pile sections is required to prevent excessive bending stresses developing on subsequent re-driving.

There are some benefits from pre-stressing concrete piles. Tensile stresses which can be set up in a pile during driving are better resisted, and the pile is less likely to be damaged during handling in the casting yard and when being pitched.

Bending stresses which can occur during driving are also less likely to produce cracking. However, the ultimate strength in axial compression is decreased as the level of pre-stress is increased, and pre-stressed piles are therefore more vulnerable to damage from striking obstructions during driving.

They are also difficult to shorten and special techniques have to be employed. As a result they are most suitable for a constant-length application.

FOUNDATION PILES TYPES BASICS AND TUTORIALS

DIFFERENT TYPES OF FOUNDATION PILES TUTORIALS
What Are The Different Types of Foundation Piles?


It used to be possible to categorize the various types of pile and their method of installation, using a simple division into ‘driven’ or ‘bored’ piles. This is adequate in many situations, but does not satisfactorily cope with the many different forms of pile now in use.

A more rigorous division into ‘displacement’ or ‘non-displacement’ piles overcomes this difficulty to some extent, but some piles are installed by a combination of these methods and their description may require qualification.

In the displacement (generally driven) pile, soil is displaced radially as the pile shaft penetrates the ground. There may also be a component of movement of the soil in the vertical direction.

Granular soils tend to become compacted by the displacement process, and clay soils may heave, with little immediate volume change as the clay is displaced.

Piles of relatively small cross-sectional area, such as steel ‘H’ section piles or open pipe piles, are termed ‘low displacement piles’, and the effects of compaction or soil heave are reduced. This can be advantageous if long lengths of pile are to be driven through granular deposits, if the piles are at close centres, or if clay heave is a problem.

In the non-displacement (generally bored) pile, lateral stresses in the ground are reduced during excavation and only partly reinstated by concreting. Problems resulting from soil displacement are therefore eliminated, but the benefit of compaction in granular soils is lost and in all soils spoil is produced which may be costly to remove from a site, especially if it is contaminated.

The displacement of the soil by a pile during installation is therefore a fundamental property, and its recognition in any classification of pile type is clearly advantageous. Little-used types such as pre-formed screw piles can also be covered by the (low) displacement classification, whereas they could not be correctly termed ‘driven piles’.

In a further development of the screw pile that is becoming more frequently employed, especially on contaminated sites where it reduces or eliminates the production of spoil, a hollow screw-form auger is rotated into the ground and the bore filled with concrete as it is back-rotated out or retracted without rotation.

The two main categories of pile types may be classified further according to whether pre-formed units are used, and whether the pre-formed unit is used as temporary support for the ground and withdrawn during concreting or left in place.

For nondisplacement piles, factors such as pile diameter and underreaming are introduced to the classification, as they have a bearing on the method of installation, and particularly

FOUNDATION ENGINEERING BASICS AND TUTORIALS

FOUNDATION ENGINEERING BASIC INFORMATION

What Is Foundation Engineering

The title foundation engineer is given to that person who by reason of training and experience is sufficiently versed in scientific principles and engineering judgment (often termed "art") to design a foundation. We might say engineering judgment is the creative part of this design process.

The necessary scientific principles are acquired through formal educational courses in geotechnical (soil mechanics, geology, foundation engineering) and structural (analysis, de-sign in reinforced concrete and steel, etc.) engineering and continued self-study via short courses, professional conferences, journal reading, and the like.

Because of the heterogeneous nature of soil and rock masses, two foundations—even on adjacent construction sites—will seldom be the same except by coincidence. Since every foundation represents at least partly a venture into the unknown, it is of great value to have access to others' solutions obtained from conference presentations, journal papers, and textbook condensations of appropriate literature.

The amalgamation of experience, study of what others have done in somewhat similar situations, and the site-specific geotechnical information to produce an economical, practical, and safe substructure design is application of engineering judgment.

The following steps are the minimum required for designing a foundation:

1.       Locate the site and the position of load. A rough estimate of the foundation load(s) is usually provided by the client or made in-house. Depending on the site or load system complexity, a literature survey may be started to see how others have approached similar problems.

2.       Physically inspect the site for any geological or other evidence that may indicate a potential design problem that will have to be taken into account when making the design or giving a design recommendation. Supplement this inspection with any previously obtained soil data.

3.       Establish the field exploration program and, on the basis of discovery (or what is found in the initial phase), set up the necessary supplemental field testing and any laboratory test program.

4.       Determine the necessary soil design parameters based on integration of test data, scientific principles, and engineering judgment. Simple or complex computer analyses may be involved. For complex problems, compare the recommended data with published literature or engage another geotechnical consultant to give an outside perspective to the results.

5.       Design the foundation using the soil parameters from step 4. The foundation should be economical and be able to be built by the available construction personnel. Take into account practical construction tolerances and local construction practices. Interact closely with all concerned (client, engineers, architect, contractor) so that the substructure system is not excessively overdesigned and risk is kept within acceptable levels. A computer may be used extensively (or not at all) in this step.

The foundation engineer should be experienced in and have participation in all five of the preceding steps. In practice this often is not the case. An independent geotechnical firm specializing in soil exploration, soil testing, design of landfills, embankments, water pollution control, etc. often assigns one of its geotechnical engineers to do steps 1 through 4.

The output of step 4 is given to the client—often a foundation engineer who specializes in the design of the structural elements making up the substructure system. The principal deficiency in this approach is the tendency to treat the design soil parameters—obtained from soil tests of variable quality, heavily supplemented with engineering judgment—as precise numbers whose magnitude is totally inviolable.

Thus, the foundation engineer and geotechnical consultant must work closely together, or at least have frequent conferences as the design progresses. It should be evident that both parties need to appreciate the problems of each other and, particularly, that the foundation design engineer must be aware of the approximate methods used to obtain the soil parameters being used. This understanding can be obtained by each having training in the other's specialty.

To this end, the primary focus of this text will be on analysis and design of the interfacing elements for buildings, machines, and retaining structures and on those soil mechanics principles used to obtain the necessary soil parameters required to accomplish the design. Specific foundation elements to be considered include shallow elements such as footings and mats and deep elements such as piles and drilled piers.