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.

CAISSON STRUCTURES BASIC INFORMATION AND TUTORIALS

Caissons are box-like structures which are similar in concept to cofferdams but they usually form an integral part of the finished structure. They can be economically constructed and installed in water or soil where the depth exceeds 18„000.

There are 4 basic types of caisson namely:

1 . Box Caissons

2. Open Caissons

3. Monolithic Caissons

4. Pneumatic Caissons

The first three(3) used in water, usually of precast concrete and used in water being towed or floated into position and sunk. The last one is for land caissons of the open type and constructed in-situ.

Pneumatic Caissons ~ these are sometimes called compressed air caissons and are similar in concept to open caissons. They can be used in difficult subsoil conditions below water level and have a pressurised lower working chamber to provide a safe dry working area.

Pneumatic caissons can be made of concrete whereby they sink under their own weight or they can be constructed from steel with hollow walls which can be filled with water to act as ballast. These caissons are usually designed to form part of the finished structure.

FOUNDATION EXCAVATION BASIC AND TUTORIALS

There are many different types of excavations performed during the construction of a project. For example, soil may be excavated from the cut or borrow area and then used as fill.

Another example is the excavation of a shear key or buttress that will be used to stabilize a slope or landslide. Other examples of excavations are as follows:

1. Footing Excavations. This type of service involves measuring the dimension of geotechnical elements (such as the depth and width of footings) to make sure that they conform to the requirements of the construction plans. This service is often performed at the same time as the field observation to confirm bearing conditions.

2. Excavation of Piers. As with the excavation of footings, the geotechnical engineer may be required to confirm embedment depths and bearing conditions for piers. Figure 1 presents typical steps in the construction of a drilled pier.

FIGURE 6.44 Typical steps in the construction of a drilled pier: (a) dry augering through self-supporting cohesive soil; (b) augering through water bearing cohesionless soil with aid of slurry; (c) setting the casing.


3. Open Excavations. An open excavation is defined as an excavation that has stable and unsupported side slopes.

4. Braced Excavations. A braced excavation is defined as an excavation where the sides are supported by retaining structures. Figure 6.45 shows common types of retaining systems and braced excavations.

Common types of retaining systems and braced excavations. (From NAVFAC DM-7.2, 1982.)

GROUND FILLING AND COMPACTION BASIC AND TUTORIALS

Scarcity of good building land will often necessitate building on areas of fill. A variety of materials can be found in filled sites, ranging from quarry and mining waste to household and industrial refuse. Sites filled with refuse can give rise to problems of internal combustion, methane gas and other toxic chemicals; therefore building on these should be avoided whenever possible.

If the fill is fairly shallow then the most sensible option is to use piled foundations. The augured pile described earlier is often not suitable in fill if large stones and rubble are likely to be encountered and an alternative method is to use a driven pile.

One option is to use a driven pile made up of individual hollow pre-cast concrete sections, typically 300 400mm diameter. Using a special crane the pile is driven down into the ground adding extra sections as necessary. It has reached its correct depth when repeated hammer blows only produce minimal downward movement of the pile; this is known as a ‘set’ and is specified by an engineer.

As the fill naturally consolidates over the years there may be a downward force on the piles due to the friction of the ground against the pile sides. This ‘down-drag’ is rarely even and the resulting differential movement can cause cracking of the building.

In practice it is difficult to sleeve the whole length of a pile and several manufacturers prefer to coat the pile sections with a bituminous compound during manufacture. Under slow rates of strain the bituminous compound acts as a viscous fluid and reduces the down-drag (or up-lift) on the pile.

Deeper fill is best dealt with by the use of rafts and, as explained earlier, the raft spreads the load from the walls over the whole ground floor area. Some movement is to be expected and it is therefore essential to make sure that the services which enter or leave the property have flexible connectors immediately adjacent to the external wall.

Rafts, when designed for poor-quality ground, or ground where subsidence is expected, can be very expensive and have to be designed by structural engineers. However, they are a fast form of construction with minimal excavation and are sometimes also used on soft clays as an alternative to the reinforced wide strip foundation.

The pictures show a simple raft foundation formed from 150mm reinforced concrete slab and a more complicated raft foundation with downstand beams.

Occasionally it is possible to provide some form of ground treatment and use traditional strip foundations. On very large housing sites this can be cheaper than the use of rafts or piling. There are a variety of methods which attempt to increase the stability and bearing capacity of the ground. One method, called vibro compaction, involves the use of a crane-mounted poker which is driven into the ground.

A spinning eccentric weight inside the poker causes it to vibrate and this helps to compact the surrounding ground. The poker is then slowly extracted from the ground at the same time as sand is pumped through the poker to fill the void.

The operation is then repeated at 2 or 3m intervals to form a regular grid across the site. Vibro-replacement is another ground treatment; a treatment more suitable for cohesive soils. A special poker or piling rig forms a grid of stone columns in the ground, at the same time compressing the surrounding soil and increasing its density.

The stone piles act as weak columns transferring the building loads to a firmer strata. An alternative form of ground treatment is called dynamic compaction. This sounds a very grand title but in fact just involves dropping a weight of several tonnes on to the ground from a crane. It is not suitable if there are existing buildings in the immediate vicinity.

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.

BEARING CAPACITY BASICS AND TUTORIALS

BEARING CAPACITY BASIC INFORMATION
What Is Bearing Capacity?

Some designers, when in a hurry, tend to want simple ‘rules of thumb’ (based on local experience) for values of bearing capacity. But like most rules of thumb, while safe for typical structures on normal soils, their use can produce uneconomic solutions, restrict the development of improved methods of foundation design, and lead to expensive mistakes when the structure is not typical.

For typical buildings:

(1) The dead and imposed loads are built up gradually and relatively slowly.
(2) Actual imposed loads (as distinct from those assumed for design purposes) are often only a third of the dead load.
(3) The building has a height/width ratio of between 1/3 and 3.
(4) The building has regularly distributed columns or load bearing walls, most of them fairly evenly loaded.

Typical buildings have changed dramatically since the Second World War. The use of higher design stresses, lower factors of safety, the removal of robust non-load-bearing partitioning, etc., has resulted in buildings of half their previous weight, more susceptible to the effects of settlement, and built for use by clients who are less tolerant in accepting relatively minor cracking of finishes, etc.

Because of these changes, practical experience gained in the past is not always applicable to present construction. For non-typical structures:

(1) The imposed load may be applied rapidly, as in tanks and silos, resulting in possible settlement problems.
(2) There may be a high ratio of imposed to dead load. Unbalanced imposed-loading cases – imposed load over part of the structure – can be critical, resulting in differential settlement or bearing capacity failures, if not allowed for in design.
(3) The requirement may be for a tall, slender building which may be susceptible to tilting or overturning and have more critical wind loads.
(4) The requirement may be for a non-regular column/ wall layout, subjected to widely varying loadings, which may require special consideration to prevent excessive differential settlement and bearing capacity failure.

There is also the danger of going to the other extreme by doing complicated calculations based on numbers from unrepresentative soil tests alone, and ignoring the important evidence of the soil profile and local experience. Structural design and materials are not, as previously stated, mathematically precise; foundation design and materials are even less precise.

Determining the bearing capacity solely from a 100 mm thick small-diameter sample and applying it to predict the behaviour of a 10 m deep stratum, is obviously not sensible – particularly when many structures could fail, in serviceability, by settlement at bearing pressures well below the soil’s ultimate bearing capacity.

Bearing capacity
Probably the happy medium is to follow the sound advice given by experienced engineers in the British Standard Institution’s Code of practice for foundations, BS 8004. There they define ultimate bearing capacity as ‘the value of the gross loading intensity for a particular foundation at which the resistance of the soil to displacement of the foundation is fully mobilized.’ (Ultimate in this instance does not refer to ultimate limit state.)

The net loading intensity (net bearing pressure) is the additional intensity of vertical loading at the base of a foundation due to the weight of the new structure and its loading, including any earthworks. The ultimate bearing capacity divided by a suitable factor of safety – typically 3 – is referred to as the safe bearing capacity.

It has not been found possible, yet, to apply limit state design fully to foundations, since bearing capacity and settlement are so intertwined and influence both foundation and superstructure design. Furthermore, the superstructure itself can be altered in design to accommodate, or reduce, the effects of settlement. A reasonable compromise has been devised by engineers in the past and is given below.

SOIL CLASSIFICATION METHODS IN FOUNDATION DESIGN BASICS AND TUTORIALS

SOIL CLASSIFICATION METHODS IN FOUNDATION DESIGN BASIC INFORMATION
What Are The Methods Of Classifying Soils In Foundation Design?

It is necessary for the foundation engineer to classify the site soils for use as a foundation for several reasons:

1. To be able to use the database of others in predicting foundation performance.
2. To build one's own local database of successes (or any failures).
3. To maintain a permanent record that can be understood by others should problems later develop and outside parties be required to investigate the original design.
4. To be able to contribute to the general body of knowledge in common terminology via journal papers or conference presentations. After all, if one is to partake in the contributions of others, one should be making contributions to the general knowledge base and not be just a "taker."


The Unified Soil Classification System (USCS) of Table 2-1 is much used in foundation work. A version of this system has been standardized by ASTM as D 2487 (in Volume 04.08: Soil and Rock; Dimension Stone; Geosynthetics). The standardized version is similar to the original USCS as given by Casagrande (1948) but with specified percentages of sand or gravel passing specific sieves being used to give the "visual description" of the soil.

The original Casagrande USCS only classified the soil using the symbols shown in Table 2-1 (GP, GW, SM, SP, CL, CH, etc.), based on the indicated percentages passing the No. 4 and No. 200 sieves and the plasticity data. The author has always suggested a visual description supplement such as the following:

It is evident in this table that terms "trace" and "with" are somewhat subjective. The soil color, such as "blue clay," "gray clay," etc., is particularly useful in soil classification.

In many areas the color—particularly of cohesive soils—is an indication of the presence of the same soil stratum as found elsewhere. For example the "soft blue clay" on the soil profile of Fig. 2-4 for Chicago has about the same properties at any site in the Chicago area.

In foundation work the terms loose, medium, and dense, , and consistency descriptions such as soft, stiff, very stiff, etc., are also commonly used in foundation soil classification. Clearly, all of these descriptive terms are of great use to the local geotechnical engineer but are somewhat subjective.

That is, there could easily be some debate over what is a "medium" versus a "dense" sand, for example. The D 2487 standard removed some of the subjectiveness of the classification and requires the following terminology:

< 15% is sand or gravel use name (organic clay, silt, etc.)
15% < x < 30% is sand or gravel describe as clay or silt with sand, or clay or silt with gravel
> 30% is sand or gravel describe as sandy clay, silty clay, or gravelly clay, gravelly silt

The gravel or sand classification is based on the percentage retained on the No. 4 (gravel) sieve or passing the No. 4 and retained on the No. 200 (sand) sieves. This explanation is only partial, as the new standard is too lengthy to be presented in detail.

Although not stated in D 2487, the standard is devised for using a computer program3 to classify the soil. Further, not all geotechnical engineers directly use the ASTM standard, particularly if their practice has a history of success using the original USC system.

FIVE (5) MAJOR FACTORS THAT AFFECT THE ENGINEERING PROPERTIES OF SOILS

MAJOR FACTORS THAT AFFECT THE ENGINEERING PROPERTIES OF SOILS
What Are The 5 Major Factors That Affect The Engineering Properties of Soils?


Most factors that affect the engineering properties of soils involve geological processes acting over long time periods. Among the most important are the following.

1. Natural Cementation and Aging

All soils undergo a natural cementation at the particle contact points. The process of aging seems to increase the cementing effect by a variable amount. This effect was recognized very early in cohesive soils but is now deemed of considerable importance in cohesionless deposits as well.

The effect of cementation and aging in sand is not nearly so pronounced as for clay but still the effect as a statistical accumulation from a very large number of grain contacts can be of significance for designing a foundation. Care must be taken to ascertain the quantitative effects properly since sample disturbance and the small relative quantity of grains in a laboratory sample versus site amounts may provide difficulties in making a value measurement that is more than just an estimate.

Field observations have well validated the concept of the cementation and aging process. Loess deposits, in particular, illustrate the beneficial effects of the cementation process where vertical banks are readily excavated.

2. Overconsolidation

A soil is said to be normally consolidated (nc) if the current overburden pressure (column of soil overlying the plane of consideration) is the largest to which the mass has ever been subjected. It has been found by experience that prior stresses on a soil element produce an imprint or stress history that is retained by the soil structure until a new stress state exceeds the maximum previous one.

The soil is said to be overconsolidated (or preconsolidated) if the stress history involves a stress state larger than the present overburden pressure.

Overconsolidated cohesive soils have received considerable attention. Only more recently has it been recognized that overconsolidation may be of some importance in cohesionless soils. A part of the problem, of course, is that it is relatively easy to ascertain overconsolidation in cohesive soils but very difficult in cohesionless deposits.

The behavior of overconsolidated soils under new loads is different from that of normally consolidated soils, so it is important— particularly for cohesive soils—to be able to recognize the occurrence.

3. Mode of Deposit Formation

Soil deposits that have been transported, particularly via water, tend to be made up of small grain sizes and initially to be somewhat loose with large void ratios.

They tend to be fairly uniform in composition but may be stratified with alternating very fine material and thin sand seams, the sand being transported and deposited during high-water periods when stream velocity can support larger grain sizes.

These deposits tend to stabilize and may become very compact (dense) over geological periods from subsequent overburden pressure as well as cementing and aging processes.

Soil deposits developed'where the transporting agent is a glacier tend to be more varied in composition. These deposits may contain large sand or clay lenses. It is not unusual for glacial deposits to contain considerable amounts of gravel and even suspended boulders.

Glacial deposits may have specific names as found in geology textbooks such as moraines, eskers, etc.; however, for foundation work our principal interest is in the uniformity and quality of the deposit. Dense, uniform deposits are usually not troublesome. Deposits with an erratic composition may be satisfactory for use, but soil properties may be very difficult to obtain.

Boulders and lenses of widely varying characteristics may cause construction difficulties. The principal consideration for residual soil deposits is the amount of rainfall that has occurred. Large amounts of surface water tend to leach materials from the upper zones to greater depths. A resulting stratum of fine particles at some depth can affect the strength and settlement characteristics of the site.

4. Quality of the Clay

The term clay is commonly used to describe any cohesive soil deposit with sufficient clay minerals present that drying produces shrinkage with the formation of cracks or fissures such that block slippage can occur.

Where drying has produced shrinkage cracks in the deposit we have a fissured clay. This material can be troublesome for field sampling because the material may be very hard, and fissures make sample recovery difficult. In laboratory strength tests the fissures can define failure planes and produce fictitiously low strength predictions (alternatively, testing intact pieces produces too high a prediction) compared to in situ tests where size effects may either bridge or confine the discontinuity.

A great potential for strength reduction exists during construction where opening an excavation reduces the overburden pressure so that expansion takes place along any fissures. Subsequent rainwater or even local humidity can enter the fissure so that interior as well as surface softening occurs.

A clay without fissures is an intact clay and is usually normally consolidated or at least has not been over consolidated from shrinkage stresses. Although these clays may expand from excavation of overburden, the subsequent access to free water is not so potentially disastrous as for fissured clay because the water effect is more nearly confined to the surface.

5. Soil Water

Soil water may be a geological phenomenon; however, it can also be as recent as the latest rainfall or broken water pipe. An increase in water content tends to decrease the shear strength of cohesive soils. An increase in the pore pressure in any soil will reduce the shear strength.

A sufficient increase can reduce the shear strength to zero—for cohesionless soils the end result is a viscous fluid. A saturated sand in a loose state can, from a sudden shock, also become a viscous fluid. This phenomenon is termed liquefaction and is of considerable importance when considering major structures (such as power plants) in earthquake-prone areas.

When soil water just dampens sand, the surface tension produced will allow shallow excavations with vertical sides. If the water evaporates, the sides will collapse; however, construction vibrations can initiate a cave-in prior to complete drying.

The sides of a vertical excavation in a cohesive soil may collapse from a combination of rainfall softening the clay together with excess water entering surface tension cracks to create hydrostatic water pressure. In any case, the shear strength of a cohesive soil can be markedly influenced by water.

Even without laboratory equipment, one has probably seen how cohesive soil strength can range from a fluid to a brick-like material as a mudhole alongside a road fills during a rain and subsequently dries. Ground cracks in the hole bottom after drying are shrinkage (or tension) cracks.

FOUNDATION CLASSIFICATIONS AND SELECT DEFINITION BASICS AND TUTORIALS

FOUNDATION CLASSIFICATIONS AND SELECT DEFINITION BASIC INFORMATION
What Are Structure Foundations?


Foundations may be classified based on where the load is carried by the ground, producing:

Shallow foundations—termed bases, footings, spread footings, or mats. The depth is generally D/B < 1 but may be somewhat more. Refer to Fig. 1-la.

Deep foundations—piles, drilled piers, or drilled caissons. Lp/B > 4+ with a pile illustrated
in Fig. l-\b.

Figure 1-1 illustrates general cases of the three basic foundation types considered in this text and provides some definitions commonly used in this type of work. Because all the definitions and symbols shown will be used throughout the text, the reader should give this figure careful study.

The superstructure brings loads to the soil interface using column-type members. The loadcarrying columns are usually of steel or concrete with allowable design compressive stresses on the order of 14O+ MPa (steel) to 1O+ MPa (concrete) and therefore are of relatively small cross-sectional area. The supporting capacity of the soil, from either strength or deformation considerations, is seldom over 1000 kPa but more often on the order of 200 to 250 kPa.

This means the foundation is interfacing two materials with a strength ratio on the order of several hundred. As a consequence the loads must be "spread" to the soil in a manner such that its limiting strength is not exceeded and resulting deformations are tolerable. Shallow foundations accomplish this by spreading the loads laterally, hence the term spread footing.

Where a spread footing (or simply footing) supports a single column, a mat is a special footing used to support several randomly spaced columns or to support several rows of parallel columns and may underlie a portion of or the entire building. The mat may also be supported, in turn, by piles or drilled piers.

Foundations supporting machinery and such are sometimes termed bases. Machinery and the like can produce a substantial load intensity over a small area, so the base is used as a load-spreading device similar to the footing.

Deep foundations are analogous to spread footings but distribute the load vertically rather than horizontally. A qualitative load distribution over depth for a pile is shown in Fig. 1-1 b. The terms drilled pier and drilled caisson are for the pile type member that is constructed by drilling a 0.76+-m diameter hole in the soil, adding reinforcing as necessary, and backfilling the cavity with concrete.

A major consideration for both spread footings (and mats) and piles is the distribution of stresses in the stress influence zone beneath the foundation [footing or pile tip (or point)].

The theoretical distribution of vertical stress beneath a square footing on the ground surface is shown in Fig. IAa. It is evident that below a critical depth of about 5B the soil has a negligible increase in stress (about 0.02qo) from the footing load.

This influence depth depends on B, however. For example, if B = 0.3 m, the critical stress zone is 5 X 0.3 = 1.5 m, and if B = 3 m, the zone is 15 m for a zonal influence depth ratio of 1 : 10. Because these B values are in a possible range beneath a large building, any poor soils below a depth of 2 m would have a considerable influence on the design of the wider footings.

Any structure used to retain soil or other material (see Fig. 1-lc) in a geometric shape other than that naturally occurring under the influence of gravity is a retaining structure.

Retaining structures may be constructed of a large number of materials including geotextiles, wood and metal sheeting, plain or reinforced concrete, reinforced earth, precast concrete elements, closely spaced pilings, interlocking wood or metal elements (crib walls), and so on. Sometimes the retaining structure is permanent and in other cases it is removed when it is no longer needed.

The foundations selected for study in this text are so numerous that their specialized study is appropriate. Every building in existence rests on a foundation whether formally designed or not. Every basement wall in any building is a retaining structure, whether formally designed or not.

Major buildings in areas underlain with thick cohesive soil deposits nearly always use piles or drilled caissons to carry the loads vertically to more competent strata, primarily to control settlement. Note that nearly every major city is underlain by clay or has zones where clay is present and requires piles or caissons.

Numerous bridges have retaining structures at the abutments and spread foundations carrying the intermediate spans. Usually the abutment end reactions are carried into the ground by piles. Harbor and offshore structures (used primarily for oil production) use piles extensively and for both vertical and lateral loads.

SPECIAL PROBLEMS OF CLAY SOIL IN FOUNDATION OF BUILDING BASICS AND TUTORIALS

PROBLEMS OF CLAY WHEN PRESENT ON THE FOUNDATION OF BUILDING
What Are The Problems Of Clay As Foundation?


Special Problems of Clay Soils
The majority of clay soils can cause foundation problems as they slowly change in volume due to increases or decreases in water content. This change is related to the season with the ground expanding in the winter and
contracting in the summer.

This seasonal change, which may be in the order of + or - 30mm at ground level, can affect the clay to a depth of about a metre, with the ground below this level having a fairly stable moisture content.

Where clay soils contain trees the problem is more severe. Trees and heavy vegetation draw a considerable amount of water from the ground during the growing season.

A mature poplar takes up as much as 1000 litres of water per week. In long hot summers with little or no rainfall the tree will continue to draw moisture out of the ground and the clay will shrink.

This, of course, is in addition to the seasonal drying mentioned above. If buildings are sited near individual or groups of trees serious cracking in the walls can occur as a result of ground movement.

To prevent this movement from affecting strip foundations they must be deeper than the tree roots. An alternative, of course, is to site the buildings well clear of the trees.

Where trees have been removed from clay soils the opposite problem occurs. As the ground slowly regains moisture it will expand and this can continue for a period of up to 10 years.


The pressure that dry clay develops when reabsorbing moisture is likely to be greater than that imposed by the building load and upward movement of the structure will occur.

If houses are built on the site before this ground expansion is complete, cracking will occur in the walls and foundations; the swelling will be uneven because it will be concentrated around the removed tree.

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.

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, design 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.

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

COMPUTER PROGRAMS IN FOUNDATION ANALYSIS AND DESIGN BASICS AND TUTORIALS

FOUNDATION ANALYSIS COMPUTER PROGRAMS BASIC INFORMATION
What Are Foundation Analysis Computer Programs?

A large number of foundation engineering problems can be efficiently analyzed and/or designed using a digital computer. Particular advantages of using a computer accrue from these features:

1. One is able to try a range of problem variables to obtain a feel for the effect of specifying, or using, a particular set of soil parameters.

2. One can avoid having to use tabulated data or plotted curves, which usually require interpolation and excessive simplification of the foundation model.

3. One can minimize computational errors from these sources:

a. Erroneous key entry when using a calculator. The bad entry is (or should be) output to paper using a computer so the input can be checked.

b. Omission of computational steps. A working computer program usually includes all the design steps. A set of hand computations may not include every step for any number of reasons (forget, not be aware of, carelessness, etc.).

c. Calculator chip malfunction not readily detected except by using two calculators. Computer chips are often internally checked on power-up, or output is so bad that chip errors are visually detected.

4. With output to a printer one has a paper record of the problem for office files without the necessity of transcribing data from intermediate steps. This avoids copy errors such as 83 for 38 and the like.

The major disadvantage of using a computer program is that it is difficult to write a first generation, error-free program of real use in a design office. Program usability tends to increase with each revision (or history) level.

With the current wide availability of computer programs—many, such as those on the included diskette, having a "history"—the advantages gained from program use far exceed any perceived disadvantages.

The author suggests that both geotechnical and foundation engineers should use computer programs whenever possible—and certainly be aware of what computer program(s) each is likely to use for the given project.

This statement is made with full awareness of the possibility of program errors (or "bugs"). Fortunately, most geotechnical software is task-specific so that the possibility of program errors or their not being detected is not so likely as for some of the large finite-element or structural analysis programs that purport to solve a wide range of tasks.

In any case, the author cannot recall a single reported foundation design failure that can be attributed to a bad4 computer program. It should be evident that computer programs vary widely in perceived quality, perceived quality being defined here as problem limitations and "ease of use." Both users and programmers should be aware that it is difficult to predefine the full range of problem parameters likely to be encountered in practice, so nearly any geotechnical program of significant value is likely to have some built-in limitations.

Ease of use is highly subjective and depends more on user familiarity with a program than how easy it really is to use—many users like pulldown menus and graphics whereas others are quite content without these features. As a final comment on computer programs, be aware that although business applications and games usually have a market in the hundreds of thousands, geotechnical programs have a potential market of only a few thousand.

This small market means geotechnical software is likely to be more expensive than other software and, to minimize development costs, it is not likely to have many so-called user-friendly features.

One should routinely check the output from any computer program used for design or analysis. The user is responsible for his or her design since it is impossible to write a computer program with any usefulness that cannot be misused in some manner. Primarily for this reason most computer programs are sold or licensed with a disclaimer making the user responsible.

Fortunately, most computer programs can be written to be somewhat self-checking, either by writing back the input data or by providing output that can be readily identified as correct (or incorrect) if the user understands or knows how to use the program. It should go without saying that, if you do not know much about the specific problem being designed or analyzed, you should first do some preliminary study before using a computer program on it.

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.