DESIGN PROCEDURES FOR A BUILDING FOUNDATION (STEP BY STEP)

Design Procedure of foundation

Good design must not only be safe but must aim to save construction costs, time and materials. The following procedures should help to achieve this and an ‘educated’ client will recognize the importance of funding this work with a realistic fee.

1. DECIDE THE LOCATION OF COLUMNS & FOUNDATION AND TYPE OF LOADS ACTING ON THEM.(E.X DEAL LOAD,  LIVE LOAD OR WIND LOAD)

On the building plan, the position of columns and loadbearing walls should be marked, and any other induced loadings and bending moments. The loads should be classified into dead, imposed and wind loadings, giving the appropriate partial safety factors for these loads.

2. ESTIMATE ALLOWABLE BEARING PRESSURE OF SOIL USING GROUND INVESTIGATION REPORT.

From a study of the site ground investigation (if available), the strength of the soil at various depths or strata below foundation level should be studied, to determine the safe bearing capacity at various levels. These values – or presumed bearing values (from any standards or codes) in the absence of a site investigation – are used to estimate the allowable bearing pressure.

3. DECIDE DEPTH OF FOUNDATION

The invert level (underside) of the foundation is determined by either the minimum depth below ground level unaffected by temperature, moisture content variation or erosion – this can be as low as 450 mm in granular soils but, depending on the site and ground conditions, can exceed 1 m – or by the depth of basement, boiler house, service ducts or similar.

4. CALCULATE FOUNDATION AREA

The foundation area required is determined from the characteristic (working) loads and estimated allowable pressure. This determines the preliminary design of the types or combination of types of foundation. The selection is usually based on economics, speed and buildability of construction.

5. DETERMINE VARIATION IN VERTICAL STRESSES

The variation of vertical stress w.r.t depth is determined, to check for possible over-stressing of any underlying weak strata.

6. CALCULATE SETTLEMENT

Settlement calculations should be carried out to check that the total and differential settlements are acceptable. If these are unacceptable then a revised allowable bearing pressure should be determined, and the foundation design amended to increase its area, or the foundations should be taken down to a deeper and stronger stratum.

7. COST CONTROL

Before finalizing the choice of foundation type, the preliminary costing of alternative superstructure designs should be made, to determine the economics of increasing superstructure costs in order to reduce foundation costs.

8. CONSIDER TIME

Alternative safe designs should be checked for economy, speed and simplicity of construction. Speed and economy can conflict in foundation construction – an initial low-cost solution may increase the construction period. Time is often of the essence for a client needing early return on capital investment. A fast-track programme for superstructure construction can be negated by slow foundation construction.

9. VARIATION IN GROUND CONDITION

The design office should be prepared to amend the design, if excavation shows variation in ground conditions from those predicted from the site soil survey and investigation.

PRACTICAL PROBLEMS TO KEEP IN MIND BEFORE FOUNDATION DESIGN

Foundation design

There are, in foundation design, a number of practical construction problems and costs to be considered.

The chief ones are:

1. The foundations should be kept as shallow as possible, commensurate with climatic effects on, and strength of, the surface soil; particularly in waterlogged ground. Excavation in seriously waterlogged ground can be expensive and slow.
2. Expensive and complex shuttering details should be avoided, particularly in stiffened rafts. Attention should be paid to buildability.
3. Reduction in the costs of piling, improvements in ground treatment, advances in soil mechanics, etc. have considerably altered the economics of design, and many standard solutions are now out-of-date. There is a need to constantly review construction costs and techniques.
4. Designers need to be more aware of the assumptions made in design, the variability of ground conditions, the occasional inapplicability of refined soil analyses and the practicality of construction.
5. The reliability of the soil investigation, by critical assessment.
6. Effect of construction on ground properties, i.e. vibration from piling, deterioration of ground exposed by excavation in adverse weather conditions, removal of overburden, seasonal variation in the water-table, compaction of the ground by construction plant.
7. Effect of varying shape, length and rigidity of the foundation, and the need for movement and settlement joints.
8. After-effects on completed foundations of sulfate attack on concrete, ground movements due to frost heave, shrinkable clays, and the effects of trees; also changes in local environment, e.g. new construction, re-routing of heavy traffic, installation of plant in adjoining factories causing impact and vibration.
9. Fast but expensive construction may be more economic than low-cost but slow construction to clients needing quick return on capital investment.
10. Effect of new foundation loading on existing adjoining structures.

These practical considerations are illustrated by the following examples.

EXAMPLE 1: EXCAVATION IN WATERLOGGED GROUND

A simple example of excavation in waterlogged ground exemplifies the problems which may be encountered. At the commencement of a 1–2 m deep underpinning contract in mass concrete, groundwater was found to be rising much higher and faster than previous trial pits had indicated. The circumstances were such that a minipiling contractor was quickly brought onto site, and speedily installed what was, at face value, a more costly solution, but proved far less expensive overall than slowly struggling to construct with mass concrete while pumping. As will be well-known to many of our readers, few small site pumps are capable of running for longer than two hours without malfunctioning!

EXAMPLE 2: VARIABILITY OF GROUND CONDITIONS

On one site a varying clay fill had been placed to a depth of roughly 2 m over clay of a similar soft to firm consistency.

Since a large industrial estate was to be developed on the site in numerous phases by different developers, a thorough site investigation had been undertaken. Nevertheless, on more than one occasion, the project engineer found himself looking down a hole of depth 2 m or greater, trying to decide if a mass concrete base was about to be founded in fill or virgin ground, and in either case whether it would achieve 100 kN/m2 allowable bearing pressure or not. This emphasizes the importance of engineers looking at the ground first-hand by examining the trial pits rather than relying on the site investigation report from the relative comfort of their desk.

EXAMPLE 3: RELIABILITY OF THE SOILS INVESTIGATION

On one site a contractor quoted a small diameter steel tube pile length of 5 m (to achieve a suitable set), based upon a site investigation report. In the event his piles achieved the set at an average of 22 m (!), so obviously cost complications ensued. In addition to this, one of the main difficulties was convincing the contractor to guarantee his piles at that depth, as he was understandably concerned about their slenderness.

EXAMPLE 4: DETERIORATION OF GROUND EXPOSED BY EXCAVATION

An investigation by the authors’ practice of one particular failure springs to mind as an example. Part of a factory had been demolished exposing what had been a party wall, but a 20 m length of this wall was undermined by an excavation for a new service duct and a classic failure ensued. The exposed excavation was then left open over a wet weekend, resulting in softening of the face and a collapse occurred early on the Monday.

So often the most catastrophic of failures are as a result of these types of classic textbook examples, which could be prevented by the most basic precautions.

EXAMPLE 5: EFFECT OF NEW FOUNDATION ON EXISTING STRUCTURE

A new storage silo was to be constructed within an existing mill, and the proposal was to found it on a filled basement, in the same way that the adjacent silo had been 20 years before. The authors’ practice was called in for their opinion fairly late in the day, with the steel silo already under fabrication.

After investigation of the fill, the client was advised to carry the new silo on small diameter piles through the fill down to bedrock. This would thereby avoid placing additional loading into the fill, and thus causing settlement of the existing silo.

WHAT CONDITIONS REQUIRE USE OF PILE FOUNDATION?

Piles are structural members that are made of steel, concrete, or timber. They are used to build pile foundations, which are deep and which cost more than shallow foundations. Despite the cost, the use of piles often is necessary to ensure structural safety. The following list identifies some of the conditions that require pile foundations:

CONDITIONS THAT REQUIRE THE USE OF PILE FOUNDATION

1. COMPRESSIBLE OR WEAK UPPER SOIL LAYER

When one or more upper soil layers are highly compressible and too weak to support the load transmitted by the superstructure, piles are used to transmit the load to underlying bedrock or a stronger soil layer, as shown in Figure-a. When bedrock is not encountered at a reasonable depth below the ground surface, piles are used to transmit the structural load to the soil gradually. The resistance to the applied structural load is derived mainly from the frictional resistance developed at the soil-pile interface. (See Figure-b)

Fig-a

Fig-b

2. PRESENCE OF HORIZONTAL FORCES

When subjected to horizontal forces (see Figure-c), pile foundations resist by bending, while still supporting the vertical load transmitted by the superstructure. This type of situation is generally encountered in the design and construction of earth-retaining structures and foundations of tall structures that are subjected to high wind or to earthquake forces.

Fig-c

3. PRESENCE OF EXPANSIVE SOILS

In many cases, expansive and collapsible soils may be present at the site of a proposed structure. These soils may extend to a great depth below the ground surface. Expansive soils swell and shrink as their moisture content increases and decreases, and the pressure of the swelling can be considerable. If shallow foundations are used in such circumstances, the structure may suffer considerable damage. However, pile foundations may be considered as an alternative when piles are extended beyond the active zone, which is where swelling and shrinking occur. (See Figure-d) Soils such as loess are collapsible in nature. When the moisture content of these soils increases, their structures may break down. A sudden decrease in the void ratio of soil induces large settlements of structures supported by shallow foundations. In such cases, pile foundations may be used in which the piles are extended into stable soil layers beyond the zone where moisture will change.

Fig-d

4. SUBJECTED TO UPLIFTING FORCES

The foundations of some structures, such as transmission towers, offshore platforms, and basement mats below the water table, are subjected to uplifting forces. Piles are sometimes used for these foundations to resist the uplifting force. (See Figure-e)

Fig-e

5. SOIL EROSION

Bridge abutments and piers are usually constructed over pile foundations to avoid the loss of bearing capacity that a shallow foundation might suffer because of soil erosion at the ground surface. (See Figure-f) Although numerous investigations, both theoretical and experimental, have been conducted in the past to predict the behavior and the load-bearing capacity of piles in granular and cohesive soils, the mechanisms are not yet entirely understood and may never be. The design and analysis of pile foundations may thus be considered somewhat of an art as a result of the uncertainties involved in working with some subsoil conditions. This chapter discusses the present state of the art.

Fig-f

HOW TO INCREASE DURABILITY OF CONCRETE PILES?

DURABILITY OF CONCRETE PILES

Properly mixed concrete compacted to a dense impermeable mass is one of the most permanent of all constructional material and give little cause of concern about its long-term durability in a non aggressive environment. However, concrete can be attacked by sulphate and sulfuric acidoccurring naturally in soils, by corrosive chemicals which may be present in industrial waste in fill materials and by organic acids and carbon dioxide present in ground water as a result of decaying vegetable matters. Attack by sulphates is a disruptive process whereas the action of organic acids or dissolved carbon dioxide is one of leaching. Attack by sulphuric acid combines features of both processes. The severity of attack by soluble sulphates must be assessed by determining the soluble sulphate content and the proportions of the various cat ions present in an aqueous extract of the soil. These determinations must be made in all cases where the concentration of sulphate in a soil sample exceeds 0.5%.

A dense, well compacted concrete provides the best protection against   the attack by sulphates on concrete piles, pile cap and ground beams. The low permeability of dense concrete prevents or greatly restricts the entry of the sulphates in to the pore spaces of the concrete. For this reason high strength precast concrete piles are most favorable type to use. However they are not suitable for all the site conditions and bored cast in situ / driven cast in situ piles if adopted must be designed to achieve the required degree of impermeability and resistance to aggressive action. Neither high alumina cement nor super sulphated cement is favored for piling work. Instead, reliance is placed on the resistance of dense impermeable concrete made with a low water cement ratio. Coating of tar or bitumen on the surface, metal sheeting or glass fibre wrapping impregnated with bitumen may be adopted.

Pile caps and ground beams can be protected on the underside by a layer of heavy gauge polythene sheeting laid on a sand carpet or on blinding concrete.   The vertical sides can be protected after removing the form work by applying hot bitumen spray coats, bituminous paint, trowelled on mastic asphalt or adhesive plastic sheeting.

Precautions against the aggressive action by sea water on concrete need only be considered in respect of precast concrete piles. Cast in situ concrete is used only as a hearting to steel tubes or cylindrical precast concrete shell pills. For precast concrete piles for marine condition, a minimum ordinary portland cement content of 360 kg/m3 and a maximum water cement ratio of 0.45 by weight should be adopted.

WHAT ARE THE REQUIREMENTS OF BENTONITE (POWDER AND SUSPENSION) USED IN PILING WORK AS PER IS-2911

The bentonite powder and bentonite suspension used for pilling work shall satisfy the following requirements:

Bentonite

1. The liquid limit of bentonite when tested in accordance with IS 2720 (Part 5) shall be 400 percent or more.
2. The bentonite suspension shall be made by mixing it with fresh water using a pump for circulation. The density of the freshly prepared bentonite suspension shall be between 1.03 and 1.10 g/ml depending upon the pile dimensions and the type of soil in which the pile is to be bored. The density of bentonite after contamination with deleterious material in the borehole may rise upto 1.25 g/ml. This should be brought down to at least 1.12 g/ml by flushing before concreting.
3. The marsh viscosity of bentonite suspension when tested by a marsh cone shall be between 30 to 60 stoke; in special cases it may be allowed up to 90s.
4. The pH value of bentonite suspension shall be between 9 and 11.5.

UNCERTAINTIES INVOLVED IN PLATE LOAD TEST

Even though plate load tests are very commonly used, they do not represent the behavior of prototype foundations totally for the following reasons:

1. The zone of influence for prototype foundation is larger and deeper than for small size plate. Thus, in stratified soil deposits the soils at greater depths will have a significant influence on the bearing capacity and settlement of prototype foundation. Whereas, for the plate its behavior is controlled chiefly by the upper strata. The figure shown below shows the effect of size of foundation and plate, on the zone of influence.

   

   PLT

2. In truly uniform cohesive deposit the ultimate bearing capacity of the plate will be nearly equal to the ultimate bearing capacity of the foundation under undrained conditions of loading. But in such cases the settlement of the plate will not represent the long-term consolidation settlement of the foundation.
3. In granular and frictional soils the bearing capacity increases with the width of the foundation. Plate load test must be carried out on different sizes of plates to infer the effect of width of foundation on bearing capacity.
4. If the plate load test is carried out within the capillary zone the plate can bear higher ultimate load since the depth of capillary zone is more than or of the same order of magnitude as the size of the plate. The effect of capillary zone is, however, small in actual foundations, due to their large size. To avoid this error, the plate load test must be carried out at the water-table level.

HOW TO CALCULATE PILE LOAD CAPACITY USING STATIC CONE PENETRATION TEST?

PILE LOAD CAPACITY CALCULATION USING STATIC CONE PENETRATION (SCPT) TEST DATA

The safe load capacity of bored cast in situ piles can be calculated using the test data obtained from static cone penetration test as described below.

We know that the ultimate load capacity of pile consist of two terms, i.e.

• End bearing resistance
• Skin friction resistance

So we need to calculate the above two parameters, thereafter by adding these two parameters we can obtain the ultimate load capacity of pile.

STEP-1 (CALCULATE END-BEARING RESISTANCE OF PILE)

The ultimate end bearing resistance of pile is calculated using the formula given below.

Where,

qu = Ultimate end bearing resistance, in kN/m2

qc0 = Average static cone resistance over a depth of 2D below the pile tip, in kN/m2

qc1 = Minimum static cone resistance over the same 2D below the pile tip, in kN/m2

qc2 = Average of the envelope of minimum static cone resistance values over the length of pile of 8D above the pile tip, in kN/m2

D = Diameter of pile

STEP-2 (CALCULATE SKIN-FRICTION RESISTANCE)

For different types of soils, the ultimate skin friction resistance (fs), in kN/m2, can be obtained using the table given below.

Type of soil	Skin friction resistance (fs), in kN/m2
qc less than 1000 kN/m2	qc/30 to qc/10
Clay	qc/25 to 2qc/25
Silty sand & silty clay	qc/100 to qc/25
Sand	qc/100 to qc/50
Coarse sand & gravel	qc/150 to qc/100

Where, qc is cone resistance in kN/m2.

STEP-3 (CALCULATE SAFE PILE LOAD CAPACITY)

Add the ultimate end bearing resistance (qu) and ultimate skin friction resistance (fs) as calculated in step-1&2 respectively, and divide it with a suitable factor of safety (minimum 2.5) to get the safe load capacity of pile, i.e.

Qsafe = (qu+fs) / F.O.S.

Where,

Qsafe = Safe load capacity of pile

qu = Ultimate end bearing resistance of pile

fs = Ultimate skin friction resistance of pile

F.O.S. = Factor of safety of pile (usually 2.5)

REFERENCE

IS: 2911-Part-1-Sec-2

HOW TO AVOID FOUNDATION FAILURE IN BLACK COTTON SOIL?

BLACK COTTON SOIL

Black cotton soil has a tendency to shrink and swell excessively. When these type of soil come in contact with water, they swell and when becomes dry, it shrinks. This alternate process of swelling and shrinking results in the differential settlement of foundation which in turn causes cracks in building. The cracks thus formed are sometimes 15 to 20 cm wide and 2.5 to 4.5 m deep.

Therefore necessary precautions need to be taken during construction to avoid any damage to building foundation.

Following precaution should be employed during construction on black cotton soil.

PRECAUTIONS TO BE TAKEN

1. The maximum load on black cotton soil should be limited to 5 tonnes/m2. If there is a chance for water to come in contact with foundation, then the load should be limited to 4.9 tonnes/m2.
2. Foundation should be placed at a depth where the cracks cease to extend. The minimum depth of foundation should be at least 1.5 m.
3. The main wall of the building must be provided with all round reinforced concrete ties or bands.
4. Reinforced concrete ties or bands having 10 to 15 cm deep should be placed at plinth level, lintel level and eaves level.
5. In case the depth of black cotton soil is only 1 m to 1.5 m, then completely remove the entire black cotton soil and place the foundation below that depth.
6. Try to avoid direct contact of black cotton soil with foundation material. This can be achieved by making wider trenches for foundation and filling spaces on either side of the foundation masonry with sand or morroum.
7. Ram the bed of the foundation trench to make it farm and hard. On this rammed bed, spread a thick layer of morroum (i.e. 30 cm) in two layers, each layer being 15 cm. each layer should be water and rammed properly to get highest possible density. On this compacted layer of morroum, place either sand or stone upto desired height where concrete foundation bed has to be made.
8. In case of important structures, raft foundation should be provided.
9. For less important structures (such as boundary wall construction), the foundation should preferably taken at least 15 cm below the depth at which cracks in soil cease to occur.
10. Construction should be done in dry season.
11. For main walls or for load bearing walls, the width of the trench should be dug 40 cm wider than width of foundation. Then fill the space on either side of the trench (i.e. 20 cm in each side) with coarse sand. This is done to separate the foundation masonry from direct contact with black cotton soil. In case of compound wall this width of sand filling can be reduced to 15 cm on each side.
12. Under reamed pile foundation is also a good choice of foundation in black cotton soil.

WHAT ARE THE ADVANTAGES & DISADVANTAGES OF PRECAST CONCRETE PILES?

PRECAST CONCRETE PILES – ADVANTAGES & DISADVANTAGES

Precast concrete piles are cast, cured and stored in a yard before they are installed in the field mostly by driving.

ADVANTAGES OF PRECAST CONCRETE PILES

1. Reinforcement used in the pile is not liable to change its place or get disturbed
2. The defects in pile can be easily identified after the removal of forms, and these defects (such as presence of cavity or hole) can be repaired before driving the pile.
3. The cost of manufacturing will be less, as a large number of piles are manufactured at a time.
4. Precast concrete piles can be driven under water. If the subsoil water contains more sulphates, the concrete of cast in situ piles would not set. Thus precast concrete piles have added advantage in such a circumstance.
5. Precast concrete piles are highly resistant to biological and chemical actions of the sub soil.
6. Better quality control can be implemented as compared to bored cast in situ piles.
7. These piles can be constructed in various cross-sectional shapes such as circular, octagonal or square.

DISADVANTAGES OF PRECAST CONCRETE PILES

1. These piles are usually very heavy. So special equipments are required for handling and transportation.
2. Sufficient care must be taken at the time of transportation, otherwise piles may break.
3. For embedding these piles in field heavy pile driving equipment is required.
4. These piles are costly as extra reinforcement is required to bear handling and driving stresses.
5. The length of the pile is restricted since it depends upon the transport facility.
6. Once constructed, it is not possible to increase the length of the pile (as per the site demand)
7. If the pile is found to be too long, during driving, it is difficult and uneconomical to cut. Also cutting of extra length results in the wastage of material.
8. Driving these piles created a lot of noise pollution.

REFERENCE

Building Construction by Dr. B.C. Punima

15 DIFFERENT TYPES OF LOADS ON BUILDING (IN SHORT)

TYPES OF LOADS

External loads on a structure may be classified in several different ways. In one classification, they may be considered as static or dynamic.

1. Static loads are forces that are applied slowly and then remain nearly constant.
2. One example is the weight, or dead load, of a floor or roof system.
3. Dynamic loads vary with time. They include repeated and impact loads.
4. Repeated loads are forces that are applied a number of times, causing a variation in the magnitude, and sometimes also in the sense, of the internal forces. A good example is an off-balance motor.
5. Impact loads are forces that require a structure or its components to absorb energy in a short interval of time. An example is the dropping of a heavy weight on a floor slab, or the shock wave from an explosion striking the walls and roof of a building.
6. External forces may also be classified as distributed and concentrated.
7. Uniformly distributed loads are forces that are, or for practical purposes may be considered, constant over a surface area of the supporting member. Dead weight of a rolled-steel I beam is a good example.
8. Concentrated loads are forces that have such a small contact area as to be negligible compared with the entire surface area of the supporting member. A beam supported on a girder, for example, may be considered, for all practical purposes, a concentrated load on the girder.
9. Another common classification for external forces labels them axial, eccentric, and torsional.
10. An axial load is a force whose resultant passes through the centroid of a section under consideration and is perpendicular to the plane of the section.
11. An eccentric load is a force perpendicular to the plane of the section under consideration but not passing through the centroid of the section, thus bending the supporting member.
12. Torsional loads are forces that are offset from the shear center of the section under consideration and are inclined to or in the plane of the section, thus twisting the supporting member.
13. Also, building codes classify loads in accordance with the nature of the source.
14. Dead loads include materials, equipment, constructions, or other elements of weight supported in, on, or by a building, including its own weight, that are intended to remain permanently in place.
15. Live loads include all occupants, materials, equipment, constructions, or other elements of weight supported in, on, or by a building and that will or are likely to be moved or relocated during the expected life of the building.
16. Impact loads are a fraction of the live loads used to account for additional stresses and deflections resulting from movement of the live loads.
17. Wind loads are maximum forces that may be applied to a building by wind in a mean recurrence interval, or a set of forces that will produce equivalent stresses.
18. Snow loads are maximum forces that may be applied by snow accumulation in a mean recurrence interval.
19. Seismic loads are forces that produce maximum stresses or deformations in a building during an earthquake.