PIPING FAILURES IN SAND CUTS.

Sheet piling is used for cuts in sand and the excavation must be dewatered by pumping from the bottom of the excavation. Sufficient penetration below the bottom of the cut must be provided to reduce the amount of seepage and to avoid the danger of piping.

Piping is a phenomenon of water rushing up through pipe-shaped channels due to large upward seepage pressure. When piping takes place, the weight of the soil is counteracted by the upward hydraulic pressure and as such there is no contact pressure between the grains at the bottom of the excavation. Therefore, it offers no lateral support to the sheet piling and as a result the sheet piling may collapse. Further the soil will become very loose and may not have any bearing power.

It is therefore, essential to avoid piping. For further discussions on piping.

Permeability and Seepage. Piping can be reduced by increasing the depth of penetration of sheet piles below the bottom of the cut.

Soil Samples and Soil Profiles.

It is a wise precaution to take more soil samples than necessary to determine the ground conditions (and increasing the frequency of samples does not proportionally increase the cost of the soil survey). It is not however necessary to test every single sample. If the surface soil is weak and underlain by good rock or dense gravel there may be little point in testing the weak surface soil if piling down to the good strata is proposed.

Soil profiles (section through boreholes) are extremely helpful in enabling the designer to visualize the ground
conditions. This valuable aid is, in the authors’ opinion, too often given inadequate attention in site investigations.

Many foundation failures can be traced back to faulty  visualization of the ground conditions due to inadequate soil profiles or misinterpretation of them. A typical soil profile is shown in Fig. 3.7.

Most experienced designers would tend to study the soil profile first before reading the site report, studying the test results and checking other data. This makes for efficiency, better assessment of site conditions, improved judgement of data, it warns of problems and can indicate the need for possible further investigation.

Some typical misinterpretations or inadequate data leading to false conclusions and similar errors are shown in Fig. 3.8 (see also Figs 2.28, 2.29 and 2.31).

Fig. 3.7 Soil profile for a typical site.

Fig. 3.8 Misinterpretation of soil profile 

(Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983),

Fig. 2.28 Mistaken bedrock.

Fig. 2.29 Unchecked fault.

Fig. 2.31 Folded strata mistaken for level strata.

Soil Investigation.

A soil survey can range from a few trial pits inspected by the designer and the soil untested by laboratory analysis to an extensive borehole investigation with deep and numerous bores and extensive sampling and testing of the soil usually by specialist investigation contractors.

The factors affecting the investigation are the amount of existing information available, the known uniformity or likely variability of the sub-soil in the area, the foundation loading and the type of structure, the general topography and likely groundwater conditions of the site.

Subsidiary factors such as the amount of time and money available, the site access and other matters should not inhibit the planning of a thorough (and as reliable as is  reasonably possible) investigation.

No matter what kind of investigation is carried out, the authors, from experience, recommend the digging of  trial pits as a first stage. Trial pits have over the past few decades fallen into almost contemptuous dismissal by some  with the increased sophistication of boring techniques, increased cost of labour in digging pits and increased awareness of the limitations of pits (e.g. they do not detect underlying soft soils which can be affected by foundation loading). But during the same period there has been increased adaptability, mobility, etc., of relatively small excavators. Such machines can easily excavate and backfill a dozen pits, or trenches, in a day to a depth of 3–6 m and can be hired on a daily basis at cost-effective rates. The cost of replacing services damaged during excavation can be substantial, especially in the case of optical fibre cables, and the responsibility for adequate insurance cover should therefore not be overlooked.

1 Borehole layout  Three bores are the minimum necessary to determine the dip of a plane strata (where known with confidence to be plane) and as a rough guide this is the minimum for a proposed investigation (it is almost self-evident not to have too many!)...

2 Trial pit layout  Trial pits should be located near to the proposed or existing foundations but not so close as to adversely affect foundation excavation or to disturb existing underground services and drains. They should straddle the proposed site of the building to give...

3 Hand augers  Hand augers are sometimes used in preliminary reconnaissance since the equipment is light, cheap and immediately available, and so that overall, time can be saved in planning a full survey. They can, in soft to firm soils, bore a hole about 150 mm diameter to a depth of 3–4 m and provide disturbed samples of the soil. They can be used in restricted spaces, which is useful in investigating foundation failure below a confined basement. However the work can be physically hard, somewhat slow and very difficult, or impossible, in stony clays and gravels.

4 Boring  Most bores are carried out using light cable percussion plant backed up, when necessary, with rotary coring and other equipment and attachments. The cable percussion rig commonly uses an 8 m high tripod and employs a friction winch to raise and lower the boring tubes and tools. Rotary coring is used when hard shales, boulders or rock strata are encountered.

There is an increasing variety of plant, sampling methods and tools, with particular advantages in cost, quality of sampling, speed of operation, use in conditions of limited access or headroom, etc., and the choice of rig is affected by the likely soil conditions to be encountered.

5 Backfilling of trial pits and boreholes  If bores and particularly pits are positioned sufficiently close to the proposed structure so as to affect foundation excavation then they should be carefully backfilled. A strip footing founded on firm clay and passing over an inadequately compacted backfilled trial pit is effectively passing over a soft-spot. A borehole can sometimes act as an artesian well or as a seepage point. Trial pits or trenches should be backfilled in layers with controlled compaction. Boreholes should be backfilled, as the casing is withdrawn, with selected excavated material and punned with a weighted shell. Grouting boreholes is sometimes necessary with 4 : 1 cement : bentonite. The quality of backfilling of trial pits is however often unreliable and if the pits are close to the foundation they should be re-excavated along with the foundation excavation and backfilled again after completion of foundation construction.

6 Soil sampling  Samples of the soil are taken from boreholes and trial pits so that the soil can be described and tested. There are two types of samples:

• Disturbed samples. Samples taken from boring tubes or hand excavated from the sides and bottom of trial pits where the soil structure is disturbed  i.e. broken up, cut, pressed, etc. These samples are placed in airtight jars (similar to screw lid jam-jars), labelled to identify the borehole or pit number, the position of the sample, the number given to it in the records, and the date taken.

Failure to label samples in standard format will obviously lead to confusion at the laboratory so the label must be secure and the information noted on it must be legible and written in waterproof ink.

Disturbed samples are tested to determine, mainly,  the type and description of the soil. The sampling and
testing of disturbed samples is relatively inexpensive and the test results are used to determine the test
programme of undisturbed samples.

If the disturbed samples are to be used to determine the moisture content of the soil it is important that the
sample jar should be completely filled by the sample to prevent it drying out. As a further precaution the air- tight cap should be wound round by a water-resistant
tape.

• Undisturbed samples. The term undisturbed  is somewhat  of a misnomer for even with refined equipment it is difficult to obtain a true undisturbed sample. Certainly, undisturbed samples are generally superior to disturbed samples in representing more closely the actual in situ structure and moisture content of the soil.

The soil structure and moisture content are important factors in soil strength and behaviour under load. Disturbed soil is trimmed from the ends of the sample tubes, the ends are then covered by foil and waxed before screwing on the tube cap or lid. Labels, giving the same information as for disturbed samples, should be placed both inside  the cap and outside the tube.

Undisturbed samples are tested to determine mainly the strength and behaviour of the soil. Undisturbed  samples are relatively expensive to obtain and test and it is generally not necessary to test all the samples. Nevertheless it is advisable to obtain at least one sample for each stratum at each borehole. The test programme is fully determined after study of borehole logs and soil profiles.

7 Storage of samples  Preferably samples should be sent to the testing laboratory immediately – and this, of course, is not always possible.  If they are just left lying around the site they could be sub- ject to drying out, impact, etc. so they should be carefully stacked and stored in a cool and somewhat moist site hut or container box.

8 Frequency of sampling  The soil investigation engineer, preferably with the design engineer’s report on site study, reconnaissance and trial pit findings if available, can decide on an economic frequency
of sampling. Generally undisturbed soil samples should be taken at 1.5 m intervals and at change of stratum level and disturbed samples taken at 1 m intervals. This is not a rigid rule and should be varied to suit soil and foundation conditions. When trial pits have not been excavated, these intervals should be halved from ground level to 2–3 m below the anticipated depth of foundation excavation. It is at or near ground level that the soil is usually most variable due to exposure to weather, change in moisture conditions and
variations in the water-table level.

The foreman driller should keep a log noting the type (classification) of soil, its depth, change of stratum level,
position of obstructions, changes of soil conditions within a stratum, groundwater level, seepage and similar information. Experienced and reliable foremen drillers are becoming, unfortunately, rarer and it is essential that the soil survey investigator backs up the foreman’s observations by adequate inspection visits by site supervision engineers.

The log should give a continuous description of the soil in the borehole from ground level to base of bore. It is important that the foreman is aware of the standard classification and description used in References 3 and 4 and does not solely employ (the often colourful) local terms such as cowbelly, sludge, mucky clay, cobbly clay. While these terms may be well-known to local engineers they can be unfamiliar and totally misleading to others. The local terms are often an invaluable guide to experienced local engineers in describing the soil and its properties and it would be a pity in some cases if these were to die out. Where there is a mixture of clay, silt and sand the MIT (Massachusetts Institute of Technology) classification should be used (see Fig. 3.3).

Fig. 3.3 The MIT classification for clay, silt and sand.

9 Appointment of specialist soil investigator  Most design offices do not have sufficient demand for soil
investigations to warrant the capital costs of obtaining site and laboratory equipment, nor the current costs of employing site and laboratory personnel. It is therefore generally necessary to appoint specialist firms – and this may not always be as easy as it might appear.

The work should be carried out by competent soil survey specialists of good reputation, staffed by experienced engineers (and drillers) who will not only supervise the borings but also the testing and can be relied upon to report accurately and advise soundly on their findings. The specialist firm should carry adequate indemnity. In the past a number of excellent firms have been driven out of business by cut-throat competition from ‘cowboy’ firms savagely undercutting sensible rates. This is a deplorable situation which could cost the client, in the end, far more than has been  saved by employing such firms. (On more than one occasion the authors’ practice has been asked to investigate foundation failures and found that borehole logs are a  complete fabrication – because they were not done!)

There should be detailed discussion between the design engineer and the soil specialist on the survey specification, cost and time. Soil specialists may not have wide experience of foundation design, behaviour of structures, economics of alternative designs, construction difficulties, etc., so the discussion is essential for reliable investigations.

It is also strongly advisable for the design engineer in  person to inspect the boring during progress to see firsthand the condition of the soil samples and sampling methods.

Changes of Soil Properties During Excavation.

The soil at level 1, below ground level – see Fig. 1.9 – is subject to pressure, and thus consolidation, due to the weight of the soil above, and is in equilibrium. If the overlying soil is removed to form a basement then the pressure, and consolidation effects, at level 1 are also removed. The unloaded soil, in this condition, is known as over-consolidated, and is likely to recover from the consolidation and rise in level (heave). This can be likened to the elastic recovery of contraction on a column when its load is removed.

Fig. 1.9 Heave following removal of overburden.

Structures: Ground treatment (geotechnical processes)

Soil properties can change under the action of superstructure loading. It compacts, consolidates and drains, and so becomes denser, stronger and less prone to settlement.

These improvements can also be induced by a variety of geotechnical processes  before construction. The ground can be temporarily loaded before construction (preconsolidated), hammered by heavy weights to compact it (dynamic consolidation), vibrated to shake down and reduce the voids ratio (vibro stabilization), the soil moisture drained off (dewatering, sand wicks), the voids filled with cementitious material (grouting, chemical injection), and similar techniques.

Imported material (usually sandy gravel) can be laid over weak ground and compacted so that the pressure from column pad foundations can be spread over a greater area.

Imported material can also be used to  seal contaminated sites. Imported soils can also be laid and compacted in thin (say 150 mm) layers with polymer nets placed between each layer. The composite material, known as reinforced soil, has been widely used in retaining walls and embankments.

These techniques are discussed in detail in later. The development of these techniques has made it possible to build economically on sites which, until recently, were too difficult and expensive to be considered as building land.

Temporary geotechnical processes can be used to ease excavation. Typical cases are:

(1) Temporary dewatering to allow the excavation to be carried out in the dry,

(2) Chemical injection, freezing, grouting and the like to
maintain sides of excavations, etc.

Permanent processes are employed to improve the ground properties by:

(1) Compaction (making the soil denser and thus stronger), and

(2) Consolidation and drainage processes to reduce the magnitude of settlement. (Such measures are discussed in detail later.)

Interaction of Superstructure and Soil.

The superstructure, its foundation, and the supporting soil should be considered as a structural entity, with the three elements interacting.

Adjustments to the superstructure design to resist the effects of bearing failure and settlements, at minor extra
costs, are often more economic than the expensive area increase or stiffening of the foundations. Some examples from the authors’ practice are given here to illustrate these adjustments. Adjustments to the soil to improve its properties are briefly discussed later. The choice of foundation type is outlined later. Adjustments and choices are made to produce the most economical solution.

Example 1: Three pinned arch   The superstructure costs for a rigid-steel portal-frame shed are generally cheaper than the three pinned arch solution (see Fig. 1.2). Differential settlement of the column pad bases will however seriously affect the bending moments (and thus...(more)

Example 2: Vierendeel superstructure   The single-storey reinforced concrete (r.c.) frame structure shown in Fig. 1.3 was founded in soft ground liable to excessive sagging/differential settlement. Two main solutions were investigated: (1) Normal r.c. superstructure founded on...(more)

Example 3: Prestressed brick diaphragm wall and Composite deep beams   Prestressed brick diaphragm wall A sports hall was to be built on a site with severe mining subsidence. At first sight the economic superstructure solution of a brickwork ...(more)

Example 4: Buoyancy raft   A four-storey block of flats was to be built on a site where part of the site was liable to ground heave due to removal  of trees. The sub-soil was of low bearing capacity overlying dense gravel. The building plan was amended to incorporate ...(more)

FOUNDATIONS ON GEOTEXTILE-REINFORCED SOIL.

LABORATORY MODEL TEST RESULTS

Results of a limited number of model tests conducted in the early to mid-1980's to determine the bearing capacity of surface foundations (that is, Df =0) resting on geotextile-reinforced soils can be found in the literature. Guido et al. [4] reported results of several laboratory model tests for a square surface foundation measuring 0.31 m × 0.31 m (B × B) and supported by a loose to medium sand reinforced with multiple layers of nonwoven melt-bonded geotextile (size b ×b). Figure 7.13 shows the geometric parameters of  the problem under consideration, and Fig. 7.14 shows some of the results of these tests.  For the tests reported in Fig. 7.14, the following parameters apply: relative density of sand, Dr = 50%; width of geogrid layers, b = 0.62 m; b/B = 0.2; u/B = 0.5;  and h/B = 0.25.

Similar model test results on a continuous surface foundation supported by a saturated clay (Φ = 0 condition) reinforced by heat-bonded nonwoven geotextile were reported by Sakti and Das [5]. The load-settlement curves for some of these tests are given in Fig. 7.15. For these tests the following parameters apply:  width, B = 76.2 mm; undrained cohesion of clay, cu = 22.5 kN/m2, and u/B = h/B = 0.33. The tests clearly show that the ultimate bearing capacity of foundations increases when geotextile reinforcement is used.

COMMENTS ON GEOTEXTILE REINFORCEMENT Figures 7.14 and 7.15 show  that geotextile reinforcement contributes  to the increase in ultimate bearing capacity of foundations on sand and saturated clay. However, at low settlement level of the foundation, geotextile reinforcement hardly contributes to the load bearing capacity. This is...(more)

FIGURE 7.13   Foundation on geotextile-reinforced soil

FIGURE 7.14 Model test results of Guido et al. [4] on geotextile-
reinforced sand for a square surface foundation

FIGURE 7.15 Model test results of Sakti and Das [5] on geotextile-reinforced
saturated clay for a continuous surface foundation

REFERENCES.

DESIGN PROCEDURE FOR A CONTINUOUS FOUNDATION ON A GRANULAR SOIL REINFORCED.

Following is a step-by-step procedure for designing a continuous foundation on a granular soil reinforced with metallic strips.

Step 1. Establish the following parameters

a. Foundation
- Net load per unit length, Q
- Depth, Df
- Factor of safety, FS, against bearing capacity failure on unreinforced soil
- Allowable settlement, Se

b. Soil
- Unit weight, '
- Friction angle, %
- Modulus of elasticity, Es
- Poisson’s ratio, µs

c. Reinforcement ties
- Width, w
- Soil-tie friction angle, Φµ
- Factor of safety against tie pullout, FSP
- Factor of safety against tie break, FSB

Step 2. Assume values of B, u, h, and number of reinforcement layers N.
Note that the depth of reinforcement,  d, from the bottom of the foundation

Step 3. Assume a value of LDR = wn

Step 4. Determine the allowable bearing capacity, q'all  , on unreinforced sand, or

Step 5. Determine the allowable bearing capacity, q'all   , based on allowable settlement. From Eq. (5.41),

The magnitude of I7 for continuous foundations can be taken to be approximately 2 for this calculation.

Step 6. The smaller of the two allowable bearing capacities (that is, q'all  or q''all  ) is equal to qo.

Step 7. Calculate qR (load per unit area of the foundation on reinforced soil) as

Step 8. Calculate T for all layers of reinforcement using Eq. (7.1). 

Step 9. Calculate the magnitude of FP /T for each layer to see if FP /T >=  FSP.

If FP /T < FSP , the length of reinforcing strips may have to be increased by substituting X´ (>X) in Eq. (7.5) so that FP /T is equal toFSP

 

Step 10. Use Eq. (7.2) to obtain the thickness of the reinforcement strips.

Step 11. If the design is unsatisfactory, repeat Steps 2 through 10.

TABLE 2.1   Terzaghi’s Bearing Capacity Factors—Eqs. (2.32), (2.33), and (2.34)

REFERENCES.

SOIL STABILIZATION BY INJECTION OF SUITABLE GROUTS.

Grouting is a process whereby fluid like materials, either in suspension, or solution form, are
injected into the subsurface soil or rock.

The purpose of injecting a grout may be any one or more of the following:

1. To decrease permeability.
2. To increase shear strength.
3. To decrease compressibility.

Suspension-type grouts include soil, cement, lime, asphalt emulsion, etc., while the solution type grouts include a wide variety of chemicals. Grouting proves especially effective in the following cases:

1. When the foundation has to be constructed below the ground water table. The deeper the foundation, the longer the time needed for construction, and therefore, the more benefit gained from grouting as compared with dewatering.
2. When there is difficult access to the foundation level. This is very often the case in city work, in tunnel shafts, sewers, and subway construction.
3. When the geometric dimensions of the foundation are complicated and involves many boundaries and contact zones.
4. When the adjacent structures require that the soil of the foundation strata should not be excavated (extension of existing foundations into deeper layers).

Grouting has been extensively used primarily to control ground water flow under earth and masonry dams, where rock grouting is used. Since the process fills soil voids with some type of stabilizing material grouting is also used to increase soil strength and prevent excessive settlement.

Many different materials have been injected into soils to produce changes in the engineering properties of the soil. In one method a casing is driven and injection is made under pressure to the soil at the bottom of the hole as the casing is withdrawn. In another method, a grouting hole is drilled and at each level in which injection is desired, the drill is withdrawn and a collar is placed at the top of the area to be grouted and grout is forced into the soil under pressure. Another method is to perforate the casing in the area to be grouted and leave the casing permanently in the soil.

Penetration grouting may involve portland cement or fine grained soils such as bentonite or other materials of a paniculate nature. These materials penetrate only a short distance through most soils and are primarily useful in very coarse sands or gravels. Viscous fluids, such as a solution of sodium silicate, may be used to penetrate fine grained soils. Some of these solutions form gels that restrict permeability and improve compressibility and strength properties.

Displacement grouting usually consists of using a grout like portland cement and sand mixture which when forced into the soil displaces and compacts the surrounding material about a central core of grout. Injection of lime is sometimes used to produce lenses in the soil that will block the flow of water and reduce compressibility and expansion properties of the soil. The lenses are produced by hydraulic fracturing of the soil.

The injection and grouting methods are generally expensive compared with other stabilization techniques and are primarily used under special situations as mentioned earlier. For a detailed study on injections, readers may refer to Caron et al., (1975).

SAND COMPACTION PILES AND STONE COLUMNS.

Sand Compaction Piles
Sand compaction piles consists of driving a hollow steel pipe with the bottom closed with a collapsible plate down to the required depth; filling it with sand, and withdrawing the pipe while air pressure is directed against the sand inside it. The bottom plate opens during withdrawal and the sand backfills the voids created earlier during the driving of the pipe. The in-situ soil is densified while the pipe is being withdrawn, and the sand backfill prevents the soil surrounding the compaction pipe from collapsing as the pipe is withdrawn. The maximum limits on the amount of fines that can be present are 15 percent passing the No. 200 sieve (0.075 mm) and 3 percent passing 0.005 mm. The distance between the piles may have to be planned according to the site conditions.

Stone Columns
The method described for installing sand compaction piles or the vibroflot described earlier can be used to construct stone columns. The size of the stones used for this purpose range from about 6 to 40 mm. Stone columns have particular application in soft inorganic, cohesive soils and are generally inserted on a volume displacement basis.

The diameter of the pipe used either for the construction of sand drains or sand compaction piles can be increased according to the requirements. Stones are placed in the pipe instead of sand, and the technique of constructing stone columns remains the same as that for sand piles.

Stone columns are placed 1 to 3 m apart over the whole area. There is no theoretical procedure for predicting the combined improvement obtained, so it is usual to assume the foundation loads are carried only by the several stone columns with no contribution from the intermediate ground (Bowles, 1996).

Bowles (1996) gives an approximate formula for the allowable bearing capacity of stone columns as

Stone columns should extend through soft clay to firm strata to control settlements. There is no end bearing in Eq. (21.11) because the principal load carrying mechanism is local perimeter shear.

Settlement is usually the principal concern with stone columns since bearing capacity is usually quite adequate (Bowles, 1996). There is no method currently available to compute settlement on a theoretical basis.

Stone columns are not applicable to thick deposits of peat or highly organic silts or clays (Bowles, 1996). Stone columns can be used in loose sand deposits to increase the density.

SOIL IMPROVEMENT.

INTRODUCTION
General practice is to use shallow foundations for the foundations of buildings and other such structures, if the soil close to the ground surface possesses sufficient bearing capacity. However, where the top soil is either loose or soft, the load from the superstructure has to be transferred to deeper firm strata. In such cases, pile or pier foundations are the obvious choice.

There is also a third method which may in some cases prove more economical than deep foundations or where the alternate method may become inevitable due to certain site and other environmental conditions.

This third method comes under the heading foundation soil improvement. In the case of earth dams, there is no other alternative than compacting the remolded soil in layers to the required density and moisture content.

The soil for the dam will be excavated at the adjoining areas and transported to the site. There are many methods by which the soil at the site can be improved. Soil improvement is frequently termed soil stabilization, which in its broadest sense is alteration of any property of a soil to improve its engineering performance. Soil improvement

1. Increases shear strength
2. Reduces permeability, and
3. Reduces compressibility

The methods of soil improvement considered in this chapter are

1. Mechanical compaction
2. Dynamic compaction
3. Vibroflotation
4. Preloading
5. Sand and stone columns
6. Use of admixtures
7. Injection of suitable grouts
8. Use of geotextiles

SAFE BEARING PRESSURE FROM EMPIRICAL EQUATIONS BASED ON CPT VALUES FOR FOOTINGS ON COHESIONLESS SOIL.

The static cone penetration test in which a standard cone of 10 cm2 sectional area is pushed into the soil without the necessity of boring provides a much more accurate and detailed variation in the soil as discussed in Chapter 9. Meyerhof (1956) suggested a set of empirical equations based on the

Terzaghi and Peck curves (1948). As these equations were also found to be conservative, modified forms with an increase of 50 percent over the original values are given below.

An approximate formula for all widths

where qc is the cone point resistance in kg/cm2 and qs in kPa.

The above equations have been developed for a settlement of 25 mm.

Meyerhof (1956) developed his equations based on the relationship qc = 4Ncor kg/cm2  for penetration resistance in sand where Ncor is the corrected SPT value.

EMPIRICAL EQUATIONS BASED ON SPT VALUES FOR FOOTINGS ON COHESIONLESS SOILS.

Footings on granular soils are sometimes proportioned using empirical relationships. Teng (1969)
proposed an equation for a settlement of 25 mm based on the curves developed by Terzaghi and
Peck (1948). The modified form of the equation is

Meyerhof (1956) proposed the following equations which are slightly different from that of Teng



Experimental results indicate that the equations presented by Teng and Meyerhof are too conservative.

Bowles ( 1 996) proposes an approximate increase of 50 percent over that of Meyerhof which can also be applied to Teng's equations. Modified equations of Teng and Meyerhof are,

Teng's equation (modified),

If the tolerable settlement is greater than 25 mm, the safe bearing pressure computed by the above equations can be increased linearly as,

where q's = net safe bearing pressure for a settlement S'mm, qs = net safe bearing pressure for a settlement of 25 mm.

DESIGN CHARTS FROM SPT VALUES FOR FOOTINGS ON SAND.

The methods suggested by Terzaghi et al., (1996) for estimating settlements and bearing pressures of footings founded on sand from SPT values are based on the findings of Burland and Burbidge (1985). The SPT values used are corrected to a standard energy ratio. The usual symbol Ncor is used in all the cases as the corrected value.

Formulas for Settlement Calculations.
The following formulas were developed for computing settlements for square footings.

For normally consolidated soils and gravels

If the footing is established at a depth below the ground surface, the removal of the soil above the base level makes the sand below the base preconsolidated by excavation. Recompression is assumed for bearing pressures up to preconstruction effective vertical stress q'o at the base of the foundation. Thus, for sands normally consolidated with respect to the original ground surface and for values of qs greater than q'o, we have,

 Figure 13.4 Thickness of granular soil beneath foundation contributing to
settlement, interpreted from settlement profiles (after Burland and Burbidge 1985)

It may be noted here that the ground water table at the site may lie above or within the depth of influence Zl Burland and Burbidge (1985) recommend no correction for the settlement calculation even if the water table lies within the depth of influence Zl. On the other hand, if for any reason, the water table were to rise into or above the zone of influence Zl after the penetration tests were conducted, the actual settlement could be as much as twice the value predicted without taking the water table into account.


Chart for Estimating Allowable Soil Pressure
Fig. 13.5 gives a chart for estimating allowable bearing pressure qs (on settlement consideration)
corresponding to a settlement of 16 mm for different values of TV (corrected). From Eq. (13.6), an
expression for q may be written as (for normally consolidated sand)

Figure 13.5 Chart for estimating allowable soil pressure for footing on sand on the
basis of results of standard penetration test. (Terzaghi, et al., 1996)

The chart m Fig. 13.5 gives the relationships between B and Q. The value of qs may be obtained from Q for any given width B. The Q to be used must conform to Eqs (13.12), (13.13) and (13.14).

The chart is constructed for square footings of width B. For rectangular footings, the value of qs should be reduced in accordance with Eq. (13.10). The bearing pressures determined by this procedure correspond to a maximum settlement of 25 mm at the end of construction.

It may be noted here that the design chart (Fig. 13.5b) has been developed by taking the SPT values corrected for 60 percent of standard energy ratio.

Field Plate Load Tests - Foundations.

The plate load test is a semi-direct method to estimate the allowable bearing pressure of soil to induce a given amount of settlement. Plates, round or square, varying in size, from 30 to 60 cm and thickness of about 2.5 cm are employed for the test.

The load on the plate is applied by making use of a hydraulic jack. The reaction of the jack load is taken by a cross beam or a steel truss anchored suitably at both the ends. The settlement of the plate is measured by a set of three dial gauges of sensitivity 0.02 mm placed 120° apart. The dial gauges are fixed to independent supports which remain undisturbed during the test. Figure 13.2a shows the arrangement for a plate load test. The method of performing the test is essentially as follows:

1. Excavate a pit of size not less than 4 to 5 times the size of the plate. The bottom of the pit should coincide with the level of the foundation.
 
2. If the water table is above the level of the foundation, pump out the water carefully and keep it at the level of the foundation.
 
3. A suitable size of plate is selected for the test. Normally a plate of size 30 cm is used in sandy soils and a larger size in clay soils. The ground should be levelled and the plate should be seated over the ground.

4. A seating load of about 70 gm/cm2 is first applied and released after some time. A higher load is next placed on the plate and settlements are recorded by means of the dial gauges.

Observations on every load increment shall be taken until the rate of settlement is less than 0.25 mm per hour. Load increments shall be approximately one-fifth of the estimated safe bearing capacity of the soil. The average of the settlements recorded by 2 or 3 dial gauges shall be taken as the settlement of the plate for each of the load increments.

5. The test should continue until a total settlement of 2.5 cm or the settlement at which the soil
fails, whichever is earlier, is obtained. After the load is released, the elastic rebound of the
soil should be recorded.

Figure 13.2a Plate load test arrangement

From the test results, a load-settlement curve should be plotted as shown in Fig. 13.2b. The allowable pressure on a prototype foundation for an assumed settlement may be found by making use of the following equations suggested by Terzaghi and Peck (1948) for square footings in granular soils.

in which Sf, and Sp are expressed in inches and B in feet.

The permissible settlement Sf, for a prototype foundation should be known. Normally a
settlement of 2.5 cm is recommended. In Eqs (13.la) or (13.2) the values of Sf, and bp are known.

The unknowns are Sp and B. The value of Sp for any assumed size B may be found from the equation. Using the plate load settlement curve Fig. 13.3 the value of the bearing pressure corresponding to the computed value of Sp is found. This bearing pressure is the safe bearing pressure for a given permissible settlement Sf.

The principal shortcoming of this approach is the unreliability of the extrapolation of Eqs (13. la) or (13.2).

Since a load test is of short duration, consolidation settlements cannot be predicted. The test gives the value of immediate settlement only. If the underlying soil is sandy in nature immediate settlement may be taken as the total settlement. If the soil is a clayey type, the immediate settlement is only a fraction of the total settlement. Load tests, therefore, do not have much significance in clayey soils to determine allowable pressure on the basis of a settlement criterion.

Plate load tests should be used with caution and the present practice is not to rely too much on this test. If the soil is not homogeneous to a great depth, plate load tests give very misleading results.

Assume, as shown in Fig. 13.2c, two layers of soil. The top layer is stiff clay whereas the bottom layer is soft clay. The load test conducted near the surface of the ground measures the characteristics of the stiff clay but does not indicate the nature of the soft clay soil which is below.

The actual foundation of a building however has a bulb of pressure which extends to a great depth into the poor soil which is highly compressible. Here the soil tested by the plate load test gives results which are highly on the unsafe side.

A plate load test is not recommended in soils which are not homogeneous at least to a depth equal to 1 1/2  to 2 times the width of the prototype foundation.

Plate load tests should not be relied on to determine the ultimate bearing capacity of sandy soils as the scale effect gives very misleading results. However, when the tests are carried on clay soils, the ultimate bearing capacity as determined by the test may be taken as equal to that of the foundation since the bearing capacity of clay is essentially independent of the footing size.

Figure 13.2c Plate load test on non-homogeneous soil

Foundations: Allowable and Safe Bearing Pressures.

The theories used in that chapter are based on shear failure criteria. They do not indicate the settlement that a footing may undergo under the ultimate loading conditions. From the known ultimate bearing capacity obtained from any one of the theories, the allowable bearing pressure can be obtained by applying a suitable factor of safety to the ultimate value. When we design a foundation, we must see that the structure is safe on two counts. They are,

1. The supporting soil should be safe from shear failure due to the loads imposed on it by the superstructure, 2. The settlement of the foundation should be within permissible limits.

Hence, we have to deal with two types of bearing pressures. They are,

1. A pressure that is safe from shear failure criteria,
2. A pressure that is safe from settlement criteria.

For the sake of convenience, let us call the first the allowable bearing pressure and the second the safe bearing pressure.

In all our design, we use only the net bearing pressure and as such we call qna the net allowable bearing pressure and qs the net safe bearing pressure. In designing a foundation, we use the least of the two bearing pressures. We learnt that qna is obtained by applying a suitable factor of safety (normally 3) to the net ultimate bearing capacity of soil. In this chapter we will learn how to obtain qs. Even without knowing the values of qna and qs, it is possible to say from experience which of the two values should be used in design based upon the composition and density of soil and the size of the footing. The composition and density of the soil and the size of the footing decide the relative values of qna and qs.

The ultimate bearing capacity of footings on sand increases with an increase in the width, and in the same way the settlement of the footing increases with increases in the width. In other words for a given settlement 5p the corresponding unit soil pressure decreases with an increase in the width of the footing. It is therefore, essential to consider that settlement will be the criterion for the design of footings in sand beyond a particular size. Experimental evidence indicates that for footings smaller than about 1.20 m, the allowable bearing pressure q is the criterion for the design of footings, whereas settlement is the criterion for footings greater than 1.2 m width.

The bearing capacity of footings on clay is independent of the size of the footings and as such the unit bearing pressure remains theoretically constant in a particular environment. However, the settlement of the footing increases with an increase in the size. It is essential to take into consideration both the shear failure and the settlement criteria together to decide the safe bearing
pressure.

However, footings on stiff clay, hard clay, and other firm soils generally require no settlement analysis if the design provides a minimum factor of safety of 3 on the net ultimate bearing capacity of the soil. Soft clay, compressible silt, and other weak soils will settle even under moderate pressure and therefore settlement analysis is necessary.

Soil Investigation.

A detailed study of the subsurface soil should be made as outlined in TM 5-818-1.  The scope of this
investigation depends on the nature and complexity of the soil, and size,   functional intent, and cost of the structure.  These parameters are frequently the consolidated-drained friction angle  N for cohesionless soil, undrained shear strength C for cohesive soil, soil elastic modulus E for undrained loading, soil dry unit weight,
and the groundwater table elevation.  Refer to TM 5-818-1 for guidance on evaluating these parameters. Consolidation and  potential heave characteristics may also be required for clay soils and  the needed parameters may be evaluated following procedures presented in TM 5-818-7.  Other tests associated with soil minvestigation are:

(1) In situ tests. The standard penetration te (SPT) according to ASTM D 1586 and the cone penetration tesi (CPT) according to ASTM D 3441 may be perfonned to estimate stnngth parameters from guidance in TM 5-818-1.

(2) Soil sampling. Most soil data are obtained from results of laboratozy tests on specimens from disturbed and relatively undisturbed samples. Visual classification of soil is necessary to roughly locate the different soil strata as a function of depth and lateral variation.

(3) Location and sampling depth. Borings should be spaced to define the lateral geology and soil nonconformities. It may be sufficient to limit exploration to a depth that includes weathered and fissured material, to bedrock, or to depths influenced by construction. For individual drilled shafts, depths of at least five tip diameters beneath the tip of the deepest element of end-bearing foundations should be investigated. For driven pile groups, a much deeper investigation is appropriate and should extend a minimum of 20 feet or two pile group widths beneath the tip of the longest anticipated pile, or (o bedrock, whichever is less. These depths are the minimum required to provide sufficient data for settlement analysis. The potential for settlement should be checked to ensure compliance with design specifications.

(4) Selection of soil parameters. Results of laboratoiy and in situ tests should be plotted as a function of depth to determine the characteristies of the subsurface soils. Typical plots include the friction angle Ø  for sands, undrained shear slrength Cu  for clays, and the elastic modulus Es. These data should be grouped depending on the geological interpretation of the subsoil of similar types. Each soil type may be given icpresentative values of strength, stiffness, and consolidation or swell indexes for estimating soil settlement or heave. Soil strength parameter could be estimated from established correlations from laboratory testing.

(a) Classification. Soil classification characteristics should be applied to estirnate soil strength arid other parameters from guidance in TM 5-818-1. Data such as gradation from sieve analysis, Atterberg hmits, water content, and specific gravity should be detemined from tests on disturbed specimens. Refer to ASTM D 2487 for soil classification procedures.

(b) Strength. Soil strength parameters are required to evaluate vertical and lateral load capacity. The strength of cohesive soil may’ be determined from triaxial test results performed cm undisturbed soil specimens at confining pressures equal to the in situ total vertical overburden pressure σv. The unconsolidated undrained Q test will determine the undrained shear strength (cohesion) Cu of cohesive soils. The effective friction angle Ø' and cohesion of overconsolidated soils may be determined from results of R tests with pore pressure measurements using a confming pressure similar to the effective overburden pressure Ø'. However, analyses are usually performed assuming eithcr cohcsive or cohcsionless soil. Mean strength values within (he zone of potential failure may be selected for pile capacily artalysis. Refer to TM 5-818- 1 and NAVFAC DM-7. 1, “Soil Mechanics,” for further details.

(c) Elastic modulus. Young’s Ilastic modulus Es, is required for evaluation of vertical displacements of the deep foundation. The Es, may be estimated as the initial slope from the stress-strain curves of strength test results performed on undisturbed soil specimens. The Es, for clay may be estimated from (he undrained shear strength Cu, the overconsolidation ratio, and the plasticity index (PI) shown in Figure 1 -8. The Es typically vanes from 100 to 400 kips per square foot (ksf) for soft clay, 1,000 to 2,000 ksf for stiff clay, 200 to 500 ksf for bose sand, and 500 to 1 ,000 ksf for dense sand.

 

The value of ks is recommended to be about 40, 150, and 390 ksf/ft for loose, medium, and dense dry or moist sands, respectively, and 35, 100, and 210 ksf/ft for submerged sands after FHWA-RD-85- 106, “Behavior of Piles and Pile Groups UnderLateral Load.” The value of ks is also recommended to be about 500, 1,700, and 5,000 ksf/ft for stiff clays with average undrained shear strength of 1 to 2, 2 to 4, and 4 to 8 ksf respectively.

Figure  1-8. Variation  Kcu  for clay with respect to undrained shear strength and overconsolidation ratio

Feasibility Study and Site Conditions.

Feasibility study. A reconnaissance study should be performed to determine the requiriements of a deep foundation designs, and the scope of in situ soil and foundation load tests.  Required cost estimates and schedules to conduct the soil investigation, load tests, and construction should be prepared and updated as the project progresses.

Site conditions.  Examination of the site includes history, geology, visual inspection of the site and adjacent area, and local design and construction experience.  Maps may provide data on wooded  areas, ponds, streams, depressions, and evidence of earlier  construction that can influence soil moisture and groundwater level.  Existence of former solid waste disposal sites within the construction area should be checked.  Some forms of solid waste, i.e., old car bodies and boulders, make installation of deep  foundations difficult or result in unacceptable lateral deviation  of driven piles.  Guidance on determining potential problems of deep foundations in expansive clay is given in TM 5-818-7, “Foundations in Expansive Soils.”  Special attention should be payed to the following aspects of site investigation:

(1)  Visual study.  A visual reconnaissance should check for desiccation  cracks and nature of the surface soil.  Structural damage  in nearby structures which may have resulted from excessive settlement of compressible soil or heave of expansive soil should be recorded.  The visual study should also determine ways  to provide proper drainage of the site and allow the performance of earthwork that may be required for construction.

(2) Accessibility. Accessibility to the site and equipment mobility also influence selection of construction methods. Some of these restrictions are on access, location of utility lines and paved roads,  location of obstructing structures and trees, and topographic and trafficability features of the site.

(3)   Local experience.  The use of local design and construction experience can avoid potential problems with certain types of foundations and can provide data on successfully constructed foundations.  Prior experience with and applications of deep foundations in the same general area should be  determined.  Local building codes should be consulted, and successful experience with recent innovations should be investigated.

(4)   Potential problems with driven piles.  The site investigation should consider sensitivity of existing structures and utilities to ground movement caused by ground vibration and surface heave of driven piles.

The condition of existing structures mprior to construction should be documented with sketches and photographs.

Site and Soil Investigations - The Foundation Selected.

The foundation selected depends on functional requirements of the structure and results of the site investigation. Site investigation is required to complete foundation selection and design and to select the most efficient construction method.

The first phase of the investigation is examination of site conditions that can influence foundation performance and construction methodology. The seond phase is to evaluate characteristics of the soil profile to determine the design and the construction method. These phases are accomplished by the  following:


a. Feasibility study.  A reconnaissance study should be performed to determine the requiriements of a deep foundation designs, and the scope of in situ soil and foundation load tests.  Required cost estimates and schedules to conduct....

b. Site conditions.  Examination of the site includes history, geology, visual inspection of the site and adjacent area, and local design and construction experience.  Maps may provide data on wooded  areas, ponds, streams, depressions, and evidence of earlier  construction that can influence soil moisture and groundwater level....

c. Soil investigation  A detailed study of the subsurface soil should be made as outlined in TM 5-818-1.  The scope of this investigation depends on the nature and complexity of the soil, and size,   functional intent, and cost of the structure.  These parameters are frequently the consolidated-drained friction angle  N for cohesionless soil, undrained shear strength C for cohesive soil, soil elastic modulus E for undrained loading, soil dry unit weight,and the groundwater table elevation....

THE GENERAL BEARING CAPACITY EQUATION.

The bearing capacity Eq. (12.6) developed by Terzaghi is for a strip footing under general shear failure. Eq. (12.6) has been modified for other types of foundations such as square, circular and rectangular by introducing shape factors. Meyerhof (1963) presented a general bearing capacity equation which takes into account the shape and the inclination of load. The general form of equation suggested by Meyerhof for bearing capacity is

Hansen (1970) extended the work of Meyerhof by including in Eq. (12.27) two additional factors to take care of base tilt and foundations on slopes. Vesic (1973, 1974) used the same form of equation suggested by Hansen. All three investigators use the equations proposed by Prandtl (1921) for computing the values of Nc and Nq wherein the foundation base is assumed as smooth with the angle a = 45° + 0/2 (Fig. 12.6). However, the equations used by them for computing the values of Nγ are different. The equations for Nc, Nq and Nγ are

Table 12.2 The values of Nc, Nq, and Meyerhof (M), Hansen (H) and Vesic (V) Nγ Factors

Table 12.3 Shape, depth and load inclination factors of Meyerhof, Hansen and

Table 12.2 gives the values of the bearing capacity factors. Equations for shape, depth and inclination factors are given in Table 12.3. The tilt of the base and the foundations on slopes are not considered here.

In Table 12.3 The following terms are defined with regard to the inclination factors
 

Qh = horizontal component of the inclined load
Qu = vertical component of the inclined load

The general bearing capacity Eq. (12.27) has not taken into account the effect of the water table position on the bearing capacity. Hence, Eq. (12.27) has to be modified according to the position of water level in the same way as explained in Section 12.7.