Augered Piles - Characteristics.

The augered pile is usually constructed by screwing a rotary auger into the ground. The material is either augered out in a similar manner to that of a carpenter’s bit and the open hole filled with concrete or alternatively an auger with a hole down its centre is used and a cement grout injected under pressure down the hole during withdrawal of the auger. Early problems experienced with voids left by rapid withdrawal of the auger before properly filling the pile shaft have now been overcome by the use of computer controlled rigs which monitor concrete pressure and give a continuous readout for quality control purposes. Augers can be used to drill large-diameter holes in a wide range of soils, the range having been extended by the use of  bentonite slurry to assist the support of the sides of the hole in soft silts and clays. In addition, the large-diameter auger can be used with under-reaming tools to enlarge the end bearing base of the pile (see Fig. 9.37).

Probably one of the most successful auger methods is the use of the hollow tube auger in soft silt, etc., where
water and squeezing of soft silts in the surrounding ground can cause necking problems for many other systems, i.e. squeezing in of the pile shaft due to side pressure. The use of the hollow auger and injected sand cement grout can produce good-quality piles in these soft and difficult conditions at competitive prices, particularly where large numbers of piles are involved. For small numbers of piles the on-site cost of the rig and grouting plant can prove to be prohibitively expensive.

Fig. 9.37 Under-reamed augered pile.

Bored Piles - Characteristics.

The bored pile is usually formed by using a simple cable percussion rig. The soil is removed by shell and auger and the hole filled with in situ reinforced concrete as required (see Fig. 9.36).

For filled sites or soft clay sites overlying stiff clay or rock, small to medium bored piles often prove to be economic.

The relatively small on-site cost of bored piles means that smaller sites can be piled more economically than they can using a driven piling system. The bored pile is not usually economic in granular soils where removal and disturbance of surrounding ground can cause excessive removal of soil and induce settlement in the surrounding area. During  piling operations the hole can be lined with a casing which can be driven ahead of the bore to overcome difficulties caused by groundwater and soft sub-soil but sometimes difficulties of withdrawing the casing after casting can prove expensive.

Fig. 9.36 Bored pile.

Driven Cast In Situ Piles.

Driven cast in situ piles use steel, or precast concrete, driving tubes which are filled with in situ concrete after driving.

Variations in pile lengths can be more easily accommodated using segmental liners. The piles can be cast accurately to the required length and the driving or liner tube can be driven in short lengths. In some cases the tube is left in position permanently and in other cases the tube is withdrawn and used to tamp the concrete by lifting and dropping the liner tube. In other situations the tube is with-drawn and vibrated as the concrete is poured and additional compaction achieved by impact to the surface of the wet concrete. Driven piles can therefore have a smooth or irregular side surface depending on the method of driving and this results in differing friction and mechanical keying to the surrounding soil which varies depending on the pile type and sub-soil conditions. Again large rigs are required for driving cast in situ piles and hardstanding requirements can prove expensive.

In piling systems where the liner tubes are withdrawn there is a danger that the tube can lift the upper portion of in situ concrete leaving a void a short distance below the surface or squeezing during withdrawal can cause necking. This can happen where the mix is not carefully controlled or where the liner tube is not withdrawn at a steady slow rate.

Driven cast in situ piles, however, can prove to be economic for sands, gravels, soft silts and clays, particularly when large numbers of piles are required. For small numbers of piles the on-site cost can prove expensive. Driven precast piles and driven cast in situ piles can prove particularly suitable where groundwater or soft inclusions occur in the sub-strata.

Driven Precast Piles.

Driven precast piles can be used in areas where the soils, through which the pile is to be driven, are relatively soft and unobstructed and where the length of pile required can be determined to a reasonable accuracy. The piles can be cast to any suitable cross-section, i.e. square, rectangular, circular, hexagonal, etc. The shape and protection to the point of the pile is determined from the end bearing requirements and driving conditions. The pile head and the reinforcement are designed to take account of the pile-driving impact loads. Some disadvantages of this method of piling are that the pile can be damaged in a location out of sight during driving and the pile can be displaced if it meets an obstruction such as a boulder in the ground. In addition the accuracy of the estimated length is only proved on site and short piles can be difficult to extend and long piles can prove to be expensive and wasteful. A further disadvantage is the relatively large rig required for driving and the need for hardstandings that are often required to provide a suitable surface for the pile-driving plant.

Concrete Piles - Uses.

Concrete piles are generally used to transfer loads through an unsuitable bearing material to a deeper load-bearing strata. This is achieved either by skin friction and end bearing or end bearing alone (see Fig. 9.34).

There are many different types and systems of piles,  however the main types are:

(1) Driven precast piles.
(2) Driven cast in situ piles.
(3) Bored piles.
(4) Augered piles.

These piles can also be divided into either displacement or replacement methods dependent on the system of driving, i.e. either removal of material, termed  replacement, or  wedging apart of material, termed  displacement. Typical examples of these types are shown in Fig. 9.35.

Fig. 9.34 Typical concrete pile.

Fig. 9.35 Concrete pile types.

Stone/Gravel Piles.

The stone, or gravel, pile is mainly used as a means of strengthening sub-strata by introducing a series of stone columns using vibration or jetting methods which compact the ground around the stone and replace the void created with a compacted stone column (see Fig. 9.33).

There are basically three main applications which require quite different design judgement and approaches to site testing, and these aspects are dealt with in detail later. In general terms, stone or gravel piles are used in areas of soft sub-strata, or fill, where sufficient upgrading of thebearing capacity or reduction in differential settlement can be achieved. In such situations the stone/gravel pile is  usually much cheaper and in some situations much more suitable than the concrete pile alternative. For example, the gravel pile has a particular advantage in mining areas where the use of concrete piles could result in the foundation picking up unacceptable ground strains and/or possibly the piles shearing off during subsidence due to the brittle form and limited capacity to resist horizontal ground strain. The gravel pile can be used incorporating a slipplane between the top of the pile and the underside of the foundation in a manner similar to that described for the slip-plane raft and Fig. 9.25.

Fig. 9.33 Stone/gravel piles (vibro).

Fig. 9.25 Slip plane raft.

Pile Foundations - Uses.

Piles are generally used as a means of transferring loads down through unsuitable bearing strata either by skin friction and end bearing or end bearing only into a firm layer at greater depth (see Fig. 9.31).

There are many different types of piles including concrete – in situ and precast; steel; timber; stone. The cross-section of the pile and the installation method vary significantly.

In addition to transferring loads to greater depths below surface level the stone pile system (vibro-stabilization) can be used to upgrade the bearing capacity of the sub-strata (see Fig. 9.32).

The distribution of load through the sub-soil varies with the various types of pile and different installation methods.

Some piles are suitable for sandy soils, others for clay soils or end bearing into rock. The aim however is generally the same and that is to provide an economic means of support for the foundation and its loads. The various pile types and/or systems have advantages and disadvantages which make each pile more suitable and competitive for particular situations and soil conditions. There is perhaps a danger of the designer, having selected a competitive system on the first piling job, making the assumption that it is also the appropriate pile system to use on future contracts and ignoring the fact that the competitive tender probably related as much to the site and ground conditions as it did to anything else.

There is, therefore, a need for designers to understand the various types of pile, their best application, and possible limitations, etc., in order to provide good engineering solutions for design purposes. The following sections which describe the various types will assist the engineer in his choice of suitable pile systems and applications.

Fig. 9.31 Typical pile foundation.

Fig. 9.32 Stone pile (vibro).

Jacking Raft.

The jacking raft is used in areas where the expected subsidence would tilt or distort the structure to an unacceptable degree and where re-levelling of the raft produces an  economic and viable foundation for the design conditions.

The jacking raft is used in locations of excessive or unpredictable subsidence, for example, in areas subjected to brine or other mineral extraction. A typical jacking raft for a domestic property is shown in Fig. 9.30.

Fig. 9.30 Jacking Raft.

Buoyancy (or ‘Foating’) Raft.

A buoyancy raft is similar to a cellular raft and is a deep raft with large voids. The main weight of removed earth is replaced with practically weightless voids of the raft (see Fig. 9.29). Basement accommodation can be provided in this form of construction. Basement slabs together with retaining walls form the raft.

It is used for heavily loaded structures in areas of low ground-bearing capacity.

Fig. 9.29 Buoyancy raft.

Beam Strip Raft.

The beam strip raft consists of (ground-bearing) downstand beams in two or more directions which support the heavy uniform or point loads from the structure. The beams are tied together by a ground-bearing slab supported on the hardcored dumplings, i.e. the raised areas of hardcore protruding up between the beam lines (see Fig. 9.28).

This raft is mainly used in areas of either mining activity or soft alluvial deposits where a stiffened beam is required  on the main load lines. The tying of the ground floor slab into the beams prevents lateral distortions of the beam and evens out any local differential settlements. This type of raft is more economic than the cellular form and is used where conditions are not as severe.

Fig. 9.28 Beam strip raft.

Lidded Cellular Raft.

The lidded cellular raft is very similar in profile to the cellular raft and is used in similar locations, i.e. severe
mining conditions, areas of poor ground where the raft will be subjected to large bending moments, etc. The main  difference however is the use of a lighter form of upper  slab designed to be separate to the main foundation (see Fig. 9.27).

The detail at the seating of the upper floor depends upon the need for re-levelling and the possible number of times adjustments to line and level may be necessary.

Fig. 9.27 Lidded cellular raft.

Cellular Raft.

A cellular raft consists of an arrangement of two-way interlocking foundation beams with a ground bearing slab at  the underside and a suspended slab at the top surface. The upper and lower slabs are usually incorporated within the beams to form I sections. The intersecting beams effectively break the large slab into two-way spanning continuous small panels (see Fig. 9.26).

The top slab is cast using precast soffits or other forms of permanent formwork such as lightweight infill blocks.

These rafts are used on sites subject to severe mining activity or in areas of poor ground where large bending
moments are to be resisted. They are also used in locations where a valuable increase in bearing capacity can be achieved by the removal of the overburden and where deep foundation beams are required.

Fig. 9.26 Cellular raft.

Slip-Plane Raft.

The slip-plane raft consists of a concrete raft constructed on a slip-plane layer, such as sand of known friction or shear resistance, which is located between the raft and the substrata. The slip-plane is constructed in sufficient thickness to ensure that a straight failure plane could occur under excessive longitudinal ground strain (see Fig. 9.25). The depth of penetration of the raft into the ground is kept to a minimum to avoid picking up loading from ground strains.

However, the depth below finished ground level must take account of potential frost heave.

Fig. 9.25 Slip plane raft.

Blanket Raft.

The blanket raft consists of a concrete crust raft constructed on a stone blanket which in turn is built up in layers off the reduced sub-strata level (see Fig. 9.24). The crust raft and blanket interact to support and span the loading over any localized soft spots or depressions. The main difference between this and the crust raft is the introduction of the stone blanket. This blanket effectively disperses any heavy point and edge loads or imbalance of load. Composite action between the crust raft and the stone blanket is the basis of the action and design of this foundation system.

Fig. 9.24 Blanket raft.

Nominal Crust Raft.

Nominal crust raft
A nominal crust raft is basically a ground-bearing reinforced concrete floor slab with nominal thickenings around the edges. Internal thickenings are sometimes incorporated in the raft (see Fig. 9.23).

The slab acts as a surface crust to the sub-strata thus evening out any small local differential settlement movements which could result from variations in imposed loading on the top of the slab and/or local variations in settlement characteristics of the sub-soil. The design is generally carried out either by sizing the raft from previous experience or by calculation based upon nominal assumptions.

Fig. 9.23 Nominal crust raft.

Crust raft

The crust raft is a stiffer version of the nominal crust raft.

The ground slab and thickening which form the crust are combined into a total raft design. Heavier loads on soil of low bearing capacity determine the size and depth of the thickenings. The thickness of the slab is dictated by the overall raft design which generally exceeds the nominal slab requirements.

Surface Spread Foundations.

Surface spread foundations consist mainly of rafts and are generally used where the normal ground bearing
sub-strata is relatively poor and the depth to suitable loadbearing soils is excessive or the load-carrying capacity of the soil deteriorates with depth. Surface spread foundations are therefore employed to distribute the superstructure/substructure loads over a large area of the ground thus reducing the contact bearing pressure. Since most structures also require a ground floor slab it is usually  economic to incorporate it with the foundation into one structure/element. This can be done by making the upper surface of the raft foundation coincide with the top surfaceof the floor slab. A simple example is shown in Fig. 9.22.

Surface spread raft foundations are often adopted in areas of active mining as the best means of resisting excessive distortion, tensile and compressive forces, etc., resulting from the ground subsidence. These and other types of surface spread foundations are discussed in the following sections.

It should be noted that rafts do not necessarily distribute the loads as a uniform contact pressure to the sub-strata, on the contrary, most rafts are relatively flexible foundations and will have higher contact pressure under loaded points and edge thickenings than below the main slab areas.

Fig. 9.22 Typical raft foundation.

⇒ Nominal crust raft
⇒ Blanket raft
⇒ Slip-plane raft
⇒ Cellular raft
⇒ Lidded cellular raft
⇒ Beam strip raft
⇒ Buoyancy (or ‘?oating’) raft
⇒ Jacking raft

Cantilever Balanced Pad Foundations.

The cantilever balanced foundation consists of a ground beam picking up loading from the superstructure and
cantilevering out over a pad foundation with the pads designed, theoretically, to have uniform bearing stress (see Fig. 9.20).

The need for a cantilever arrangement can be produced by restrictions from adjacent buildings or existing services (see Fig. 9.21).

Fig. 9.20 Cantilever balanced foundation.

Fig. 9.21 Cantilever balanced foundation.

Holed Balanced Pad Foundations.

The holed balanced foundation is a pad type foundation supporting a number of loads and transferring the load  to the bearing strata in a relatively uniform fashion. The allowable variation in bearing pressure and differential settlement is again determined from the ground conditions and sensitivity of the superstructure.

The resultant load and its position are determined for the critical load case.

While with the rectangular base the balancing is done by varying the cantilever and with the trapezoidal base by varying the end dimensions, in this case the balancing is done by forming a hole in the base positioned so as to move the centroid of the base to coincide with that of the resultant load (see Fig. 9.19).

Fig. 9.19 Holed balanced pad foundation.

Trapezoidal Balanced Pad Foundations.

The trapezoidal balanced foundation is used in similar circumstances to the rectangular balanced foundation.

Adjusting the width of each end of the pad in relation to the load supported can produce a more economic solution.

This is particularly useful where two point loads of different sizes need to be supported and a relatively uniform bearing pressure is required (see Fig. 9.17). It is also useful where adjustments by cantilever action  are not possible, for example, where two different column loads on the edge of opposite building lines require support (see Fig. 9.18).

Fig. 9.17 Trapezoidal balanced pad foundation.

Fig. 9.18 Trapezoidal balanced pad foundation.

Rectangular Balanced Pad Foundations.

A typical rectangular balanced foundation supporting two point loads from a sensitive structure that has only a small tolerance to accommodate differential settlement is shown in Fig. 9.16. The problem has been overcome by adjusting the cantilevered ends of the base to produce a constant ground bearing pressure for the load conditions.

Fig. 9.16 Rectagular balanced pad foundation.

Balanced Pad Foundations.

Balanced pad foundations are used where a number of loads are required to be supported on a single pad and where excessive variations in pressure could produce unacceptable differential movement. They consist of reinforced concrete pad bases designed for the critical design loading with the aim of keeping the differential ground stresses  and hence settlements to an acceptable level. This requirement could be the result of a sensitive sub-strata and/or  a sensitive superstructure over. There are a number of  different types of balanced pad foundations which include rectangular, trapezoidal, holed and cantilever, and these are described in the following sections.

Deep Mass Concrete Pads.

Deep mass pads consist of mass concrete pads cast with their soffit at depths in excess of 1.5–2 m. They are generally used where a suitable ground bearing strata is relatively deep and where the piling alternative is more expensive, i.e. a small number of pads are required or access for piling is difficult and expensive. Deep mass pads tend to be of two types, one being constructed up to high level using a basic cross-section and the other using a reduced and shuttered cross-section for the upper levels (see Fig. 9.14).

An alternative to concrete for the upper reduced crosssection is to construct a brick pier off the mass concrete pad (see Fig. 9.15). This solution has the advantage of avoiding the need for expensive shuttering and can result in an overall saving. If brickwork is adopted it is necessary that the pad size provides the necessary working space for the bricklayers to build the pier.

Fig. 9.14 Deep mass concrete pad.

Fig. 9.15 Deep mass concrete pad with brick pier.

Deep Reinforced Concrete Pads.

Deep reinforced concrete pads are similar in cross-section to the shallow reinforced pad but are constructed at depth in situations where the suitable sub-strata is not available  at high level. Such pads are not often economic and more cost-effective mass concrete bases or piles and caps are often used. However in some situations they can prove  to be a suitable solution – see Fig. 9.13 which indicates a typical example of such a use.

Fig. 9.13 Deep reinforced concrete pad.

Shallow Reinforced Concrete Pads.

Reinforced concrete pads are similar to the mass concrete pads but for the same conditions can be thinner when reinforced with steel. The reduction in thickness is made possible by the introduction of reinforcement on the tensile face of the pad which increases the pad’s resistance to bending moment (see Fig. 9.12).

Fig. 9.12 Shallow reinforced concrete pad.

Shallow Mass Concrete Pads.

Shallow mass pads consist of mass concrete pads supporting point loads from columns, piers, etc. (see Fig. 9.10).

They are used for varying conditions of sub-strata where suitable load-bearing soils exist at shallow depths below the effects of frost and general weathering. They are particularly economic where the side of the excavation can be used as a shutter and where a suitable depth of mass can be accommodated to disperse the load without the need for reinforcement. The general assumption for load dispersion is as mentioned previously i.e. a 45° spread through the mass concrete (see the typical example shown in Fig. 9.11).

Fig. 9.10 Shallow mass concrete pad.

 

Fig. 9.11 Load spread on mass concrete pad.

Inverted T Beam Strips.

The inverted T beam strip fulfils the same function as the rectangular beam strip but the cross-section is modified to an inverted T so that the flanges reduce the contact pressure on the ground (see Fig. 9.9).

Fig. 9.9 Inverted beam strip.

Rectangular Beam Strips.

Rectangular beam strips consist of rectangular reinforced ground beams which are designed to be of sufficient width to reduce the bearing pressures on the sub-strata to an acceptable value. The beam is required to be of sufficient cross-section to resist the induced bending moments and shear forces in the longitudinal direction. The beam is reinforced with either ladder reinforcement or caged reinforcement to suit the design conditions. Figure 9.8 shows a typical beam strip supporting point loads.

Fig. 9.8 Typical beam strip.

Stone Trench Fill.

Stone trench fill consists of stone deposited into the open trench excavation and compacted in layers. It is particularly useful in areas where poor quality sands, sandy silts, etc., exist. The material immediately below the topsoil is often suitable for the general floor slab loading but not for the more heavily loaded external and internal strip loadings.

Suitable sub-strata for the strip loads often exists at a  shallow depth and stone trench fill can be used down to these levels (see Fig. 9.7).

Fig. 9.7 Stone trench fill.

Concrete Trench Fill.

Concrete trench fill consists of a mass concrete strip cast into the open trench making use of the trench sides as a shutter (see Fig. 9.5).

Concrete trench fill is often used where strip loads are required to be transferred to relatively shallow depths
through soft material which is capable of standing up, without extra support, for at least a period adequate to cater for the construction sequence to be adopted. The trench fill can embrace requirements for heavy loads going down to rock or light loads on soft sub-strata (see Fig. 9.6).

The requirement for working space within the trench  for bricklayers is not a factor in determining the width of excavation with this method. Pouring concrete to within 150 mm of ground level overcomes this consideration.

Fig. 9.5 Concrete trench fill.

Fig. 9.6 Concrete trench fill.

Concrete Strips – Plain and Reinforced.

The concrete strip footing replaced the corbelled masonry in more recent constructions. In plain (unreinforced) strip footings the thickness is determined by the requirement for the line of dispersion to pass through the side of the footing as shown in Fig. 9.1. The width of the trench must also allow working space for the bricklayers to build the masonry off the footing.

The profile of the reinforced concrete strip is similar to the unreinforced strip except that it can generally be made thinner in relation to its projections since it no longer relies upon an approximate 45° line of load dispersion. The strip is often reinforced with a fabric or lattice reinforcement.

The longitudinal bars are the main bars selected to suit the longitudinal bending expected on the strip and the cross bars designed to cater for the cantilever action on the projections (see Fig. 9.4).

Fig. 9.1 Typical strip footing.

Fig. 9.4 Reinforced strip footing.

Masonry Strips.

Masonry strips are rarely used these days, however they can be adopted where good quality sub-strata exists and the raw materials for masonry construction are cheap and abundant. The wall is increased in width by corbelling out the masonry to achieve the required overall foundation width as shown in Fig. 9.3.

It should be noted that it can be important, particularly when using masonry strips in clay or silt sub-strata, to bed the masonry units in mortar and to completely fill all joints.

The reason for filling the joints is mainly to prevent the strip footing acting as a field drain with the water flowing along the surface of the formation level and through the open joints of masonry. The authors have found clear evidence  of induced settlement due to softening of the clay surface below dry random rubble strips (dry random rubble being a term for dry stacking without mortar and not dry meaning no moisture).

Fig. 9.3 Masonry strip footing.

STRUCTURES: Strip footings.

Strip footings are used under relatively uniform point loads or line loads. The main structural function of the strip is  to disperse the concentration of load sideways into an increased width of sub-strata in order to reduce the bearing stress and settlement to an acceptable limit. A cross-section through an unreinforced concrete strip footing showing the assumed dispersion of load is shown in Fig. 9.1.

A further major structural function is to redistribute the loads in the longitudinal direction where the loading is
non-uniform or where the sub-strata resistance is variable (see Fig. 9.2). The width of the strip is usually decided by calculating the width required to limit the bearing stress and choosing the nearest excavator bucket size up from that dimension. From a construction point of view, the strip depth is used as a means of levelling out irregularities in the trench bottom and the width has to absorb the excavation
tolerances which would be unacceptable for the setting out of walls etc.

There are a number of different types of strips which include masonry strips; concrete strips – plain and reinforced; concrete trench fill –  and stone trench fill; reinforced beam strips – Rectangular Beam Strips  and Inverted T Beam Strips, and these are described in the following sections.

Fig. 9.1 Typical strip footing.

Fig. 9.2 Strip footing load spread distribution.

Group One – Strip and Pad Foundations.

Strip footings and pad bases are used to deliver and spread superstructure loads over a suitable area at foundation  (formation) level. The foundation is required to be stiff enough to distribute the loadings onto the sub-strata in a uniform manner.

1 Strip footings  Strip footings are used under relatively uniform point loads or line loads. The main structural function of the strip is  to disperse the concentration of load sideways into an increased width of sub-strata in order to reduce the bearing stress and....

2 Masonry strips  Masonry strips are rarely used these days, however they can be adopted where good quality sub-strata exists and the raw materials for masonry construction are cheap and abundant. The wall is increased in width by corbelling out the masonry to...

3 Concrete strips – plain and reinforced
4 Concrete trench fill
5 Stone trench fill
6 Rectangular beam strips
7 Inverted T beam strips
8 Pad bases
9 Shallow mass concrete pads
10 Shallow reinforced concrete pads
11 Deep reinforced concrete pads
12 Deep mass concrete pads
13 Balanced pad foundations
14 Rectangular balanced pad foundations
15 Trapezoidal balanced pad foundations
16 Holed balanced pad foundations
17 Cantilever balanced pad foundations

Foundation Types: Strip and pad foundations, Surface spread foundations, Piled foundations, Miscellaneous elements and forms.

• Strip and pad foundations,
• Surface spread foundations,
• Piled foundations,
• Miscellaneous elements and forms.

The design of foundations involves the use of many different combinations of structural elements and foundation types which in turn vary to perform a wide variety of functions. It is therefore not surprising that the foundation scene has grown into a jumble of rather poorly defined elements and forms. In addition to providing guidance on the elements and forms available, this chapter suggests a more clearly defined terminology in an attempt to help clarify the issue. It is possible therefore that even the experienced engineer may at first find some of the terms unfamiliar. However, the authors have found that with use, the terms prove to be of great assistance. Since this chapter  covers modern developments in foundation design this has resulted in the introduction of further new terms. Wherever existing terms clearly define the structural element or  foundation form they have been retained, but more vague definitions such as ‘rigid raft’, etc. have been deliberately omitted, since such terms are in danger of misinterpretation and cover a widely varying group of foundations.

In addition to the design of the foundations to support the applied loads, without excessive settlement and distortion, there is a need to resist or prevent the effects of frost-heave and/or shrinkage and swelling of sub-strata. The many different loads and conditions demand different solutions, however the foundation types can generally be defined and the main types are described in this chapter. Various references are made in the text to relative costs. Appendix M should be consulted for more detailed cost guidance.

Site Investigation Report.

The report should contain the information gained in reconnaissance, survey, investigation, testing and soil survey recommendations and the design engineer’s recommendations. Since the report is the property of the client his permission should be obtained for its distribution to invited main and appropriate specialist sub-contractors and any public authority collecting soil data.

The report will contain a mass of information which must be presented in an orderly, easily digested manner and written in clear, unambiguous, good English. Since most  of the intended readers are mainly visually orientated, the use of photos, maps, soil profiles, borehole logs and other visual aids is to be recommended as is the tabulation of test results and other information. The report is not a thesis nor a scientific treatise, but a factual report with comments, opinions and recommendations based on the interpretation

of the facts from experience. The facts and opinions must be clearly separated. Since the report is likely to be subject to hard and frequent usage it is advisable to bind it between stiff covers rather than merely stapling a mass of A4 sheets.

The script, drawings and layout should be checked and  re-checked just as carefully as calculations and drawings from the design office.

A recommended procedure is as follows:

(1) Collect data, categorize it and rough out a preliminary draft.
(2) Edit the draft and seek methods of visual presentation and tabulation.
(3) Polish re-draft and check for improvements in pres- entation, check for typing errors and appearance.

1 Factors affecting quality of report  The restraints of time and funding that need to be allowed for in the investigation have been discussed in earlier  sections. There are other factors which can affect the quality of the investigation, recommendations and the engineering judgement. Among those which may affect some engineers are:

(1) Uncritical acceptance of well-presented opinion, results of sophisticated (but not necessarily relevant) tests and over- and unqualified respect for some specialists.
(2) Allowing site difficulties to dictate the investigation in an attempt to keep the investigation simple and cheap.
(3) Lack of recognition that piling and other foundation techniques can be used to economic advantage even on good sites.
(4) Lack of recognition that some fills, possibly upgraded by ground improvement techniques, can provide an adequate and economic bearing strata.
(5) Lack of appreciation that advances in structural design can accommodate relatively high settlements.
(6) Under-estimation of the importance of the designer,  at least, visiting the site during the investigation or  dismissal of trial pits as unscientific or out-dated.

2 Sequence of report  Foundation reports follow the normal sequence of items of engineering reports in having a title, contents list,  synopsis, introduction, body of the report, conclusions and recommendations. Lengthy descriptions of tests and similar matters are best dealt with in appendices and the test results tabulated in the body of the report. The client tends to read the synopsis and recommendations; the main and sub-contractors concentrate on the body of the report and the design office on its conclusions and recommendations.

If the brief imposed such limitations on cost and time allocation for the investigation that the engineer was not able to carry out an adequate survey this should be tactfully pointed out. It should also be made clear in such cases that the engineer’s conclusions and recommendations are qualified – this is unfortunately advisable in the present  litigatious climate.

3 Site description  This, as far as possible, should be given on small-scale plans showing site location, access and surrounding area. The proposed position of the buildings and access roads should be shown. The site plan should also show the general layout and surface features, note presence of existing buildings, old foundations and previous usage, services, vegetation, surface water, any subsidence or unstable slopes, etc.

Written description of the site exposure (for wind speed regulations) should be given together with records of any flooding, erosion and other geographical and hydrographic information.

Geological maps and sections should, when they are necessary, be provided, noting mines, shafts, quarries, swallowholes and other geological features affecting design and construction.

Photographs taken on the site, preferably colour ones, can be very helpful and should be supplemented by aerial  photographs if considered necessary.

4 The ground investigation

(1) Background study and location of holes. This should give  a full account of the desk-top study, examination of  old records, information from local authorities, public utilities and the like, and the field survey. It should detail the position and depth of trial pits and boreholes, equipment used and in situ testing and information.

(2) Boreholes, trial pits and soil profiles. This section will be mainly a visual presentation of the logs and profiles together with colour photographs of the trial pits.

Where possible, written information should be given in note form on the soil profiles.

(3) Soil tests. This should list the site and laboratory tests drawing attention to any unusual, unexpected or special results. The results of the tests should be tabulated, for ease of reference, and diagrams of such information as particle size distribution, pressure–void ratio curves and Mohr’s circles should be given. If such form of presentation is not fully adequate then test descriptions and results should be given in an appendix.

5 Results  This must give details of ground conditions, previous use of site, present conditions, groundwater and drainage pattern.

The tests must give adequate information to determine the soil’s bearing capacity, settlement characteristics, behaviour during and after foundation construction and, where necessary, its chemical make-up and condition.

6 Recommendations  This is both comment on the facts and also opinions based on experience; the difference should be made clear. Since the discussion is usually a major part of the report it should
be broken down into sections for ease of reference and readability.

The first section should briefly describe the proposed main and subsidiary structures and their loading, a description and assessment of the ground conditions and the types of appropriate foundations.

The second section should advise on foundation depths, pressures, settlements, discuss alternatives giving advantages, disadvantages and possible problems keeping in mind cost and buildability considerations.

Typical main recommendations are:

(1) Safe bearing capacities at various depths, estimates  of total and differential settlement and time-span of
settlement.
(2) Problems of excavation (fills, rock, water ingress, toxic and combustible material).
(3) Chemical attack on concrete and steel by sulfates and chlorides or acids within soil.
(4) Flotation effect on buoyant or submerged foundations.
(5) Where the proposed structure houses plant which could vibrate or impact shock the soil, the effect on the soil must be assessed.
(6) Details of any necessary geotechnical processes to improve the soil’s properties.
(7) Where piling is necessary, information must be given on founding level, possible negative skin friction, obstructions, appropriate type and installation of piles and the effects of piling on adjacent constructions and existing buildings.
(8) Where a foundation is subject to lateral loading, the magnitude and position of the loading must be given
together with the skin friction between the soil and the passive resistance of the soil.
(9) Where retaining walls are required, information is needed on active pressure, passive resistance, sur-
charge, factor of safety against slip circle failure, possible landslides or slips.
(10) Where road construction is involved requiring CBR values, etc., though this is outside the scope of this book.

The final section should give firm recommendations on the foundation type or types to be adopted.

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.

Recording Information – Trial Pit and Borehole Logs and Soil Profiles.

Before embarking on expensive laboratory testing of soil samples it is advisable to record (log) the information gained on site in order to plan the test programme. To facilitate the reading of logs and boreholes the soils and rocks should be indicated by standardized symbols. Widely accepted diagrammatic symbols are given in Fig. 3.4.

A typical trial pit log of the engineer’s observations is given in Fig. 3.5.

A borehole log should give details of the foreman driller’s log, the observations of the supervising engineer and the results of any site tests. A typical borehole log is shown in Fig. 3.6.

Trial pits, trenches and boreholes should be given reference numbers, located on plan, their ground level noted and the date of excavation recorded. It is advisable to record the  following additional information:

(1) Type of rig, diameter and depth of bore or width of bucket.
(2) Diameter and depth of any casing used and why it was necessary.
(3) Depth of each change of strata and a full description of the strata. (Was the soil virgin ground or fill?)
(4) Depths at which samples taken, type of sample and sample reference number.
(5) In situ test depth and reference number.
(6) The levels at which groundwater was first noted;  the rate of rise of the water; its level at start and end of
each day. (When more information on permeability, porewater pressure, and the like is required, then it is vitally important that the use of piezometers should  be considered.)
(7) Depth and description of obstructions (i.e. boulders), services (drains) or cavities encountered.
(8) Rate of boring or excavation (useful to contractors and piling sub-contractors as such information gives some guidance in ease of excavation or pile driving).
(9) Name of supervising engineer.
(10) Date and weather conditions during investigation.

Fig. 3.4 Recommended symbols for soils and rocks

Fig. 3.5 Typical trial pit log (Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983),

Fig. 3.6 Example of a typical borehole log (BS 5930, Fig. 22).

Field (site) Testing of Soils.

No matter how carefully soil samples are taken, stored, transported to a laboratory and tested, some disturbance is possible and even likely – and therefore many engineers prefer the alternative of testing the soil in situ. As with  sampling techniques there have been advances in sophistication and variety of field testing techniques and the most common types are briefly described here.

Site testing has come a long way from kicking the clay at the bottom of a trial pit with the heel of the investigator’s shoe – though this can still be a useful, if crude, assessment when carried out by an experienced engineer familiar with local conditions.

In foundation design less is known of soil as a structural material than is known of concrete and steel. It is not possible to analyse and forecast, with certainty, the stresses in the soil or the soil’s reaction to those stresses, since the foundation loading can only be a reasonable assessment.

Foundation design is therefore based not solely on analysis but also needs the application of sound engineering judgement.

In a sensible and valuable search to understand the material it must be tested and some researchers have devoted their careers to this essential cause. In each of the following field and laboratory tests there has been extensive research, literally thousands of learned papers and many international conferences – some devoted to just one test, for example, see References 5 and 6. It is not possible therefore in a  book on foundation design to discuss fully in depth any one test; discussion is limited to the broader considerations.

Furthermore the site and laboratory testing of soils is  the contractual responsibility of the soil survey specialist.

Hence the following sections outline and summarize the tests and the main references are given for designers wishing for more detailed information. Experience is necessary to estimate what and how to test, the test results need engineering judgement in assessing their application and relevance and in forecasting estimated behaviour – for none  of the tests give scientifically accurate results applicable to the actual strata under the real pressure. The theories, as in structural theory, are based on simplifying assumptions not fully related to the reality of practice. But to dismiss tests and theory and rely on outdated  rules of thumb  methods is inappropriate to modern structures and is as foolish as blind faith in science.

1 Standard Penetration Test (SPT)  The SPT is a useful method of indicating the relative  density of sands and gravels. It is based on the fact that the denser the sand or gravel the harder it is to hammer a peg into it. A standard weight is dropped a defined distance on a tube, with either a split tube or a cone head (cone penetration test, CPT), placed in the borehole. The tube is driven 450 mm into the soil and the number of hammer-blows taken to drive the tube into the last 300 mm of soil is termed its  N  value. Care in interpreting the result is particularly  necessary where boulders, very coarse gravel or bricks in backfill may be present, for the measurement may be of the resistance of the obstruction and not of the soil.

Approximate values of the relationship between sand properties and  N  values are given in Table 3.3 and a
summary of the test is given in Table 3.4. CIRIA Publication  The Standard Penetration Test (SPT): Methods and Use(7) is a comprehensive reference.

2 Vane test  If a garden spade is driven into clay and then rotated it will effectively shear the clay and the higher the shear resistance of the clay then the greater the force (torque) required to rotate the spade. This is the principle of the vane test.

The vane is a cruciform of four blades fixed to the end of the boring tube’s rod. It is pushed into the undisturbed soil at the base of the borehole or trial pit and the torque required to rotate the vane is measured. Table 3.5 gives a summary of the test.

When the height of the vane is twice its diameter, D(m), the relationship between shear strength of the soil, τ, and the maximum applied torque, M(kN m), is generally:

3 Plate bearing test  A plate, of known area, can be placed at the bottom of a trial pit or borehole and loaded. The settlement of the soil under load can be measured and also the pressure required to cause
shear failure of the soil. The test is summarized in Table 3.6.

4 Pressuremeters  A pressuremeter could be considered as basically a vertical plate test. If an expanding cell is placed in a borehole and pumped up to exert pressure against the sides of the bore then the stronger the soil the greater the pressure required to expand the cell. Summaries of different pressuremeters are given in Table 3.7.

5 Groundwater (piezometers and standpipes)  The presence of moisture in, and the magnitude of moisture content of, soils has a pronounced effect on soil properties and behaviour. Since the moisture content can vary so too can the soil. It is essential therefore to investigate the ground-water conditions and possible variation. Groundwater variations are likely on coastal, estuarine and tidal river sites; sites subject to artesian conditions and variable water-table levels; sites with permeable granular soil where bored piles or bentonite diaphragm walls are to be used, and particularly sites founded on fills.

The rate of seepage of groundwater into pits and bores together with level and variations in level should be
recorded. Piezometers or standpipes should be employed when groundwater problems are anticipated.

A standpipe can at its simplest be the open borehole, and the outline of the test is summarized in Table 3.8.
Piezometers, of varying sophistication, are basically perforated tubes lined internally with porous tubing, and details are summarized in Table 3.9.

6 Other field tests  There are a number of developments, refinements and adjustments to the above tests as well as geophysical  tests, aerial infra-red photography, video photography in boreholes, etc. These newer tests can sometimes be less expensive, less time-consuming and yield more information than the traditional tests. The interested reader should refer to specialist soil mechanics literature for details.

 Table 3.3 Relationship between N values and sand properties

 Table 3.4 Standard Penetration Test (Weltman, A.J. & Head, J.M.,  

Site Investigation Manual, CIRIA (1983)

 Table 3.5 Vane test (Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983)

Table 3.6 Plate bearing test (Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983)

 Table 3.7 Pressuremeter test (Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983)

 Table 3.8 Open borehole test (Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983)

Table 3.9 Piezometer test (Weltman, A.J. & Head, J.M., Site Investigation Manual, CIRIA (1983)