Lecture Notes - Foundation Engineering - Teacher: Dr Gurmel S Ghataora, Dr David Chapman PDF

Preview

Summary

Foundation Engineering - Teacher: Dr Gurmel S Ghataora, Dr David Chapman...

Description

SCHOOL OF ENGINEERING Department of Civil Engineering

Lecture notes on Foundation Engineering Module CE5FOEa MSc programmes in Geotechnical Engineering and Geotechnical Engineering and Management

Principal Lecturer Dr Gurmel S Ghataora ([email protected])

Note that the following lectures notes are based on material originally prepared by Dr J Bilam.

Lecture notes on Pile Testing, Construction of Piles, Down-drag on Piles and Laterally Loaded Piles are attached to the end of these notes. Lecture notes on engineering in earthquake zones will be supplied separately.

CONTENTS

Aims of the course Part 1 Introduction Shallow foundations Bearing capacity Settlement Part 2 Piles foundations Single piles Bearing capacity Settlement Pile groups Bearing capacity Settlement Faults in piles Pile tests Piled design for lateral loading Appendix A - EU/C U ratio Appendix B - Module Description

VOLUME 2 Earthquake engineering VOLUME 3 Pile design for lateral loading and downdrag

AIMS OF THE COURSE The aim of this course is to give the student a basic understanding of bearing capacity and settlement of both shallow and deep (piled) foundations, including pile groups. It will also give the student an introduction into concepts of designing foundations and other geotechnical structures subjected to earthquake type loading. LEARNING OUTCOMES (1) Have enough knowledge to confidently design both shallow and deep foundations. (2) Have an appreciation of the procedures involved with designing structures in earthquake areas.

2

USEFUL REFERENCES Das B. (2011) Principles of Foundation Engineering, SI Edition, Cengage Learning Dowrick D. J. (1977) Earthquake resistant design, New York, Willey. Ken Fleming, Austin Weltman, Mark Randolph and Keith Elson (2009) Piling Engineering, 3rd Edition. file:///C:/Users/ghataogs/Downloads/PilingEngineering%203rd%20Edition%25282001 %2529.pdf NHBC Foundation (2010) Efficient design of piled foundations for low-rise housing (NF21) Published by BRE on behalf of NHBC. http://www.nhbcfoundation.org/Efficientdesignofpiledfoundations/tabid/427/Default.aspx

Reese L.C. and Van Impe W.F. (2001) Single Piles and Pile Groups Under lateral Loading, Taylor and Francis/ Balkema Publishers. ISBN: 90 5809 348 4. Seed H., (1975) Earthquake effects on foundation systems, Chapter 25 in Foundation Engineering Handbook (ed H.F. Winterkorn and H.Y. Fang), pp. 700-732. Michael Tomlinson and John Woodward (2008) Pile design and construction practice – 5th ed., Taylor & Francis http://www.menglim498.files.wordpress.com/2013/04/piledesign-and-construction.pdf John Woodward (2005) An Introduction to Geotechnical Processes, Taylor and Francis, ISBN 0-415 28645-X Zeevaert L. (1983) Introduction to earthquake problems in building foundations, Chapter 12 in Foundation Engineering for Difficult Subsoil Conditions, pp. 489-595, New York, van Nostrand Reinhold.

OUTLINE PROGRAMME Timetable for the lectures will be supplied separately

3

PART 1 – SHALLOW FOUNDATIONS 1.0

DESIGN CRITERIA

The purpose of a foundation is to transmit a structural load to the ground. To achieve this purpose four design criteria must be satisfied: 1)

Bearing capacity

There should be a sufficient margin of safety against shear failure of the ground.

2)

Settlement and deflexion

The settlement (vertical movement) or deflexion (horizontal movement) should not cause damage to the structure, affect its intended use, or impair its visual appearance.

3)

Construction

It should be possible to build the foundation by available methods of construction.

4)

Cost

The cost should be the minimum needed to satisfy criteria 1, 2 and 3. P

Fig. 1.1 Shallow foundation supporting a vertical central force

Ground level

D q

Foundation level

B

Shallow foundation (or footing): A base with D/B < 2. Breadth B and length L. The plan dimensions of a rectangular foundation: B is always the shorter dimension. Foundation level (FL): The level of the underside of a foundation. Gross bearing pressure: q gross =

weight of base and column + weight of backfill above base plan area A of base

Overburden pressure: Total overburden pressure = γD; Effective overburden pressure = γ D - u Where γ is the bulk unit weight of the soil and u is the pore water pressure at FL Net bearing pressure q = qgross - overburden pressure. This is the increase in vertical stress on the soil at FL and it should be used in all foundation design calculations. For solid foundations q = P/A is a sufficiently close estimate, where P is the applied vertical force at ground level and A is the plan area of the foundations. In other cases, such as hollow foundations, basements and buried tanks, q should be calculated from qgross and the overburden pressure. Ultimate bearing capacity qult is the net bearing pressure that would cause shear failure of the ground. Safe bearing pressure qsafe = qult/F, where F is a factor of safety. 4

Allowable bearing pressure qall is the design value of q such that both bearing capacity and settlement criteria are satisfied.

2.0 DEFINITIONS OF P ILES Ultimate load Q ult is the force on a pile head at which the surrounding ground would fail in shear. Working load Pw is the recommended load on a pile head such that F is satisfactory and the predicted settlement is within the tolerance of the structure. Safe load Qsafe = Q ult/F, where F is a factor of safety. Note that in some cases pile group behaviour overrides individual behaviour: for example, the settlement of a group of piles with average load Q is always greater than that of a single pile at the same load Q. 3.0 UNCERTAINTY AND F ACTOR OF SAFETY The use of a factor of safety reduces the probability of failure to an acceptable level: it does not prevent failure. Presuming that there are no mistakes in analysis, there are three reasons for uncertainty: 1. Knowledge of soil properties 2. Knowledge of the applied loading 3. Imperfections in available methods of analysis Item 3 contains the least uncertainty: analytical methods are far from perfect and they need to be improved but, when correctly chosen and applied, the inaccuracy is relatively small. The greatest uncertainty is in Item 1.

Probability

Figure 1 shows the meaning of the conventional factor of safety .Both the demand D (i.e. the loading) and the capacity C (i.e. the strength) are variable quantities. In design we assign fixed values D* and C* to them, and in bearing capacity calculation F = C* ID*. But there is still a risk that D>C (or F1.5

Very high

alluvial clays

0.3 - 1.5

High

Weathered OC clays

0.1 - 0.3

Medium

Unweathered OC clays

0.05 - 0.1

Low

HOC clays; soft rocks; some boulder clays

50MPa For OC sands (and gravels?) (OCR > 2) E' = 5qc qc < 50MPa E' = 250MPa qc > 50MPa (see Lunne, T. and Christoffersen, H.P. (1985) Interpretation of cone penetrometer data for offshore sands. Norwegian Geotechnical Institute, Publication No. 156: 1-12)

7. Plate-loading tests and pressuremeter tests.

36

14.3 Differential Settlement With careful sampling and testing, total settlement (wi + wc ) can often be predicted with tolerable accuracy. Differential settlement is much harder to predict, because the difference in settlement between two points in a structure is likely to be about the same as the error in predicting their total settlements. A further complication is that the differential settlement of a building structure is partly determined by its own stiffness: it is a 'soil-structure interaction' problem. Analysis is possible for a bare frame placed complete on an elastic soil, but the practical reality is that the frame takes time to build, and the insertion of walls and partitions vastly changes its stiffness. Empirical methods are therefore commonly used, in which the likely differential settlement is assessed from the maximum total settlement. A common rule is that the likely maximum differential settlement is about half of the maximum total settlement. Definitions

L

H

∆ δ

Deflexion Ratio ∆/L

Relative rotation or angular distortion

(b)

Figure 14.1 Relative rotation and deflection ratio Suggested limiting values of δ/L and ∆/L Structure

Framed building and reinforced load bearing wall

Damage Structural Cracking in walls and partitions

Limiting values of δ/L 1/250 1/200 1/500 1/500 (1/1400 1/1000 for end bays)

1/150 1/300 - 1/500

1/150 1/500

SOURCE (1) Skempton, A.W. & MacDonald, D.H. (1956) The allowable settlement of buildings. Proc. Instn Civ. Engrs Part 3, Vol. 5: 727-84 (2) Meyerhof, G.G. (1947) The settlement analysis of building frames. The Struct. Engr, Vol. 25: 309. (3) Polshin, D.E. & Tokar, R.A. (1957) Maximum allowable non-uniform settlement of structures. Proc. 4th Int. Conf. Soil Mech. Fndn Engng (London), Vol. 1: 402 (4) Bjerrum, L. (1963) Discussion. Proc. Europ. Conf. Soil Mech. Fndn Engng (Wiesbaden), Vol. 2: 16-17

37

Structure

Damage

Unreinforced load- bearing walls

Cracking by sagging

Limiting values of ∆/L 0.0004

L/H = 3 0.0003-0.0004

Cracking by hogging Source

L/H = 1: 0.0004 L/H = 5: 0.0008 L/H = 1: 0.0002 L/H = 5: 0.0004

(1)

(2)

(3)

Reference Burland, J.B. & Wroth, C.P. (1974 ) Settlement of buildings and associated damage. Proc. Conf. Settlement of Structures (Cambridge): 611-654. Cambridge, Pentech.

15 Creep (or secondary consolidation) settlement At present creep settlement can be estimated only by empirical means. For clays, Simons (1974) suggests the following: coeff. of creep compression Cα = 0.00018 x water content (%) Then the estimated creep settlement is

wt =

σ ' +σ 'z H Cα log vo σ ' vo (1 + e 1 )

where H is the thickness of the layer, e1 is the initial voids ratio, σ'vo is the initial effective overburden pressure and σz' is the increase in effective stress caused by the foundation. (The initial voids ratio e1 can be obtained from the water content x specific gravity.) Suppose the water content of a 1m-thick clay layer is 30%, the initial overburden pressure is 50kPa and the stress increase is 100kPa. Then the voids ratio is about 0.81, Ca is 0.0054 and wt = 1.4mm. Simons, N.E. (1974) Normally consolidated and lightly overconsolidated cohesive materials. Proc. Conf. Settlement of Structures (Cambridge): 500-530. Cambridge, Pentech

38

15.1

Classification of damage caused by settlement

The following table (Tomlinson et al. 1978) is useful when assessing damage caused by settlement. Category

Degree of damage

Description of damage

Crack width (mm)

Very slight

Hairline cracks

< 0.1

1

Very slight

Fine cracks easily repaired during normal decorating. Perhaps isolated slight fracturing in building. Cracks rarely visible in external brickwork

≤1

2

Slight

Cracks easily filled. Redecoration probably needed. Recurrent cracks can be masked by suitable linings. Cracks not always visible externally. Some external repointing may be needed to ensure water-tightness. Doors and windows may stick slightly.

≤5

3

Moderate

Cracks need some opening-up and can be patched by a mason. Repointing of external brickwork and possibly a small amount of brickwork to be replaced. Doors and windows sticking. Service pipes may break. Weathertightness often impaired.

5 to 15, or several cracks ≥ 3

4

Severe

Extensive repairs needed, such as breaking-out and replacing sections of brickwork especially over doors and windows. Window and door frames distorted; floors sloping noticeably. Walls bulging or leaning noticeably; some loss of bearing in beams. Service pipes disrupted.

15 to 25, but depends on number of cracks

5

Very severe

Major repairs; partial or complete rebuilding. Beams lose bearing; walls lean badly and need shoring. Windows broken by distortion. Danger of instability.

>25, but depends on number of cracks

Notes Observations of damage should take account of location in the building, and the function of the building. Crack width is only one aspect of degree of damage. Deviations from the horizontal or vertical > 1/100 are clearly visible; deviations >1/150 are undesirable. Tomlinson, M.J., Driscoll, R. and Burland, J.B. (1978) Foundations of low-rise buildings. The Structural Engineer, Vol. 56A: 161-173.

39

PART 2 - PILE FOUNDATIONS 1 WHAT ARE PILE FOUNDATIONS? Piles are long slender members that are constructed in the ground to transmit load to competent ground at depth. They may also be used for constructing structures over water. Piles may be made of wood, steel, concrete or plastic. Steel and concrete piles are most common. Pile may either driven or cast in-situ , or they be a combination of the two. They are generally vertical, but may be raked to cater for lateral loading. Pile foundations have been used by many civilizations. Steel piles have been used since the 1800s and concrete piles since about 1900. More recently, piles made of plastic composites have been used in specialist applications.

2

PILE DESIGN

Piled foundations may be needed if: 1) the soil is too weak or compressible for shallow foundations; 2) a structure has to be supported over water; 3) tension forces are to be resisted; 4) horizontal forces have to be resisted; 5) a structure has to be underpinned; or 6) a piled foundation is cheaper or more convenient than other types. Piles may be 'end-bearing' or 'frictional' (see Figure 2.1).

Figure 2.1

40

3 MAIN T YPES OFP ILE Main Types of Pile are shown in Figure 3.1.

Non- Displacement (OR REPLACEMENT)

Displacement

LARGE DISPLACEMENT

PREFORMED Solid r.c. sections R.c. sectional piles Timber piles Closed-ended tubes

LOW DISPLACEMENT

Normal bored piles: cylindrical hole drilled and filled with in situ r.c. (diameter up to 1m)

Rolled steel sections (such as 'H' piles)

Continuous flight auger (CFA) piles: hole drilled by continuous auger and filled with concrete through the auger stem Large-diameter bored piles (diameter up to about 6m) Large-diameter under-reamed bored piles (i.e. with enlarged bases)

DRIVEN, CAST-INPLACE

Closed-ended sectional concrete tubes later filled with in situ concrete (e.g. West's) Hole is formed by driving steel tube, then filled with concrete as tube is withdrawn (e.g. Franki)

Other Types VIBRO-REPLACEMENT STONE COLUMNS Hole formed by jetting and displacement, then filled with broken stone MINI- AND MICRO-PILES. These have diameters 75mm to 200mm and may be driven or bored

Figure 3.1

Classification of piles of piles by Fleming et al. 2008 is shown in Figure 3.2

41

Figure 3.2 Classification of piles (After Weltman and Little (1977) in Fleming et al. (2008) Piling Engineering, Taylor and Francis, Oxford.

42

4 CHOOSING A SUITABLE PILE TYPE Choosing a pile depends of many factors including ground conditions, load that need to be supported, availability of plant, materials and expertise amongst many. Types of piles method of construction together with their applications and limitations are described below. a) Preformed piles (concrete, steel) can be seen complete before driving. The 'set' often gives visible evidence of ground resistance, though this can be misleading especially in saturated silts and in chalk. Preformed piles are good in water-bearing permeable soils. Concrete has to be of a high-quality, so it has good resistance to normal chemical attack. Preformed concrete and steel piles are strong in bending, so they are good where lateral forces have to be resisted. Driving is noisy. Ground heave may occur in clay soils. Lateral displacement of soils may affect nearby retaining walls. Driving may cause a local rise in pore water pressure in fine-grained soils: this could affect nearby slopes. Length can be reduced by cutting off some of the pile, but extension of concrete piles is difficult and costly (steel piles can be extended easily by welding). Substantial obstructions cannot be penetrated, and boulders may deflect the pile off line. Preformed piles (except for mini-piles) cannot be driven in low headroom. b) Driven, cast-in-place piles can be adjusted to length during driving. Base enlargement is possible with some types (e.g. Franki pile). Driving noise is reduced with some types when an internal hammer is used. If a temporary tube has to be driven, withdrawal may cause defects (with the Franki pile this is unlikely because the concrete is hammered). Ground water seepage and chemical aggression can affect freshly constructed piles. Pile driving can disrupt nearby fresh piles. Ground heave may occur in clay soils. Lateral displacement of soils may affect nearby retaining walls. Driving may cause a local rise in pore water pressure in finegrained soils: this could affect nearby slopes. c) Low-displacement piles cause little soil displacement, heave, or pore water pressure increase. Steel piles have high bending strength, and they can penetrate some obstructions. Length adjustment is easy. They are good as raking piles. Boulders can deflect piles off line. Driving is noisy. Over-driving should be avoided when toeing-in to weak rocks such as shale, mudstone and siltstone. d) Bored, cast-in-place piles are best in firm to stiff clays. The soil profile can be observed during drilling. Large diameters are possible, and under-reaming increases load capacity. Obstructions can be penetrated during drilling (but at a cost). Construction noise is less than for most driven piles. Construction is possible in low headroom. Bored piles are difficult to construct in water-bearing permeable strata. The use of casing is the cause of most types of defect. Bored piles are susceptible to chemical attack (though sulfate-resisting cement can be used to overcome this). Under-reaming is impossible in cohesionless soils. d) Continuous flight auger (CFA) piles have a wide range of applications. Construction is possible in permeable water-bearing soils, without the need for a tremie, and the presence of the auger removes the need for casing. Care is needed to ensure that the rate of withdrawal of the auger is matched by the supply of concrete, otherwise gaps may be created in the pile shaft. Most rigs now have flow meters and monitoring systems to check this balance, and a printout is produced as a permanent record for each pile.

43

Examples of some pile types are shown in Figures 4.1 to 4.2 below.

4.1

4.2

Precast and driven cast in place piles

Raymond, steel and Franki piles

Recommended reading: Chapter 3. Ken Fleming, Austin Weltman, Mark Randolph and Keith Elson (2009) Piling Engineering, 3rd Edition. [NOTE THAT CONTENTS OF THIS CHAPTER ARE EXAMINABLE]

44

5 PILE ACTION The action of pile in the ground (see Fig 5.1) The ultimate load (more loosely, the bearing capacity) of a single pile is the sum of the end resistance...