Pile Design and Construction

Design of pile foundation

Pile Design and Construction

বাংলাদেশের মাটিতে পাইল ফাউন্ডেশন দরকার হয় বেশীরভাগ ক্ষেত্রে। থিওরি এবং বাস্তবতার আলোকে Cast in situ pile এবং precast driven pile এর ডিজাইন কিভাবে করতে হয় এবং এর কোয়ালিটি কন্ট্রোল কিভাবে করা যায়, এই বিষয়গুলি অধ্যাপক জাহাঙ্গীর আলম উনার 20 বছরের অভিজ্ঞতা থেকে বোঝানোর চেষ্টা করেছেন। যেহেতু বাংলাদেশের 70%  জায়গায়ই সফট সয়েল, সুতরাং বাংলাদেশের কোন সিভিল ইঞ্জিনিয়ার যেন পাইল ডিজাইন এ ভুল না করে – এটাই আমাদের প্রত্যাশা। অনেক সয়েল টেস্ট রিপোর্টে দেওয়া পাইল ক্যাপাসিটি ভুল হওয়ার সম্ভাবনা থাকে। এই কোর্সটি করলে পাইল ক্যাপাছিটি ক্যালকুলেশনের AASHTO Method এর উপর যে এক্সেল শিট তৈরি করা আছে সেটা ফ্রি দেওয়া হবে।

Recent alluvial deposits (soft silt and clay, loose sand) exist all over Bangladesh. Most of the construction requires pile foundation here. Both theory and practical aspects of designing bored pile (or drilled pile, drilled shaft, cast-in-situ pile) and precast driven pile (hammer driven or push pile) are taught by Professor Dr. Jahangir Alam from his 20+  years of experience in teaching, designing and research. Construction or pile installation methodology and quality control of construction were also covered in this short course. More than 70% land area of Bangladesh has soft soil. So, we hope and expect that Professional Civil Engineers of Bangladesh would not do any mistakes in pile foundation design. Most of the subsoil investigation report contains erroneous pile capacity. After completing this short course, you will get an EXCEL PROGRAM of PILE CAPACITY CALCULATION using AASHTO METHOD.

What you'll learn

  • Load carrying mechanism of pile
  • Load capacity of driven pile (Tomlinson)
  • Load capacity of bored pile (AASHTO)
  • Downdrag of pile and its remedy
  • Structural design of pile
  • Quality control of cast in situ piling
  • Soil identification and USCS classification
  • Correlations of soil strength parameters
  • Shear strength of soil

Special Gift with The Course

  • EXCEL PROGRAM of PILE CAPACITY CALCULATION using AASHTO METHOD

Prerequisite / Eligibility

  • BSc, MSc or PhD in Civil Engineering
  • Level 4 or 4th year student of bachelor’s in civil engineering

Course Teacher

Professor Dr. Jahangir Alam, Department of Civil Engineering, BUET, Dhaka, Bangladesh

Brief Biography of Course Teacher

Education

  • PhD in Geotechnical Earthquake Engineering, the University of Tokyo, Japan, 2005
  • MSc in Civil and Geotechnical Engineering, BUET, Dhaka, Bangladesh, 2002
  • BSc in Civil Engineering, BUET, Dhaka, Bangladesh, 1998

Biography

Professor Dr. Engr. Md. Jahangir Alam is faculty member at the Department of Civil Engineering, BUET, Dhaka-1000, since 1999. He completed his PhD from University of Tokyo in Geotechnical Earthquake Engineering in 2005 as a Monbu-Kagaku-sho Scholar. He was research fellow in Ecole Centrale Paris in 2008. Professor Jahangir did his BSc in Civil Engg with major in Structure and MSc in Civil and Geotechnical Engineering from BUET.

Professor Jahangir has multidisciplinary research interests and has publications in international journals and conferences. His current research topics are “Risk Sensitive Land Use Planning of Mega City”, Physical and Numerical Modeling of Liquefaction Hazard, Mitigation against Seismic Liquefaction, Cyclic Behavior of Non-plastic silt, Reinforced Earth, Earthquake Resistant Foundation in Soft Soil, Climate Resilient Concrete, Climate Resilient Road.

Professor Jahangir was involved in many consultancy projects where he designed/checked high rise RCC buildings, Communication towers, Jetty, Shore Pile, Embankment, Container Terminal, Ground Improvement, Bridge Foundation etc. He supervised many MSc and PhD students. He was involved in pile load testing, pile integrity testing, concrete mix design and development of laboratory and field-testing equipment.

Professor Jahangir actively involved in National and International professional bodies. He is life member and was Treasurer of Bangladesh Society for Geotechnical Engineering (BSGE), which is Bangladesh Chapter of ISSMGE. He is life fellow of Institute of Engineers Bangladesh (IEB). He is life member of Bangladesh Earthquake Society (BES). He was Treasurer of Bangladesh Earthquake Society (BES).

Certificate of Attendance

Certificate of attendance will be awarded after completion of all video lessons and quizzes

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Frequently Asked Questions (FAQ)

Detail Course Outline

Load Bearing Mechanism of Pile

  • Pile Types Based on: Construction Method (Driven or cast-in-situ pile), materials (RCC, Steel), load distribution (Friction, End bearing, Floating pile)
  • Pile driving methods: Jacking, Vibratory Driving, Hammering
  • Distribution of Skin Friction, Load-Settlement Curve, Ultimate Load Vs. Service Load
  • Pile load test and limitations

Driven Pile Design

  • Feature of driven pile: More skin friction, Densification, Instant Load capacity, Good quality control
  • Geotechnical Capacity: Capacity calculation, Factor of Safety, Axial capacity, Group capacity, Side resistance, Tip resistance, Settlement
  • Limitations of Compressive Load Test Result: Don’t consider negative skin friction, consolidation settlement, group action of piles, lateral load capacity of pile.

Bored pile (cast in situ) pile design

  • Pile capacity: compression capacity, uplift capacity
  • Factor of safety selection
  • Side resistance
  • Tip resistance
  • Group action

Quality control of cast in situ piling

  • Comparison between precast and bored RCC pile
  • Mix ratio
  • Method of construction
  • Steps of cast-in-situ piling
  • Use of casing
  • Temporary casing necessary
  • Precautions for temporary casing
  • Drilling Slurry
  • Slurry types
  • Problems of Cast in situ piling
  • Pile reinforcement
  • Installation of concrete rollers
  • Pile Cap Casting
  • Placement of concrete in drilled shaft
  • Use of retarder in tremie concrete
  • Precautions during concreting
  • Influence of construction on soil properties of clay

BNBC 2017/ BNBC 2020 provisions on pile foundation

  • Load combinations
  • Settlement
  • BNBC provision: Driven Precast Piles, Bored Pile, Pile cap, Structural capacity, Allowable Stress, Rebar, Hoops, Concrete strength, Clear cover, Rebar detailing, Group capacity, Pile spacing, Rebar Spacing, Bentonite slurry, Rabbit, Tremie concreting.

Structural Design of Piles

  • Precast Concrete Pile: Lifting, Driving, Size Vs. Length, Rebar, Hoops, Push Pile, Rebar for Hard Driving, Driving Stresses, Detailing, Pile Shoes.
  • Cast in Situ Concrete Piles: Design considering or without considering lateral load, Rebar, Pile cap design, Thickness, Strut and Tie.

Negative skin friction and its remedy

  • Definition
  • Determination
  • Structural design of pile: Settlement calculation, load combinations
  • Remedy for driven and bored pile

Soil identification and classification

  • Formation, type and identification of soils: Grain size, particle shapes.
  • Soil classification system: USCS, AASHTO

Introduction to Pile Design and Construction

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Introduction
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All documents of Pile Design and Construction

Load Bearing Mechanism of Pile

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Load Bearing Mechanism of Pile, Part -1

Load Bearing Mechanism of Pile

load bearing mechanism pile is a an important topic for civil engineers. without understanding load bearing mechanism of pile, it is not possible to learn pile foundation design.



Pile Types Based on Construction Method

1.     Driven pile (steel, timber or precast RCC pile driven by hammering)

2.     Pushed or jacked pile

3.     Driven and cast-in-place (closed end steel tube driven and then filled by concrete in place)

4.     Bored pile or cast-in-situ pile



Pile types based on materials

1.     RCC

2.     Timber

3.     Steel

4.     Soilcrete (Soil-Cement)



Pile types based on load distribution

•        Friction: More than 80% ultimate load taken by skin friction

•        End bearing: More than 80% ultimate load taken by end bearing

•        Combination of Friction & End bearing: Ultimate load taken by both skin friction and end bearing

•        Floating pile: End bearing is neglected



Pile driving methods

•        Jacking

•        Vibratory Driving

•        Hammering



Bored Pile vs. Drilled Shaft?

•        Same thing

•        Sometimes large diameter piles called shaft or pier



Ultimate Load Vs. Service Load

Mobilized Friction and End Bearing

Axial Force Distribution along the Pile



Limitations of Compressive Load Test Result

•        Don’t account negative skin friction

•        Don’t include consolidation settlement

•        Don’t consider group action of piles

•        Don’t consider lateral load capacity of pile



Pile capacity from single pile load test

Test pile is loaded until failure (2 to 4 times the design load) Service pile is loaded upto 1.5 times design load

Safe Load for Single Pile :

1.     Two thirds of the final load at which the load displacement attains a value of 12 mm unless otherwise required in a given case on the basis of nature and type of structure in which case, the safe load should be corresponding to the stated total displacement permissible

2.     Fifty (50) percent of the final load at which the total displacement equals to 10 percent of pile diameter case of uniform diameter piles and 7.5 percent of bulb diameter in case of under-reamed piles.



Pile capacity from group pile load test

Safe Load for Pile Group :

1.     Final load at which the load displacement attains a value of 25 mm unless otherwise required in a given case on the basis of nature and type of structure.

2.     Two thirds of the final load at which the total displacement attains a value of 40 mm.



Single pile vs. group pile

1.     Failure mechanism is different for single and group pile

2.     Failure may be initiated by single piles or block failure as group; factors are

•        Spacing of pile *** (more spacing >>>> single pile)

•        Soil type

•        Pile length

3.     Settlement of group is more than single pile



True Distributions of Load in Instrumented Piles

1.     The measurements are analyzed from the assumption that the “zero readings”, which are the readings taken at “zero” time.

2.     This assumption is more than a little off. It neglects the existence of locked-in loads—residual load—in the pile and is one of the sources of the myth of the so-called “critical depth”.

3.     Neglect of the residual load distribution is also the main reason for conclusions of instrumented tests that suggest shaft resistance to be smaller when the pile is loaded in tension as opposed to when it is in loaded in compression.


locked-in stress and strain, also called “residual loads”

Residual load will always develop in a pile, be it a driven or a bored pile



Direct Measurements of Residual Load for Piles Driven in Sand

•        Four instrumented, 8 m and 16 m long, 280 mm diameter precast concrete piles driven into a very loose sand

•        it both includes measurements of residual load before the start of the static loading test and of the true load distribution in the piles at the ultimate load



Residual distributions are measured before static load test in a driven pile

1.     The ultimate resistance of Pile are presented figure, showing the measured resistance Distribution labeled “True” and a curve fitted to the data by an effective stress calculation, as well as the measured distribution of residual load.

2.     Distributions are characterized by that the load in the pile increases below the pile head due to progressively increasing negative skin friction

3.     At a depth of about 6 m or slightly below, a gradual reduction of negative skin friction and transition to positive shaft resistance begins.

4.     An equilibrium (neutral plane) between downward and upward acting forces exists at a depth of about 10 m below which the transition continues with increasing positive shaft resistance.

5.     The unit negative skin friction along the upper about 6 m length of the piles corresponds to a beta-coefficient of 0.35 in an effective stress analysis.

6.     The static loading tests on Pile reached ultimate resistances for loads applied to the pile head of 510 KN.

7.     The toe resistance of Pile, starting from the locked-in value of about 50 KN, increased linearly to 110 KN.

8.     The values of total shaft resistance for Pile 400 KN.

9.     A 400-kN ultimate shaft resistance value corresponds to a beta-coefficient of 0.20 determined in an effective stress analysis.

10. The reduction of the shear stress was about 40% due to several loading and unloading during the test

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Load Bearing Mechanism of Pile, Part -2
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Load Bearing Mechanism of Pile, Part -3
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Load Bearing Mechanism of Pile, Part -4
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Load Bearing Mechanism of Pile, Part -5
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Load Bearing Mechanism of Pile, Part -6
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Load Bearing Mechanism of Pile, Part -7
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Quiz on Load bearing mechanism of pile
13 questions

Driven Pile Design

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Driven Pile Design, Part -1

Driven Pile Design




Salient feature of driven/pushed pile

·        Driven or pushed to soil >>>> K is more >>> more skin friction

·        Densification of loose sand during driving

·        End bearing is mobilized at small movement of pile toe during service load

·        Load capacity may be confirmed during installation

·        Concrete or material quality control is easy



Compare driven pile and bored pile

Driven Pile

·        End bearing is mobilized at 1-2% movement of base dia

·        Full skin friction is mobilized

·        At sandy soil, skin friction is higher than bored pile (large K)

·        Elastic settlement under service load is less

·        Consolidation settlement under service load is same for both type


Bored Pile (Cast in Situ)

·        End bearing is mobilized at 5-10% movement of base dia

·        Full skin friction is mobilized

·        At sandy soil, skin friction is less than driven pile

·        Elastic settlement under service load is more

·        Consolidation settlement under service load is same for both type



Geotechnical Capacity of Pile Vs. Structural Capacity of Pile: Limitations of Compressive Load Test Result

·        Don’t account negative skin friction

·        Don’t include consolidation settlement

·        Don’t consider group action of piles

·        Don’t consider lateral load capacity of pile



Factor of Safety (BNBC-2017) Vs. Partial Safety Factors (EC7)

ASD Vs. LRFD

WSD Vs. USD

ASD = Allowable Stress Design

WSD = Working Stress Design

LSD = Limit State Design

LRFD = Load and Resistance Factor Design



ASD in AASHTO (2002) Vs. LRFD in AASHTO (2007): The ultimate axial capacity of pile

Compression (Qult=Qs+Qt-w)

Uplifting        (Qult≤ 0.7Qs+w)

The allowable axial load shall be determined as:

Qall=Qult/FS



Factor of Safety as per BNBC 2017

Factor of Safety for Deep Foundation for Downward and Upward Load

Design Life (yrs.) ---------Good Control -------Normal Control-------- Poor Control --------V. Poor Control

> 100-----------------------------2.30-----------------------3.00-----------------------3.50-------------------4.00

25 -100---------------------------2.00-----------------------2.50----------------------2.80-------------------3.00

< 25--------------------------------1.40-----------------------2.00----------------------2.30-------------------2.80


Guidelines for Investigation, Analysis and Construction Control



Factor of Safety and Settlement

Experience shows

·        Safety factor > 2.5 will ensure that an isolated pile with a shaft diameter of not more than 600 mm driven into a coarse soil will not settle by more than 15 mm



Capacity Calculation for Cohesion less Soil

Side resistance for cohesion less soil

Unit side resistance = skin friction (stress)

qs=K_s σ'tanδ

Ks = a coefficient of horizontal earth pressure

σ'vo = average effective overburden pressure over the depth of the soil layer

δ = angle of friction at the pile/soil interface



Side resistance for cohesionless soil

·        Ks is not constant over the depth of the pile shaft

·        It depends on the relative density of the soil

·        It depends on the volume displacement of the soil by the pile


Values of the coefficient of horizontal soil stress Ks (Kulhawy)

Installation method-----------------------------Ks/K0

Driven piles, large displacement------------1.0-2.0

Driven piles, small displacement-----------0.75-1.75

Bored and cast-in-place piles----------------0.71-1.00

Jetted piles---------------------------------------0.5-0.7


Values of the angle of pile to soil friction for various interface conditions (Kulhawy)

Pile/soil interface condition------------------Angle of Pile/soil friction, δ

Smooth (coated) steel/sand-------------------------0.5φ'-0.7φ'

Rough (corrugated) steel/sand---------------------0.7φ'-0.9φ'

Precast concrete/sand ***--------------------------0.8φ'-1.0φ'

Cast-in-place concrete/sand---------------------------1.0φ'

Timber/sand------------------------------------------0.8φ'-0.9φ'



Critical Depth

·        The critical depth of piles are normally assumed as 10-20 pile diameter

·        The critical depth is a fallacy which comes from the failure to interpret the results of full and model-scale pile load tests



Tip resistance for cohesion less soil

1.      Tip resistance: Qb=qbAb

2.      Unit tip resistance: qb =N_q σ'vo

·        Nq is a bearing capacity factor

·        Related to the peak angle of shearing resistance φ^'of the soil

·        And slenderness ratio (L/B) of the pile



Side resistance for cohesive soil

Side resistance, Qs=q_s As

Unit side resistance, qs=F×αp ×C_u

F= Length factor

αp= Peak adhesion factor

α= adhesion factor = FX αp

Cu= undrained cohesion



Tip resistance for cohesive soil

·        Tip resistance, Qb= qbXA

·        Unit tip resistance, qb= CuXNc

Nc = 9, when Length ≥ 5Xwidth of pile



Group capacity of driven pile

1.      Reduction factor, ζ = 1.0

2.      The group capacity may then be computed as the lesser of

               (i) the sum of the individual capacities of each shaft in group, or

               (ii) the capacity of an equivalent pier defined in the perimeter area of the group

3.      For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction



Limitation pile capacity calculation

Densification of sand during driving is ignored

ASCE Committee on Deep Foundations report [CDF (1984)] recommends not using group efficiency as a description of group action. It suggests that group efficiency > = 1.

The reasons are

·        Soil displacement by pile volume

·        Densification of granular soil during pile driving



Perimeter and Base Area of RCC Pile

Plugged Capacity = External Skin Friction + 0.5*Full End Bearing?

Plugged Capacity = External Skin Friction + Full End Bearing?

Unplugged Capacity = External Skin Friction + Internal Skin Friction + Full End Bearing

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Driven Pile Design, Part -2
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Driven Pile Design, Part -3
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Driven Pile Design, Part -4
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Driven Pile Design, Part -5
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Driven Pile Design, Part -6
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Quiz on Driven pile design
27 questions

Mathematical Examples of Driven Pile

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Driven Pile Design, Example-1, Part-1
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Driven Pile Design, Example-1, Part-2
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Driven Pile Design, Example-2, Part-1
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Driven Pile Design, Example-2, Part-2
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Driven Pile Design, Example-2, Part-3

Quality Control of Cast In Situ Piling

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Quality Control, Part-1

QUALITY CONTROL OF CAST IN SITU PILING




Compare between precast and bored RCC pile

Precast pile

1. Slump value 50-100 mm

2. Retarder and plasticizer is not needed in general

3. Compaction by vibration is necessary

4. Clear cover > 50 mm

5. Durability should be considered

 

Bore pile

1. Slump value 150 – 200 mm

2. Retarder and plasticizer should be used

3. Compaction is prohibited

4. Clear cover > 75 mm

5. Durability should be considered



Mix ratio

Mix ratio of cast in situ piling concrete (Without plasticizer):

Cement = CEM-II, FA = Coarse Sand, CA = Stone chips or Shingles

•            1:1.25:2.0

•            Minimum fc’ = 28 Mpa (4000 psi)


Mix ratio of cast in situ piling concrete (Without plasticizer):

Cement = CEM-II, FA = Coarse Sand, CA = Stone chips or Shingles

•            1:1.5:2.5



Wet method of construction or Slurry method of construction

Excavation of bore hole can be done by:

1. Percussion method of drilling

 a. Usually done in Bangladesh

 b. Good for dia 20” – 24” and L = 90’

2. Rotary method of drilling

 a. Expensive

 b. Now used in big projects

 c. Any dia and length ok

 d. Economic for large dia



Steps of cast-in-situ piling

·        Boring

·        Insertion of rebar cage

·        Concreting through tremie pipe



Temporary casing necessary

·        when caving soil (loose sand and silt) exist at top layer

·        To maintain the piling location

·        To maintain the slurry above the ground level

·        Length of casing depends of soil layers



Precautions for temporary casing 

Cleaning after each use

Must be as smooth as possible

Casing with bonded concrete should not be allowed

Slurry column should extend well above the level of the piezometric surface so that any fluid flow is from the excavation outward



Drilling Slurry

·        Bentonite or polymer slurry may be used

·        Drilling fluid serves to put soil particles in suspension and will form a membrane or a filter cake at the walls of the borehole.

·        The membrane acts to prevent caving or collapse of the borehole.

·        Also called as drilling mud



Drilling slurry is essential

·        For piling into caving soil

·        Caving soil types

o  Loose sand and gravel

o  Nonplastic Silt


·        Important note: avoid drilling slurry if only water is enough, because

o  Bentonite slurry is expensive

o  Reduces skin friction of pile



Slurry types

·        Bentonite slurry

·        Polymer slurry



Pile reinforcement (cage)

The cage is designed to meet two requirements

·        The structural requirement for bending and for column action (sometimes slender column)

·        Stability requirements of the rebar cage during its placing, during the placing of concrete and during



Minimum longitudinal reinforcement

If the pile has sufficient axial strength using only half the gross concrete area, Ag/2, the longitudinal reinforcement ratio can be reduced to 0.5 percent of the gross concrete area, Ag.



Longitudinal bar in the cage

Maximum longitudinal reinforcement at the top if no drag load

Symmetrical arrangement of longitudinal rebar is recommended unless there is compelling reasons

Minimum 5 or preferably 6 longitudinal bars are needed

16mm dia bar is the minimum size of longitudinal bar

Clear spacing between bars is 3-5 times maximum size of CA

If a very large amount of rebar is needed, concentric multiple cages or bundled bars may be used.



Centering Devices

Two purposes:

1. Clear cover 3”

2. To flow concrete


 Rebar’s should not be used for centering devices unless they are epoxy-coated. Better solution is to use concrete roller



Pile Cap Casting

·        Break 3 ft weak concrete at top of drilled shaft / bored pile before casting pile cap

·        Use top, bottom and side reinforcements in pile cap even it is not needed by calculation

·        Use thick pile cap to ensure rigid action of pile cap to distribute column load to individual piles uniformly

·        Take care of basement construction at pile cap level



Placement of concrete in drilled shaft

Basic characteristics of concrete for drilled shafts

·        Excellent fluidity

·        SCC (Self Compacting Concrete) Compaction under self-weight

·        Resistance to segregation

·        Controlled setting



Use of retarder in tremie concrete

If the required time of pouring concrete is more than initial setting time, retarder must be used

Initial setting time >= 45 minute

Final setting time <= 420 minute

Usually initial setting time = 2 hours



Precautions for tremie concrete

·        The concrete shall contain at least 10 per cent more cement than that required for the same mix placed in the dry.

·        Slump = 150 mm to 200 mm

·        Successful placement of concrete under water requires preventing flow of water across or through the concrete.

·        The first concrete placed is physically separated from the water by using a “rabbit” or go-devil or plug in the pipe, or by having the pipe mouth capped or sealed and the pipe dewatered.

·        Once filled with concrete, the pipe is raised slightly to allow the “rabbit” to escape or to break the end seal. Concrete will then flow out and develop a mound around the mouth of the pipe. This is termed as “establishing a seal”.

·        Once the seal is established, fresh concrete is injected into the mass of existing concrete.



Precautions during concreting

·        In the “rabbit” plug approach, open tremie pipe should be set on the bottom, the “rabbit” plug inserted at the top and then concrete should be added to the tremie slowly to force the “rabbit” downward separating the concrete from the water. Once the tremie pipe is fully charged and the “rabbit” reaches the mouth of the tremie, the tremie pipe should be lifted a maximum of 150 mm (6 inch) off the bottom to allow the “rabbit” to escape and to start the concrete flowing. After this, a tremie pipe should not be lifted again until a sufficient mound is established around the mouth of the tremie.

·        Tremies should be embedded in the fresh concrete a minimum of 1.0 to 1.5 m (3 to 5 ft) and maintained at that depth throughout concreting to prevent entry of water into the pipe.

·        Rapid raising or lowering of the tremie pipe should not be allowed. All vertical movements of the tremie pipe must be done slowly and carefully to prevent “loss of seal”.

·        Underwater concrete shall be placed continuously for the whole of a pour to its full depth approved by the Engineer, without interruption by meal breaks, change of shift, movements of placing positions, and the like. Delays in placement may allow the concrete to stiffen and resist flow once placement resumes.

·        The volume of concrete in place should be monitored throughout the placement. Under runs are indicative of loss of tremie seal since the washed and segregated aggregates will occupy a greater volume. Over runs are indicative of loss of concrete.


·        Tremie pipe dia > 6 times of maximum size of CA

·        Plug of cement paste is recommended as low cost solution of initial charge



Minimum c/c spacing of piles

Drilling and extraction of casing during construction of a pile can cause upward directed shear stresses to develop on the perimeter of adjacent piles that were installed earlier, possibly damaging those shafts

So minimum c/c spacing = 3D

If closer spacing is required, sequence of shaft installation must be mentioned



Load distribution in pile

·        Initial loads are taken almost completely by skin friction at very small displacement

·        as loading continues, some load is transferred to the base of the drilled shaft

·        At the ultimate load, a sizeable portion of load is carried by end bearing but at significant amount of downward displacement



Influence of construction on soil properties of clay

·        Stress released during excavation. The placing of concrete will reimpose a stress in the clay surrounding the drilled shaft that can be greater than the in situ stress. The magnitude of the concrete stress is dependent on the slump of the concrete, and high-slump concrete is highly recommended.

·        Some chemical bond occur between clay and cement, so skin friction increases. So, Shear failure does not occur at interface but a short distance from interface



Influence of construction on soil properties of sand

A membrane of bentonite is created at the wall of the borehole

Stress released during excavation. The placing of concrete will reimpose a stress in the sand surrounding the drilled shaft that can be greater than the in situ stress. The magnitude of the concrete stress is dependent on the slump of the concrete, and high-slump concrete is highly recommended.

Some chemical bond occur between sand and cement, so skin friction increases. So, Shear failure does not occur at interface but a short distance far from interface



Quality Assurance

·        Pile integrity test – length and integrity

·        Pile load test on test piles and service piles

o  Static load test – takes time

o  Dynamic load test (PDA) - rapid



Summary of quality control of pile

·        Ensuring length

·        Cross checking the soil test bore log with drilling bored pile

·        Slump and mix ratio of concrete

·        Outlet of tremie pipe into concrete all the time

·        Maintaining clear cover

·        Prevention of caving by using bentonite slurry or any other mud

·        Cleaning the bore hole properly to ensure end bearing

2
Quality Control, Part-2
3
Quality Control, Part-3
4
Quality Control, Part-4
5
Quality Control, Part-5
6
Quality Control, Part-6
7
Quality Control, Part-7
8
Quality Control, Part-8
9
Quality Control, Part-9
10
Quality Control, Part-10

Cast in Situ Pile (Bored Pile) Design

1
Cast in situ pile design, Part-1

Geotechnical Design of Bored Pile (AASHTO: 2002)

Ref: Standard Specifications for Highway Bridges :

Published by American Association of State Highway and Transportation Officials (AASHTO)




The ultimate axial capacity of drilled shafts shall be determined in accordance with the following

Compression: Qult=Qs+Qt-w

Uplifting: Qult<0.7Qs+w

The allowable axial load shall be determined as:

Qall = Qult/FS



SIDE RESISTANCE IN COHESIVE SOIL

Ultimate side resistance may be estimated using the following:

Qs = Ultimate side resistance in soil (kip)

Sui = Incremental undrained shear strength as a function over ith depth interval (ksf)

Sui= 5*N60(kPa)

αi = Adhesion factor as a function over ith depth interval (dim)

Δzi = ith increment of shaft length (ft)

B = Diameter of the pile (ft)

N  = number of layers


Recommended Values of α and fsi for Estimation of Drilled Shaft Side Resistance in Cohesive Soil Reese and O’Neill (1988)


??=?_? ????? = (?_? ????)?^′ ??= β?



TIP RESISTANCE IN COHESIVE SOIL

Nc = Bearing capacity factor (dim)

qT = Ultimate unit tip resistance for shafts (ksf)

Sut = Undrained shear strength within 2B below shaft tip (ksf)

Su= 5*N60(kPa)

Recommended Values of qT for Estimation of Drilled Shaft Tip Resistance in Cohesionless Soil after Reese and O'Neill (1988)



Group action for cohesive soil

·        If the cap is not in firm contact with the ground, or if the soil at the surface is loose or soft, the individual capacity of each shaft should be reduced to ?=0.67 for a center-to-center (CTC) spacing of 3B and ?=1.0 for a CTC spacing of 6B

·        The group capacity then is computed as the lesser of the sum of the modified individual capacities of each shaft in the group or the capacity of an equivalent pier



Group action for cohesionless soil

·        If the cap is not in firm contact with the ground, or if the soil at the surface is loose or soft, the individual capacity of each shaft should be reduced to ?=0.67 for a center-to-center (CTC) spacing of 3B and ?=1.0 for a CTC spacing of 8B

·        The group capacity then is computed as the lesser of the sum of the modified individual capacities of each shaft in the group or the capacity of an equivalent pier



Nominal Axial Compression Resistance of Single Drilled Shafts

The factored resistance of drilled shafts, RR, shall be taken as

RR= Ultimate Resistance of single drilled shaft

Rn= Nominal Resistance of single drilled shaft

ɸ= Resistance factor

ɸT= Resistance factor for Tip location

ɸs= Resistance factor for skin friction


Shafts in cohesive soil

It may be designed by total and effective stress methods of analysis, for undrained and drained loading conditions, respectively.


Shafts in cohesionless soil

It shall be designed by effective stress methods of analysis for drained loading conditions.



SIDE RESISTANCE IN COHESIVE SOIL

α method: Based on Total Stress (Undrained and Short Term)

1.      Undrained condition

·        The ultimate unit load transfer in side resistance at any depth

·        Environmental, long-term loading or construction factors may dictate that a depth greater than 5 feet should be ignored in estimating RS

·        Recommended Values of a for Estimation of Drilled Shaft Side Resistance in Cohesive Soil (After Reese and O’Neill)

·        The following portions of a drilled shaft, should not be taken to contribute to the development of resistance through skin friction


2.      Drained condition

In drained condition, clay soil properties shall be derived from CD test



SIDE RESISTANCE IN COHESIONLESS SOIL β METHOD 

The ultimate side resistance of axially loaded drilled shafts:

The value of βi may be determined using the following:

The value of ?_? may be determined using the following:

?_?=?.?−?.???√(?_? ) ;?.?>?_?>?.?? ??? ?_??≥??

?_?=?_??/??[?.?−?.???√(?_? ) ];?.?>?_?>?.?? ??? ?_??<??


The value of β is function of depth and SPT-N value in AASHTO (LRFD) 2007

In AASHTO: 2002 β is function of depth only



TIP RESISTANCE IN COHESIVE SOIL

·        The value of Su should be determined from the results of in-situ and/or laboratory testing of undisturbed samples obtained within a depth of 2.0 diameters below the tip of the shaft

·        If the soil within 2.0 diameters of the tip has Su <0.5 ksf, the value of Nc should be multiplied by 0.67

·        Cohesive Soil ( drained)

·        In drained condition cohesive soil is considered as sand

·        SPT-N60 blow counts greater than 50 shall be treated as intermediate geomaterial (IGM) and the tip resistance, in ksf

·        This IGM concept has been excluded in AASHTO( LRFD): 2017



Nominal Axial Compression Resistance of Single Drilled Shafts

The factored resistance of drilled shafts, RR, shall be taken as

RR= Ultimate Resistance of single drilled shaft

Rn= Nominal Resistance of single drilled shaft

ɸ= Resistance factor

ɸT= Resistance factor for Tip location

ɸs= Resistance factor for skin friction



DESIGN CONSIDERATION

·        Shafts in cohesive soil

It may be designed by total and effective stress methods of analysis, for undrained and drained loading conditions, respectively.


·        Shafts in cohesionless soil

It shall be designed by effective stress methods of analysis for drained loading conditions.

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Cast in situ pile design, Part-2
3
Cast in situ pile design, Part-3
4
Cast in situ pile design, Part-4
5
Cast in situ pile design, Part-5

BNBC 2020 Provision of Pile Foundation

1
BNBC 2020 Provision of Pile Foundation, Part-1

SOILS AND FOUNDATIONS

BNBC 2017, PART 6, CHAPTER 3




Foundation Engineer vs. Geotechnical Engineer

FOUNDATION ENGINEER

A graduate Engineer with at least 5 (five) years of experience in civil engineering particularly in foundation design or construction.


GEOTECHNICAL ENGINEER

Engineer with Master’s degree in geotechnical engineering having at least 2 (two) years of experience in geotechnical design/construction or graduate in civil engineering/engineering geology having 10 (ten) years of experience in geotechnical design/construction.


Geotechnical Engineer > Foundation Engineer



Working Stress Load combinations

·        D + F  (F = fluid load)

·        D + H + F + L + T (H = earth pressure load, T = load due to temperature change)

·        D + H + F + (Lr or R)

·        D + H + F + 0.75(L + T ) + 0.75(Lr or R)

·        D + H + F + (W or 0.7E)

·        D + H + F + 0.75(W or 0.7E) + 0.75L + 0.75(Lr or R)

·        0.6D + W + H (needed to check uplifting)***

·        0.6D + 0.7E + H (needed to check uplifting)***



Load combinations for foundation design

Design Load :

a)      Shallow foundation design considering bearing capacity due to shear strength shall consider the most unfavorable effect of the following combinations of loading:

1.      D + L

2.      0.75×[D + L + (W or E)]   {0.75 = 1/1.33}

3.      0.9×D + Buoyancy Pressure

4.      0.6xD + W

b)     Shallow foundation design considering settlement shall consider the most unfavorable effect of the following combinations of loading:

1.      Sand: D + L

2.      CLAY: D + 0.5 L



Critical Depth for pile in sand

Dc=10D, for loose sand

Dc=15D, for medium dense sand

Dc=20D, for dense sand***

Applicable only in the method where no limiting value is mentioned



Important notes

Driven Precast Piles :

·        The minimum center-to-center pile spacing of 2.5B is recommended. The nominal dimensions and length of all the piles in a group should be similar. *****

·        All piles shall be braced to provide lateral stability in all directions.

·        Three or more piles connected by a rigid cap shall be considered as being braced (stable),

·        A two pile group in a rigid cap shall be considered to be braced along the axis connecting the two piles.

·        Piles supporting walls shall be driven alternately in lines at least 300 mm apart and located symmetrically under the centre of gravity of the wall load, unless effective measures are taken to cater for eccentricity and lateral forces, or the wall piles are adequately braced to provide lateral stability.

·        Individual piles are considered stable if the pile tops are laterally braced in two directions by construction, such as a structural floor slab, grade beams, struts, or walls. *****



Pile cap

1.      Pile caps shall be made of reinforced concrete.

2.      The soil immediately below the pile cap shall not be considered as carrying any vertical load.

3.      The tops of all piles shall be embedded not less than 75 mm into pile caps and the cap shall extend at least 100 mm beyond the edge of all piles.

4.      The tops of all piles shall be cut back to sound material before capping.

5.      The pile cap shall be rigid enough, so that the imposed load can be distributed on the piles in a group equitably.

6.      The cap shall generally be cast over a 75 mm thick levelling course of concrete.

7.      The clear cover for the main reinforcement in the cap slab under such condition shall not be less than 50 mm.



Structural capacity of driven precast pile section

Pile diameter/cross-section of a pile shaft at any level shall not be less than the designated nominal diameter/cross-section. The structural design of piles must consider each of the following loading conditions.

1.      Handling loads are those imposed on the pile between the time it is fabricated and the time it is in the pile driver leads and ready to be driven. They are generated by cranes, fork lifts, and other construction equipment.

2.      Driving loads are produced by the pile hammer during driving.

3.      Service loads are the design loads from the completed structures.



Maximum Allowable Stress on Driven Pile

RCC pile =0.33fc

Steel pile =0.25fy

Prestressed RCC pile =0.33fc-fpc

If Su < 10 kN/m2 Pile shall be designed As a long column



Minimum rebar in driven pile

Minimum Reinforcement in Driven Concrete Pile: The maximum bending stress is produced while handling if the pile is pitched at the head. To prevent whipping during handling, length/diameter ratio of the pile should never exceed 50. The following reinforcement provisions may not be valid for laterally loaded piles or piles for uplift resistance.

1.      Pile length < 30 times the least width : 1.00%

2.      Pile length 30 to 40 times the least width : 1.5%

3.      Pile length > 40 times the least width : 2%



Hoops in driven pile

The lateral reinforcement resists the driving stresses induced in the piles and should be in the form hoops or links of diameter not less than 6 mm. The volume of lateral reinforcement shall not be less than the following:

·        At each end of the pile for a distance of about three times the least width/diameter – not less than 0.6% of the gross volume of the pile.

·        In the body of the pile – not less than 0.2% of the gross volume of the pile.

·        The transition between closer spacing and the maximum should be gradual over a length of 3 times the least width/diameter.



Concrete of driven pile

Minimum Grades of Concrete: The minimum 28 days cylinder strength of concrete for driven piles is 21 MPa. Depending on driving stresses, the following grades of concrete should be used.

·        For hard driving – 28 MPa

·        For easy driving – 21 MPa

**** Durability must be considered to select concrete mix ratio



Bored Pile vs. Drilled Shaft?

·        Same thing

·        Sometimes large diameter piles called shaft or pier



Bored pile rebar

Minimum Reinforcement in Bored Concrete Pile

The longitudinal reinforcement shall be of high yield steel bars (min f_y = 420 Mpa) and shall not be less than:

               0.5% of A_c , for A_c ≤ 0.5 m2;

               2500 mm2 , for 0.5 m2 < A_c ≤ 1 m2;

               0.25% of A_c, for A_c > 1.0 m2;

Where, A_c is the gross cross-sectional area of the pile.

The minimum diameter for the longitudinal bars is 16 mm



Bored pile rebar detailing

1.      The assembled reinforcement cage should be sufficiently strong to sustain lifting and lowering into the pile bore without permanent distortion or displacement of bars

2.      bars should not be so densely packed that concrete aggregate cannot pass freely between them.

3.      Hoop reinforcement (for shear) is not recommended closer than 100 mm centres

4.      Minimum concrete cover to the reinforcement periphery shall be 75 mm.

5.      This guidance is only applicable for piles with vertical load.



Group capacity of bored pile or drilled shaft in cohesive soil

CAP IN FIRM CONTACT WITH GROUND

·        Reduction factor, ζ = 1.0

·        The group capacity may then be computed as the lesser of

o  the sum of the individual capacities of each shaft in group, or

o  the capacity of an equivalent pier defined in the perimeter area of the group

·        For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction


CAP IN FIRM CONTACT WITH GROUND OR GROUND IS SOFT

1.      Reduction factor, ζ = 0.67 for a center-to-center (CTC) spacing of 3B (B=pile dia)

2.      Reduction factor, ζ = 1.0 for a CTC spacing of 8B (B=pile dia)

3.      The group capacity may then be computed as the lesser of

               (i) the sum of the individual capacities of each shaft in group, or

               (ii) the capacity of an equivalent pier defined in the perimeter area of the group

For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction



Group capacity of bored pile or drilled shaft in cohesionless soil

CAP CONTACT WITH GROUND is not considered here

·        Reduction factor, ζ = 0.67 for a center-to-center (CTC) spacing of 3B (B=pile dia)

·        Reduction factor, ζ = 1.0 for a CTC spacing of 8B (B=pile dia)

·        The group capacity may then be computed as the lesser of

               (i) the sum of the modified individual capacities of each shaft in group, or

               (ii) the capacity of an equivalent pier defined in the perimeter area of the group

For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction



Group capacity of driven pile

·        Reduction factor, ζ = 1.0

·        The group capacity may then be computed as the lesser of

               (i) the sum of the individual capacities of each shaft in group, or

               (ii) the capacity of an equivalent pier defined in the perimeter area of the group

For the equivalent pier, shear strength shall not be reduced by any factor to calculate skin friction



Minimum pile spacing for bored pile

Center to Center Spacing:

Conflict with previous

If closer spacing is required, the sequence of construction shall be specified and the interaction effects between adjacent shafts shall be evaluated by the designer.



Longitudinal Bar Spacing:

·        The minimum clear distance between longitudinal reinforcement shall not be less than 3 times the bar diameter nor 3 times the maximum aggregate size.

·        If bars are bundled in forming the reinforcing cage, the minimum clear distance between longitudinal reinforcement shall not be less than 3 times the diameter of the bundled bars.

·        Where heavy reinforcement is required, consideration may be given to an inner and outer reinforcing cage.



Bentonite slurry specification

In drilling of holes for all piles, bentonite and any other material shall be mixed thoroughly with clean water.

Bentonite slurry shall meet the Specifications as shown below


Density during drilling to support excavation, greater than 1.05 g/ml, Mud density Balance (ASTM D4380)


·        Density prior to concreting, less than 1.25 g/ml, Mud density Balance (ASTM D4380)

·        Viscosity, 30 – 90 Seconds, Marsh Cone Method (ASTM D6910)

·        pH, 9.5 to 12, pH indicator paper strips or electrical pH meter (ASTM D4972)

·        Liquid limit, > 450%, Casagrande apparatus (ASTM D4318)



Drilling fluid level

Where a borehole is formed using drilling fluid for maintaining the stability of a boring, the level of the water shall maintain at a level not less than 2 m above the level of ground water.



Bored pile construction sequence

·        Augured cast-in-situ pile shall not be installed within 6 pile diameters centre to centre of a pile filled with concrete less than 24 hours old.

·        Bored cast-in-situ concrete piles shall not be drilled/bored within a clear distance of 3 m from an adjacent pile with concrete less than 48 hours old.

·        For concreting under water, the concrete shall contain at least 10 percent more cement than that required for the same mix placed in the dry.



“Rabbit” for bored pile

Successful placement of concrete under water requires preventing flow of water across or through the placement site. Once flow is controlled, the tremie placement consists of the following three basic steps:

·        The first concrete placed is physically separated from the water by using a “rabbit” or go-devil in the pipe, or by having the pipe mouth capped or sealed and the pipe dewatered.

·        Once filled with concrete, the pipe is raised slightly to allow the “rabbit” to escape or to break the end seal. Concrete will then flow out and develop a mound around the mouth of the pipe. This is termed as “establishing a seal”.

·        Once the seal is established, fresh concrete is injected into the mass of existing concrete.



Methods of tremie concreting

Tremie concreting method

·        The capped tremie pipe approach

·        The “rabbit” plug approach



Important notes

1.      Tremies should be embedded in the fresh concrete a minimum of 1.0 to 1.5 m (3 to 5 ft) and maintained at that depth throughout concreting to prevent entry of water into the pipe.

2.      Rapid raising or lowering of the tremie pipe should not be allowed.

3.      All vertical movements of the tremie pipe must be done slowly and carefully to prevent “loss of seal”.



Number of tests?

1.      Integrity Test: In order to check the structural integrity of the piles Integrity tests shall be performed on the piles in accordance with the procedure outlined in ASTM D5882. For any project where pile has been installed, integrity tests shall be performed on 100% of the piles.

2.      Axial Load Tests for Compression : 

        I.           For a major project, at least 2% of pile's axial load test for compression shall be tested in each area of uniform subsoil conditions.

      II.           The load test on a pile shall not be carried out earlier than 4 weeks from the date of casting the pile. 

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BNBC 2020 Provision of Pile Foundation, Part-2
3
BNBC 2020 Provision of Pile Foundation, Part-3
4
BNBC 2020 Provision of Pile Foundation, Part-4

Structural design of pile

1
Structural design of pile, Part-1

Structural Design of Piles




Precast Concrete Pile

The structural design of precast concrete pile is governed by

·        Lifting and handling the piles

·        The driving of the piles



Size Vs. Length

Pile size (mm square)---Maximum length (m)

250 (10 in)-----------------12 (40 ft)

300 (12 in)-----------------15 (50 ft)

350 (14 in)-----------------18 (60 ft)

400 (16 in)-----------------21 (70 ft)

450 (18 in)-----------------25 (82 ft)


Maximum length = 50 x least dimension



Minimum longitudinal rebar in driven pile

1.      Pile length < 30 times the least width : 1.0%

2.      Pile length 30 to 40 times the least width : 1.5%

3.      Pile length > 40 times the least width : 2.0%



Hoops in driven pile

The lateral reinforcement (hoops) resists the driving stresses induced in the piles and should be in the form hoops or links of diameter not less than 6 mm. The volume of lateral reinforcement shall not be less than the following:

1.      At each end of the pile for a distance of about 3 times the least width/diameter – not less than 0.6% of the gross volume of the pile.

2.      In the body of the pile – not less than 0.2% of the gross volume of the pile.

3.      The transition between closer spacing and the maximum should be gradual over a length of 3 times the least width/diameter.



Push Pile and Hoop

Push piles are driven by catching the pile at different locations. So, BNBC-2017 guideline could not address this issue

My recommendation is hoop rebar > 0.4% at the middle part

Nearly end of driving the pile, push is applied on top of pile. At this time maximum load is applied in the pile.

So, Maximum spacing of hoop at upper 3 m = 75 mm (my opinion). Hoop rebar = 0.6%

Concrete quality must be good, fc’>5000 psi



Example of hoop or lateral rebar calculation

Say, pile section = 300x300

And length = 15 m

So, first 0.9 m we have to provide 0.6% hoops

Say, 10 mm dia rebar as hoop

Clear cover = 50 mm

Length of one hoop or lateral tie = 4*(300-50-50-5-5) = 760mm

Volume of one hoop = (3.14*10*10/4 )*760 mm3 = 59660 mm3

Length of one hoop or lateral tie = 4*(300-50-50-5-5) = 760mm

Volume of one hoop = (3.14*10*10/4 )*760 mm3= 59660 mm3

Volume of concrete of first 0.9 m = 300*300*900= 81,000,000 mm3

0.6% of gross volume = 486000 mm3

Number of hoop required = 486000/59660 = 8.14

So, spacing = 900/8.14 = 110.5 mm= 110 mm c/c


It is better to use small dia rebar with closer spacing for lateral ties (hoops)



Function of Main and Hoop Rebar

Reinforcement

·        The quantity of longitudinal steel should be proportional to the stresses arising in lifting and handling

·        The research showed that the proportion of main steel did not seem to have any effect on the resistance to driving stresses



Rebar for Hard Driving

Reinforcement

·        The quantity of transverse reinforcement, where hard driving is expected, should not be less than 0.4% of the gross concrete volume

·        The proportion of main steel in the head of the pile should be 1.0 %



Pile Design for Driving Stresses

1.      Driving static load is 2-3 times greater than the service load

2.      Push piles should be designed for an axial load at least 2.5 times the service load

3.      Concrete quality must be good, fc’>5000 psi



Design without considering lateral load

·        Structural design of cast in situ/ bored pile is identical to design of RCC column if Su > 10 kPa of soil

·        Nominal capacity of pile

Pn= b*0.85*f’c*Ac+As*fy where, b= 0.85 for spiral column

·        Design capacity of pile

fPn where f= 0.75 for spiral column



Design considering lateral load

·        If lateral load is to be considered, design is difficult

·        Finite element modeling

·        As a thumb rule, upper 20 ft of pile, longitudinal reinforcement should be 1% for building upto 10 story



Bored pile rebar

Minimum Reinforcement in Bored Concrete Pile

The longitudinal reinforcement shall be of high yield steel bars (min ?_? = 420 Mpa) and shall not be less than:

0.5% of ?_?, for ?_? ≤ 0.5 m2;

2500 mm2, for 0.5 m2 < ?_? ≤ 1.0 m2;

0.25% of ?_?, for ?_? > 1.0 m2;

Where, ?_? is the gross cross-sectional area of the pile.

The minimum diameter for the longitudinal bars is 16 mm



Pile cap design

Flexural Rebar

·        Bottom: Minimum flexural rebar, minimum rebar for shrinkage and temperature crack control, rebar required for bending moment

·        Top: required flexural or D12@ 200 mm c/c (good practice)

·        Side: D16 @ 300 mm c/c (good practice)

·        Bottom bar is U shaped for anchorage (good practice)


Punching

To make rigid pile cap, provide more thickness than required

Concrete: durability need to be considered


Strut and tie model is only applicable for deep pile caps.

ACI 318-14 does not allow strut and tie model in all cases

2
Structural design of pile, Part-2
3
Structural design of pile, Part-3
4
Structural design of pile, Part-4
5
Structural design of pile, Part-5

Negative skin friction

1
Negative skin friction, Part-1

Negative skin friction and its remedy




SETTLEMENT OF A PILE

1.      The settlement of the pile head is equal to The settlement of the neutral plane plus the compression of the pile caused by the applied dead load and the dragload combined.

2.      For a driven pile, the toe movement necessary to mobilize the toe resistance is about 1 to 2 percent of the pile toe diameter.

3.      For bored piles, the movement is larger (5-10% of toe dia)

4.      Where the toe movement is too small the settlement is normally not an issue.

5.      Use (D + 0.5L + drag load) for settlement calculation****



Laod combinations

·        Use (D + 0.5L + drag load) for settlement calculation

·        Use (D + L + drag load) for structural design of pile

·        Use (D + L) and 0.75*(D + L + W / E) for bearing capacity of pile



Important notes of drag down

·        The movements associated with negative skin friction indicate that extremely small relative movements on the order of 1 mm are sufficient to generate negative skin friction.

·        The stiffness between a pile and soil is different so all piles are subjected to relative movements of this magnitude.

·        Therefore, all piles are subjected to negative skin friction.

·        In all piles, a neutral plane develops

·        To determine the location of the neutral plane, an analysis of the load distribution in the pile must first be performed.



Bearing Capacity and Settlement

·        The dragload must not be included when considering bearing capacity because the dragload is of no consequence for the analysis of soil bearing failure.

·        For bearing capacity consideration, it is incorrect to reduce the dead load by any portion of the dragload.

·        The dead load should only be reduced because of insufficient structural strength of the pile at the location of the neutral plane, where the pile is subjected to the combination of dead load and dragload.

·        Load is reduced to control the settlement of pile group. That means FoS > 3.5



Remedy for Driven and pushed pile

·        Site development well before construction works begin

·        Preloading before piling

·        Sand drain or PVD with preloading before piling

·        Lubricating pile before driving

·        Settlement is the major problem of negative skin friction. FoS > 3.5 will minimize this problem.***

·        Grade beam and floor after complete backfilling upto PL with some waiting period



Remedy for bored pile

·        Site development well before construction works begin

·        Preloading before piling

·        Sand drain or PVD with preloading before piling

·        Settlement is the major problem of negative skin friction. FoS > 3.5 will minimize this problem

·        Grade beam and floor after complete backfilling upto PL with some waiting period



Structural design of pile

Consider (D + L + Dragload) at neutral plane

Don’t reduce steel at deeper locations of pile

2
Negative skin friction, Part-2

Soil identification and classification

1
Soil identifation and classification, Part-1

SOIL IDENTIFICATION AND CLASSIFICATION




Tower of Pisa

·        Height: 54 m;

·        Max tilt: 5 m out of plumb

·        Weight: 15,700 tons;

·        Base dia = 20 m;

·        Reason: a weak clay layer at 11 m depth

·        Solution: excavation of soil from north side for about 70 tons.



Identification of Soil

Soil Properties that Control its Engineering Behavior

·        Particle Size distribution and

·        Plasticity behavior


SOIL

·        coarse-grained: Particle/Grain Size Distribution, Particle Shapes

·        fine-grained: Soil Plasticity



Clay vs. Sand/Silt

·        Clay particles are generally more platy / flaky in shape (sands are more equi-dimensional)

·        Clay particles carry surface charge

·        Amount of surface charge depends on type of clay minerals

·        Surface charges that exist on clay particles have major influence on their behavior (for e.g. plasticity)



Clay Minerals

·        Kaolinite family: Kaolinite (ceramic industry, paper, paint, pharmaceutical)

·        Smectite family: Montmorillonite (weathered volcanic ash, Wyoming Bentonite, highly expansive, used in drilling mud)

·        Illite family



Water in Clay Soil

·        All of the water held to clay particles by force of attraction is known as double-layer water

·        The innermost layer of double-layer water, which is held very strongly by clay, is known as adsorbed water. This water is more viscous than free water.



Clay Morphology

·        Scanning Electron Microscope (SEM)

·        Allows us to study morphology of clay minerals

·        Used in mineral identification



ASTM D 422

·        Gravel: passing 3-in. and retained on No. 4 sieve

·        Sand: passing No. 4 sieve and retained on No. 200 sieve

(a) Coarse sand: passing No. 4 sieve and retained on No. 10 sieve

(b) Medium sand: passing No. 10 sieve and retained on No. 40 sieve

(c) Fine sand: passing No. 40 sieve and retained on No. 200 sieve . . . . . %

·        Silt size, 0.074 to 0.005 mm

·        Clay size, smaller than 0.005 mm

·        Colloids, smaller than 0.001 mm



Engineering classification of soil

Unified Soil Classification System (ASTM)

Based on particle size distribution and Atterberg Limits


AASHTO Classification System

Based on particle size distribution and Atterberg Limits



Unified Soil Classification System (USCS), ASTM D 2487

·        G=Gravel, S=Sand,

·        M= Inorganic Silt, C=Inorganic Clay,

·        O=Organic Silts and Clays

·        PT=peat, muck and other highly organic soil

·        W=well graded, P=poorly graded

·        L=low plasticity, H=high plasticity



Factors Considered in USCS

·        Percent passing through the #200 sieve (this is the fine fraction)

·        Percent of passing through the #4 sieve

·        Uniformity coefficient and coefficient of gradation (for soils with 0-12% passing through #200 sieve)

·        LL and PI of that portion of soil passing through #40 sieve (for soils with 5% or more passing through #200 sieve)

When the percent passing through #200 sieve is 5-12%, dual symbols, such as, GW-SC, are needed

For fine grained soils, Plasticity Chart is to be used

For peat, visual-manual identification may be necessary

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Soil identifation and classification, Part-2
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Soil identifation and classification, Part-3
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Soil identifation and classification, Part-4
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Soil identifation and classification, Part-5

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Pile Design and Construction
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