P4 محاضرات هندسة الاساسات د. طارق نجيب

April 13, 2023 Deep Foundations 1

DEEP FOUNDATIONSDEEP FOUNDATIONS

4th Year Civil

FOUNDATION ENGINEERINGFOUNDATION ENGINEERING


DEEP FOUNDATIONSDEEP FOUNDATIONS

TYPES OF DEEP FOUNDATIONS:

1- PILES الخوازيق

2- CAISSONS القيسوناتبالتغويصوالحفر تنفذ خلوية أساسات

3- PIERS الدعائمذات قواعد أو كبير ذاتقطر خوازيق وهى الكبارى أساسات

يجففداخلها كبير حجم

االسكندرانى -4 اآلبار


PILE FOUNDATIONSPILE FOUNDATIONS

Piles are stiff members used to transmit surface loads to the bearing strata.

Piles are classified to two categories according to the method of load transfer:

1- End bearing piles: ارتكاز خوازيقTip point carries most of the load.

. الخازوق حمل معظم تنقل االرتكاز نقطة2- Friction Piles: احتكاك خوازيق

Side friction carries most of the load.

عن للتربة ينتقل الخازوق حمل معظمالسطحى االحتكاك طريق


Pile ApplicationsPile Applications

LowLowWeightWeight

Soft toSoft toFirm ClayFirm Clay

Large DistributedLarge DistributedWeightWeight

Very Large ConcentratedVery Large ConcentratedWeightWeight

Dense Sand

Strong RockStrong Rock


Piles are used in:Piles are used in:1- Upper soil is weak,

compressible, or could not support the surface loads.

2- The loads are tension, horizontal, or inclined.

3- Problematic soils;

Swelling soils giving tension on the pile.

Collapsing soils, adding down-drag forces on the pile.

4- Scour under bridge piers.

5- Temporary or Permanent Excavation Side Support


Types of Pile Materials

ConcreteConcreteTimberTimber Steel HSteel H CompositeCompositePre-cast ConcretePre-cast Concrete

Steel Concrete

Steel PipeSteel Pipe

Timber


Timber PilesTimber Piles

- Relatively inexpensive

- Usually limited to short lengths [though Douglas Fir does come in longer lengths (higher expense)]

- Low capacity.

- Advantages: Easy handling, Non-corrosive material, If permanently submerged then fairly resistant to decay.

- Disadvantages: May require treatment to prevent decay, insects, and borers from damaging pile. Easily damaged during hard driving and inconvenient to splice.


Steel H-PilesSteel H-Piles

- Advantages: high axial working capacity. Wide variety of sizes. Easy on-site modifications. Fairly easy to drive, minimal soil displacement, good penetration through hard materials (with shoe).

- Disadvantages: high cost, potential delays in delivery, relatively higher corrosion, noisy driving, low bearing and friction areas.


Steel Pipe Piles

May drive closed or open-ended.

Advantages: May be driven empty then filled with cheaper material (concrete). Can provide very high capacities. On-site modifications easy.

Disadvantages: similar to H-piles with additional item–more difficult driving due to soil displacement.


Precast Concrete Piles

- May be prestressed to withstand driving and handling stresses.

- Advantages: High capacity. Usually durable and corrosion resistant in many environments (not marine).

- Disadvantages: Handling, splicing, and cutting more of a problem. Transportation difficulties.


- Piles are bored using drilling rig of internal diameter allowing for casting the concrete till the required depth.

- Casting of concrete starts with withdrawing the drilling rig upwards.

- Capacity of about 100 ton- Advantages: ease of changing lengths by

cutting or slicing the shell. Material costs relatively low. Inspection possible.

- Disadvantage: not feasible in hard soils or rock. Voids in concrete may be created. Splicing problems after concreting.

Continuous Flight Auger (CFA)


- Holes are drilled in the soil then filled with bentonite slurry to support the sides.

- Steel reinforcements are then lowered to the required level.

- Concrete casting is performed from bottom to top with pumps.

- Bentonite slurry is then replaced by the concrete and the slurry is desanded for reuse.

Cast-in-place Concrete Piles


Choice of Pile TypeThe pile type is chosen according to:

- Load Capacity & Pile Spacing

- Constructability

- Soil stratigraphy

- Need for splicing or cutting

- Driving vibrations

- Driving speed.

- Performance

- Environmental suitability (corrosion)

- Availability

- Cost


Pile Load Transfer Mechanism

- Fairly complicated, though understandable.

. منتظم- غير وغالبا= معقد الحمل توزيع- Changes with changes in load because friction along

shaft is fully mobilized when pile has displaced only 5-10 mm whereas maximum point resistance is not fully mobilized until a movement of 10% of the pile diameter (or width) for driven piles or even higher in bored piles.

يتحقق- السطحى االحتكاك األقصىفى الحمل) 10-5عند ( نقطة عند األقصى الحمل ويصل مم

حدود فى أو% 10االرتكاز الخازوق قطر منعرضه.


Mobilization of Base and Shaft Resistance

Mobilization of Base and Shaft Resistance

Shaft

2 - 5mm

Base

10 - 20% diam

Total

Displacement

Load

Shaft ??


Pile Failure SurfacePile Failure Surface

Df

Ground Surface

Arching Action

B

DfPO = Df

Zone of Shear & Volume Decrease

Pult

Failure Surface Along the Shaft


End Bearing PilesEnd Bearing Piles


End bearing piles:End bearing piles:Transmit most of their Transmit most of their loads to the load bearing loads to the load bearing layer (dense sand or layer (dense sand or rock). Most of the pile rock). Most of the pile capacity inferred from capacity inferred from the end bearing point.the end bearing point.

من األكبر الجزء من ينقل األكبر الجزء ينقلطريق عن طريق الحمل عن الحمل

وهى االرتكاز وهى نقطة االرتكاز نقطةترتكز التى ترتكز الخوازيق التى الخوازيق

أو الكثيف الرمل أو على الكثيف الرمل علىالصخر.الصخر.

Pbase

Side Friction

End Bearing

Pile Load, P


Friction Piles:Friction Piles:


Friction Piles:Friction Piles: Transmit most of their Transmit most of their load through the layers load through the layers through which the piles through which the piles pass, i.e., mostly through pass, i.e., mostly through the surface friction with the surface friction with the surrounding soilsthe surrounding soils..

عن الحمل معظم عن ينقل الحمل معظم ينقلاالحتكاك االحتكاك طريق طريق

مثل مثل السطحى السطحىفى المنفذة فى الخوازيق المنفذة الخوازيق. الصرفة الطينية .التربة الصرفة الطينية التربة Pbase

End Bearing

Pile Load, P

Side Friction


Individual Pile Capacity

Methods of Estimating Pile Load Capacity:Methods of Estimating Pile Load Capacity:1- Static Analysis from the Shear Strength Parameters1- Static Analysis from the Shear Strength Parameters

خواصالقصفى- 11 من الخوازيق تحمل قدرة خواصالقصفى- تحديد من الخوازيق تحمل قدرة تحديدالتربة.التربة.

2- Dynamic Driving Formula2- Dynamic Driving Formula

دق- 22 صيغة من الخوازيق تحمل قدرة دق- تحديد صيغة من الخوازيق تحمل قدرة تحديد. المستخدمة .الخوازيق المستخدمة الخوازيق

3- Pile Load Test3- Pile Load Test

تحميل- 33 اختبارات من الخوازيق تحمل قدرة تحميل- تحديد اختبارات من الخوازيق تحمل قدرة تحديدالخوازيق.الخوازيق.

4- Field Tests: SPT, CPT, etc.4- Field Tests: SPT, CPT, etc.

االختبارات- 44 من الخوازيق تحمل قدرة االختبارات- تحديد من الخوازيق تحمل قدرة تحديدالحقلية.الحقلية.


PILE CAPACITYPILE CAPACITY1- Bearing capacity of piles from soil parameters:1- Bearing capacity of piles from soil parameters:

Static Formula Method (QStatic Formula Method (Quu = Q = Qbb + Q + Qss))

Embedded Length = D

Qu = Ultimate Bearing Capacity

Qs = fAs

f = Unit Frictional Resistance

AS = Shaft Area (Pile surface area)

qb = Unit Bearing Capacity

Ab = Area of Pile Base

Qb = qbAb


Base ResistanceBase Resistance

Qb = Ab [cbNc ] for Clay

Qb

Qb = Ab [P'ob Nq] for Sand

Ab = Area of Pile BaseNc = 9 for Piles in ClayNq = Given in Tables


Shaft ResistanceShaft Resistance

Due to cohesion or friction or both

In cohesive soils : Qsc = As . ca

In friction soils: Qsf = As .KHC P'ob tan

P'ob

KHC.P'ob

As

As = Pile Surface Area


Total Pile ResistanceTotal Pile Resistance

Qu = Qb + Qs

Qu = Ab [cb Nc] + As [ca ] For Piles in Clay

Qu = Ab [P'ob Nq] + As [KHC P'ob tan ] for Piles in Sand


Qu = Ab [ P'ob Nq ] + As [ KHC P'ob tan ]

Qu = Ab P'ob Nq + As KHc P'ob tan

Piles in SandPiles in Sand

= 20o for Steel = ¾ for Concrete = ¾ for Timber


Qu = As [KHT P'ob tan ] + W

Tension Piles in SandTension Piles in Sand

W = Pile Weight

Pile Type KHC KHT

H-Section Pile 0.5 – 1.0 0.3 – 0.5 Displacement Pile 1.0 – 1.5 0.6 – 1.0 Displacement Pile, Variable Section 1.5 – 2.0 1.0 – 1.3 Displacement Pile, with Water Jetting 0.4 – 0.9 0.3 – 0.6 Bored Pile (D < 60 cm) 0.7 – 1.5 0.4 – 1.0

Table (1): Values of KHC and KHT from the Egyptian Code


Piles in ClayPiles in Clay

Qu = Ab cbNc + As ca

Qu = Ab [cbNc] + As [ca]


Tension Piles in ClayTension Piles in ClayQu = As [ca] + W W = Pile Weight

Pile Material

Soil Consistency

Cohesion, c (kN/m2)

Adhesion, ca (kN/m2)

Very Soft 0.0 – 12.50 0.0 – 12.50 Soft 12.50 – 25.0 12.50 – 24.0 Medium Sitff 25.0 – 50.0 24.0 – 37.50 Stiff 50.0 – 100.0 37.50 – 47.50

Timber or

Concrete

Very Stiff 100.0 – 200.0 47.50 – 65.0

Very Soft 0.0 – 12.50 0.0 – 12.50 Soft 12.50 – 25.0 12.50 – 23.0 Medium Sitff 25.0 – 50.0 23.0 – 35.0 Stiff 50.0 – 100.0 35.0 – 36.0

Steel

Very Stiff 100.0 – 200.0 36.0 – 37.50

Table (2): Adhesion on Piles in Saturated Clay (Egyptian Code)


Overburden Stress P'obOverburden Stress P'ob

Qu = [Ab P'ob Nq] + [AsKHC P'ob tan ]

Meyerhof Method : P'ob = 'z

Vesic Method : critical depth, zc

for z < zc : P'ob = 'zfor z > zc : P'ob = 'zc

zc/d is a function of after installation Suggested value = 20 d


Max Limit on End Bearing?

Some suggest a limit on end bearing to match experience.

Problems with that approach:more complex than that; need to

consider both strength and compressibility of the soil

friction angle varies with effective stress

Over-consolidation causes changes in bearing capacity


Nq from the Egyptian Code

o 25 30 35 40 Nq 15 30 75 150

Table (3): Nq Values Vs for Sand, Egyptian Code.

for Displacement Pile = ( (before construction) + 40o)/2

for Bored Pile = (before construction) – 3o

Nc for ClayNc = 9.0 for calculating the end bearing resistance of piles in clay.


EXAMPLE

Determine the allowable capacity for the concrete bored pile shown in Figure.Pile Diameter D = 0.50 mPile Length L = 14.0 m

Medium stiff clay:C = 30 kN/m2

Ca = 25 kN/m2

sat = 18 kN/m3

Dense Sand: = 40o sat = 19 kN/m3

Nq = 150, KHC = 1.0

12.00 m

2.00 m

April 13, 2023 Deep Foundations

SOLUTIONSide Friction:Side Friction:Qs = As [ca + KHC P'ob tan ]qs in clay:qs-c = ca = 25 kN/m2

Qs-clay = ca [DLc]= 25 [*0.50*12.0]= 25 *18.85 = 471.25 kN

qs in sand:qs-s = KHC P'ob tanCritical depth Zc= 20 * 0.50 = 10.00 mP'ob = 8 * 10 = 80 kN/m2

Lc = 12.0 m

Ls =2.0 m

P'bo distribution

10.0 m10.0 m


SOLUTION = 3/4 = 30o

Qs-s = 1.0 * 80 * 0.578 = 46.24 kN/m2

Qs-s = qs [DLs]= 46.24 [*0.50*2.0] = 145.27 kN

Total side friction:Qs = Qs-c + Qs-s = 471.25 + 145.27 = 616.52 kNEnd Bearing Resistance:qb = P'ob Nq = 80 * 150 = 12000 kN/m2

Qb = qb * Ab = qb * D2 = 12000 * 0.196= 2356.2 kN

Ultimate Pile Capacity = 616.52 + 2356.2 = 2973 kN


SOLUTIONUltimate Pile Capacity Qult = 2973 kNAllowable Pile CapacityQall = Qult/F.S.Qall = 2973/3.0 = 991 kN

= 99.10 ton

Check of Concrete Capacity:Pc = fc (Ac + 1.14 * n * As)

= 5000 (0.196 + 1.14 * 10 * 0.00196)= 5000 * (0.218) = 1090 kN= 109 ton > 99.10 (Qall-soil) (O.K.)


Pile DrivingPile Driving


Closed End Diesel Hammer


The Fundamental Pile Driving FormulaThe Fundamental Pile Driving Formula

shW

R

s R hW

ResistanceSoilofWorkEnergyHammer

RR

WW

SS

hh


Single ActingSingle Acting Double ActingDouble Acting


Diesel HammerDiesel Hammer Vibratory HammerVibratory Hammer


Where:Where:QQallall = Allowable pile capacity, (kN); = Allowable pile capacity, (kN);

WWrr = Weight of hammer, kN;= Weight of hammer, kN;

HH = Height of hammer fall, m;= Height of hammer fall, m;SS = Amount of pile penetration/blow, (mm);= Amount of pile penetration/blow, (mm);CC = Constant = 25 for drop hammer= Constant = 25 for drop hammer

= 2.50 for steam hammer.= 2.50 for steam hammer.

Engineering News Formula (F.S. = 6):Engineering News Formula (F.S. = 6):

PILE CAPACITY2- From Pile Driving Formula

C S 6H. W1000

Q rall


Determine the allowable pile capacity for a pile of Determine the allowable pile capacity for a pile of diameter D = 0.30 m, driven by a steam hammer, diameter D = 0.30 m, driven by a steam hammer, knowing that:knowing that:Average penetration per blow = 17 mmAverage penetration per blow = 17 mmHammer rating Wr.H = 40 kNHammer rating Wr.H = 40 kN

Example:Example:

PILE CAPACITY2- From Pile Driving Formula

ton34.2 kN 341.9

2.50 17 640 * 1000

Qall

Solution:Solution:


PILE CAPACITY3- Static Pile Load – Test Setup

Reaction Beam

Stiffeners

PlateLoad Cell

Spherical Bearing

Ram

Hydraulic JackBourdon Gage

Dial GageLVDT

Mirror

Scale

Test Pile

Grade

Bracket Attached to PileWire


Typical Arrangement for Load Testing a Pile or Drilled Shaft

Reaction BeamReaction Beam

JackJackDial

GageDial

Gage

Test Pile or Drilled ShaftTest Pile or

Drilled Shaft

Support Beam

Support Beam

Anchor Pile or Drilled ShaftAnchor Pile or Drilled Shaft


Pile Static Load – Test Setup


Pile Capacity from Static Pile Load Test

Loading is performed up to a test load of 150% (or 200%) of the pile design load, with steps of 25% of the design load. The ultimate capacity is determined by many methods, for example, the Modified Chen Method.

يساوى اختبار حمل عند الخازوق اختبار 150يتمعلى%) 200أو% ( وذلك التصميمى الحمل من

خطوة كل مقدار الحمل% 25خطوات منتحمل. قدرة أقصى تحديد ويتم التصميمى

طريقة باستخدام .Chenللخازوق المعدلة


Pile Capacity from Static Pile Load Test, Load-Settlement Curve


Pile Capacity from Static Pile Load Test, Modified Chen Method

x = 6.801

b

y = 3.70


Pile Capacity from Static Pile Load Test, Modified Chen Method

Ultimate Pile Capacity from Static Pile Load Test, Using Modified Chen Method:The loading curve is drawn as follows:Horizontal axis = measured settlement ();Vertical axis = measured settlement () /load (P).

b 1.21

Qult Where:Qult = Ultimate pile capacity, (kN);1.2 = Safety factorb = Slope of the /Load Vs. curve.


Pile Capacity from Pile Load TestThree conditions should be satisfied from the pile load test:I- QI- Qultult ≥ 2.00 Q ≥ 2.00 Qallall (D.L. + L.L.) (D.L. + L.L.)

Dead + Live LoadsQult ≥ 1.75 Qall (D.L. + L.L. + W.L.)

Dead + Live + Wind LoadsQult ≥ 1.50 Qall (D.L. + L.L. + W.L.+Eq.L.)

Dead + Live + Wind + Earthquake Loads

1.50 Qat Settlement

Q 1.25at Settlement

all

all

II- Measured II- Measured at 1.25 test load ≤ 1.50 at 1.25 test load ≤ 1.50 at Q at Qallall


Pile Capacity from Pile Load Test

A EL P

* 0.50 D 0.02 max

III- Measured III- Measured ≤ ≤ maxmax

Where:Where:maxmax = maximum pile settlement, (m); = maximum pile settlement, (m);

DD = Pile diameter, (m);= Pile diameter, (m);PP = Pile test load, (ton);= Pile test load, (ton);LL = Pile length, (m);= Pile length, (m);EE = Modulus of elasticity of concrete, (t/m= Modulus of elasticity of concrete, (t/m22))AA = Pile cross-section area.= Pile cross-section area.



Settlement (mm) Date

Load (kN)

S1 S2 S3 S4

Avg. Sett. (mm)

Loading 9/9/04 0.0 0.00 0.00 0.00 0.00 0.00

250.0 0.59 0.66 0.56 0.58 0.60 500.0 0.88 1.00 0.89 0.93 0.93 750.0 1.85 1.98 1.86 1.92 1.90 1000.0 3.12 3.28 3.20 3.26 3.21 1250.0 4.16 4.34 4.27 4.37 4.29 1500.0 10.11 10.46 10.21 10.41 10.30

Unloading 10/9/04 1250.0 10.11 10.46 10.41 10.21 10.30

1000.0 10.01 10.29 10.07 10.27 10.16 750.0 9.70 9.97 9.80 9.99 9.87 500.0 9.20 9.45 9.35 9.03 9.26 250.0 8.53 8.78 8.79 9.97 8.77 0.0 6.65 6.82 6.83 6.84 6.79

Example:Determine the ultimate pile capacity for a pile of diameter D = 0.60 m, and Length L = 10.50 m, with the test results shown in the table, knowing that the design load is 1000 kN, test load = 1500 kN.


Pile Capacity from Pile Load Test Solution:

I- QI- Qultult 1532 > 1.50 Q 1532 > 1.50 Qallall (1500 kN) (1500 kN)

kN 1532 3.70 * 20.11000 * 6.80

b 1.2

1 Qult

QQultult using modified Chen Method: using modified Chen Method:

1.50 1.34 3.214.29

Qat Settlement

Q 1.25at Settlement

all

all

II- Measured II- Measured at 1.25 test load ≤ 1.5 at 1.25 test load ≤ 1.5 at Q at Qallall



A EL P

* 0.50 D 0.02 max

III- III- maxmax ≤ ≤ at maximum test load: at maximum test load:

0.283 *10*140

10.50 * 150 * 0.50 0.60 * 0.02

4max

maxmax = 0.012 + 0.002 = 0.014 m = 14 mm = 0.012 + 0.002 = 0.014 m = 14 mm

at test load = 10.30 mm < 14.0 mm (at test load = 10.30 mm < 14.0 mm (maxmax) O.K.) O.K.

E = 140 t/cmE = 140 t/cm22 = 140 * 10 = 140 * 1044 t/m t/m22


PILE CAPACITY4- In-Situ Soil Test Methods

The pile capacity can be determined from The pile capacity can be determined from the field tests as follows:the field tests as follows:

1- Standard Penetration Test (SPT).1- Standard Penetration Test (SPT).

2- Cone Penetration Test (CPT).2- Cone Penetration Test (CPT).


Pile Capacity from SPTQallall = 90 N ( R2) + N' (2 RL), (kN)

Qallall = Pile working load, F.S. = 2.5 for end bearing and 2.0 for side friction.

N = Average SPT, 2R below tip and 6R above tip,

N ≤ 50 (Preferably ≤ 30).

N' = Average SPT in sand layers, along the pile shaft.

R = Pile radius, (m).

L = Pile length within the sand layers, (m).For bored piles, Qallall = 50 up to 100% of that values according

to the pile type and construction method.


Pile Capacity from SPTExample:Determine the

allowable pile capacity for a bored pile constructed in the soil log shown in the table. Knowing that the pile diameter is 50 cm, and the pile length is 9.0 m.

D SPT

1 8

2 11

3 14

4 23

5 28

6 31

7 33

8 36

9 41

10 53

11 55

Clay

Sand


Pile Capacity from SPTD SPT

1 8

2 11

3 14

4 23

5 28

6 31

7 33

8 36

9 41

10 53

11 55

Clay

Sand

Solution:

Qallall = 90 N ( R2) + N' (2 RL), (kN)

N = (N8+N9+N10)/3

= (36+41+50)/3 = 42

N' = (N4+N5+N6+N7+N8+N9)/6

= (23+28+31+33+36+41)/6 = 32

Qall = 90 N ( R2) + N' (2 RL), (kN)

= 90*42*0.196+32*(2**0.25*6)

= 740.9+301.6 = 1042.5 = 104 ton


CPT Correlations

The CPT is very similar to driving piles therefore this test is a good predictor of capacity.

Pile capacity is determined from correlations based on CPT, as presented in the Egyptian Code.


Pile Capacity from CPT

Qallall = 1/3 qc ( R2) + ½ fc (2 RL), (kN)Qallall = Pile working load, F.S. = 3.0 for end bearing and 2.0 for

side friction.

qc = Average CPT tip resistance, 6R below tip and 12R

above tip, qc not exceeding 150 kg/cm2.

= Factor relating the pile diameter to the cone diameter,

assumed = 0.70.

fc = Average CPT side friction along the pile, not exceeding

1.0 kg/cm2.

R = Pile radius, (m).

For bored piles, Qallall = 50 up to 100% of that values according to the pile type and construction method.


Pile Groups

Piles are usually arranged in groups under the columns.

The number of piles in a group is determined as follows:

all

c

Q1.10 * P

N

Where:N = No. of piles;Pc = Column Load;Qall = Allowable load of

a single pile.


Arrangement of Pile GroupsThe spacing between piles in a group can be assumed based on the following:1- Driven piles need higher spacing than bored piles.2- Friction piles need higher spacing than end bearing piles.3- Minimum spacing (S) between piles is 2.5.4- Maximum spacing (S) between piles is 8.0.


S

2 Piles

S

3 Piles S

4 Piles

S

S5 Piles

S S6 Piles

S S

S7 Piles


S S

S

8 Piles

SS

S S9 Piles


Negative Skin Friction Q

L1

Granular Fill

L2

Fc

Fg

Soft Clay

Rock

Original Ground Surface

Downdrag force occurs when the Downdrag force occurs when the soil surrounding the pile settles soil surrounding the pile settles more than the pile itself.more than the pile itself.

Occurred in the following Occurred in the following conditions:conditions:1- Recent fill over very soft soil.1- Recent fill over very soft soil.2- Soils undergoing consolidations.2- Soils undergoing consolidations.3- Lowering of ground water table 3- Lowering of ground water table may cause subsidence and may cause subsidence and consequently –ve skin friction.consequently –ve skin friction.


Negative Skin Friction

Negative Skin Friction is calculated as follows:: كالتالى والخوازيق التربة بين السلبى االحتكاك حساب يتم

-ve skin friction = Weight of soil bounded within the soft layers;+ Pile group surface area of the soft layers * pile-soil

adhesion (Ca)بمجموعة = خارجيا= المحاطة التربة وزن السلبى االحتكاك

الخوازيق +المالصقة الخوازيق لمجموعة السطحية المساحة

الضعيفة للطبقاتبها * المحيطة والتربة الخوازيق بين االلتصاق معامل

The negative skin friction per pile should not exceed the pile skin friction value.

قدرة الواحد للخازوق السلبى االحتكاك يتعدى ال بحيث. الخازوق لذلك السطحى االحتكاك


n m 90n 1) - (m + m 1)-(n

- 1 = Ge

Where:Ge = Group efficiency; = tan-1 (D/S) in degrees;D = Pile diameter (m);S = Pile spacing (m);n = Number of piles in a row;m = Number of pile rows

Efficiency of Pile GroupsEfficiency of Pile Groups


Determine the number, arrangement, and group efficiency of a pile group to support a column load of 520 ton, knowing that the pile capacity is 110 ton, the pile diameter is 60 cm.

ExampleExample

S S6 Piles

Solution:Solution:No. of Piles = (520 * 1.10)/110

= 5.20 take 6 pilesAssume that the pile spacing S = 4 D = 2.40 m = tan-1(D/S ) = tan-1(0.25)

=14.04o


0.818 0.182 - 1

2 * 3 * 90

3 1) - (2 + 2 1)- (3 14.04 - 1 = Ge

n = 3 الواحد الصف فى الخوازيق عددm = 2 الخوازيق صفوف عدد

ExampleExample

S S6 Piles

Pile group capacity:Qg = N * Qall * Ge

Qg = 6 * 110 * 0.818 = 539.88 ton≈ 540 < 570, considered O.K.


Pile Groups Under Eccentric Loads


ex

ey

H

V R

Pile groups under eccentric loads


2y

2x

vy

y .V.e

x

.xV.e

nV

P

Where:

V = Total vertical force on the group

ex = Eccentricity in the x-direction

ey = Eccentricity in the y-direction


Settlement of Pile Groups in Sand (After Skempton)

FoundationFoundation

Zone of Settlement

Zone of Settlement

Ground Prestressed by Pile Driving

Ground Prestressed by Pile Driving

Single Pile Load TestSingle Pile Load Test


Settlement for End Bearing Pile GroupsSettlement for End Bearing Pile Groups

Soft Clay

Sand

Soft Clay

1H:2V H1

Hc

nQa

L

BAnQa

A, B = Pile Group Dimensions


Settlement of Friction Pile GroupsSettlement of Friction Pile Groups

nQa

ABnQa

*H

1H:2V

L 32

L

B, A = Pile Group Dimensions


Pile Group Settlement AnalysisPile Group Settlement Analysis

Where:H = Settlement H = Layer thickness Cc = Compression Index eo = Initial voids ratio PO = Overburden Pressure at the middle of the

consolidating layer. P = Change in Pressure at the middle of the

consolidating layer

o

o

o

cc P

ΔPPLog

e1C

HΔH


Structural Design of the Pile Cap


Failure Modes in the Pile Cap

1- Punching, column or pile.

2- Flexural, due to high tensile stresses.

Design of the Pile CapDesign of the Pile Cap


Failure by Punching

Failure due to high tensile

stressesMinimum thickness of the pile cap = Minimum thickness of the pile cap = 80 cm to assure even distribution of 80 cm to assure even distribution of settlements over the piles, or 2.25 pile settlements over the piles, or 2.25 pile diameter D.diameter D.


The concrete section and area steel are determined The concrete section and area steel are determined from the max. moment, then check of shear and from the max. moment, then check of shear and punching is performed.punching is performed.

1:1 1:1d/2 d

Critical Sec. for Shear

Critical Sec. for Moment

d/2 d

P4 محاضرات هندسة الاساسات د. طارق نجيب

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