osp mans edu eg deepfoundation ch6 htm

pdfcrowd.comopen in browser PRO version Are you a developer? Try out the HTML to PDF API

CHAPTER 1

CHAPTER 2

CHAPTER 3

CHAPTER 4

CHAPTER 5 CHAPTER 6

CHAPTER 7

CHAPTER 8

CHAPTER 9

CHAPTER 10

Axial Pile Capacity

6.1 IntroductionSelection of pile geometry:

• In order to reach a safe and economic design, the designershould seek the suitable diameter and length of the used pilessuch that the excess number of piles used is minimum.

• Length of end bearing piles is chosen such that they are restedon a firm stratum.

• Length of friction piles is determined from the load to betransmitted to soil.

• The cross-sectional dimensions of piles are determined fromthe properties and strength of the used material.

http://osp.mans.edu.eg/deepfoundation/ch10.htm










http://pdfcrowd.com/html-to-pdf-api/?ref=pdf

http://pdfcrowd.com/customize/

http://pdfcrowd.com/redirect/?url=http%3a%2f%2fosp.mans.edu.eg%2fdeepfoundation%2fch6.htm&id=ma-151122145439-6e923966

http://pdfcrowd.com


The Ultimate axial capacity of piles depends on both the pile and thesurrounding soil. The two major parameters that control the axialcapacity of piles are:

1- Pile Geometry (Length and Cross-sectional Area)

2- Soil Strength.

Loads on piles are transformed to the surrounding soil either throughfriction between the pile shaft and the surrounding soil or by endbearing at the pile tip or both. This depends on the strength of thesurrounding soil.

Loads on piles are transformed to the surrounding soil either throughfriction between the pile shaft and the surrounding soil or by endbearing at the pile tip or both. This depends on the strength of thesurrounding soil.




http://pdfcrowd.com


The behaviour of piles under load:

Piles are designed that calculations and prediction of carryingcapacity is based on the application of ultimate axial load in theparticular soil conditions at the site at relatively short time afterinstallation.

When pile is subjected to gradually increasing compressive load inmaintained load stages, initially the pile-soil system behaves in alinear-elastic manner up to point ( A ) on the settlement-load diagramand if the load is realised at any stage up to this point the pile headrebound to its original level.

When the load is increase beyond point ( A ) there is yielding at, orclose to, the pile-soil interface and slippage occurs until point ( B ) isreached, when the maximum skin friction on the pile shaft will havebeen mobilised.

If the load is realised at this stage the pile head will rebound to point (C ), the amount of permanent settlement being the distance OC. Whenthe stage of full mobilisation of the base resistance is reached ( pointD ), the pile plunges downwards with out any farther increase of load,or small increases in load producing large settlements.




http://pdfcrowd.com





http://pdfcrowd.com


Typical Load –Movement Curve




http://pdfcrowd.com


• At what load will the pile fail?

• How much will pile deflect under service loads?

· Prediction:

On basis of Site investigation and laboratory testing.

· Verification:

By some method of loading tests




http://pdfcrowd.com


• Ultimate capacity is the load required to cause failure, whetherby excessive settlement or irreversible movement of the pilerelative to the soil.

• Allowable capacity is the ultimate capacity divided by a factor ofsafety

• The pile ultimate load capacity can be determined by the followingmethods :

1- Pile Load Tests (load test on piles at the site)

(باستخدام نتائج تجارب تحمیل الخوازیق بالموقع)

2- Pile Driving Formulae

(باستخدام الصیغ الدینامیكیة لخوازیق الدق)

3- Correlations with SPT or CPT data

(باستخدام نتائج االختراق اإلستاتیكیة أو الدینامیكیة)

4- Correlations based on soil shear strength parameters.

6.2 Pile Load tests

Pile load test are usually carried out that one or some of the following




http://pdfcrowd.com


reasons are fulfilled:

1. To obtain back-figured soil data that will enable other piles tobe designed.

2. To confirm pile lengths and hence contract costs before theclient is committed to over all job costs.

3. To counter-check results from geotechnical and pile drivingformulae.

4. To determine the load-settlement behaviour of a pile, especiallyin the region of the anticipated working load that the data can beused in prediction of group settlement.

5. To verify structural soundness of the pile.

Limitations of Pile Load Tests:

It is important to recognize that a load test performed on a single piledose not :

* account for long-term settlement

* take into account downdrag from settling soils

* take into account the effect of group action

The above must be considered when using load test results to designor analyze pile foundations




http://pdfcrowd.com


Test loading:

There are four types of test loading:

1- Compression test 2- Lateral-load test

3- Uplift test 4- Torsion-load test

Performing pile load tests is the most reliable method to measureaxial capacity of piles as it is performed in situ taking field conditionsinto consideration.

Compression tests (Static Load Tests)

• The most precise – if not always the most accurate – method ofdetermining the ultimate upward or downward load capacity ofa deep foundation.

• Static load tests, however, are time consuming and expensive;must be used judiciously.

• Object of the test is to develop a load-displacement curve, fromwhich the load capacity can be determined.




http://pdfcrowd.com


The design load shall be defined as 50% of the failure load (factor ofsafety = 2 ). The failure load for the pile shall be defined as follows:

For piles 60 cm (24 inches) or less in diameter or width, the failureload of a pile tested under axial compressive load is that load whichproduces a settlement at failure of the pile head equal to:

Sf = Δ + (4 + 0.008 D)

Where: Sf = Settlement at failure (mm).

D = Pile diameter or width (mm).

Δ = Elastic deformation of total pile length (mm).

For piles greater than 60 cm in diameter or width:

S f = Δ + D/30

The top elevation of the test pile shall be determined immediately afterdriving and again just before load testing to check for heave. Any pilewhich heaves more than 6 mm shall be redriven or jacked to theoriginal elevation prior to testing. Unless otherwise specified in the




http://pdfcrowd.com


contract, a minimum 3‑day waiting period shall be observed betweenthe driving of any anchor piles or the load test pile and thecommencement of the load test.

The pile capacity may increase (soil setup) or decrease (relaxation)after the end of driving. Therefore, it is essential that static loadtesting be performed after equilibrium conditions in the soil have re-established. Static load tests performed before equilibriumconditions have re-established will underestimate the long term pilecapacity in soil setup conditions and overestimate the long termcapacity in relaxation cases. For piles in clays, specifications shouldrequire at least 2 weeks or longer to elapse between driving and loadtesting. In sandy silts and sands, 5 days to a week is usuallysufficient. Load testing of piles driven into shale should also bedelayed for at least 2 weeks after driving.

• Failure of the pile is defined in to two ways that as the load at whichthe pile continues to move downward without further increase inload, or according to the B.S., the load which the penetrationreaches a value equal to one-tenth of the diameter of the pile at thebase.




http://pdfcrowd.com


Compression Test :

• Stress-controlled tests are performed by applying vertical loads tothe pile and observing or measuring the vertical pile displacement.

• The load increments are typically: 25, 50, 75, 100, 125, 150, 175, and200% of the estimated capacity (the design load of the pile), untilexcessive pile displacement (failure) is observed.

• Piles are loaded to 150% to 200% of their theoretical axial capacityusing a hydraulic jack.

• Loads are applied according to the following table:




http://pdfcrowd.com


Load (% of Design load) Time (hrs)25% 150% 175% 1

100% 3125% 3150% 12125% ¼100% ¼75% ¼50% ¼25% ¼

Interpretation of Load Test:

The load displacement curve generated from the pile load test is usedto determine the allowable pile capacity. The following methods havebeen used to define failure:

· Offset Limit Method (Davisson 1972): The failure load is definedas the load corresponding to a movement which exceeds theelastic compression of the pile, when considered as a freecolumn, by a value of 0.15 inches (4 mm) plus a factordepending on the diameter of the pile (D/120), where D is thediameter of the pile in inches. AASHTO (1992) and FHWArecommend that the offset method be used to determine thefailure load.




http://pdfcrowd.com


· De Beers Method (Fellenius, 1980): The load displacementvalues are plotted on a double logarithmic scale, where thevalues may be shown to fall on two straight lines. Theintersection of the lines corresponds to the failure load.

· Slope and Tangent (Butler and Hoy, 1977): The failure load isdefined as the load at the intersection of a line tangent to theinitial straight line portion of the load displacement curve and aline tangent to the load displacement curve where the slope ofthe line reaches 0.05 inches/ton).

The results of a pile load test are typically plotted as load versusdisplacement (movement of the pile butt). The scale of the plot shouldbe arithmetic and should be selected so that the slope of the elasticdeformation of the pile is inclined at an approximate angle of 20°.

The elastic deformation of a pile may be determined using thefollowing equation:

Δ = ( Q L) / (AE)

Where: Δ = Elastic deformation

Q = Test load

L = Pile length




http://pdfcrowd.com


A = Pile cross-sectional area

E = Modulus of elasticity of pile material

This equation is accurate for end bearing piles where no stresstransfer occurs along the length of the pile. Timber piles, however,are typically friction piles or a combination of friction and endbearing. The elastic deformation will, therefore, typically be less thanthat determined from the above equation

The failure load (offset limit method) of a timber pile is the load thatproduces a movement of the pile butt (head) (Sf) equal to:

S f = Δ + ( 4 + 0.008 D ) For piles diameter or width ≤ 60cm

S f = Δ + ( 4 + 0.008 D ) For piles diameter or width ≤ 60cm

Where: Sf = movement of the pile head (mm)

D = Pile diameter (mm)

Δ = Elastic deformation (mm)




http://pdfcrowd.com


Figure 6-1 presents a typical pile load test load movement curve. Theelastic deformation and the offset limit failure criteria are also plotted.The intersection of the failure criterion line and load movement lineyields the ultimate capacity of the pile.

Figure 6-1 Typical static pile load test results

• Results of load-settlement and load-time curves are recorded andtheir curves are drawn.




http://pdfcrowd.com


• Modified Chin method is used to analyze the data

Davisson’s Method




http://pdfcrowd.com


Davisson’s Method

90 % Criterion (Brinch Hansen, 1963)The failure load is defined as the load at which the movement is twicethat obtained for 90% of that load.

This criterion has been proposed for use with the constant Rate ofPenetration test (CPR) regardless of soil type.

At the Constant Rate of Penetration test (CPR), the test load is carriedto failure and the pile head is forced to settle at constant rate of:




http://pdfcrowd.com


0.4 mm/min for friction pile in clay.

2.0 mm/min for end bearing pile in sand.

Brinch Hansen’s Method

Modified Chin’s Method




http://pdfcrowd.com


Qult = 1 / (1.2b)

تحدید الحمل والهبوط المسموح به للخازوق من نتائج تجارب التحمیل:

یجب أن یتوافر الشرطان التالیان معًا فى تقدیر الحمل التصمیمى المسموح به للخازوق:

أال یقل الحمل األقصى عن ضعف الحمل التصمیمى المسموح به ( وعن -1وعلى أال یقل عن مرة والمیتة والریاح ) 1.75 الحمل التصمیمى المسموح به ( نتیجة األحمال الحیة وفى حالة عدم حدوث أخذ تأثیر الزالزل فى الحساب ونصف الحمل التصمیمى المسموح به فى حالة القیمتین المتحصل علیهما من طریقتى تشین المعدلة 1970 إنهیار واضح یقدر الحمل األقصى كمتوسط

وبرینك هانسن 1963.

التصمیمى المسموح به Q عن 2% من - أال یزید الهبوط بعد 12 ساعة من وضع مرة ونصف الحمل 2قطر الخازوق مضافًا إلیه الهبوط المرن كما هو موضح بالمعادلة التالیة :

SQ = 0.02 d + 0.5 QL/AE

على أال یتعارض ذلك مع قیمة الهبوط وفارق الهبوط المسموح بهما للمنشأ.




http://pdfcrowd.com


حیث:

d قطر الخازوق،

Qمرة ونصف الحمل التصمیمى،

Lطول الخازوق،

Aمساحة مقطع الخازوق ،

طبقا للكود المصرى المرونة لمادة الخازوق وتحدد قیمته للخوازیق سابقة الصب معامل E للخرسانة :

E = 14000 √ fcu kg/cm2

حیث fcu هى مقاوم الضغط الممیزة للخرسانة.

أما بالنسبة للخوازیق الخرسانیة المصبوبة فى مكانها فیؤخذ 140 طن /سم2.

وال تسرى المعادلة السابقة على الخوازیق الخاصة مثل الخوازیق صغیرة األقطار والخوازیق الشریطیة.

Test Setup:

n Reaction System

n Loading & Load Measurement

n Reference System & Displacement Measurements




http://pdfcrowd.com





http://pdfcrowd.com


Reaction Pile Test Disadvantages

1- Reaction piles may pull out

2- Flexible system stores energy during tests




http://pdfcrowd.com





http://pdfcrowd.com


6.3 Pile Driving Formulae• Pile driving formulae are mainly used to estimate the ultimate axial




http://pdfcrowd.com


load of driven piles.

• These formulae depend on correlations between the weight of theused hammer, the resulting settlement and the height of drop.




http://pdfcrowd.com


Engineering news formula:

Highly Formula (1925):

C = C c + Cp + C g ,

C g = 0.50 % of the pile diameter




http://pdfcrowd.com


Efficiency of different types of hammers

Type of Hammer Power Efficiency, kDrop hammer ( triggered fall ) 1.0Steam or compressed air hammer 0.9Drop hammer (winch operated) 0.8Diesel hammer 0.6 - 0.8

Cushion type Coefficient ofRestitution, e

Young’ s modulus, Ec

(MN/m2)Micarta plastic 0.8 3x103

Greenheart oak 0.5 3x102

Other timber 0.3 2x102

6.4 Correlations with SPT and CPT tests

6.4.1- Correlations with SPT data

Standard penetration test (S.P.T) اختبار االختراق القیاسى




http://pdfcrowd.com





http://pdfcrowd.com


Empirical correlations with the results of SPT data usually take thefollowing form:

where AN and BN are empirical numbers, and depend on the units offs, and N = SPT value at the point under consideration.

(حمل التشغیل) مرتكز (kNبالكیلو نیوتن) طبقًا للكود المصرى لألساسات فإن قدرة تحمل خازوق إزاحةتشغیل الخازوق ویتضمن معامل أمان قدرة 2.5 لمقاومة ارتكاز فى تربه غیر متماسكة الحبیبات.(حمل

الخازوق، 2 لمقاومة االحتكاك. )

حیث :

طبقة التربة المؤثرة على حمل االرتكاز القیمة المتوسطة لعدد الدقات فى تجربة االختراق القیاسى فى Nوالممتدة لمسافةd اسفل قاعدة الخازوق و 3d أعلى نقطة االرتكاز – بحیث ال تزید عن 50 دقة/قدم.

طول الخازوق داخل الطبقة أو القیمة المتوسطة لعدد الدقات فى تجربة االختراق القیاسى على N¯الطبقات غیر متماسكة الحبیبات.

l طول اختراق الخازوق للطبقة غیر متماسكة الحبیبات بالمتر.

d قطر الخازوق بالمتر.




http://pdfcrowd.com


d قطر الخازوق بالمتر.

في حالة خوازیق التثقیب المنفذة بطریقة الحفر فإن حمل تشغیل الخازوق یتراوح بین 50% و%100من القیمة المحسوبة بالمعادلة المذكورة أعاله وذلك طبقًا لنوعیة خازوق التثقیب المستخدم وطریقة تنفیذه.

6.4.2 Correlations with CPT tests

Static Cone Penetrometer Test (C.P.T)




http://pdfcrowd.com


n Static Cone penetrometer test (C.P.T) measures:




http://pdfcrowd.com


1- Cone Resistance, qc

2- Sleeve Resistance, fsc




http://pdfcrowd.com


Typical CPT results

Empirical correlation with the results of CPT data often takes thefollowing form:




http://pdfcrowd.com


Where Aq = empirical number, and qc = cone penetration resistance atthe point under consideration, and

where Cq = empirical factor, and qcb = average cone resistancearound the pile tip.

In most practical methods of design, an upper limits are placed on thevalues of fs (fsl) and fb (fbl), these being dependent on the type of soiland the type of pile.

للكود المصرى لألساسات فإن قدرة تحمل خازوق إزاحة (بالكیلو نیوتن kN) (حمل التشغیل) مرتكز طبقًا لمقاومة ارتكاز تشغیل الخازوق ویتضمن معامل أمان قدرة 3 (حمل فى تربه غیر متماسكة الحبیبات.

الخازوق، 2 لمقاومة االحتكاك ):

حیث :

مرات قطر الخازوق أعال منسوب المقاومة المتوسطة إلختراق المخروط اإلستاتیكي في مسافة 6 qcاإلرتكاز و 3مرات هذا القطر أسفل منسوب اإلرتكاز حیث ال تزید قیمة qc عن 150كجم/سم2.




http://pdfcrowd.com


تزید عن 100كیلو نیوتن/م2 المتوسطة المقاسة لإلحتكاك الجانبي بطول الخازوق بحیث ال القیمة fc1.0) كجم/سم2) ویمكن في حالة عدم قیاسها تقدیرها كنسبة تبلغ 0.005 من قیمqc المناظرة.

القیاسي إال أنه یجب مراعاة أن نتائج اإلختبار بعدم وجود العیوب المصاحبة إلختیار اإلختراق یتمیز هذا دقیقة في حالة التربة الرملیة الكثیفة جدًا أو في الطبقات اإلستاتیكي CPT ال تعتبر إختبار المخروط خازوق إزاحة مرتكز في رمل سائب إلى الرملیة المحتویة على نسبة من الزلط ویمكن تقدیر قدرة تحمیل

كثیف أو طمى غیر لدن باستخدام نتائج تجربة المخروط اإلستاتیكي طبقًا للعالقة السابقة.

الخازوق یتراوح بین 50 حالة خوازیق التثقیب المنفذة بطریقة الحفر فإن حمل تشغیل كما أنه أیضا في % و 100 % من القیمة المحسوبة بالمعادلة المذكورة أعاله وذلك طبقًا لنوعیة خازوق التثقیب المستخدم

وطریقة تنفیذه.

6.5 Low Strain Integrity Methods




http://pdfcrowd.com


Low strain integrity tests examine the response of a pile to a smallhammer blow at the pile head. The induced stress wave travels downthe pile shaft and reflected waves from significant changes in pileshaft acoustic impedance are registered by a transducer held againstthe pile head. The measured signals are digitized by the test unit,stored internally and downloaded onto PC for hard copy reporting.




http://pdfcrowd.com


n Ideal as quality control for pile integrity.

n All piles on site can be tested quickly and efficiently.

n Faults can be detected immediately on site.

n Pile lengths as well as necking & bulging can be detected.

n Practical & inexpensive.

n Over 200 piles per day may be tested depending on access andpile head condition.

n Modern, battery operated data acquisition systems.

Data downloaded to PC for quick reporting




http://pdfcrowd.com





http://pdfcrowd.com

osp mans edu eg deepfoundation ch6 htm

Documents

pilesoil system

pilesoil interface

pile shaft

pile tip

pile headrebound

soil strength

pile geometry length

axial pile capacity