Piles Foundation

Pile Construction and Testing Techniques

Group Members Tanveer Younas (2015-MS-CES-24) Atta Muhammad (2013-MS-CES-27)

Contents  Introduction to Piles  Function of Piles  Types of Piles  Construction Techniques for Piling  Comparison of Different Piles  Problems in pile construction and remedial

measures  Testing Techniques in Piles  Conclusion

Introduction to Piles Definition A pile is a vertical structural element of a deep foundation, driven deep into the ground at the building site. For Deep Foundations Df/B > 4

Pile foundation- need and functions Pile foundation is required when the soil bearing capacity is not sufficient for the structure to withstand. This is due to the soil condition or the order of bottom layers, type of loads on foundations, conditions at site and operational conditions.

Pile foundation- need and functions (cont.) Purpose of pile foundation is  To transmit the buildings loads to the foundations and the ground soil layers whether these loads vertical or inclined.  To install loose cohesion less soil through displacement and vibration.  To control the settlements.  To increase the factor of safety for heavy loads buildings

Types of Piles Piles are classified on the basis of i. Material of construction ii. Type of soil iii. Load transmitting characteristics iv. Construction method .W.r.t material of construction, piles are .Timber Piles .Steel Piles .Concrete Piles

Types of Piles  Once

soil data is obtained from soil exploration boreholes at different depths, selection of pile foundation type depends upon following factors.  Soil conditions  Nature of loads  Number of piles to be used  Cost of construction

Types of Piles (cont.)  There

are three types of piles based upon load transfer mechanism  End bearing piles  Friction bearing piles  Combination of end bearing and friction bearing piles.

Types of Piles (cont.) Based upon construction techniques, piles may be classified into  Driven piles  Cast-in-situ piles  Driven and cast-in-situ piles. 

Pile installation methods  The

installation process and method of installations are equally important factors as of the design process of pile foundations. pile installation methods are  Installation by pile hammer  Installation by mechanical auger In order to avoid damages to the piles, during design, installation Methods and installation equipment should be carefully selected.

Pile driving methods Methods of pile driving can be categorized as follow:  Dropping weight  Explosion  Vibration  Jacking (restricted to micro-piling)  Jetting 

Driven piles vs. jetted piles, a comparison In case of driven piles, hard soils may offer sufficient resistance to penetration of pile making the action of pile driving difficult.  In such situation, jetting may be used.  “Pile jetting” is a technique that is frequently used in conjunction with, or separate from, pile driving equipment for pile placement.  Pile jetting utilizes a carefully directed and pressurized flow of water to assist in pile placement

Driven piles vs. jetted piles, a comparison (cont.)  Pile jetting  Liquefies the

soil at the pile tip during pile

placement  reduces the friction and interlocking between adjactant sub-grade soil particles around the jet.  Thus, decreases the bearing capacity of soil underneath the pile tip, pile descends easily under its own weight.

Problems in pile construction and remedial measures Different problems may occur during pile construction.  Overbreak Formation of cavities outside nominal diameter of pile is known as over break.  It may occur in cohesion less soils below water table  Reduced skin friction and end bearing.

Overbreak (cont.) Measure: It may be overcome by using temporary casing or drilling under bentonite drilling fluid.

Overbreak

Base of Borehole  To

develop available end bearing, It is essential that bottom of borehole is clean and undisturbed.  In clayey soils  In sandy soils Measure: To avoid falling of debris, cover the borehole properly before concreting.

Effect of water table In dry drilling of borehole, ingress of small quantity of water from sides of borehole reduces  Skin friction  Bearing from bottom of borehole Measure: In this condition, concreting should be carried out with tremmie pipe otherwise, segregation of concrete may occur. 

Design of Reinforcement  Major

problem in design of pile reinforcement is its detailing.  A minimum spacing of 100mm is required otherwise, concrete will not flow outside the cage and proper cover could not be provided.  A few large diameter bars are preferred in place of small diameter bars.  The problem is visible during concreting when cage drags downward into the borehole.

Lowering of Reinforcement  Steel

cage should be lowered after providing proper laps so that there is no slippage of cage while lowering.  When weight of cage is more in deep piles, cage should be welded at the laps before lowering.

Lowering of Reinforcement

Steel cage properly welded and lapped before lowering

Placing of Concrete  Concrete

is placed in borehole using tremmie in order to avoid segregation  Concreting process should be continuous and should be completed without any break  If slight delay occurs between batches then cement slurry may come over surface causing weakness of pile shaft.  If prolonged delay occurs between batches then concrete may approach final setting time.

Extracting temporary casing  After

concreting, extraction of temporary casing may cause problems particularly if delays occur and partial separation of pile shaft may occur.  This is due to  Use of dirty and dented casing  Delay in extracting  Use of poor workable mix

Extracting temporary casing

Faulty removal of casing

Problems in soft ground  In

soft ground having undrained shear strength less than 15 KN/m2 causing squeezing of pile section near the head because lateral pressure of soil is more than the lateral pressure of concrete.

Squeezing of section in soft ground

Testing techniques in piles  The

success of piling depends upon the load carrying capacity and integrity of pile. If one can somehow, find the load carrying capacity and integrity of pile then the performance of piling can be understood in a better way.

Testing techniques in piles (cont.)  These

days, with the invent of modern computers different modern methods of pile testing have been devised which give more reliable information regarding the load carrying capacity and integrity of pile.  Some methods of pile testing are being enlisted here.

Testing techniques in piles (cont.)  Static

load method  Dynamic load method  Sonic integrity method

Testing techniques in piles (cont.)  Static

Load Method  Static load testing is the method by which the load displacement characteristics of a pile can be determined. All piles are suited to testing in this way. Description  In order to apply a known load to the test pile some form of reaction is necessary

Static load Method  The

most commonly used methods are Kent ledge or tension pile reaction, dependent upon ground conditions. Other methodologies can be adopted according to site requirements e.g. in areas of restricted access or headroom.  Once adequate reaction has been provided, the testing is carried out using a hydraulic jack and calibrated digital load cell to a previously agreed procedure.

Static load Method (cont.)  Time,

load, temperature and displacement data are usually recorded.  Reaction loading frames and Kent ledge assemblies allow tests of up to 4000kN to be carried out. Measurement of pile response to load is measured by digital load cells and electronic linear variable displacement transducers.

Static load Method (cont.)

Pile head measurement Abingdon, Oxford

at

Kent ledge test-frame on site at Belvedere, South London

Dynamic load Method  Dynamic

load testing is suitable for all types of piles, but is mostly employed on pre-cast concrete  This technique determines load bearing characteristics of pile including skin friction and end bearing.  Pile integrity, pile stresses, driving and load displacement behavior can also be determined.

Dynamic load Method (cont.)  Description  In

order to dynamically test a pile, pile must be restruck using a pile hammer.  Two strain transducers and accelerometers are firmly attached to the face of pile near to head.  As pile is restruck, instrument measures the force and acceleration of pile.This information is relayed to pile driving analyser which gives information about pile capacity to an experienced test operator.

Dynamic load Method (cont.)  As

test takes a few minutes per pile so, a number of piles may be tested in one visit.  The aim of method is to produce a model that resembles as closely as possible to a model which is truly replica of recorded data.  Once this has been achieved the model gives bearing capacity, load settlement behavior together with additional information as required.

Dynamic load Method (cont.)

Pile prepared for testing on site

Pile driving analyzer for collection and analysis of test data

Sonic integrity test  Integrity

testing is primarily used on continuous flight auger (CFA) or RB continuous helical displacement (CHD) and other wet bored piling techniques.  The method is fast and reliable, allowing a large number of piles to be tested in a single site visit. The technique is used to determine the reliability, morphology and quality of construction of the piling method.

Sonic integrity test  Description  The

pile to be tested must be sufficiently cured, free of latence and trimmed to sound concrete, preferably to final cut-off level.  Using a small hand-held hammer, a series of low strain acoustic shock waves are passed down the pile. As it does so, the wave rebounds where changes in impedance occur. This rebound or echo is then recorded by a small accelerometer, held against the pile head.

Sonic integrity test 

The response is stored digitally and a graphical representation displayed and plotted, usually against time, for immediate inspection.

Sonic integrity testing equipment

Static & Dynamic testing of piles A case-study  This

case study was done by S.Abe , G.Likins and C.M. Morgano  Traditionally, static methods were used for pile testing, which now-a-days are being replaced by dynamic testing as this case study depicts.  As a part of deep foundation institute program in North Carolina, a demonstration of pile driving and pile testing was performed.

Static & Dynamic testing of piles A case-study A

305mm prestressed pile of length 16.5m was driven into sandy and clayey silts by a conmaco 65E5 hammer to a final penetration resistance of 5blows/inch.  Upon restrike, three days later resistance increased to 10 blows/25mm indicating soil setup. On a WEAP bearing graph,restrike blow count indicated that capacity came out to be 1735 KN (390 kips).

Static & Dynamic testing of piles A case-study  Then

pile was tested dynamically using a pile driving analyzer and results were further analyzed by CAPWAP method.  This analysis indicated a capacity of 1870 KN (420 kips).  After that a static test was performed using ASTM D-1143, pile was loaded to maximum load of 1840KN and had a davisson offset limit load of 1820 KN.

Static & Dynamic testing of piles A case-study

Conclusion  For

pile construction, an experienced engineer is required who can tackle the problems at site in the most effective way.  A good supervision at the site is required in order to maintain better performance of piling.  A suitable test is to be devised as per site conditions, sensitivity of the project and economy available.