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Shaft and resistance of bored piles

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Page 1: Shaft and resistance of bored piles
Page 2: Shaft and resistance of bored piles

Presented by Presented by

Mehwish RiazMehwish Riaz

2007-MS-GEO-062007-MS-GEO-06

Relative Proportions of shaft Relative Proportions of shaft and base resistance of bored and base resistance of bored piles in Punjab submerged piles in Punjab submerged

sandssands

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ContentsContents

IntroductionIntroduction Soil ProfileSoil Profile Construction MethodConstruction Method Load Test ProgrammeLoad Test Programme Evaluation of ultimate LoadEvaluation of ultimate Load Resistance along Pile Shaft and BaseResistance along Pile Shaft and Base Probable ReasoningProbable Reasoning ConclusionConclusion

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IntroductionIntroduction

Project is up gradation of NHA(Mianchannu-Sahiwal Project is up gradation of NHA(Mianchannu-Sahiwal Section)Section)

25 m deep boreholes at canal bank at bridge site.25 m deep boreholes at canal bank at bridge site. Pile foundations of 660 mm diameter and 13-18 m Pile foundations of 660 mm diameter and 13-18 m

length were recommended with allowable load (from length were recommended with allowable load (from load test result) of 140-155 tonnes.load test result) of 140-155 tonnes.

Full Scale Load tests were performed up to 2-2.5 Full Scale Load tests were performed up to 2-2.5 times the initial design load.times the initial design load.

Results showed premature failure at less then design Results showed premature failure at less then design load as well as absence of adequate base resistance.load as well as absence of adequate base resistance.

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Soil ProfileSoil Profile

Formation of Formation of relatively relatively heterogeneous heterogeneous quaternary soil quaternary soil deposits deposits comprising a comprising a combination of combination of alluvium and loess alluvium and loess is present.is present.

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Construction MethodConstruction Method

Pile testing was distributed in the form of Pile testing was distributed in the form of packages 6 (a) and 6 (b) packages 6 (a) and 6 (b)

For package 6 (a) construction of test piles was For package 6 (a) construction of test piles was done by wash boring method.done by wash boring method.

For package 6 (b) construction was undertaken For package 6 (b) construction was undertaken by reverse rotary method.by reverse rotary method.

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Load Test ProgrammeLoad Test Programme

6 (a)

Bridge 1310 tonnes

Bridge Kassowal 280 tonnes

Bridge 1 310 tonnes

Bridge Kassowal 280 tonnes

6 (b)

Bridge 4310 tonnes

Bridge 6 310 tonnes

Bridge 7310 tonnes

Bridge 8310 tonnes

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Evaluation of Ultimate /Failure LoadEvaluation of Ultimate /Failure Load

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Resistance along Pile Shaft and BaseResistance along Pile Shaft and Base

Ideal way to measure ultimate skin Friction (USF) Ideal way to measure ultimate skin Friction (USF) mobilized at onset of failure is the measurement of mobilized at onset of failure is the measurement of base resistance by placing load cell and then base resistance by placing load cell and then subtracting the value from total load.subtracting the value from total load.

For this study, USF is determined by two methodsFor this study, USF is determined by two methods

(a) evaluation of load corresponding to 6 mm butt (a) evaluation of load corresponding to 6 mm butt settlementsettlement

(b) estimation of skin component by van weel’s (b) estimation of skin component by van weel’s graphical constructiongraphical construction

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Drawing a line parallel to the yield zone of the Drawing a line parallel to the yield zone of the load-settlement curve, from the originload-settlement curve, from the origin

Skin friction values by both these methods Skin friction values by both these methods agree closely with each other and their average agree closely with each other and their average is slightly short of AASHTO failure loadis slightly short of AASHTO failure load

This suggests that after mobilization of full This suggests that after mobilization of full skin resistance, the pile base does not take skin resistance, the pile base does not take appreciable load in materials termsappreciable load in materials terms

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These bored piles took only about 8 % of the These bored piles took only about 8 % of the load at the base, at failure up to 92 % of load load at the base, at failure up to 92 % of load was resisted by the shaft.was resisted by the shaft.

Percentage proportion of shaft and base Percentage proportion of shaft and base resistance components for the two kind of resistance components for the two kind of borings were as followsborings were as follows

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Probable ReasoningProbable Reasoning The increase in shaft resistance proportions is The increase in shaft resistance proportions is

understandable, owing to high degree of roughening understandable, owing to high degree of roughening of borehole walls by the tool movement.of borehole walls by the tool movement.

Piles of highly irregular skin would have resulted Piles of highly irregular skin would have resulted after casting, which yielded high skin friction.after casting, which yielded high skin friction.

Insitu soils at the pile tip horizons were not weak, so Insitu soils at the pile tip horizons were not weak, so the most probable reason for less base resistance is the most probable reason for less base resistance is very method of construction.very method of construction.

It creates looseness at the base of bore hole resulting It creates looseness at the base of bore hole resulting into higher settlements before base resistance can be into higher settlements before base resistance can be mobilized.mobilized.

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So the less base resistance may b due So the less base resistance may b due to……………….to……………….

Looseness of soil at the base by the movement of the Looseness of soil at the base by the movement of the bit and improper cleaning of the boreholes before bit and improper cleaning of the boreholes before pouring concrete.pouring concrete.

During lowering of steel cage, assemblage of loose During lowering of steel cage, assemblage of loose soils scratched from the borehole walls at the base.soils scratched from the borehole walls at the base.

Inadequate control over boiling of sand during Inadequate control over boiling of sand during boring.boring.

Introduction of loose soils from borehole within pile Introduction of loose soils from borehole within pile concrete resulted into necking.concrete resulted into necking.

Segregation or honey combing in the poured Segregation or honey combing in the poured concrete.concrete.

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Conclusion Conclusion In normal works in Pakistan, the boring of piles with In normal works in Pakistan, the boring of piles with

wash boring and reverse rotary excessively loosens wash boring and reverse rotary excessively loosens the soils at the base of hole and thus reduce the base the soils at the base of hole and thus reduce the base resistance components to negligible proportions.resistance components to negligible proportions.

The conventional pile capacity forecast method fails The conventional pile capacity forecast method fails to predict the skin friction and end bearing resistance to predict the skin friction and end bearing resistance components. components.

The design method should be carefully adopted The design method should be carefully adopted giving due consideration to the method of giving due consideration to the method of construction.construction.

There is need for more stringent construction There is need for more stringent construction supervision.supervision.

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