Termination Criteria of Bored Pile Subjected to Axial Loading - Dr.K.premalatha

Termination Criteria of Bored Pile Subjected to Axial Loading

Dr. K. Premalatha,Associate Prof & Head i/c,Division of Soil Mechanics and Foundation Engg,Department of Civil Engg,Anna University,Chennai-25.

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Outline

Introduction Need for the Study Objective Literature Review Methodology Validation of the Software Load Carrying Capacity of the Pile Analysis Input parameters for the Analysis Mesh Generation Calculation and Output




Outline (Contd..) Case Study Presentation of Results Load-Settlement Curves Maximum Settlement Vs the rock socket length of the

pile Influence of the Diameter of the Pile Influence of the Rock-Pile Interface strength

Reduction Influence of the Rock strata

Conclusion: Effective Rock Socket Length proposed Comparison between IS:14593:1998 and Numerical

Analysis Results.




Introduction

In the present day scenario the increase in theconstruction of heavily loaded structures,necessitate the transfer of loads to the underlyingrock mass.

Rock socketed piles find its use in such applications.Rock socketed piles are commonly used for thesuperstructures such as high-rise buildings, longspan bridges, tower structures, etc.,

The research have been started in the late 1960’s inCanada and USA and kindled the researchers inAustralia, South Africa and parts of UK in 1970’s.Even to this present day, this area of study is stillan interesting field to explore.




Need for the Study

The determination of socketing length of the pile atpresent is mostly based on few factors affecting thepile mechanism, which sometimes give a better designand sometimes an uneconomical design.

In IS:14593-1998, it is mentioned that the partiallymobilized frictional resistance offered by theoverlying soil may be neglected. But there may be someinfluence of the overlying soil on the socket length.

Too much of socketing length results in over-run ofproject time and also there will be waste of money.

Therefore the effect of all the possible factorsinvolved need to be studied in proposing guidelines forthe design of rock socket length of the pile.




ObjectiveThe aim of this project is to propose guidelines forthe termination criteria of the rock socketed pileusing PLAXIS-3D software, based on the influenceof

Capacity requirement of the pile Thickness and properties of the overlying soil Characteristics of the founding strata Rock-Pile interface strength

for axial load acting on the pile in sandy soil overlyingthe rock.




Literature Review

The Capacity Requirement of the PileThe load carrying capacity of the piles socketed in rock can beestimated based on

The uniaxial compressive strength of rock, Pressuremeter tests results, Shear strength parameters of rock and Structural strength of pile (IS 14593: 1998).

The safe pile capacities can be determined based on the pile load testsas per IS:14593- 1998 and IS 2911 – Part 4.




Thickness and Properties of Overlying Soil

1. Zhang et al. (2011) studied the mechanism of bearing behaviour at the tip ofa pile embedded in rock, the depth effect with respective to the embedmentratio and overburden factor.The pile of 1m diameter socketed in homogeneous rock layer for differentrock socket length from 0 to 8m was considered for the study. The study wascarried out using the generalized non linear unified strength criterion andcompared results with numerical simulation.Result: The ultimate end bearing capacity increases with the overburden anddecreases with the rock damage.

2. For rock- socketed piles in IS: 14593-1998, it is stated that the partiallymobilized frictional resistance offered by the soil may be neglected.

Characteristics of the founding strata

1.Cole and Stroud (1977) classified rocks in terms of scale shear strength,Standard Penetration values SPT of rock and breakability, penetration andscratch test, and are listed in Table.




Scale of Strength and SPT Values for Rocks (Source: Srinivasamurthy (2009))




2. Rowe et al. (1984) gave correlation for rock mass modulus based on the average unconfined compressive strength for weak rock deposits.

Rock mass modulus = 215 √qu MPa The correlation can be used for initial estimate of rock mass

modulus.

3. Long (2000) studied the uncertainties involved in the empiricalmethods for the design of rock – socketed piles in hard rocks.

The load test results of twenty four piles at twelve sites locatedthroughout Ireland were back analyzed.

The diameter of the piles varies from 0.17m to 0.8m and the socketlengths range from 0.25D to 10D

Suggestion: Fracture spacing index or rock quality designation is animportant parameter for determining ultimate skin friction than theunconfined compressive strength of hard rocks.




Summary 1:Capacity requirement of the pile:1. The capacity requirement of pile can be arrived using

IS- 14593:1998, IS-2911(Part-I/Sec 2), IS-2911(Part-IV)

Thickness and properties of the overlying soil:1. As per IS:14593:1998, the partially mobilized frictional resistance

mobilized by the overlying soil may be neglected.2. The ultimate end bearing capacity of the rock socketed piles increases

with increase in the overburden factor.

Characteristics of the founding strata:1. Rocks classification exists based on shear strength, SPT values of

rocks and breakability, penetration test.2. Correlation for Rock mass modulus = 215 √qu MPa3. Uncertainties are there in the use of empirical methods. Rock quality

designation also plays role in determining ultimate skin friction otherthan the unconfined compressive strength.




Rock-Pile Interface Strength and Socket Roughness1. Malinverno A. (1990) proposed Roughness–Length method (RLM)

for estimating the fractal dimension and used to analyse the socket roughness statistics such as mean roughness and standard deviation of the roughness.

2. Seidel et al. (2001), predicted the side resistance of a rock by parametric study

Simulation program ROCKET was used The important factor involved was shaft resistance coefficient




3. Seidel et al. (2001), proposed a relationship between the meanroughness height and the unconfined compressive strength.

The upper and lower bound roughness guidelines proposed by theauthor based on the shaft resistance coefficient is shown inFigure.




4. Kodagoda et al. (2002) studied the adhesive behaviour of thepile-rock socket wall interface using the Finite Element AnalysisProgramme (FEAP).

The pile material (concrete) and the surrounding rock materialwere considered as isotropic linear elastic materials. Thediameter of the pile was 300mm.

The parametric analyses were done for an axial load of 100 kN,over a range of socket length from 1D to 4D, the interface bondmodulus was varied from 105 to 108 kN/m2/m to model thebehaviour of a smooth to a very rough socket wall.

The rock strength varied from weaker to harder rock. Results:

In rough socket wall, the increase in the rock socket lengthincreased the load transfer by shear as compared to the loadtransfer to the base. As the strength of the rock increases, alarge proportion of the applied load is resisted by the socket wallshear. The author also emphasized the usefulness of finiteelement analyses in the study of the rock-socketed piles, ascompared to the difficult task of simulating the exact materialproperty in real situations.




5. Nam et al. (2008) studied the effect of drilling tools on the unitside resistance of rock socketed drilled shafts.

The roughness of the borehole wall made by drilling tool, weremeasured using Laser Borehole Roughness Profiler (LBRP).

LBRP works on the principle of Laser Triangulation. Roughness measurement accuracy was reported as 0.5mm for

both the vertical and radial directions.

Drilling Tools Employed The effect of drilling tools such as core barrel and auger on unit

side resistance of rock socketed drilled shafts was also studiedby Nam et al. (2008).

Field load tests were performed at three sites Hampton, DentonTap and Rowlett Creek on clay shale and limestone. The diameterof the pile is 760mm. The socket lengths for the three sites are8D, 7D and 6D respectively

Result: The borehole roughness of the core barrel test hole wasrougher than the auger test hole.

Socket roughness drilled by the core barrel was about 30%rougher than the auger, regardless of the rock type.




Summary 2:Rock-Pile Interface Strength and Socket Roughness:1. The socket roughness can be determined using Roughness-Length method.2. Side resistance of the shaft is determined using a factor, Shaft

resistance coefficient which is based on the roughness of the socket,method of construction, rock strength.

3. Based on the shaft resistance coefficient the upper and lower boundroughness guidelines were proposed by Seidel et al. (2001)

4. In rough socket wall, the increase in the rock socket length increased theload transfer by shear as compared to the load transfer to the base. Asthe strength of the rock increases, a large proportion of the applied loadis resisted by the socket wall shear.

5. The roughness of the borehole wall were measured using Laser BoreholeRoughness Profiler (LBRP), which works on the principle of LaserTriangulation. Accurate to 0.5mm for both the vertical and radialdirections were measured.

Drilling Tools Employed1. Socket roughness drilled by the core barrel was about 30% rougher than

the auger, regardless of the rock type.




Nature of loading and Socketing length1. Gandhi et al. (1987) described the study of the axial load transfer

by means of instrumentation with load sensors in bored cast-in-situpiles. Pile of 500mm diameter, socketed in weathered rock andhard rock for about 6D and 1D length respectively was studied.

Results were compared with FEM analysis program consideringspring support from various layers.

Result: The pile was supported predominantly by the frictionalresistance offered by the weak rock.

2. Radhakrishnan et al. (1989) reported the load transfer behaviourof rock- socketed piles under axial loading by means ofinstrumentation.

The nature of the rock deposits was sedimentary rocks consistingof highly weathered sandstone, silt stone and shale. Socket lengthswere 13D, 2D, 5D in highly fractured siltstone and 2D in hardshale. The piles were tested for twice the working load.

Result: It was not beneficial to have a socket length in excess oftwo pile diameters for a loading range of 2.8MN to 10MN, unlesspile settlement criteria could not be satisfied.




3. In IS: 14593-1998, the suggested minimum length of socket forpreliminary design of pile under compression varies from 1D to 4D,with respect to the rock type and its quality.

4. Hoonil (2007) investigated the load-transfer behaviour of rock–socketed drilled shafts based on field tests and numerical analysis.

Full scale field tests were conducted on nine rock-socketed testshafts. The diameter of the pile is 1m. The rock socket depthvaried from 2D to 9D.

Finite element analysis was performed to study the effect ofcoupled soil resistance on the total pile displacement. The coupledsoil resistance denotes the effect of shear load transfer behaviourof soil on pile-toe settlement.

Result: The effect of coupled soil resistance exists and theprediction of load deflection of drilled shafts based on coupled soilresistance found to be in good agreement with the trend observedby in-situ measurements.




5. Karthigeyan et al. (2012) studied the effect of combinedloading on the lateral response of piles in homogeneous clayeyand sandy soils.

The study was carried out using three-dimensional finiteelement software GEOFEM3D.

Result: The influence of combined loading on the lateralresponse has significantly increased the lateral capacity oflong piles in sandy soil even up to 30 times the pile width.

In clayey soil, there was no significant effect for piles withlength beyond 15 times the pile width. The design bendingmoment is influenced by the combined loading.

6. Srinivasamurthy et al. (2009) reviewed the existing methods ofdetermination of rock socketing length in axially loaded boredpiles and attempted to formulate a practical approach todetermine the length of the socket of bored piles in rock.

The importance of the consideration of both frictioncomponent and end bearing on rocks in the determination ofsocket length was also emphasized by the author.




7. Zhang et al. (2009) investigated the axial load transfer behaviourof rock socketed shafts based on 99 field test results collectedfrom various published reports. The shafts are 1.4 to 96.3m longand have diameter between 0.5 and 1.5m. The rock-socket lengthsare from 0 to 18.8 metres.

Result: The end bearing resistance in the design of rock socketedshafts should be considered, because even at relatively smallerdisplacement in the working range of 5-15mm, up to 25% of theload is transferred to the base and the load transmitted increaseswith time due to creep.

8. Naveen et al. (2011) used a finite element software PLAXIS- 2Dfor the numerical simulation of field load tests. The test pilediameter is 1.2m, socketed in soft weathered rock up to a lengthof 7.5D.

The author simulated the field load test in this software to studythe pile behaviour under vertical loads.

Result: Reasonable agreement exists between the results of fieldload test and PLAXIS 2D modelling.

Soft weathered rocks can offer more skin resistance.




9. Karthigeyan et al. (2012) examined the influence of rock socket lengths onthe lateral response of piles embedded in homogeneous sandy soil overlyingrock using 3D finite element software GEOFEM3D. The width of the pile is1m square pile of length 9m.

The factors considered are socket length variation from 0 to 3 times widthof the pile, and the pile fixity conditions.

Results: The lateral deflections of the pile decreases with increase in rocksocket lengths only up to a certain limit of socketing.

Limiting depth – From Maximum lateral load capacity point of view – equal towidth of pile for both free and fixed head pile conditions.From Maximum lateral deflection point of view- 0.8B for free head piles and0.5B for fixed head piles.

10. Dai et al. (2012) investigated the axial load transfer in rock-socketed pilesby analyzing a database of results from 120 load tests collected frompublished papers and reports. The pile lengths ranges from 6 to 120m andthe diameter varied between 0.5 to 3.0m.

Result: The shaft resistance increases with the increase in the rock socketlength and in turn the base resistance decreases.




Summary 3:Nature of loading and Socketing length1. Weak rock supports the axially loaded rock socketed pile by

offering frictional resistance.2. Socket length in excess of two pile diameters for loading range of

2.8MN to 10MN was not beneficial in rock- socketed piles onsedimentary rocks under axial loading.

3. In IS: 14593-1998, the suggested minimum length of socket forpreliminary design of pile under compression varies from 1D to 4D,with respect to the rock type and its quality.

4. The influence of combined loading on the lateral response hassignificantly increased the lateral capacity of piles in sandy soil. Inclayey soil, there was no significant effect for piles with lengthbeyond 15 times the pile width. The design bending moment isinfluenced by the combined loading. The literature available oncombined loading is scanty.




5. The end bearing resistance in the design of rock socketedshafts should be considered, because even at relativelysmaller displacement in the working range of 5-15mm, up to25% of the load is transferred to the base and the loadtransmitted increases with time due to creep.

6. PLAXIS 2D software can simulate the pile behaviour.7. The lateral deflections of the pile decreases with increase

in rock socket lengths only up to a certain limit ofsocketing. Limiting depth – From Maximum lateral loadcapacity point of view – equal to width of pile for both freeand fixed head pile conditions. From Maximum lateraldeflection point of view- 0.8B for free head piles and 0.5Bfor fixed head piles.

8. Based on the database study the shaft resistanceincreases with the increase in the rock socket length and inturn the base resistance decreases.




Methodology

Method to be followed for the present study isNumerical analysis using PLAXIS 3D Foundationsoftware

Parameters that are varied for the presentanalysis are(i) Thickness and properties of the overlying soil(ii) Rock strength and socket length(iii) Pile dimensions(iv) Rock-Pile interface strength




The details of the above different parameters that areconsidered for the present analysis are listed below:

Parameter Details

Type of overlying soil - Different density of sand is consideredfor the comparison of the results.Loose sand, medium dense sand and densesand

Thickness of overlying soil - 5m , 10m , 15m and 20mRock type (based on strength) -

Weak rock, moderately weak rock,moderately hard rock, hard rock andweathered rock

Socket length - 1D to 8DPile diameter - 600mm, 750mm and 1000mmPile length - vary depending on thickness of overlying

soil and socket lengthNature of Loading - Axial loadingRock-pile Interface Strength Reduction Factor - 0.25, 0.5, 0.75




Material (Depth)

Layer 1 Layer 2 Layer 3

Clay (0-6m)

Soft weathered rock (6-20m)

Concrete Pile(15m)

Model Mohr –Coulomb Mohr – Coulomb Linear Elastic

Unit Weight, γ [kN/m3] 21.00 22.00 25.00

Young’s Modulus, E [kN/m2] 40E3 10E5 30E6

Poisson’s Ratio, υ [-] 0.30 0.33 0.20

Cohesion, Cu [kN/m2] 30.00 50.00 -

Friction Angle, φ [º] 20.00 25.00 -

VALIDATION OF THE SOFTWARE AND MODELLINGAxially Loaded Pile

Soil Parameters (Naveen et al. 2011)




Input – Plan view and Isometric view (PLAXIS 3D)




Output – Deformed mesh, Settlement contour (PLAXIS 3D)




Deformed mesh and displacement contour (Naveen et al. 2011)PLAXIS 2D




Load- Settlement data

Methods Vertical Settlement (mm)

PLAXIS 2D analysis(Naveen et al. 2011)

4.91

PLAXIS 3D analysis(Validation analysis)

4.88




VALIDATION OF THE SOFTWARE AND MODELLING

Laterally Loaded Pile

Pile, Soil and Rock properties considered in the analysis (Karthigeyan et al. 2012)

Pile DetailsSoil and Rock Details

Sand Foundation rock

Size ‘B’: 1.0m x 1.0m Angle of internal frictionφ = 30º φ = 40º

Length ‘Lp’ = 6m in soil + 3B depth in rock Dilation angle ψ = 0 º ψ = 0 º

Type of pile: concrete

Young’s modulusEs = 20 MPa

Young’s modulus Er = 50000 MPa

Grade of concrete: M25

Young’s modulusEp = 25000 MPa

Poisson’s ratio (μp) = 0.15 Poisson’s ratio (μs) = 0.30 μr = 0.24




Input – Plan view and Isometric view (PLAXIS 3D)




Output – Deformed mesh and settlement contour (PLAXIS 3D)

Methods Lateral Deflection (mm)

GEOFEM- 3D(Karthigeyan et al. 2012) 7.2

PLAXIS 3D(Validation analysis) 7.6

Lateral Deflection of Piles




Load Carrying Capacity of the Pile

The axial load carrying capacity of the pile on rock is calculated asper IS 14593:1998.

The axial load of the pile in granular soil and the structuralcapacity of the pile are estimated using the provisions in IS: 2911

(Part I / Sec 2) and IS 456:2000 respectively.

Profile Method

Sand + weak rockSand + moderately weak rockSand + weathered rock

- lesser of (Frictional resistance offered bysoil + Shear strength of rock) and Structuralcapacity of pile

Sand + moderately hard rockSand + hard rock

- lesser of (Frictional resistance offered bysoil + Uniaxial compressive strength of rock)and Structural capacity of pile




Load Carrying Capacity of the Pile (Contd..)

From the above calculations, it is observed that the design loadfor weak rock and weathered rocks is due to the resistance of soilor rock. For the other types of rocks, the design load is thestructural capacity of the pile. Since the partial mobilization offrictional resistance of soil for 10m, 15m and 20m overlying sanddeposit in the load carrying capacity exceeds the structuralcapacity of the pile, the axial load estimated for 5m overlyingthickness of sand is considered for arriving at the load-settlementrelationship.




Applied Axial Load on the Pile in Numerical Analysis

Diameter of the Pile Profile Applied Axial Load (kN)

600mm

Sand + Weak rock 3700

Sand + Moderately weak rock 4200

Sand + Moderately hard rock 4200

Sand + Hard rock 4200

Sand + Moderately weathered rock 1360

Sand + Highly weathered rock 1360

750mm Sand + Weak rock 5600

1000 mm Sand + Weak rock 9200




Analysis

Two layered soil-rock profile

Loose to dense overlying sand layer

Weak to Hard rock stratum, weathered rocks

Water table well below the rock stratum

Influence of the diameter of the pile and therock-pile interface strength reduction arestudied

Total number of analyses done are 828

Input Parameters for the analysis

Mesh Generation

Calculation and Output




Soil-Rock Profiles Selected for the Numerical Analysis




Diameter R inter Thickness of Soil Layer Soil Type Rock Type

600 mm

1.0 5m,10m,15m, 20m loose sand weak rock1.0 5m,10m,15m, 20m loose sand moderately weak rock1.0 5m,10m,15m, 20m loose sand moderately hard rock1.0 5m,10m,15m, 20m loose sand hard rock

1.0 5m,10m,15m, 20m loose sand moderately weathered rock

1.0 5m,10m,15m, 20m loose sand highly weathered rock1.0 5m,10m,15m, 20m medium dense sand weak rock1.0 5m,10m,15m, 20m medium dense sand moderately weak rock1.0 5m,10m,15m, 20m medium dense sand moderately hard rock1.0 5m,10m,15m, 20m medium dense sand hard rock

1.0 5m,10m,15m, 20m medium dense sand moderately weathered rock

1.0 5m,10m,15m, 20m medium dense sand highly weathered rock1.0 5m,10m,15m, 20m dense sand weak rock1.0 5m,10m,15m, 20m dense sand moderately weak rock1.0 5m,10m,15m, 20m dense sand moderately hard rock1.0 5m,10m,15m, 20m dense sand hard rock

1.0 5m,10m,15m, 20m dense sand moderately weathered rock

1.0 5m,10m,15m, 20m dense sand highly weathered rock




Soil-Rock Profiles Selected for the Numerical Analysis (Contd..)

Diameter R inter Thickness of Soil Layer Soil Type Rock Type

600mm 0.75 5m,10m,15m, 20m medium dense sand weak rock



750mm 1 5m,10m,15m, 20m medium dense sand weak rock

1000mm 1 5m,10m,15m, 20m medium dense sand weak rock




Soil Properties Selected for the Analysis

Parameter Loose sand Medium dense sand Dense sand

Material model Mohr-Coulomb Mohr-Coulomb Mohr-Coulomb

Unit weight, γ 16 kN/m3 18 kN/m3 20 kN/m3

Young’s modulus, E 15 MPa 20 MPa 45 MPa

Poisson’s ratio, ν 0.3 0.3 0.3

Cohesion, Cu - - -

Friction Angle, φ 30º 34º 40º




Rock Properties for the Numerical Analysis

Parameter weak rock

moderately weak rock

moderately hard rock

hard rock

moderately weathered

rock

highly weathered

rock

Material model

Mohr-Coulomb

Mohr-Coulomb

Mohr-Coulomb

Mohr-Coulomb

Mohr-Coulomb

Mohr-Coulomb

Unit weight, γ

20 kN/m3 21.5 kN/m3 25 kN/m3 27

kN/m3 21 kN/m3 20.5 kN/m3

Young’s modulus, E 420 MPa 700 MPa 35000

MPa50000 MPa 180 MPa 150 MPa

Poisson’s ratio, ν 0.2 0.2 0.15 0.15 0.2 0.3

Cohesion, Cu

1.3 MPa 3.65 MPa 10 MPa 30 MPa 0.45 MPa 0.12 MPa

Friction Angle, φ - - - - - 45º




The mesh element comprises of Quadratic 15-node wedgeelements.

The mesh is generated with coarser elements for restrictingthe run-time.

The soil cluster around the pile is refined further for betteraccuracy.

The pile is modelled as linear elastic material. Soil and rock aremodelled as elastic perfectly- plastic material based on Mohr-coulomb model.

The material type is drained behaviour. However, the resultsare same for both drained and undrained conditions for dry sand.

Mesh Generation




Mesh Arrangement in PLAXIS 3D

P ile

L p+10m

A xial L oad

L p=T otal length of the P ileD =D iam eter of the P ile

40D

20D




Soil – Rock Layer and Bored Rock Socketed Pile

Sand

R ock0D to 8D

P

P-A xial LoadD -D iam eter of the P ile

5m to 20m

Lp




The analyses are carried out in three phases. Initial Phase Phase – I Phase – II

as in PLAXIS 3D Foundation software. The maximum number of iterations is 50, up to which

the loading process occurs. PLAXIS Output program gives the settlement and

stresses in terms of tables and contour. PLAXIS Curves program is used to generate the

load-settlement curves.

Calculation and Output




Case Study

S o ft M ud N < 1

L o ose to M ed iu m de nsem e d ium to co arse sa nd N = 10

M e d iu m d en sem ed iu m to c oa rse san d N = 22

M ed ium de nse to de nsem ed iu m to c oa rse san d N = 4 1

S o u nd G ra n ite B e d R o ck

1 .45 m0m

19 .4 5 m

33 .4 5 m

5 5 .8 m

1 10 .8 m1 15 .1 0 m




Soil – Rock Properties for the Field Bore Log Profile

Layer Soft Mud Loose to medium dense sand

Medium dense sand

Medium dense to dense sand

Granite

Average SPT N value

>1 10 22 41 -

Unit weight (kN/m3) 17 – 18 14 – 18 17 -20 17 - 22 24 -27

Cohesion (MN/m2) < 0.0125 - - - 140 -

210Friction

angle, φ - 28 – 34 30 – 40 33 – 50 -

Young’s Modulus, E (MN/m2)

4 - 20 10.5 – 24 17.25 –27.60

34.50 –55.20

30000 -50000




To compare the load-settlement curves generated from PLAXIS-3Dsoftware, analysis was carried out for the profile shown in the previousslide.

The material properties chosen from both the available minimum andmaximum range of values and analyses were carried out.

The load-settlement curves from numerical analyses and that of thefield curve are compared.

Settlement for minimum range – 31.4 mmSettlement for maximum range – 39.9 mmSettlement from field curve – 32.2 mm

From the results it is clear that the sand and rock properties selectedfor the analysis are valid. Further study can be done for other profiles.

Theoretical solution as per Poulos & Davis (1980), gave the settlementvalue as 20 mm, which is lesser than the numerical and field values.

Case Study (Contd..)




Case Study (Contd..)

05

1015202530354045

0 5000 10000 15000 20000 25000 30000 35000 40000 45000

Load (kN)

Settl

emen

t (m

m)

Minimum valuesField curveMaximum values




Sand overlying Rock Stratum Load – Settlement Curves Maximum settlement Vs Rock Socket length

Results pertaining to the influence of the increase in diameter of the pile on the settlement

Results related to the rock – pile interface strength reduction on the pile settlement

From the results the effective rock socket length is proposed for different thickness and different densities of sand overlying the rock.

Presentation of Results




Load – Settlement Curves

0

500

1000

1500

2000

2500

3000

3500

4000

0 2 4 6 8 10 12 14 16settlement (mm)

Axi

al lo

ad (k

N) (Ls/D) = 0

(Ls/D) = 1(Ls/D) = 2(Ls/D) = 3(Ls/D) = 4(Ls/D) = 6(Ls/D) =7(Ls/D)=8

0

500

1000

1500

2000

2500

3000

3500

4000

0 2 4 6 8 10 12 14 16 18 20settlement (mm)

Axi

al lo

ad (k

N)

(Ls/D) = 0(Ls/D) = 1(Ls/D) = 2(Ls/D) = 3(Ls/D) = 4(Ls/D) = 5(Ls/D) = 6(Ls/D) = 7(Ls/D) = 8

Typical load-settlement curve for 5m loose sand overlying weak rock

Typical load-settlement curve for 20m loose sand overlying weak rock





Typical load-settlement curve for 5m loose sand overlying moderately weak rock

Typical load-settlement curve for 20m loose sand overlying moderately weak rock

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 2 4 6 8 10 12settlement (mm)

Axi

al lo

ad (k

N) (Ls/D) = 0

(Ls/D) = 1(Ls/D) = 2(Ls/D) = 3(Ls/D) = 4(Ls/D) = 5(Ls/D) =6(Ls/D)=7(Ls/D)=8

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 2 4 6 8 10 12 14 16 18settlement (mm)

Axi

al lo

ad (k

N) (Ls/D) = 0

(Ls/D) = 1(Ls/D) = 2(Ls/D) = 3(Ls/D) = 4(Ls/D) = 5(Ls/D) = 6(Ls/D) = 7(Ls/D) = 8





Typical load-settlement curve for 5m loose sand overlying moderately hard rock

Typical load-settlement curve for 20m loose sand overlying moderately hard rock

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5settlement (mm)

Axi

al lo

ad (k

N)

(Ls/D) = 0

(Ls/D) = 1

(Ls/D) = 2

(Ls/D) = 4

(Ls/D) = 6

(Ls/D) = 8

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 2 4 6 8 10 12 14

settlement (mm)

Axi

al lo

ad (k

N)

(Ls/D) = 0

(Ls/D) = 1

(Ls/D) = 2

(Ls/D) = 4

(Ls/D) = 6

(Ls/D) = 8





Typical load-settlement curve for 5m loose sand overlying hard rock

Typical load-settlement curve for 20m loose sand overlying hard rock

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

settlement (mm)

Axi

al lo

ad (k

N)

(Ls/D) = 0(Ls/D) = 1(Ls/D) = 2(Ls/D) = 4(Ls/D) = 6(Ls/D) = 8

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 2 4 6 8 10 12 14

settlement (mm)

Axi

al lo

ad (k

N)

(Ls/D) = 0

(Ls/D) = 1

(Ls/D) = 2

(Ls/D) = 4

(Ls/D) = 6

(Ls/D) = 8





Typical load-settlement curve for 5m loose sand overlying moderately weathered rock

Typical load-settlement curve for 20m loose sand overlying moderately weathered rock

0

200

400

600

800

1000

1200

1400

1600

0 2 4 6 8 10 12 14 16 18 20

settlement (mm)

Axi

al lo

ad (k

N)


0200

400600800

10001200

14001600

0 1 2 3 4 5 6 7 8settlement (mm)

Axi

al lo

ad (k

N)

(Ls/d) = 0(Ls/D) = 1(Ls/D) = 2(Ls/D) = 3(Ls/D) = 4(Ls/D) = 5(Ls/D) = 6(Ls/D) = 7(Ls/D) = 8





Typical load-settlement curve for 5m loose sand overlying highly weathered rock

Typical load-settlement curve for 20m loose sand overlying highly weathered rock

0

200

400

600

800

1000

1200

1400

1600

0 2 4 6 8 10 12settlement (mm)

Axi

al lo

ad (k

N) (Ls/D) = 0

(Ls/D) = 1(Ls/D) = 2(Ls/D) = 3(Ls/D) = 4(Ls/D) = 5(Ls/D) = 6(Ls/D) = 7(Ls/D) = 8

0

200

400

600

800

1000

1200

1400

1600

0 1 2 3 4 5 6 7 8settlement (mm)

Axi

al lo

ad (k

N)





Load-Settlement Curves The curves are non-linear for weak rock, moderately weak

rock, weathered rock and highly weathered rocks. The non-linearity of the curves decreases with the increasing overlyingsoil thickness and rock socket length. Also the non-linearitydecreases with the increase in the density of soil i.e. loosesand to dense sand.

The curves are completely linear for moderately hard andhard rocks irrespective of the rock socket length andthickness of the overlying sand layer.

Settlement of the pile decreases with the increasing densityof the sand layer.

Settlement of the pile increases with the increase in thethickness of the overlying soil layer for weak rock, moderatelyweak rock, moderately hard rock and hard rock.

Settlement of the pile decreases with the increase in thethickness of the overlying soil layer for moderately and highlyweathered rocks. This may be due to the higher frictionalresistance mobilized by the weathered rock when compared tothe overlying sand layer.




Load-Settlement Curves Settlement of the pile decreases with the

increasing socket length in weak rock, moderately weak rock and weathered rocks.

There is not much settlement variation for pile in moderately hard and hard rocks. However, the settlement for pile resting on rock is lesser than the settlement at 1D socket. Beyond this socket length the settlement reduces, but the variation is not significant.

Settlement reduction for highly weathered rocks is about 50% from 0D to 1D rock socket length in 5m thick overlying sand for all the densities.

The influence of the rock socket length on settlement decreases with the increase in the overlying soil thickness.




Maximum Settlement Vs Rock Socket Length

Typical curve for Maximum settlement of the pile Vs Rock Socket Length in Loose Sand overlying Weak rock

0

4

8

12

16

20

0 2 4 6 8 10Socket Length / diameter (Ls/d)

Settl

emen

t (m

m) loose sand-5m

loose sand-10m

loose sand-15m

loose sand-20m





Typical curve for Maximum settlement of the pile Vs Rock Socket Length in Loose Sand overlying Moderately Weak rock

02468

1012141618


Settl

emen

t (m

m)

Loose sand-5mLoose sand-10mLoose sand-15mLoose sand-20m





Typical curve for Maximum settlement of the pile Vs Rock Socket Length in Loose Sand overlying Moderately Hard rock

0

2

4

6

8

10

12

14


Settl

emen

t (m

m) Loose sand-5m

Loose sand-10mLoose sand-15mLoose sand-20m





Typical curve for Maximum settlement of the pile Vs Rock Socket Length in Loose Sand Overlying Hard rock

0

2

4

6

8

10

12

14


Settl

emen

t (m

m)






Typical curve for Maximum settlement of the pile Vs Rock Socket Length in Loose Sand overlying Moderately weathered rock

0

2

4

6

8

10

12

0 2 4 6 8 10Socket Length/diameter (Ls/d)

Settl

emen

t (m

m)






Typical curve for Maximum settlement of the pile Vs Rock Socket Length in Loose Sand overlying Highly Weathered rock

0

4

8

12

16

20

0 2 4 6 8 10Socket Length/diameter (Ls/d)

Settl

emen

t (m

m)






To propose the maximum socket length, a threshold valuefor the difference in the settlement ratio is observed.Settlement ratio is the ratio between the settlements fornD socket length to that of the settlement for pile restingon rock, where n varies from 1 to 8. The threshold value isassumed as 0.03 (settlement ratio).

Based on the threshold value for settlement ratio, themaximum rock socket length is observed. This isrepresented as the Effective Rock Socket Depth.

For 5m thick loose sand overlying weak rock, the effectiverock socket length is 4D. Similarly for 10m, 15m and 20mthe effective rock socket length is 4D, 3D and 3Drespectively.

Similarly for different rock types and sand layers,effective rock socket lengths are observed.




Influence of the Diameter of the Pile

Analyses were done for 750mm and 1000mmdiameter pile in medium dense sand overlying weakrock.

0

4

8

12

16

20

500 600 700 800 900 1000 1100Diameter(mm)

Settl

emen

t(mm

)

L/D=0L/D=1L/D=2L/D=3L/D=4L/D=5L/D=6L/D=7L/D=8

Typical Relationship between the Settlement and Diameter of the Pile in 5m Medium dense sand overlying Weak rock




Influence of the Pile Diameter

The increase in the diameter of the pile increases the pile settlement.

The increase in the diameter of the pile does not change the Effective rock socket length required. It is similar to profile for 600mm diameter pile in medium dense sand overlying weak rock.

It is observed that the influence of the diameter of the pile decreases with the increase in the rock socket length and the thickness of the overlying soil.




Influence of Rock-Pile Interface Strength Reduction

Analyses were done for 600mm diameter pile in medium dense sandoverlying weak rock, by reducing the rock-pile interface strengthfrom 25% to 75%.

In PLAXIS 3D, the reduction is represented as R-inter factor.

Typical Relationship between the Settlement and Rock-Pile Interface Strength Reduction factor in 5m thick medium dense sand overlying

weak rock

0

4

8

12

16

0.00 0.20 0.40 0.60 0.80 1.00 1.20

R-inter

Settl

emen

t(mm

)

L/D=0L/D=1L/D=2L/D=4L/D=6L/D=8




Influence of the Rock-Pile Interface Strength

The settlement of the pile increases when the rock-pileinterface strength is reduced.

Interface strength reduction does not alter the effectiverock socket length for the medium dense sand depositoverlying weak rock.

The increment in settlement for reduction in interfaceshear strength for a given socket length is maximum for5m thick overlying soil than 20m thick sand layer.

For higher overburden thickness, the influence ofinterface strength reduction on settlement decreases withincrease in the socket length. This may be due to thelesser load transferred to the rock socket for 20m thickoverlying sand layer when compared to the 5m thickoverlying sand layer.




Influence of the Rock Strata

To understand the effect of the rock strata on socketlength, the load-settlement curves for varyingthickness of overlying sand layer are studied.

Typical Relationship between the Settlement and Rock socket length in 5m thick loose sand overlying rock

0

5

10

15

20

0 2 4 6 8 10

socket length / diameter

Settl

emen

t (m

m) weak rock

mod weak rockmod hard rockhard rockmod weathered rockhigh weathered rock




Influence of the Rock Strata

The settlement of the pile increases with the increase inthe thickness of the overlying soil layer for weak rock,moderately weak rock, moderately hard rock and hardrock.

The variation in settlement is not significant in moderatelyhard and hard rocks.

The settlement of pile decreases with the increase in thethickness of the overlying soil layer for weathered rocks.

Settlement reduces with increase in the socket length forall the rock types, but maximum for highly weathered rock.

In weathered rocks the settlement reduction is higher for1D rock socket length in 5m and 10m thick overlying soil.

Irrespective of the rock type the influence of rock socketlength on settlement decreases, with increase in theoverlying soil thickness




Conclusion




Effective Rock Socket Length for Loose sand / Medium dense sand

Type of RockEffective Rock Socket length for Loose

sand / Medium dense sand

5m 10m 15m 20m

Weak rock 4D 4D 3D 3D

Moderately weak rock 3D 3D 2D 2D

Moderately Hard rock 1D 1D 1D 1D

Hard rock 1D 1D 1D 1D

Moderately weathered rock 3D 3D 2D 2D

Highly weathered rock 4D 4D 3D 2D




Effective Rock Socket Length for Dense sand

Type of RockEffective Rock Socket length for Dense

sand

5m 10m 15m 20m

Weak rock 2D 2D 2D 2D

Moderately weak rock 2D 2D 2D 2D

Moderately Hard rock 1D 1D 1D 1D

Hard rock 1D 1D 1D 1D

Moderately weathered rock 3D 3D 2D 2D

Highly weathered rock 3D 3D 2D 2D




Effective rock socket length required for theprofiles with loose sand and medium dense sand issame.

Effective socket length required for profile withmedium dense sand is greater than the profilewith dense sand as the overlying soil. This may bedue to the higher frictional resistance mobilizedby the dense sand layer, which in turn decreasesthe requirement of the longer socket length.

The increase in the thickness of dense sand layerhas no influence on the effective rock socketdepth in weak rock, moderately weak rock,moderately hard rock and hard rock.

Effective Rock Socket Length




Comparison between IS guidelines and Numerical Analysis results

Rock type

Effective Rock Socket Length required as per

IS:14593-1998

Numerical Analysis results

Loose sand / Medium dense sand Dense sand

5m 10m 15m 20m 5m 10m 15m 20m

Sound, relatively homogenous rock 1D to 2D 1D 1D 1D 1D 1D 1D 1D 1D

Soft rocks, Sedimentary rocks (sandstone)

3D to 4D 4D 4D 3D 3D 2D 2D 2D 2D

Moderately weathered rock 2D to 3D 3D 3D 2D 2D 3D 3D 2D 2D

Highly weathered rock - 4D 4D 3D 2D 3D 3D 2D 2D




Comparison between IS guidelines and Numerical Analysis Results

From the comparison, it is observed that the thicknessof sand overlying sound rock has no influence on theeffective rock socket length. Rock socket length of 1D isfound to be sufficient against the IS: 14593-1998provision of 2D.

The IS provision of 3D to 4D for soft rocks iscomparable with the required rock socket length for thecase of overlying loose sand and medium dense sand.

But for the dense sand overlying soft rock, 2D is foundto be sufficient when compared with the IS provision of3D to 4D.




Comparison between IS guidelines and Numerical Analysis Results

The increase in thickness of the sand layer overlyingmoderately weathered rock reduces the effective rocksocket length from 3D to 2D, irrespective of the densityof the sand.

For Highly weathered rock, there is no suggested rocksocket length in IS:14593-1998. The increase in thedensity of the overlying soil layer decreases theeffective rock socket length from 4D to 3D. And forincreasing sand layer thickness from 5m to 20m, thesocket length decreases from 4D to 2D in loose sand /medium dense sand layer and 3D to 2D in dense sandlayer.

However, the conclusion is based on the numericalanalyses alone. It has to be verified using field testsresults.




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Thank you




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