07-Cp4 vs Ec7 (Dr t g Ng)

GeoSS GEOTECHNICAL SOCIETY

OF SINGAPORE (GeoSS)

Dr T G Ng Golder Associates (Singapore) Pte Ltd

Guide on Determination of Characteristic Value and CP4 vs EC7 in Bored Pile Design

SCOPE OF PRESENTATION

1. Introduction

2. Geotechnical parameters and characteristic values in EC7

3. CP4 vs EC7 in Design of Bored Pile Site Investigation Structural Design Geotechnical Design Load Test

4. Conclusion

INTRODUCTION

Introduction: Distinction between Principles and Application Rules

C1.4(1) Distinction is made between Principles and Application Rules, depending on the character of the individual clauses

C1.4(2) The Principles comprises:

General statements and definitions for which there is no alternative

Requirements and analytical models for which no alternative is permitted unless specifically stated

C1.4(3) The Principles are preceded by the Letter P

C1.4(4) The Application Rules are examples of generally recognised rules, which follow the Principles and satisfy their requirements.

C1.4(5) It is permissible to use alternatives to the Application Rules given in this standard, provided it is shown that the alternative rules accord with relevant Principles and are at least equivalent with regard to the structural safety, serviceability and durability, which would be expected when using the Eurocodes.

Introduction: Distinction between Principles and Application Rules

Distinction between Principles and Application Rules (SS EN 1997-1: 2010)

Eurocode 7 : Geotechnical design

Designers are responsible to ensure structural safety, serviceability and durability of the designs.

Designers are responsible for the planning of the geotechnical investigation

Designers are accountable for their decisions, i.e. specification of field and laboratory tests, determination geotechnical design parameters and characteristic values etc.

2 briefing/dialogue sessions were held in Nov 2014 to raise awareness to the designers on key aspects on geotechnical investigations and recommendations on how to determinate characteristic values

GeoSS EC7 Work Group

GeoSS Site Investigation Task Force

Chairman: Seh Chong Peng

Members: Poh Chee Kuan, Kiefer Chiam, Kyaw Kyaw Zin, Dr M. Karthieyan, Dr Cai Jun Gang, Akira Wada, Arturo Taclob, Suresh Kumar, Gao JianSheng, Kevin Quan, Khin Latt, Kyi Yu, Cheong Kok Leong, James Tsu, Aung Moe, Tan Yong Beng, Ariff

DETERMINATION OF GEOTECHNICAL PARAMETERS AND CHARACTERISTIC VALUES

From ground investigations and lab tests

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GEOTECHNICAL PARAMETERS


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SPT N values

cu=5N


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SPT N values

cu=5N

How to obtain characteristic values?

CHARACTERISTIC VALUE

EN 1997-1 C2.4.5.2(2)P defines the characteristic value as being selected as cautious estimate of the value affecting the occurrence of the limit state

Each word and phrase in this clause is important: Selected emphasizes the importance of engineering

judgement Cautious estimate some conservatism is required Limit state the selected value must relate to the limit state

(failure mechanism) Applicable geotechnical parameters from GeoSS EC7 Guide:

Applicable Geotechnical Parameters

tan j Effective angle of shearing resistance

c Effective cohesion value

cu Undrained shear strength

N SPT N values

qc CPT qc values


SS EN1997-1 Clause 2.4.5.2(4)P states, the selection of characteristic values for geotechnical parameters shall take account of the following: geological and other background information, such as data

from previous projects; the variability of the measured property values and other

relevant information, e.g. from existing knowledge; the extent of the field and laboratory investigation; the type and number of samples; the extent of the zone of ground governing the behaviour of

the geotechnical structure at the limit state being considered; the ability of the geotechnical structure to transfer loads

from weak to strong zones in the ground.


SS EN1997-1 Clause 2.4.5.2(10) suggested statistical methods to determine characteristic ground values. When applying statistical methods, the designer should consider the following: adequacy and quality of geotechnical investigations distribution of sampling/testing highly variable non-conforming nature of geomaterials allowing the use of a priori knowledge of comparable ground

properties, applying engineering judgement.


For most limit state cases where the soil volume involved is large, the characteristic value should be derived such that a cautious estimate of the mean value is a selection of the mean value of the limited set of geotechnical parameter values, with a confidence level of 95% (moderately conservative parameters); where local failure is concerned, a cautious estimate of the low value is a 5% fractile (inferior parameters).

Examples of aplication using statistical methods are available in Annex E and Annex F of the GeoSS EC7 Guide

Xk = mx - 0.5sX (upper bound equivalent to 95% mean reliable) Xk = mx 1.65sX (lower bound equivalent to low value 5% fractile) where

Schneider(1999) Method

Ck = characteristic value

mC = mean value

sX = standard deviation

n = number of samples

Characteristic Values by Statistical Method

CP4 (Current Practice) vs EC7 in Design of Bored Pile

CP4 vs EC7 in Design of Bored Pile

Site Investigation Design Structural Geotechnical

Load Test

CP4 vs EC7 in Design of Bored Pile Site Investigation

Current Practice EC7

BCA /IES /ACES ADVISORY NOTE 1/03 (a) The number of boreholes should be

the greater of (i) one borehole per 300 sq m or (ii) one borehole at every interval

between 10m to 30m, but no less than 3 boreholes in a project site.

(b) Boreholes should go more than 5

metres into hard stratum with SPT blow counts of 100 or more than 3 times the pile diameters beyond the intended founding level.

GeoSS EC7 Guide Table 2.2 SS EN 1997-2 Annex B

CP4 vs EC7 in Design of Bored Pile Design (Structural)

CP4 EC7

Allowable concrete compressive stress, sc = 0.25 fcu < 7.5MPa Pile working load, Qst = sc . Ac

SS EN 1992-1: NRd,p = Acfcd,p > NEd = 1.35Gk + 1.5Qk fcd,p = cc,p fck/ gc,f acc,p= 0.85 (reinforced); acc,p= 0.60 (un-reinforced) gc,f = gc x kf = 1.5 x 1.1 = 1.65 fck = 0.8 fcu Reinforced NRd,p = Ac x 0.412 x fcu Un-Reinforced NRd,p = Ac x 0.291 x fcu cast in place piles without permanent casing. Ac should be taken as: - if dnom < 400 mm d = dnom - 20 mm - if 400 dnom 1000 mm d = 0.95.dnom - if dnom > 1000 mm d = dnom - 50 mm

Structural Working Load

Case 1: fcu = 35MPa EC7 (Factored capacity, NRd,p)

EC7 (Service load) Avg. Load Factor = 1.4

WL by CP4

dnom Anom d Ac Reinfored Un-Reinf Reinfored Un-Reinf sc = 7.5MPa

(mm) (m2) (mm) (m2) (kN) (kN) (kN) (kN) (kN)

800 0.503 760 0.454 6543 4619 4674 3299 3770

900 0.636 855 0.574 8282 5846 5915 4176 4771

1000 0.785 950 0.709 10224 7217 7303 5155 5890

1100 0.950 1050 0.866 12490 8816 8921 6297 7127

1200 1.131 1150 1.039 14982 10576 10702 7554 8482

1300 1.327 1250 1.227 17701 12495 12644 8925 9955



WL by CP4



800 0.503 760 0.454 7478 5279 5342 3771 3770

900 0.636 855 0.574 9465 6681 6761 4772 4771

1000 0.785 950 0.709 11685 8248 8346 5892 5890

1100 0.950 1050 0.866 14274 10076 10196 7197 7127

1200 1.131 1150 1.039 17123 12087 12230 8633 8482

1300 1.327 1250 1.227 20230 14280 14450 10200 9955

Structural Working Load




WL by CP4



800 0.503 760 0.454 6543 4619 4674 3299 3770

900 0.636 855 0.574 8282 5846 5915 4176 4771

1000 0.785 950 0.709 10224 7217 7303 5155 5890

1100 0.950 1050 0.866 12490 8816 8921 6297 7127

1200 1.131 1150 1.039 14982 10576 10702 7554 8482

1300 1.327 1250 1.227 17701 12495 12644 8925 9955



WL by CP4



800 0.503 760 0.454 7478 5279 5342 3771 3770

900 0.636 855 0.574 9465 6681 6761 4772 4771

1000 0.785 950 0.709 11685 8248 8346 5892 5890

1100 0.950 1050 0.866 14274 10076 10196 7197 7127

1200 1.131 1150 1.039 17123 12087 12230 8633 8482

1300 1.327 1250 1.227 20230 14280 14450 10200 9955

Example for 1000mm dia. pile



CP4 EC7

As 0.5% Ac SS EN 1992-1: 9.8.5(3)

Arrangement of reinforcements to allow free flow of

concrete. Min. diameter for longitudinal bars not be less than 16

mm. At least 6 longitudinal bars. Clear distance between bars should not exceed 200

mm measured along the periphery of the pile.

Min. area of longitudinal reinforcement


Clear distance between bars

should not exceed

200 mm

measured along

the periphery of

the pile.



dnom Anom d Ac Dia no of As As/Ac As/Anom Clear spacing at

(mm) (m2) (mm) (m2) (mm) rebar (cm2) (%) (%) periphery of pile (mm)

1000 0.785 950 0.709 16 13 26.1 0.37% 0.33% 222

1000 0.785 950 0.709 16 14 28.1 0.40% 0.36% 205

1000 0.785 950 0.709 16 15 30.2 0.43% 0.38% 190





1000 0.785 950 0.709 16 13 26.1 0.37% 0.33% 222

1000 0.785 950 0.709 16 14 28.1 0.40% 0.36% 205

1000 0.785 950 0.709 16 15 30.2 0.43% 0.38% 190



800 0.503 760 0.454 16 13 26.1 0.58% 0.52% 172

900 0.636 855 0.574 16 13 26.1 0.46% 0.41% 197

1000 0.785 950 0.709 16 15 30.2 0.43% 0.38% 190

1100 0.950 1050 0.866 16 16 32.2 0.37% 0.34% 197

1200 1.131 1150 1.039 16 18 36.2 0.35% 0.32% 191

1300 1.327 1250 1.227 16 19 38.2 0.31% 0.29% 196

CP4 vs EC7 in Design of Bored Pile Design (Geotechnical)

Current Practice

Where, Qs - Ultimate Total Skin Friction Resistance Qb - Ultimate End Bearing Capacity Qa - Allowable geotechnical capacity WL - Working load DL - Dead load LL - Live load

Qa1 = Qs/2.5 + Qb/2.5

Qa2 = Qs/2 + Qb/3

Qa3 = Qs/1.5

Qa = Min (Qa1, Qa2, Qa3)

Qa > WL = = (DL + LL)


Current Practice Qa1 = Qs/2.5 + Qb/2.5Qa2 = Qs/2 + Qb/3

Qa3 = Qs/1.5


Qa > WL = = (DL + LL)

Qt = Qs + Qb

Qs = 0.6 Qt

Qb = 0.4 Qt

Qa (1) = 0.24 Qt + 0.16 Qt

Qa (1) = 0.4 Qt

Qt = 2.5 Qa

Qa (2) = 0.3 Qt + 0.133 Qt

Qa (2) = 0.43 Qt

Qt = 2.31 Qa

Qt = Qs + Qb

Qs = 0.4 Qt

Qb = 0.6 Qt

Qa (1) = 0.16 Qt + 0.24 Qt

Qa (1) = 0.4 Qt

Qt = 2.5 Qa

Qa (2) = 0.2 Qt + 0.200 Qt

Qa (2) = 0.40 Qt

Qt = 2.50 Qa


Qt = Qs + Qb

Qs = 0.4 Qt

Qb = 0.6 Qt

Qt(1) = 2.5 Qa

Qt(2) = 2.5 Qa

Qs = 0.6 Qt

Qb = 0.4 Qt

Qt(1) = 2.5 Qa

Qt(2) = 2.31 Qa

Qa1 = Qs/2.5 + Qb/2.5

Qa2 = Qs/2 + Qb/3

Qa3 = Qs/1.5


Qa > WL = = (DL + LL)


EC7 Alternative Method Model Factor

With ULT ; =

;1.2

+;

1.2

Without ULT ; =

;1.4

+;

1.4

With Pile Load Test ; =

;1.2

+;

1.2

Without Pile Load Test ; =

;1.4

+;

1.4

EC7 Alternative Method Partial Resistance Factor

gb and gs depends on which approach. Generally,

DA1-1, no factor on resistance (factor =1) DA1-2, some factors on resistance (refer Table A.NA.7)


gb and gs depends on which approach. Generally,

DA1-1, no factor on resistance (factor =1) DA1-2, some factors on resistance (refer Table A.NA.7)


Design values of actions, Fd

Fd = gG Gk + gQ Qk

where gG and gQ are partial factor Generally,

DA1-1 higher factor DA1-2, lower factor


Fcd (ACTION)

Assume:

Dead Load (DL) = a x Column Load (CL)

Live Load (LL) = (1-a) x CL

So:

Fcd = gG;dst x DL + gQ;dst x LL

Fcd = gG;dst x a x CL + gQ;dst x (1-a) x CL

Fcd = ( gQ;dst + (gG;dst - gQ;dst) x a ) x CL

EC7 Alternative Method

Hence,

Fcd Rcd

; + ; ; =

+

(1)

=

= ; + ; ;

2

+

= ; + ; ;

+


EC7 Alternative Method vs CP4


EC7 Alternative Method vs CP4

without ULT, MF = 1.4; With ULT, MF=1.2 without WLT, higher R4 factor; with WLT, lower R4 factor

CP4 vs EC7 in Design of Bored Pile Load Test

Current Practice EC7 (Alternative Method)

BCA /IES /ACES ADVISORY NOTE 1/03 (a) ULT - 1 number or 0.5% of the total

piles, whichever is greater. (b) WLT - 2 numbers or 1% of working

piles installed or 1 for every 50 metres length of proposed building, whichever is greater.

(c) Non-destructive integrity test - 2 numbers or 2% of working piles installed, whichever is greater.

CP4 proof loads, usually 2x Pile design load, in certain conditions proof load of 1.5x may be used. The number of piles to be tested usually 1% to 2% of the working piles

NA to SS EN 1997-12010 A.3.3.2 - The value of the model factor should be 1.4, except that it may be reduced to 1.2 if the resistance is verified by a maintained load test taken to the calculated, unfactored ultimate resistance. - The lower partial resistance factor, g in R4 may be adopted (a) if serviceability is verified by load tests (preliminary and/or working) carried out on more than 1% of the constructed piles to loads not less than 1.5 times the representative load for which they are designed,


Current Practice EC7 (Alternative Method)

Allowable settlement CP4 7.5.4.4 For working pile load test for which the pile is usually tested to 1.5 to 2.0 times working load, the allowable maximum settlement measured at the pile top under full test load is generally taken as 15mm or 25mm respectively. > The load test does not affect the FOS on geotechnical capacity or Design Zoning

Representative load Suggestion 1

SLS load = 1.0 Gk + 1.0 Qk Allowable settlement follows CP4 Suggestion 2 Follow DA1-2,

Fd = 1.0 Gk + 1.3 Qk Allowable settlement adjust accordingly Maximum test load and allowable

settlement shall be specified clearly on the drawing.

Load test affect geotechnical design


Design Zoning by Ref BH

How ULT & WLT affect the MF & R4 for each design zone?


Option 1


Option 2

NO WLT 1.5xWLT

1.5xWLT

NO WLT

ZONE A ZONE B

ZONE C (worst profile of same geological formation) ZONE D

Less Favourable Resistance Factors (R4)

MF = 1.2

More Favourable Resistance Factors(R4)

MF = 1.2

More Favourable Resistance Factors(R4)

MF = 1.2

Less Favourable Resistance Factors (R4)

MF = 1.2

ULT

CONCLUSION

CONCLUSION

The 1st Principle - Designers are responsible to ensure structural safety, serviceability and durability of the designs for the structures.

To fulfil the 1st Principle, Designers are responsible for the planning of the geotechnical investigation which include Preliminary, Design and Control Investigations

Guidelines and recommendations in Informative Annexes are available in EC7-1 and EC7-2 for reference by Designers to decide on specifications of field and laboratory tests, no of BH, field and lab tests etc

Characteristic values shall be determined from derived values for design purposes.

Guidelines on GI and Methods to determine Characteristic values are provided in GeoSS EC7 Guide

CONCLUSION

Structural Design o Allowable concrete compressive stress of 7.5MPa and

As>0.5%Ac in CP4 has been removed. o More comprehensive design considerations in terms of partial

load factors on geometry, material, reinforcement spacing, permanent casing etc shall be taken.

o Structural capacity varies for reinforced and un-reinforced concrete section.

Geotechnical Design o Alternative method is closer to current design practice o The geotechnical design is governed by DA1-2 o The quantity and allowable settlement for ULT and WLT remain

the same as current practice o With comprehensive ULT and WLT, proper GI and determination

of characteristic values, EC7 generally resulting in more economical design as compared with CP4

REFERENCES

http://eurocodes.jrc.ec.europa.eu/

REFERENCES

THANK YOU NG Tiong Guan Executive Director/Principal Golder Associates (Singapore) Pte Ltd 18 Ah Hood Road, #10-51, Hiap Hoe Building @ Zhongshan Park, Singapore 329983 T: +65 6546 6318 | D: +65 6885 9388 | M: +65 9797 6846 | E: [email protected] | www.golder.com

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07-Cp4 vs Ec7 (Dr t g Ng)

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