Guide on Determination of Characteristic Value and CP4 vs EC7 in Bored Pile Design
Dr T G Ng Golder Associates (Singapore) Pte Ltd
GeoSS
GEOTECHNICAL SOCIETY OF SINGAPORE (GeoSS)
SCOPE OF PRESENTATION 1. Introduction 2. Geote Geotech chni nica call param paramet eter erss and char character acteris isti ticc values in EC7 3. CP4 CP4 vs vs EC7 EC7 in Desi Design gn of Bor Bored Pile Pile I nvestigation tigation Site Inves Structural Design Geotechnical Design Load Test 4. Conclusion
SCOPE OF PRESENTATION 1. Introduction 2. Geote Geotech chni nica call param paramet eter erss and char character acteris isti ticc values in EC7 3. CP4 CP4 vs vs EC7 EC7 in Desi Design gn of Bor Bored Pile Pile I nvestigation tigation Site Inves Structural Design Geotechnical Design Load Test 4. Conclusion
INTRODUCTION
Introduction: Distinction between Principles and Application Rules •
•
C1.4(1) Distinction is made between Principles 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
Introduction: Distinction between Principles and Application Rules •
•
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.
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
GEOTECHNICAL PARAMETERS From ground investigations and lab tests s e u l a v d e v i r e D c i t s i r e t c s a e r u a l h a C v n s g i e u s l e a D v
GEOTECHNICAL PARAMETERS SPT N values
s e u l a v d e v i r e D c i t s i r e t c s a e r u a l h a C v n s g i e u s l e a D v
cu=5N
From ground investigations and lab tests
GEOTECHNICAL PARAMETERS SPT N values
s e u l a v d e v i r e D c i t s i r e t c s a e r u a l h a C v n s g i e u s l e a D v
From ground investigations and lab tests
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
’
Effective angle of shearing resistance
c’
Effective cohesion value
cu
Undrained shear strength
N
SPT N values
qc
CPT qc values
CHARACTERISTIC VALUE 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.
CHARACTERISTIC VALUE 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.
CHARACTERISTIC VALUE •
•
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
Characteristic Values by Statistical Method Schneider(1999) Method Xk = mx - 0.5sX (upper bound equivalent to 95% mean reliable) X k = m x – 1.65s X (lower bound equivalent to low value 5% fractile)
where
Ck = characteristic value
mC = mean value sX = standard deviation n = number of samples
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
GeoSS EC7 Guide Table 2.2
(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.
SS EN 1997-2 Annex B
(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.
CP4 vs EC7 in Design of Bored Pile – Design (Structural) Structural Working Load CP4 Allowable concrete compressive stress, c = 0.25 f cu < 7.5MPa
Pile working load, Q st = c . Ac
EC7 SS EN 1992-1: NRd,p = Acf cd,p > NEd = 1.35Gk + 1.5Q k f cd,p = αcc,p f ck/ 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 f ck = 0.8 f cu Reinforced NRd,p = Ac x 0.412 x f cu Un-Reinforced NRd,p = Ac x 0.291 x f cu cast in place piles without permanent casing. Ac should be taken as: - if d nom < 400 mm d = d nom - 20 mm - if 400 ≤ d nom ≤ 1000 mm d = 0.95.d nom - if d nom > 1000 mm d = d nom - 50 mm
CP4 vs EC7 in Design of Bored Pile – Design (Structural) Structural Working Load Case 1: f cu = 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
Case 2: f cu = 40MPa
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
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
CP4 vs EC7 in Design of Bored Pile – Design (Structural) Example for 1000mm dia. pile Case 1: f cu = 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
Case 2: f cu = 40MPa
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
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
CP4 vs EC7 in Design of Bored Pile – Design (Structural) Min. area of longitudinal reinforcement CP4 As ≥ 0.5% Ac
EC7 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.
CP4 vs EC7 in Design of Bored Pile – Design (Structural)
Clear distance between bars should not exceed 200 mm measured along the periphery of the pile.
CP4 vs EC7 in Design of Bored Pile – Design (Structural) Min. area of longitudinal reinforcement 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
CP4 vs EC7 in Design of Bored Pile – Design (Structural) Min. area of longitudinal reinforcement 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
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)
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
Qa1 Qa2 Qa3 Qa Qa
Where, Qs Qb Qa WL DL LL
= = = = >
-
Qs/2.5 + Qb/2.5 Qs/2 + Qb/3 Qs/1.5 Min (Qa1, Qa2, Qa3) WL = = (DL + LL)
Ultimate Total Skin Friction Resistance Ultimate End Bearing Capacity Allowable geotechnical capacity Working load Dead load Live load
CP4 vs EC7 in Design of Bored Pile – Design (Geotechnical) Current Practice
Qt Qs Qb
Qt Qs Qb
= = =
= = =
Qs + Qb 0.6 Qt 0.4 Qt
Qs + Qb 0.4 Qt 0.6 Qt
Qa1 Qa2 Qa3 Qa Qa
= = = = >
Qs/2.5 + Qb/2.5 Qs/2 + Qb/3 Qs/1.5 Min (Qa1, Qa2, Qa3) WL = = (DL + LL)
Qa (1) Qa (1) Qt
= = =
0.24 Qt 0.4 Qt 2.5 Qa
+
0.16 Qt
Qa (2) Qa (2) Qt
= = =
0.3 Qt 0.43 Qt 2.31 Qa
+
0.133 Qt
Qa (1) Qa (1) Qt
= = =
0.16 Qt 0.4 Qt 2.5 Qa
+
0.24 Qt
Qa (2) Qa (2) Qt
= = =
0.2 Qt 0.40 Qt 2.50 Qa
+
0.200 Qt
CP4 vs EC7 in Design of Bored Pile – Design (Geotechnical) Qa1 Qa2 Qa3 Qa Qa
= = = = >
Qs/2.5 + Qb/2.5 Qs/2 + Qb/3 Qs/1.5 Min (Qa1, Qa2, Qa3) WL = = (DL + LL)
Qt Qs Qb Qt(1) Qt(2)
= = = = =
Qs + Qb 0.4 Qt 0.6 Qt 2.5 Qa 2.5 Qa
Qs Qb Qt(1) Qt(2)
= = = =
0.6 0.4 2.5 2.31
Qt Qt Qa Qa
CP4 vs EC7 in Design of Bored Pile – Design (Geotechnical) EC7 Alternative Method – Model Factor With ULT
Without ULT
; =
; =
; 1.2 ∗ ; 1.4 ∗
+
+
; 1.2 ∗ ; 1.4 ∗
EC7 Alternative Method – Partial Resistance Factor With Pile Load Test
Without Pile Load Test
• •
; =
; =
; 1.2 ∗ ; 1.4 ∗
+
+
; 1.2 ∗ ; 1.4 ∗
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) • •
CP4 vs EC7 in Design of Bored Pile – Design (Geotechnical) • •
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) • •
CP4 vs EC7 in Design of Bored Pile – Design (Geotechnical) Design values of actions, Fd Fd = gG Gk + gQ Q k where gG and gQ are partial factor Generally, DA1-1 higher factor DA1-2, lower factor •
• •
CP4 vs EC7 in Design of Bored Pile – Design (Geotechnical) EC7 Alternative Method Fcd (ACTION) Assume: Dead Load (DL) =
x Column Load (CL)
Live Load (LL) = (1- ) 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
Hence, Fcd ≥ R cd
; + ; − ; = + (1 −) = = + − + − ;
;
= ; + ; − ; − +
;
2
CP4 vs EC7 in Design of Bored Pile – Design (Geotechnical) EC7 Alternative Method vs CP4
CP4 vs EC7 in Design of Bored Pile – Design (Geotechnical) 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 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
EC7 (Alternative Method) 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, 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,…
CP4 vs EC7 in Design of Bored Pile – Load Test Current Practice
EC7 (Alternative Method)
Allowable settlement
Representative load
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.
Suggestion 1 SLS load = 1.0 G k + 1.0 Q k Allowable settlement follows CP4
> The load test does not affect the FOS on geotechnical capacity or Design Zoning
Suggestion 2 Follow DA1-2, Fd = 1.0 Gk + 1.3 Q k Allowable settlement adjust accordingly Maximum test load and allowable settlement shall be specified clearly on the drawing. Load test affect geotechnical design
CP4 vs EC7 in Design of Bored Pile – Load Test Design Zoning by Ref BH
How ULT & WLT affect the MF & R4 for each design zone?
CP4 vs EC7 in Design of Bored Pile – Load Test Option 1
CP4 vs EC7 in Design of Bored Pile – Load Test Option 2 ZONE A
ZONE B
NO WLT
1.5xWLT
Less Favourable Resistance Factors (R4)
More Favourable Resistance Factors(R4)
MF = 1.2
MF = 1.2 ULT
1.5xWLT
NO WLT
More Favourable Resistance Factors(R4)
Less Favourable Resistance Factors (R4)
MF = 1.2
MF = 1.2
ZONE C (worst profile of same geological formation)
ZONE D
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
REFERENCES
REFERENCES
http://eurocodes.jrc.ec.europa.eu/
