pile foundation EC7-ex1+2
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Transcript of pile foundation EC7-ex1+2
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Pile foundations
Design approaches according to Eurocode 7
Structural Engineering Master program
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Section 7 Eurocode 7
is addressing to:
type of the load-transfer characteristic for the pile of interest -end-bearing piles (piloi purttori pe vrf), shaft-bearing piles(piloi flotani), piles subjected to tension (piloi solicitai latraciune), and transversaly loaded piles (piloi ncrcaitransversal),
type of the piling technology intended to perform on site -driven, jacking or screw piles (piloi pui n oper prin batere,presare sau nurubare), CFA or bored piles (piloi forai cu neccontinuu sau forare clasic), with or without injections (cu saufr injectri).
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Pile foundation failure issues Failure by reaching one of the following limit states:
ULS Ultimate Limit States Overall stability (pierderea stabilitii generale)
Bearing resistance failure (epuizarea capacitii portante a fundaiei pe piloi)
Up-lift or tensile resistance failure (ridicarea sau rezistena la traciune insuficient a fundaiei pe piloi)
Transverse failure in the ground (cedarea terenului datorit ncrcrii transversale a fundaiei pe piloi)
Structural failure of the pile in compression, tension, bending, buckling or shear (cedarea structural a pilotului la compresiune, ntindere, ncovoiere, flambaj sau for tietoare)
SLS Service Limit States
Excessive settlement (tasare excesiv)
Excessive heave (ridicare excessiv)
Excessive lateral movement (deplasare lateral excesiv)
Vibrations (vibraii inacceptabile)
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pile load tests
When decision over performing pile load tests ismandatory?
A pile type or a piling technology is intended that lackscomparable experience;
Piles are subjected to loads for which both theory andexperience are not providing enough confidence for that specificproject;
During piling, the resulting pile is strongly devianting and totallyunfavorable from the anticipated behaviour.
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Pile resistance - R
can be established from:
Ground test results (in situ tests, laboratory tests),
Pile load tests (Static Load Test SLT, Dynamic Load Test DLT),
Rk,d resistance as characteristic, design value for ULS onaxially loaded piles - compressive or tensileresistance failure;
Rtr,k,d resistance as characteristic, design value for anyULS on transversally loaded piles
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Pile load tests represent valid input data for assessing pile resistances
when subjected to various load configuration;
confirm or disagree the results from the analyticalcalculations;
performed on trial piles, individually;
on one or several piles from the group piles aresubjected to the design load, and potentially increasedup to 30%;
single pile outside the pile group area this pile issubjected to load increments until failure occurs;
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Intended results from pile load tests
Evaluation over the benefits of using a specific pilingtechnology;
To establish the response for a representative pile wheninteracting with the surrounding soil, regarding values ofboth settlement and limit load;
Evaluate the entire pile foundation behaviour.
Decision over the number of piles to be tested, selectionof their location and the appropriate moment to start thetest.
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Static load tests SLTthe bench-mark of pile performance
the direct measurement of pile headdisplacement in the response to a physicallyapplied test load in various configurations(compression, lateral, tension);
http://www.loadtest.co.uk
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axially loaded pile
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laterally loaded
pile
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Dynamic Load Test
30 tons hammer with a fewseparated impacts;
forces and motions are recorded byPile Driving Analyzer (PDA) pilecapacity as pile resistance;
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Pile test single pile
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Pile load test
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Geotechnical restriction
Fd (Ftr,d )
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Characteristic value of compressive or tensile resistance Rk derived from values R (measured values Rm or calculated values Rcal)
Rk = R/, as the correlation factor
Rk = Min {Rm, mean/1 ; Rm,min/ 2}
from static load tests
Rk = R/, as the correlation factor
Rk = Rbk+Rsk = (Rb,cal+Rs,cal)/ = Rcal/ = Min {Rcal, mean/3 ; Rcal,min/ 4}
from ground test results and calculation rules
Rbk = qbk Ab and Rsk = qsik Asian alternative procedure using base resistance
qbk and shaft friction qsik from values of ground parameters
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Characteristic values
from dynamic load tests
dynamic impact tests;
pile driving formulae;
wave equation analysis;
Rk = Min {Rm,mean/5 ; Rm,min/ 6}
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For the ULS, the three possible design approaches
use different sets of partial safety factors:
Design approach 1 with two combinations: for piles: Combination 1: A1 + M1 (= 1) + R1 Combination 2: A2 (= 1) + (M1 (= 1) or M2) + R4 (M1 for pile resistance, M2 for unfavorable actions like negative skin
friction or transversal loads)
Design approach 2: A1 + M1 (= 1) + R2 safety factors on loads and resistances
Design approach 3: A2 (= 1) + M2 + R3 (= 1) (A1 for loads from the structure without influence of soil material
parameters)
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Design values of the pile resistance:
Rd = Rk/t or
Rd = Rbk/ b + Rsk/s
Design value of the applied compression/tension load:
Fd = t Fk
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Design example 1 pile foundationsexample from the workshop on Eurocode 7 Roger Frank, Cermes-ENPC
bored pile, 600mm diameter
ULS compressive resistance failure
design load on an individual pile
Fd
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partial safety factors on Actions - A
Actions A1 A2
Permanent loads, unfavorable 1.35 1.00
Permanent loads, favorable 1.00 1.00
Variable loads, unfavorable 1.50 1.30
Variable loads, favorable 0.00 0.00
DA1 C1 Fd = 1.35 x 1200 + 1,5 x 200 = 1920 kN
DA1 C2 Fd = 1.00 x 1200 + 1,3 x 200 = 1460 kN
DA2 Fd = 1.35 x 1200 + 1,5 x 200 = 1920 kN
DA3 Fd = 1.35 x 1200 + 1,5 x 200 = 1920 kN
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partial safety factors on Materials - M
Materials M1 M2
Angle of internal friction tan() 1.00 1.25
Cohesion c 1.00 1.25
Undrained cohesion cu 1.00 1.40
Unit weight 1.00 1.00
k = 350 d = tan
-1 [(tank)/M] = tan-1[(tan350)/1.25] =
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partial safety factors on Resistances - R
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Characteristic values of qbk and qsk
qbk = v0 x Nq Nq = e
x tantan2(/4 + '/2) v0 = 2 + (L-2)(- w) qsk = h tan = K0 v0 tan = 0.5(1-sin) v0 tan
L = ?
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Design lengths for different Design Approaches
Design Approach pile length (m) OFS = Rk/FkDrainedcondition
DA1 (Combination 1)
DA1 (Combination 2)
DA2
DA3
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Design situation
pile group piles are located to work as single piles (no group effect considered);
precast piles, driven into the ground;
pile diameter 0.4m, pile length 15m;
two pile load tests performed beyond 0.1 pile diameter;
the allowable settlement is 10mm;
actions from the structure above the ground level: permanent vertical load 20,000kN, variable vertical load 5,000 kN
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Geotechnical restriction
Fd
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actions
Fd = k Gk+ Q Qk = k 20,000+ Q 5,000
Actions A1 A2
Permanent loads, unfavorable 1.35 1.00
Permanent loads, favorable 1.00 1.00
Variable loads, unfavorable 1.50 1.30
Variable loads, favorable 0.00 0.00
DA1 C1 Fd = 1.35 x 20,000 + 1.5 x 5,000 = .. kN
DA1 C2 Fd = 1.00 x 20,000 + 1.3 x 5,000 = kN
DA2 Fd = 1.35 x 20,000 + 1.5 x 5,000 = kN
DA3 Fd = 1.35 x 20,000 + 1.5 x 5,000 = kN
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Design example 2
example from the workshop on Eurocode 7 T.L.L. Orr, Trinitiy College, Dublin
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Pile group resistance
Rk = R/, as the correlation factor
Rk = Min {Rm, mean/1 ; Rm,min/ 2}
from static load tests
Rk,single pile =?
Rk,pile group = N x Rk, single pile/R
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For the ULS, the three possible design approaches
use different sets of partial safety factors:
Design approach 1 with two combinations: for piles: Combination 1: A1 + M1 (= 1) + R1 (= 1) Combination 2: A2 (= 1) + (M1 (= 1) or M2) + R4 (M1 for pile resistance, M2 for unfavorable actions like negative skin
friction or transversal loads)
Design approach 2: A1 + M1 (= 1) + R2 safety factors on loads and resistances
Design approach 3: A2 (= 1) + M2 + R3 (= 1) (A1 for loads from the structure without influence of soil material
parameters)
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No. of piles for different Design Approaches
Design Approach ULS no. of piles
DA1 (Combination 1)
DA1 (Combination 2)
DA2
DA3
Design Approach SLS