Analysis of inuence of creep and damage on stress resultants and deformations in diaphragm walls

Transcription

1 Analysis of inuence of creep and damage on stress resultants and deformations in diaphragm walls Andrzej Truty & Krzysztof Podle± Cracow University of Technology ZACE Services Ltd Lausanne / 19

2 Motivation Typical design approach 1 Assume that structure is elastic 2 Use sophisticated models for subsoil (HSs) 3 Apply partial loading coecient to the resulting internal forces (γ G = 1.35 for instance) 4 Dimension reinforcement basing on modied values of internal forces 5 This way the serviceability limit state is not suciently reproduced as creep and cracking are not taken into account (in the EC2 for isolated members one may use the nominal stiness or nominal curvature approach) Ref. ZSoil Day (2015) Hartmann, Obrzud (2015), Obrzud, Hartmann and Podle± (Studia Geotechnica 2016) 2 / 19

3 Consistent FEA approach This aspect was already discussed by Obrzud and Hartman (2015) Simplied approach (EC2) like nominal stiness/curvature is dedicated to isolated members Diaphragm wall should not be treated in this manner Nonlinear soil and elastic structure safe estimation of internal forces For deep excavations the above approach may yield very high values of bending moments In several cases doubled bars must be placed in the cross section Question: is this amount of reinforcement really needed? 3 / 19

4 Design criteria 1 Ultimate limit state (easy to check with FEA) 2 Serviceability limit states Deections + limited rigid body movements (?) (easy with FEA) Crack opening (very important!) (requires extra postprocessing on FEA results using EC2 standard) Crack opening is induced by strain dierence in concrete and steel In standard beam/shell models strain compatibility is assumed 4 / 19

5 Verication of the initial design by fully consistent analysis 1 Soil is nonlinear (HSs model is used) 2 Structure is nonlinear (ZSoil version of Lee Fenves model is used including EC2 creep) 3 Reinforcement is added basing on the initial design 4 Consolidation model is used (sensitivity to the seepage coecient must be checked!) 5 After completing the analysis design criteria are checked at each point of the wall 6 Here optimization can be run 5 / 19

6 Checking the ultimate limit state (elastic/inelastic) Coupled action of M and N forces In diaphragms e = M/N is usually large Therefore only M = γ G M while Ñ = N 6 / 19

7 Some aspects concerning subsoil modeling In situ stress conditions (OCR/q POP approach) p vs σ 3 stress dependency in HSs model 1 σ 3 stress dependency is applicable only to cohesive soils 2 p stress dependency must be used for sands (available as custom in ZSoil) Sensitivity to the uncertainty of seepage coecient (consolidation model) 7 / 19

8 Some aspects concerning subsoil modeling OCR(z) = σ z(z) + q POP σ z(z) In most cases oedometer test is not reliable to assess preconsolidation stress SDMT test is preferable 8 / 19

9 Why SDMT?: modeling 3D DMT test Updated Lagrangian is used (ALE is not needed) Size of the FE model has to be as small as possible (large CPU time) K o = 0.5, σ o,v = 100 kpa, Eur ref = kpa, m = 0.5, E ref 50 = kpa, Eoed ref = kpa, OCR = 1, φ = 30 o, ψ = 5 o 9 / 19

10 Why SDMT?: Animation of DMT test Animation video 10 / 19

11 Why SDMT?: Pressure-deection curve I D 6, K D 3, E D kpa M DMT kpa E ur = kpa E ur,insitu kpa 11 / 19

12 Case study 2 rows of prestressed anchors 17m long (spacing 1.6m in Z-direction) (prestress forces 750 kn / 650 kn) Diaphragm wall panel width 6.5m Excavation depth 16m Subsoil: overconsonsolidated quaternary silty clays (Warsaw, Poland) 12 / 19

13 Case study Installation procedure is not considered here 2 models are analyzed (elastic structure / inelastic+creep) Reinforcement 50cm 2 /m internal face, 30 cm 2 /m external face 13 / 19

14 Envelopes of bending moments just after completing excavation Elastic structure Inelastic structure 14 / 19

15 Envelopes of bending moments 30 days after completing excavation Elastic structure Inelastic structure 15 / 19

16 Envelopes of bending moments 100 days after completing excavation Elastic structure Inelastic structure 16 / 19

17 Wall deections Elastic structure Inelastic structure 17 / 19

18 Mismatch between measurement and prediction for Berlin wall 18 / 19

19 Conclusions Nonlinear soil - elastic structure conservative design There is still some margin of safety if the fully consistent modeling is used (cracks must be under control) Larger cracks will be localized in small parts of the structure (here extra reinforcement can be added) Cracking in the structure changes stiness in adjacent subsoil zones Real soil testing (SDMT + triax) recommended 19 / 19