Post on 30-Jun-2018
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Selected aspects of designing deep excavations
Goal
recall for less experienced modelers
exchange users’ experience
Compare different approaches to serviceability state analysis
simplified (wall rigidity arbitrarily reduced to account for cracking in
concrete)
“true” serviceability control
Effect of analysis type in quasi-impermeable soils
steady-state (dissipated excess of pore water pressure)
consolidation (real time)
Content
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Compare different approaches to serviceability state control
Berlin Sand benchmark ..\ZSoil v2013\Full\HS Model\HS-std-Exc-Berlin-Sand-2phase.inp
Sand I
= 35°
Eur = 180 MPa
Eur /E50 = 4
Sand II
= 38°
Eur = 300 MPa
Eur /E50 = 4
Wall
h = 0.8 m
Impermeable barrier
- 3.0 m
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Simplified serviceability control
Assumptions:
• short term (creep neglected)
• reduced wall stiffness to account for concrete cracking
Results:
• intuitively a good approximation
of wall deflection and settlements
behind the wall
• pitfall:
results of internal forces
(Mk-Nk-Vk) may be carelessly
used to design concrete structures
Compare different approaches to serviceability control
Retaining wall:
Elastic-beam E = 20 GPa
(EC2 values 34 GPa)
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Compare different approaches to serviceability control
Simplified approach
Single FE run Wall model: elastic E=20 GPa Results: 1. Wall deflection & soil deformations 2. Mk = Mk (E=20 GPa)
Design of concrete structure Md = 1.35 · Mk (E=20 GPa) Reinforcement: Ad = Ad(Md)
“True” control approach
1st First FE run Wall model: elastic E=34 GPa Results: Mk = Mk (E=34 GPa)
Design of concrete structure Md = 1.35 · Mk (E=34 GPa) Reinforcement: Ad = Ad(Md)
2nd FE run TRUE CONTROL Wall model: layered beam, elasto-plastic Einit=34 GPa 1. « True » wall deflection & soil deformations
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Compare different approaches to serviceability control
Layered beam model used in
2nd FE run
s-e relationship according
to EC2 for C30
Einit = 34 GPa
fcm = 38 MPa
fctk = 0.1 MPa (neglected)
Es = 210 Gpa
fsd = 500 MPa
fsk = 435 MPa
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Compare different approaches to serviceability control
Hypothesis Simplified model
h d z
xpl
e
As
Ac
─ Contribution of the compressed rebar
is not considered
─ Tension in concrete is neglected
Initial choice : 𝜙 = 30 mm
ℎ = 0.8 m
𝑏 = 1.0 m
𝑒 = 55 mm
Given :
𝒇𝒄𝒅
𝒇𝒔𝒅
𝑨𝒔 = 𝒃 ⋅ 𝒙𝒑𝒍 ⋅ 𝒇𝒄𝒅
𝒇𝒔𝒅
𝑵𝑪 = 𝑨𝒄 ⋅ 𝒇𝒄𝒅 𝑵𝑻 = 𝑨𝒔 ⋅ 𝒇𝒔𝒅 𝑵𝑻 = −𝑵𝑪 = 𝑴𝒅
𝒛
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Compare different approaches to serviceability control
Start
Iterate
𝐴𝑠𝑑 =𝑀𝑑
𝑧 ⋅ 𝑓𝑠𝑑
𝑑 = ℎ − 𝑒 −𝜙
2 𝑧0 = 0.9 ⋅ 𝑑
𝑥𝑝𝑙 =𝑀𝑑
𝑏 ⋅ 𝑧 ⋅ 𝑓𝑐𝑑
𝑧 = 𝑑 −𝑥𝑝𝑙
2
Computation of Ad
Results E20 E34 RE
Asd+ [mm2] 3339 4016 –17%
Asd- [mm2] 1690 1792 –5.7%
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Compare different approaches to serviceability control
Simplified approach
Beam elastic E=20GPa
1st run of “true” control
Beam elastic E=34GPa
Mmax = 757 kNm Mmax = 901 kNm
Mmin =393 kNm Mmin = 416 kNm
Mmax RE = – 16%
Mmin RE = – 5.5%
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Compare different approaches to serviceability control
Simplified approach
Beam elastic E=20GPa
2nd run of “true” control
Beam elasto-plastic Einit=34GPa
uy = 1.89 cm
ux =3.88 cm
ux RE = – 5 %
uy RE = – 5.5 %
ux =4.09 cm
uy = 1.95 cm
1st run for elastic beam E=34 GPa
ux = 3.52 cm ARE = 14 %
uy = 1.75 cm ARE = 10%
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Compare different approaches to serviceability control
Discrepancies might increase with increasing amplitude of
deformations …
Is the careless use of internal efforts for wall designing based on the simplified approach consistent with design codes?
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Effective stress in ZSoil
12
Fluid is considered as a mixture of air and water (single-phase model of fluid)
p – pore water pressure
S = S(p) – degree of saturation depending on pore water pressure
suction stress if p > 0 (above GWT)
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Deformation + flow analysis (soil water retention curve)
Effect of an apparent cohesion can be easily reproduced by coupling:
o constitutive model described by effective parameters
o Darcy’s flow in consolidation analysis
o Soil water retention curve model (van Genuchten’s model)
13
+
-
p
p>0
p<0
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Effect of analysis type in quasi-impermeable soils
Berlin Sand benchmark reused
Sand replaced with impervious clay
Clay I
= 30°
Eur = 90 MPa
Eur /E50 = 4
k = 10-9 m/s
a = 0.1
Clay II
= 32°
Eur = 150 MPa
Eur /E50 = 4
k = 10-9 m/s
a = 0.1
Wall
h = 0.8 m
Impermeable barrier REMOVED
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Effect of analysis type in quasi-impermeable soils
ANALYSIS TYPE
Steady – state
excess of excess pore water pressure is
dissipated at each state (full drainage)
Consolidation
real time analysis
development of partially saturated zones due to
undrained and partially-drained conditions
Positive pore pressures
No suction effect
False GWT lowering when in equilibrium in the case of homogenous hydraulic conductivity
Null pore pressure line
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Effect of analysis type in quasi-impermeable soils
Construction stages
4 excavation steps, each includes:
I. Excavation N – N+20days(unloading over 5 days);
accounting for 3D effect
II. Realisation of prestressed anchors N+20 – N+30
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Effect of analysis type in quasi-impermeable soils
0tot
ijs
kPappSij 20)(' s
Analysis type: Consolidation
Maps: S*p
4 m Thickness of
unsaturated zone
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Effect of analysis type in quasi-impermeable soils
Steady - state
Beam elastic E=34GPa
Consolidation
Beam elastic E=34GPa
Assumed horizontal GWT (auxiliary superficial permeable layer)
Comparison in terms of J2 Strain
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Effect of analysis type in quasi-impermeable soils
Steady - state
Beam elastic E=34GPa
Consolidation
Beam elastic E=34GPa
uy– = 7.8 cm
uy– = 4.6 cm
uy– = 5.1 cm
Instantaneous
After 25 days
uy+
=9.6 cm
uy+
=4.8 cm
uy+
=4.3 cm
Result RE
uy+
+ 100 %
uy– + 70 %
ux + 66 %
ux =11.0 cm ux =6.6 cm
ux =7.2 cm
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Effect of analysis type in quasi-impermeable soils
Steady - state
Beam elastic E=34GPa
Consolidation
Beam elastic E=34GPa
Mmax = 1403 kNm Mmax = 919 kNm
Mmax RE = + 52 %
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Effect of analysis type in quasi-impermeable soils
Steady - state + simplified approach
Beam elastic E=20GPa
Consolidation + “true“ control
Beam elasto-plastic E=34 GPa
Ad=Ad(Mmax)
SERVICABILITY CONTROL
uy– = 8.2 cm
uy+
=10. cm
ux =11.8 cm uy
– = 5.3 cm
uy+
=4.9 cm
ux =7.6 cm
Result RE
uy+
+ 104 %
uy– + 55 %
ux + 55 %
Aspects of designing deep excavations
S. Hartmann, R. Obrzud, GeoMod SA
28.08.2015, Lausanne, Switzerland
Effect of analysis type in quasi-impermeable soils
Discrepancies are difficult to foresee due to a highly nonlinear
nature of the problem
In this example, the steady-state approach gives a superior
confidence limit for internal efforts with respect to consolidation
analysis
Is the “steady-state shortcut” profitable in the light of an optimal design?