Upload
buibao
View
218
Download
0
Embed Size (px)
Citation preview
DGF-2016-10-27
Interpretation of Limestone / Chalk Parameters
Applicable to Foundation Design
L Kellezi
P.B. Hansen & O. KoretaDGF: 2017-10-27
www.geo.dk
When you ask GeoMan:
• Limestone: General name for a vide variety of sedimentary rocks, composed primarily of calcium carbonate
• Chalk: A variety of limestone, composed primarily of shells of single-celled, calcium carbonate secreting creatures
• Site Investigation Data – Case History
• Estimation of Hoek-Brown (HB) Classification Rock Parameters
• Conversion of HB Rock Parameters into Mohr-Coulomb (MC) Rock Mass Parameters
• Rock Data Software
• Palmstrøm Jv Index
• Axial Bearing Capacity of Sheet Piles Installed in Chalk
• Conventional Bearing Capacity
• 2D FE Analysis
• Conclusions & Recommendations
Presentation Outline
BH-1 core drilling
www.geo.dk
0.0 m
12.9 m
1.3 m
19.5 m
Fill (Sand)
Clay till /
moræneler
Limestone /
kalksten
Overview of Site AreaSite Investigation Data
Limestone / chalk as an engineering material, has been studied and discussed since 1965,
when ICE organized the first symposium Chalk in Earthworks and Foundations.
www.geo.dk
RQD (Rock Quality Designation) =21 %, TCR =74%
RQD =10 %, TCR (Total Core Recovery)=67%
RQD =20 %, TCR=67%
RQD =0 %, TCR=77%
Site Investigation Data
High degree of fractures (S4 – S5) -strongly fractured limestone, factures <6 cm
Degree of induration (H2-H4)-slightly to strongly indurated
limestone.
www.geo.dk
• Hoek-Brown Generalized Failure Criterion
HB Failure Criterion in Principal Stresses
Non-Linear Relationship
HB Failure Criterion in space
Rock may show also significant
Tensile strength (σt)
MC Failure Criterion in space
Linear-Elastic
Perfectly Plastic
• Latest HB Update - 2002 Edition
Generalized HB Criterion
Material constant
Constant for the rock mass
Constant for the rock mass
“D” – Factor depending.on degree of disturbance
Estimation of Hoek-Brown (HB) Rock Classification Parameters
Estimation of Hoek-Brown (HB) Classification Parameters
www.geo.dk
• Uniaxial Compressive Strength of the Intact Rock (quc) (σci)
• Geological Strength Index (GSI)
• Material Constant for the Intact Rock (mi)
• Disturbance Factor (D)
• Rock Mass Modulus (Em (Erm) / Intact Rock Modulus (Ei)
Hoek-Brown Classification Parameters
Lower Bound (LB)
Upper Bound (UB)
Best Estimate (BE)
Different scenarios investigated through parameter studies
Following parameters have been considered:
Final HB Parameters
www.geo.dk
• Uniaxial Compressive Strength of the Intact Rock (quc) & Point Load Strength Index Is50
The correlation show: quc / Is50 = (15-20)
Degree of Induration – H (Danish Class. System)
Bulk dens. < 2.1 g/cm3 - H=2 (DGF 1-5)
2.1 < Bulk dens. < 2.25 - H=3 (DGF 5-20)
H=2
H=3
Degree of Induration
quc [MPa]LB UB
H2 1.0 3.0H3 5.0 8.0
H=4,5
For the top part of limestone (max. 2 m depth)
Degree of Induration
quc [MPa]LB UB BE
H2 1.0 3.0 1.5
quc of the formation (averaged)
Degree of Induration
quc [MPa]BE UB
H2,H3,H4,H5 11.3 24.2
Estimation of Hoek-Brown (HB) Classification Parameters
Hoek-Brown Classification Parameters
www.geo.dk
• Geological Strength Index (GSI)
Factor Rating
Values Rate
quc 1-5(8) 1
RQD <25% 3-8
Spacing of discontinuities
<60 mm 5-8
Condition of discontinuities
Slightly Weathered-
HighlyWeathered
25-30
Groundwater Wet 7
Joint orientation 0 0
RMR total 41-54
GSI BE 36-49
Rock Mass Rating (RMR89) –after Bieniawsky
• Material Constant (mi)
• Disturbance Factor (D)
Recommended values for Chalk = 7 ± 2
Different references (Hoek, Kaiser, Bawden)
(Average mi=8-10)
Selected values:
MaterialConstant
miLB BE6 9
Not disturbed structure considered: D=0
E. Hoek, C. Carranza-Torres and B. Corkum (2002)
Estimation of Hoek-Brown (HB) Classification Parameters
Hoek-Brown Classification Parameters
www.geo.dk
• Rock Mass Modulus (Em (Erm)
(Hoek, 2002) Previously calculated
• Intact Rock Modulus (Ei)
Layer Thickness (ti)
Induration(DGF)
quc ti*quc Ei ti*Ei
No. [m] [-] [Mpa] [MN/m] [Mpa] [MN/m]
1 0.10 H3 5-8 0.5-0.8 10000 1000
2 0.31 H2 1-3 0.3-0.9 1000 310
3 0.09 H3 5-8 0.5-0.8 10000 900
4 0.16 H2 1-3 0.2-0.5 1000 160
… …. …. …. ….. ….. …..
quc, average
Ei,average
Estimation of Hoek-Brown (HB) Classification Parameters
Hoek-Brown Classification Parameters
www.geo.dk
Estimated intact rock modulus
Based on a large set of data (Hoek & Diederichs, 2006)
MR
Ref. Paper: Composite Elasticity of Copenhagen Limestone
N. Katić & H.F. Christensen
MR ≈1000 – Copenhagen Limestone (results of several UCS tests)
& Hoek & Diederichs 2006 for micritic limestone
• Further analysis: (Hoek & Diederichs, 2006)
Estimation of Hoek-Brown (HB) Classification Parameters
Hoek-Brown Classification Parameters
www.geo.dk
• Results HB Parameters
Hoek-Brown
Classification Parameters
Geo
Best Estimate
quc [MPa] below 2 m 11.3
quc [MPa] above 2 m 1.5
σ'3,max [kPa] 1500
GSI [-] 35
mi 9
D 0
Em [MPa] (Hoek 2002) 1418
Ei [MPa] averaged (DGF) 7800
Additional Analysis
Erm [MPa] (Hoek & Diederichs 2006 ) 1281
Ei [MPa] = MR * quc 11300
(5000 – 30000) DGF
Estimation of Hoek-Brown (HB) Classification Parameters
Hoek-Brown Classification Parameters
www.geo.dk
• Results HB Parameters – In line with other investigations
“Based on a large set of available measurement of Elastic Modulus of Copenhagen Limestone”
Estimation of Hoek-Brown (HB) Classification Parameters
Hoek-Brown Classification Parameters
Conversion of HB Parameters into Mohr-Coulomb (MC)
www.geo.dk
Why we need MC parameters?
• Many geotechnical engineering analyses require MC parameters (cohesion & friction angle)
even though actual strength envelopes are often non-linear
Equivalent MC
Hoek-Brown
Criterion
• Mohr-Coulomb Equivalent Parameters
Previously calculated
Conversion of HB Parameters into Mohr-Coulomb (MC)
www.geo.dk
• Rock Data 5.003 – Rockscience References :
- Hoek, Evert et. Al. (2002): Hoek-Brown Failure Criterion – 2002 Edition
• MC Strength parameters - Palmstrøm Jv IndexReferences :
- Palmstrøm, A (2005). Measurements of and Correlations between Block Size and Rock Quality Designation (RQD)
- Pade, E. DTU MSc Thesis, 2006: Parameter study based on jointed limestone
- Foged, N., Stabell, S. & Foged, B. (2011): Rock mass characterization for tunnelsin limestone in Copenhagen
and Malmö area,
- Foged, N. (2008): Rock Mass Characterization in Limestone, IngeniørgeologiskFjeldklassifikation,
Foredrag i Dansk Geoteknisk Forening 2008-04-24
www.geo.dk
Conversion of HB Parameters into Mohr-Coulomb (MC)
• Rock Data 5.003 – Rockscience
www.geo.dk
• Palmstrøm Jv Index
Pade, E. DTU MSc Thesis, 2006: Parameter study based on jointed limestone
Foged, N. (2008): Rock Mass Characterization in Limestone,
IngeniørgeologiskFjeldklassifikation,
• Results
Conversion of HB Parameters into Mohr-Coulomb (MC)
www.geo.dk
• Conventional Calculations
MC Parameters from RockData
MC Parameters from Palmstrøm Jv Index
• Plaxis 2D FE Analysis
MC Parameters from RockData
MC Parameters from Palmstrøm Jv Index
HB Parameters (σci,GSI,mi,D)
Plaxis VIP Feature – Hoek Brown Model
Axial Bearing Capacity of Sheet piles installed in Limestone
www.geo.dk
• Conventional Calculations (sheet pile installed in chalk (the clay layer is neglected)
- Using the same conventional theory applicable to steel piles
- No established practice exists for vertical bearing capacity of the sheet piles
- Discussions about the effective area!
- Is only the steel area to be taken into account? What about the plug size?
AZ 42-700 N
1400
Hwall = 499
B’ = 0.07 * Hwall = 0.035 m
• In this study, the effective end area of the sheet pile has been calculated according to:
The plug size estimated = (7-9) % of Hwall
Axial Bearing Capacity of Sheet piles installed in Limestone
www.geo.dk
• Conventional Calculations (tip (end) bearing + shaft)
Characteristic tip bearing
- p’>125 kPa, ξ = 1,5
- Nc, Nq (Tomlinson 2001)
-small, neglected (Tomlinson 2001)
Design tip bearing
Characteristic shaft resistance
(Kulhawy & Poon, 1993)
L – embedment in the limestone layer (2.1m)
Design shaft resistance
(Kulhawy & Poon, 1993)
Axial Bearing Capacity of Sheet piles installed in Limestone
www.geo.dk
• Conventional Calculations Results
c*Nc + p’*Nq ≈ c*Nc + p’*Nq
Depending only on quc
Axial Bearing Capacity of Sheet piles installed in Limestone
www.geo.dk
• Plaxis 2D FE Analysis (15 Nodes, 2D Plain Strain + Interface)
• Same Geometry – Changing Soil Parameters
MC Parameters from RockData
MC Parameters from Palmstrøm Jv Index
HB Parameters (σci,GSI,mi,D)
HB Model is more realistic
Axial Bearing Capacity of Sheet piles installed in Limestone
www.geo.dk
• Plaxis 2D FE Outputs
Axial Bearing Capacity of Sheet piles installed in Limestone
Conclusions & Recommendations
www.geo.dk
• Equivuivalent MC parameters derived from Palmstrøm Jv Index show similar FE axial bearing
capacity with HB FE Model, considered more realistic.
• The axial bearing capacity with MC parameters from RockData show much larger capacity,
compared also with the conventional methods.
• The conventional calculation for sheet piles is associated with uncertainties, deriving possibly
from the assumptions of the effective steel area
• The use of Palmstrøm Jv Index (in combination with GSI from Hoek), it is believed to be more
realistic (high friction, low cohesion).
• Further Investigations needed!
Evert Hoek, 2007
Total Design Capacity [kN/m]
MC model
(Rock-Data)
MC model
(Palmstrøm Jv Index)
HB model
(Plaxis FE HB)
807 412 304