Interpretation of Limestone / Chalk Parameters … · Plaxis VIP Feature –Hoek Brown Model Axial...

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

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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.

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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.

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• 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

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• 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

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• 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)


quc [MPa]LB UB BE

H2 1.0 3.0 1.5

quc of the formation (averaged)


quc [MPa]BE UB

H2,H3,H4,H5 11.3 24.2



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• 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)



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• 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



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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)



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• 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



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• Results HB Parameters – In line with other investigations

“Based on a large set of available measurement of Elastic Modulus of Copenhagen Limestone”



Conversion of HB Parameters into Mohr-Coulomb (MC)

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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


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• 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

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• Rock Data 5.003 – Rockscience

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• 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


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• Conventional Calculations

MC Parameters from RockData

MC Parameters from Palmstrøm Jv Index

• Plaxis 2D FE Analysis



HB Parameters (σci,GSI,mi,D)

Plaxis VIP Feature – Hoek Brown Model

Axial Bearing Capacity of Sheet piles installed in Limestone

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• 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


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• 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)


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• Conventional Calculations Results

c*Nc + p’*Nq ≈ c*Nc + p’*Nq

Depending only on quc


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• Plaxis 2D FE Analysis (15 Nodes, 2D Plain Strain + Interface)

• Same Geometry – Changing Soil Parameters



HB Parameters (σci,GSI,mi,D)

HB Model is more realistic


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• Plaxis 2D FE Outputs


Conclusions & Recommendations

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• 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

Interpretation of Limestone / Chalk Parameters … · Plaxis VIP Feature –Hoek Brown Model Axial...

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