GeoSS-BCA EC7 Briefing (19Nov2014).pdf

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GEOSS-BCA EC7 BRIEFING

ON GROUND INVESTIGATION AND

DETERMINATION OF CHARACTERISTIC VALUES

GeoSS GEOTECHNICAL SOCIETY

OF SINGAPORE

19 Nov 2014

Dr T G Ng

President GeoSS

SCOPE OF PRESENTATION

1. Introduction

2. Geotechnical investigation to EC7

3. Geotechnical parameters and characteristic

values in EC7

4. Geotechnical Design Report

5. Q&A

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INTRODUCTION

Introduction: Distinction between

Principles and Application Rules

C1.4(1) Distinction is made between Principles and

Application Rules, depending on the character of

the individual clauses

C1.4(2) The Principles comprises:

General statements and definitions for which there is

no alternative

Requirements and analytical models for which no

alternative is permitted unless specifically stated

C1.4(3) The Principles are preceded by the Letter P

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C1.4(4) The Application Rules are examples of generally recognised rules, which follow the Principles and satisfy their requirements.

C1.4(5) It is permissible to use alternatives to the Application Rules given in this standard, provided it is shown that the alternative rules accord with relevant Principles and are at least equivalent with regard to the structural safety, serviceability and durability, which would be expected when using the Eurocodes.

Introduction: Distinction between

Principles and Application Rules

Distinction between Principles and

Application Rules (SS EN 1997-1: 2010)

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Distinction between Principles and

Application Rules (SS EN 1997-2: 2010)

Eurocode 7 : Geotechnical design

Designers are responsible to ensure structural safety,

serviceability and durability of the designs.

Designers are responsible for the planning of the

geotechnical investigation

Designers are accountable for their decisions, i.e.

specification of field and laboratory tests,

determination geotechnical design parameters and

characteristic values etc.

This briefing/dialogue aims to raise awareness to the

designers on key aspects on geotechnical

investigations and recommendations on how to

determinate characteristic values

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GEOTECHNICAL INVESTIGATIONS

TO EC7

Geotechnical categories

EN 1997-1 C2.1(8) to C2.1(21)

To establish geotechnical design, structures are

classified into Geotechnical Categories 1, 2 or 3

according to:

- complexity of the structure,

- complexity of the ground conditions

- complexity of the loading

- level of risk that is acceptable for the purpose of

the structure

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Geotechnical Categories related to geotechnical hazard and vulnerability levels (Geotechnical Design to Eurocode 7; Orr & Farrell, 1999)

Factors to be

Considered

Geotechnical Categories

GC1 GC2 GC3

Geotechnical

Hazards/risk

Low Moderate High

Ground

conditions

Known from comparable experience

to be straightforward. Not involving

soft, loose or compressible soil,

loose fill or sloping ground.

Ground conditions and properties

can be determined from routine

investigation and tests.

Unusual or exceptionally difficult

ground conditions requiring non

routine investigations and tests.

Groundwater

situation

No excavations below water table,

except where experience indicates

this will not cause problems.

No risk of damage without prior

warning to structures due to

groundwater lowering or drainage.

No exceptional water tightness

requirements

High groundwater pressures and

exceptionally groundwater conditions,

e.g. multi-layered strata with variable

permeability.

Regional

seismicity

Areas with no or vary low

earthquake hazard

Moderate earthquake hazard

where seismic design code (EC8)

may be used

Areas of high earthquake hazard

Influence of the

environment

Negligible risk of problems due to

surface water, subsidence,

hazardous chemicals, etc

Environmental factors covered

routine design methods

Complex or difficult environmental

factors requiring special design

methods

Vulnerability Low Moderate High

Natural and size

of the structure

and its

elements

Small and relatively simple

structures or construction.

Insensitive structures in seismic

areas

Conventional types of structures

with no abnormal risks

Very large or unusual structures and

structures involving abnormal risks.

Very sensitive structures in seismic

areas

Surroundings Negligible risk of damage to or from

neighbouring structures or services

and negligible risk of life

Possible risk of damage to

neighbouring structures or services

due, for example, to excavation or

piling

High risk of damage to neighbouring

structures or services


Geotechnical Categories related to geotechnical hazard and vulnerability levels (Geotechnical Design to Eurocode 7; Orr & Farrell, 1999)

Geotechnical Categories

GC1 GC2 GC3

Expertise

required

Person with appropriate comparable

experience

Experienced qualified person Experienced geotechnical specialist

Geotechnical

Investigations

Qualitative investigations including

trial pits

Routine investigations involving

borings, field and laboratory tests

Additional more sophisticated investigations

and laboratory tests

Design

procedures

Prescriptive measures and simplified

design procedures. E.g. design

bearing pressure based on

experience or published presumed

bearing pressures. Stability or

deformation calculations may not be

necessary

Routine calculations for stability

and deformations based on design

procedures in EC7

More sophisticated analyses

Examples of

structures

small and relatively simple structures

Landed housing on footings in firm

residual soil; single storey sheds;

linkways; roadside drain

Spread foundations; raft

foundations; pile foundations;

bridge piers and abutments;

embankments and earthworks;

ground anchors and other tied-

back systems

Canal; shallow; walls and other

structures retaining or supporting

soil or water < 6m height;

excavations < 6m depth; tunnels in

hard, non-fractured rock/

competent soils.

Infrastructure projects for rail and road

tunnels; utilities tunnels of more than 3 m

in diameter; airport terminal buildings; port

structures or major maritime structures;

dam; dikes; foundation in limestone areas

for mid to high density development;

foundation for highrise of more than 10

storey on reclaimed land, or soft soils with

combined thickness of soft soils of more

than 10 m; foundation for buildings of 30

storey or more; deep basement

excavation

6m depth; retaining wall of

more than 6 m height.

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Designers guide to Eurocode 7: Geotechnical Design (Frank et al. 2013)

Geotechnical investigations

EN 1997-1

EN 1997-2

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EC7-1 Section 3: Geotechnical Data

EC7-2 Section 2: Planning of ground investigations

Groundwater investigation

Gathering of all relevant information about the site

Ground investigation

Preliminary investigation (conceptual design) desk

studies & site inspection

Design investigation (detailed design) specify

relevant investigation methods i.e. field tests/ lab tests

to justify choice of foundations, geotechnical works

Control investigation (construction stage) - Verification

of choice of foundation method and design procedure,

control of ground improvement works and stability

during construction

Geotechnical investigations

Preliminary investigations

EN 1997-2 C2.3

Assess suitability of site in comparison with

alternative sites

Assess suitable positioning of structure

Evaluate the possible effects of the proposed works

on surroundings, such as neighbouring buildings,

structures and sites

Walk-over surveys, desk studies of previous site

investigations

Plan the design and control investigations

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Design investigations

EN 1997-2 C2.4

To provide all the information required for the design of temporary and

permanent works

Identify any difficulties that may arise during construction

Include drilling, field tests, laboratory tests, groundwater measurement

Design investigations

EN 1997-2 C2.4

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Structures Type Number of investigation points

recommendedBuildings

Up to 10 stories high

More than 10 stories high

15m to 40m grid, minimum 1 BH per block, and 3 BHs

per site

10m to 30m grid, 1 BH per 300sqm, minimum 2 BHs per

block, and 3 BHs per site

Large area 60 m grid per BH, at designers discretion

Roads, railways, canals, pipelines, inland

dikes

1 BH every 20 to 200m

ERSS, retaining wall < 6m high

ERSS, retaining wall >= 6m high



Tunnelling in built-up area

Tunnelling in green field area



Dam, costal dikes, weirs 1 BH every 25 to 75m along vertical sections

Road Bridges, tower stacks, heavy

machinery foundation

2 to 6 BHs per foundation

Design investigations No of BH

* From BCA advisory note

Design investigations Depth of BH

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where

DF is the pile base diameter; and

bg is the smaller side of the rectangle

circumscribing the group of piles

forming the foundation at the level

of the pile base

Design investigations SamplingEN 1997-2

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Design investigations SamplingEN 1997-2

Design investigations SamplingREFERENCE BS EN ISO 22475-1 Geotechnical investigation and testing Sampling methods and groundwater measurements

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DETERMINATION OF

GEOTECHNICAL PARAMETERS AND

CHARACTERISTIC VALUES

From ground

investigations as

described earlier

De

rive

d v

alu

es

Ch

ara

cte

rist

ic

valu

es

De

sig

n

valu

es

GEOTECHNICAL PARAMETERS

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From ground

investigations as

described earlierD

eri

ved

va

lue

sC

ha

ract

eri

stic

valu

es

De

sig

n

valu

es


SPT N values

cu=5N

From ground

investigations as

described earlier

De

rive

d v

alu

es

Ch

ara

cte

rist

ic

valu

es

De

sig

n

valu

es


SPT N values

cu=5N

How to obtain

characteristic

values?

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WHAT IS CHARACTERISTIC VALUE?

EN 1997-1 C2.4.5.2(2)P defines the characteristic value as being

selected as cautious estimate of the value affecting the occurrence

of the limit state

Each word and phrase in this clause is important:

Selected emphasizes the importance of engineering

judgement

Cautious estimate some conservatism is required

Limit state the selected value must relate to the limit state

(failure mechanism)

For most limit state cases where the soil volume involved is large,

the characteristic value should be derived such that a cautious

estimate of the mean value is a selection of the mean value of the

limited set of geotechnical parameter values, with a confidence

level of 95% (moderately conservative parameters); where local

failure is concerned, a cautious estimate of the low value is a 5%

fractile (worst credible parameters).

Applicable for

predominantly end bearing

piles in non-competent layer,

where shaft resistance

contributed < 70% of total

pile resistance

Pile design using alternative method

APPLICATION OF CHARACTERISTIC VALUE

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Applicable Geotechnical Parameters

tan Effective angle of shearing resistance

c Effective cohesion value

cu Undrained shear strength

N SPT N values

qc CPT qc values

APPLICABLE GEOTECHNICAL PARAMETERS

EC7 only mentions characteristic values could be

obtained by statistical methods but did not provide

details. cl.2.4.5.2

If other methods are used e.g. direct estimate by

comparable experience, designer must be able to

justify his slection of characteristic values.

HOW TO OBTAIN CHAR VALUES FROM DERIVED VALUES?

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Schneider

Or statistical

Two common methods of obtaining characteristics values:

1. Schneider(1999) method

2. Statistical method

HOW TO OBTAIN CHAR VALUES FROM DERIVED VALUES?

This method would be applicable across all geotechnical categories.

Xk = mx - 0.5sX(upper bound equivalent to 95% mean reliable)

Xk = mx - sX(lower bound equivalent to low value 5% fractile)

where

Schneider(1999) Method

k = characteristic valuem = mean valuesX = standard deviationn = number of samples

Most likely industry will

adopt this method!

SCHNEIDER METHOD

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SCHNEIDER METHOD (EXAMPLE)

Assuming homogenous soil, the characteristic mean value of a geotechnicalparameter is calculated using: (EC0 D7.2)

k = m (1 knVX)

k = characteristic mean value at 95% reliable or 5% fractile, depending on the kn input

m = mean valuekn = coefficient for 95% reliable or 5% fractile mean value, a function of n, number of samples VX = coefficient of variation of parameter X,

for Vx unknown, VX = sX/mx

sX = standard variation

Hence

Xk = mx (1- kn VX)

= mx - kn sX

STATISTICAL METHOD

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Values of the coefficient kn for the assessment of a characteristic value as a 5% fractile value

Reference SS EN 1997-0

STATISTICAL METHOD

Values of the coefficient kn for the assessment of a characteristic value as a 95% reliable mean value

NOT FOUND IN EUROCODE!

From EC7 designer handbook

STATISTICAL METHOD

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STATISTICAL METHOD (EXAMPLE)

Can I use existing site investigation data

from on BS Standard after Apr 2015?

Can we use derived values from non-EC7

GI to derive Characteristic value for EC7

design?

Use of existing SI data

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Can I use existing site investigation data

from on BS Standard after Apr 2015?

Can we use derived values from non-EC7

GI to derive Characteristic value for EC7

design?

Yes, but designer should be aware of the

difference between BS and EC7 soil

description.

Use of existing SI data

DIFFERENCE BETWEEN BS & EC

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Geotechnical

Category

GI availability Recommendation for characteristic values

1 Based on available

literature e.g. geological

map, published soil

parameters, or SI of

immediate neighbour

plots

Schneider method could be adopted.

For geotechnical parameters where sample testing is

insufficient or where the values are obtained from

the GI of a neighbouring plot, the determined

characteristic value should be reduced by a further

factor of 1.2.

2 Available SI based on BS

and/or new SI to EC stds

Schneider or Statistical method

Additional GI should be conducted for geotechnical

parameters where sample testing is insufficient.

3 Available SI based on BS

and/or new SI to EC stds

Schneider or Statistical method, the latter is

recommended if >10 sets of data is available

Can we use derived values from non-EC7 GI?

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GEOTECHNICAL DESIGN REPORT


EN 1997-1 C2.8(1)P The assumptions, data, methods of calculation and

results of the verification of safety and serviceability shall be recorded in

the Geotechnical Design Report

GDR should include:

Ground Investigation Report

Presentation of all available geotechnical information

Geotechnical evaluation of the information, stating the

assumptions made in the interpretation of the test results

Description of the site and surroundings

Description of the ground conditions

Description of the proposed construction, including actions

Design values of soil and rock properties

Statement on codes and standards applied

Statement on suitability of the site

Geotechnical design calculations and drawings

Foundation design recommendations

Items to be checked during construction or requiring maintenance or

monitoring

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EN 1997-1 C2.8(4)P The GDR shall include a plan of

supervision and monitoring, as appropriate.

Item which require checking during construction or,

which require maintenance after construction shall

be clearly identified.

When the required checks have been carried out

during construction, they shall be recorded in an

addendum to the Report

Deliverables specified by EC7:

Ground Investigation report (GIR)Current practice - Site Investigation Factual Reports etc

Geotechnical Design Report (GDR) & Final Design Report

Current practice - Impact assessment due to geotechnical works,

Geotechnical Interpretation Report, design calculations and drawings

submission to BCA etc

Program for inspection, supervision and monitoringCurrent practice Advisory 01/09, Qualified Site Supervisors regime as

required by BC regulations, pile load tests as required by CP4,

instrumentation and monitoring plans etc


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Deliverables specified by EC7:

Ground Investigation report (GIR)Current practice - Site Investigation Factual Reports etc

Geotechnical Design Report (GDR) & Final Design Report

Current practice - Impact assessment due to geotechnical works,

Geotechnical Interpretation Report, design calculations and drawings

submission to BCA etc

Program for inspection, supervision and monitoringCurrent practice Advisory 01/09, Qualified Site Supervisors regime as

required by BC regulations, pile load tests as required by CP4,

instrumentation and monitoring plans etc


Current practices and regulations in-line with EC7 principles

DELIVERABLES

EC7-2 B.1 Stages of ground investigations in geotechnical design, execution of works and exploitation of the structure

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CONCLUSIONS

CONCLUSIONS

The 1st Principle - Designers are responsible to ensure

structural safety, serviceability and durability of the designs for

the structures. Structures are classified into Geotechnical

Categories 1, 2 or 3.

To fulfil the 1st Principle, Designers are responsible for the

planning of the geotechnical investigation which include

Preliminary, Design and Control Investigations

Guidelines and recommendations in Informative Annexes are

available in EC7-1 and EC7-2 for reference by Designers to

decide on specifications of field and laboratory tests, no of BH,

field and lab tests etc

Characteristic values shall be determined from derived values

for design purposes.

A comprehensive Geotechnical Design Report should be

submitted to document the entire design process

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REFERENCES

REFERENCES

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http://eurocodes.jrc.ec.europa.eu/

REFERENCES

THANK YOU

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Q & A

Documents

GeoSS-BCA EC7 Briefing (19Nov2014).pdf