1. 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
2. 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
4. 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
5. • 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
8. 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
10. 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
11. Geotechnical categories
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
12. Geotechnical categories
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.
15. 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
16. 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
17. 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
19. Structures Type Number of investigation points
recommended
Buildings –
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 designer’s discretion
Roads, railways, canals, pipelines, inland
dikes
1 BH every 20 to 200m
ERSS, retaining wall < 6m high
ERSS, retaining wall >= 6m high
1 BH every 15 to 40m
1 BH every 10 to 30m
Tunnelling in built-up area
Tunnelling in green field area
1 BH every 10 to 75m
1 BH every 20 to 200m
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
25. Design investigations – Depth of BH
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
28. Design investigations – Sampling
REFERENCE BS EN ISO 22475-1 Geotechnical investigation and testing – Sampling methods and groundwater measurements
32. From ground
investigations as
described earlier
Derived
values
Characteristic
values
Design
values
GEOTECHNICAL PARAMETERS
SPT N values
cu=5N
How to obtain
characteristic
values?
33. 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).
34. 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
35. Applicable Geotechnical Parameters
tan j’ Effective angle of shearing resistance
c’ Effective cohesion value
cu Undrained shear strength
N SPT N values
qc CPT qc values
APPLICABLE GEOTECHNICAL PARAMETERS
36. 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?
37. 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?
38. 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
Ck = characteristic value
mC = mean value
sX = standard deviation
n = number of samples
Most likely industry will
adopt this method!
SCHNEIDER METHOD
40. Assuming homogenous soil, the characteristic mean value of a geotechnical
parameter is calculated using: (EC0 D7.2)
Ck = mC (1- knVX)
Ck = characteristic mean value at 95% reliable or 5%
fractile, depending on the kn input
mC = mean value
kn = 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
41. Values of the coefficient kn for the assessment of a characteristic value as a 5%
fractile value
Reference SS EN 1997-0
STATISTICAL METHOD
42. 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
44. 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
45. 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
50. 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?
52. 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
53. GEOTECHNICAL DESIGN REPORT
• 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
54. 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 monitoring
Current 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
GEOTECHNICAL DESIGN REPORT
55. 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 monitoring
Current 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
GEOTECHNICAL DESIGN REPORT
Current practices and regulations in-line with EC7 principles
56. DELIVERABLES
EC7-2 B.1 Stages of ground investigations in geotechnical design, execution of works and exploitation of the structure
58. 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
