prEN ISO 22282-2
(Main)Geotechnical investigation and testing - Geohydraulic testing - Part 2: Water permeability tests in a borehole without packer (ISO/DIS 22282-2:2008)
Geotechnical investigation and testing - Geohydraulic testing - Part 2: Water permeability tests in a borehole without packer (ISO/DIS 22282-2:2008)
2010-07-09 EMA: ISO/CS confirmed that no text has been received, WI deleted following the 3-year timeframe exercise.
Geotechnische Erkundung und Untersuchung - Geohydraulische Versuche - Teil 2: Wasserdurchlässigkeitsversuche in einem Bohrloch unter Anwendung offener Systeme (ISO/DIS 22282-2:2008)
Die vorliegende Norm legt Anforderungen an die Bestimmung der örtlichen Durchlässigkeit in Böden und Felsen unter und oberhalb des Grundwasserspiegels in einem offenen Bohrloch mit Hilfe von Wasserdurchlässigkeitsversuchen als Teil der geotechnischen Erkundungsdienste nach EN 1997 1 und prEN 1997 2 fest.
Die Norm behandelt auch die Abschätzung der Durchlässigkeit von ungesättigten Böden.
Reconnaissance et essais géotechniques - Essais géohydrauliques - Partie 2: Essai de perméabilité dans un forage en tube ouvert (ISO/DIS 22282-2:2008)
Geotehnično preiskovanje in preskušanje - Hidrogeološke preisave - 2. del: Preskusi prepustnosti za vodo v vrtini brez zavijanja (ISO/DIS 22282-2:2008)
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
oSIST prEN ISO 22282-2:2008
01-september-2008
*HRWHKQLþQRSUHLVNRYDQMHLQSUHVNXãDQMH+LGURJHRORãNHSUHLVDYHGHO
3UHVNXVLSUHSXVWQRVWL]DYRGRYYUWLQLEUH]]DYLMDQMD ,62',6
Geotechnical investigation and testing - Geohydraulic testing - Part 2: Water permeability
tests in a borehole without packer (ISO/DIS 22282-2:2008)
Geotechnische Erkundung und Untersuchung - Geohydraulische Versuche - Teil 2:
Wasserdurchlässigkeitsversuche in einem Bohrloch unter Anwendung offener Systeme
(ISO/DIS 22282-2:2008)
Reconnaissance et essais géotechniques - Essais géohydrauliques - Partie 2: Essai de
perméabilité dans un forage en tube ouvert (ISO/DIS 22282-2:2008)
Ta slovenski standard je istoveten z: prEN ISO 22282-2
ICS:
93.020 Zemeljska dela. Izkopavanja. Earthworks. Excavations.
Gradnja temeljev. Dela pod Foundation construction.
zemljo Underground works
oSIST prEN ISO 22282-2:2008 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
---------------------- Page: 1 ----------------------
oSIST prEN ISO 22282-2:2008
---------------------- Page: 2 ----------------------
oSIST prEN ISO 22282-2:2008
EUROPEAN STANDARD
DRAFT
prEN ISO 22282-2
NORME EUROPÉENNE
EUROPÄISCHE NORM
April 2008
ICS 13.080.40
English Version
Geotechnical investigation and testing - Geohydraulic testing -
Part 2: Water permeability tests in a borehole without packer
(ISO/DIS 22282-2:2008)
Reconnaissance et essais géotechniques - Essais
géohydrauliques - Partie 2: Essai de perméabilité dans un
forage en tube ouvert (ISO/DIS 22282-2:2008)
This draft European Standard is submitted to CEN members for parallel enquiry. It has been drawn up by the Technical Committee
CEN/TC 341.
If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations which
stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other language
made by translation under the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the
same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,
Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to
provide supporting documentation.
Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without notice and
shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels
© 2008 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN ISO 22282-2:2008: E
worldwide for CEN national Members.
---------------------- Page: 3 ----------------------
oSIST prEN ISO 22282-2:2008
prEN ISO 22282-2:2008 (E)
Contents Page
Foreword.3
2
---------------------- Page: 4 ----------------------
oSIST prEN ISO 22282-2:2008
prEN ISO 22282-2:2008 (E)
Foreword
This document (prEN ISO 22282-2:2008) has been prepared by Technical Committee CEN/TC 341
"Geotechnical Investigation and Testing", the secretariat of which is held by ELOT, in collaboration with
Technical Committee ISO/TC 182 "Geotechnics".
This document is currently submitted to the parallel Enquiry.
3
---------------------- Page: 5 ----------------------
oSIST prEN ISO 22282-2:2008
---------------------- Page: 6 ----------------------
oSIST prEN ISO 22282-2:2008
DRAFT INTERNATIONAL STANDARD ISO/DIS 22282-2
ISO/TC 182/SC 1 Secretariat: DIN
Voting begins on: Voting terminates on:
2008-04-17 2008-09-17
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION • МЕЖДУНАРОДНАЯ ОРГАНИЗАЦИЯ ПО СТАНДАРТИЗАЦИИ • ORGANISATION INTERNATIONALE DE NORMALISATION
Geotechnical investigation and testing — Geohydraulic
testing —
Part 2:
Water permeability tests in a borehole without packer
Reconnaissance et essais géotechniques — Essais géohydrauliques —
Partie 2: Essai de perméabilité dans un forage en tube ouvert
ICS 93.020
ISO/CEN PARALLEL ENQUIRY
This draft International Standard is a draft standard developed within the European Committee for
Standardization (CEN) and processed under the CEN-lead mode of collaboration as defined in the
Vienna Agreement. The document has been transmitted by CEN to ISO for circulation for ISO member
body voting in parallel with CEN enquiry. Comments received from ISO member bodies, including those
from non-CEN members, will be considered by the appropriate CEN technical body. Should this DIS be
accepted, a final draft, established on the basis of comments received, will be submitted to a parallel two-
month FDIS vote in ISO and formal vote in CEN.
In accordance with the provisions of Council Resolution 15/1993 this document is circulated in
the English language only.
Conformément aux dispositions de la Résolution du Conseil 15/1993, ce document est distribué
en version anglaise seulement.
To expedite distribution, this document is circulated as received from the committee secretariat.
ISO Central Secretariat work of editing and text composition will be undertaken at publication
stage.
Pour accélérer la distribution, le présent document est distribué tel qu'il est parvenu du
secrétariat du comité. Le travail de rédaction et de composition de texte sera effectué au
Secrétariat central de l'ISO au stade de publication.
THIS DOCUMENT IS A DRAFT CIRCULATED FOR COMMENT AND APPROVAL. IT IS THEREFORE SUBJECT TO CHANGE AND MAY NOT BE
REFERRED TO AS AN INTERNATIONAL STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS BEING ACCEPTABLE FOR INDUSTRIAL, TECHNOLOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN NATIONAL REGULATIONS.
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT, WITH THEIR COMMENTS, NOTIFICATION OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPORTING DOCUMENTATION.
©
International Organization for Standardization, 2008
---------------------- Page: 7 ----------------------
oSIST prEN ISO 22282-2:2008
ISO/DIS 22282-2
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but shall
not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In the
unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.
Copyright notice
This ISO document is a Draft International Standard and is copyright-protected by ISO. Except as permitted
under the applicable laws of the user's country, neither this ISO draft nor any extract from it may be
reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, photocopying,
recording or otherwise, without prior written permission being secured.
Requests for permission to reproduce should be addressed to either ISO at the address below or ISO's
member body in the country of the requester.
ISO copyright office
Case postale 56 CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Reproduction may be subject to royalty payments or a licensing agreement.
Violators may be prosecuted.
©
ii ISO 2008 – All rights reserved
---------------------- Page: 8 ----------------------
oSIST prEN ISO 22282-2:2008
ISO/DIS 22282-2
Contents Page
Foreword .iv
1 Scope.1
2 Normative references.1
3 Terms, definitions and symbols .1
3.1 Terms and definitions .1
3.2 Symbols.1
4 Test principle .2
5 Equipment.3
6 Test procedure.3
6.1 Preparation of a test section .3
6.1.1 General .3
6.1.2 Preparation of a test section in non-stable soil and rock below the groundwater surface.3
6.1.3 Preparation of a test section in stable soil and rock .4
6.1.4 Preparation of the test section in unsaturated conditions .6
6.2 Conducting the test.7
6.2.1 General .7
6.2.2 Constant flow rate test method.7
6.2.3 Variable head test method.8
6.2.4 Constant head test method .8
7 Test results .8
7.1 Constant flow rate test method.8
7.2 Variable head test method.9
7.3 Constant head test method .9
8 Report.9
8.1 Field report.9
8.1.1 General .9
8.1.2 Record of measured values and test results.10
8.2 Test report.11
Annex A (informative) Example of record of measured values and test results.12
Annex B (informative) Interpretation of test results.13
B.1 General.13
B.2 Constant rate test method.13
B.3 Constant head test method .14
B.4 Variable head test method.15
B.4.1 Velocity graph method.15
B.4.2 Hvorslev Method.17
B.4.3 Cooper-Bredehoeft-Papadopoulos Method.18
B.4.4 Bouwer and Rice Method.20
B.5 Unsaturated soil conditions .22
Bibliography.25
© ISO 2007 – All rights reserved iii
DRAFT 2008
---------------------- Page: 9 ----------------------
oSIST prEN ISO 22282-2:2008
ISO/DIS 22282-2
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 22282-2 was prepared by Technical Committee ISO/TC 182, Geotechnics, Subcommittee SC 1, and by
Technical Committee CEN/TC 341, Geotechnical investigation and testing in collaboration. ISO 22282
consists of the following parts, under the general title Geotechnical investigation and testing — Geohydraulic
testing:
⎯ Part 1: General rules
⎯ Part 2: Water permeability tests in a borehole using open systems
⎯ Part 3: Water pressure test in rocks
⎯ Part 4: Pumping tests
⎯ Part 5: Infiltrometer tests
⎯ Part 6: Water permeability tests in a borehole using closed systems
iv © ISO 2007 – All rights reserved
DRAFT 2008
---------------------- Page: 10 ----------------------
oSIST prEN ISO 22282-2:2008
ISO/DIS 22282-2
Geotechnical investigation and testing — Geohydraulic
testing —
Part 2:
Water permeability tests in a borehole without packer
1 Scope
This standard specifies requirements for the determination of the local permeability in soils and rocks below
and above ground water level in an open hole by the water permeability tests as part of the geotechnical
investigation services according to EN 1997-1 and prEN 1997-2.
It also includes the estimation of permeability of unsaturated soils.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
EN 1997-1, Eurocode 7: Geotechnical design - Part 1: General rules.
prEN 1997-2, Eurocode 7: Geotechnical design - Part 2: Ground investigation and testing.
EN ISO 14688-1, Geotechnical investigation and classification of soils − Part 1: Identification and description.
prEN ISO 22282-2, Geotechnical investigation and testing − Geohydraulic testing − Part 2: Water permeability
tests in a borehole without packer.
EN ISO 22475-1, Geotechnical investigation and testing − Sampling by drilling and excavation methods and
groundwater measurements − Part 1: Technical principles for execution.
3 Terms, definitions and symbols
3.1 Terms and definitions
For the purposes of this document, the terms and definitions given in EN ISO 22475-1 and prEN ISO 22282-1
are applied.
3.2 Symbols
For the purposes of this document, the symbols given in Table 1 apply.
© ISO 2007 – All rights reserved 1
DRAFT 2008
---------------------- Page: 11 ----------------------
oSIST prEN ISO 22282-2:2008
ISO/DIS 22282-2
Table 1 — Symbols
Symbol Designation Unit
2
A area of the inner cross section of the casing m
c
2
A area of the water surface in the reservoir m
r
D
borehole diameter, diameter of the test section m
F shape factor m
H hydraulic head of the test m
H , H , H applied hydraulic heads m
1 2 3
H
0 distance of the water level from the ground level m
∆H change in hydraulic head m
k permeability coefficient m/s
k field saturated permeability coefficient m/s
fs
L
length (height) of the test section m
3
Q effective inflow m /s
3
Q , Q , Q water flow m /s
1 2 3
t time need to reach the equilibrium s
i
t
time s
t time at test begin s
0
4 Test principle
The test is based on the following assumption that the test section shall be isolated and located above or
below the groundwater surface.
The results may vary depending of the test type chosen (water withdrawal or injection) according to the
purpose of the test.
Three test methods are available depending of the type of loading:
-6
a) Constant flow rate test method (suitable for k-value greater than 10 m/s)
The test consists in producing a change in hydraulic head in a section of a borehole by injecting or
withdrawing a constant flow rate. The change in hydraulic head is measured against time.
-4 –9
b) Variable head test method (suitable for k-value between 10 m/s and 10 m/s)
The test consists in producing an instant change in hydraulic head in a section of a borehole. The change in
hydraulic head is measured against time.
-4 –9
c) Constant head test method (suitable for k-value between 10 m/s and 10 m/s)
The test consists in maintaining a constant hydraulic head in a section of a borehole. The flow rate is
measured against time.
2 © ISO 2007 – All rights reserved
DRAFT 2008
---------------------- Page: 12 ----------------------
oSIST prEN ISO 22282-2:2008
ISO/DIS 22282-2
5 Equipment
Beside a casing or a piezometer the following equipment is necessary:
a) water supply or plain rod for the rising head test;
b) pump or bail system for the falling head test;
c) device to determine the flow rate with an accuracy of 5 % of the measuring range;
d) device to maintain the flow rate;
e) perforated tube or/and filter material (for filter criteria see prEN ISO 22282-1);
f) device to measure the water level in the casing or piezometer with an accuracy of 0,01 m;
g) a time measuring and/or recording device , reading in s
All the equipment and measuring devices shall be periodically or before using calibrated according to their use.
6 Test procedure
6.1 Preparation of a test section
6.1.1 General
The test section shall be prepared in accordance with prEN ISO 22282-1, Annex A. During drilling the test
section and installation flushing additives shall not be used.
6.1.2 Preparation of a test section in non-stable soil and rock below the groundwater surface
Drilling shall be executed with the use of a casing. After drilling of the test section and cleaning of the borehole,
the test section shall be prepared according to one of these three alternatives (see Figure 1):
a) A perforated tube that is closed by a perforated disk at the bottom shall be installed through the casing
tube in the test section (Figure 1a). After that the casing tube shall be withdrawn by the length L shortly
beneath the upper end of the perforated section.
NOTE If the bottom of the boreholes can not be cleaned, a full disc can be used at the bottom of the tube. In this
case a specific shape factor is used (see prEN ISO 22282-1).
b) Before withdrawing the casing tube, an appropriate filter material shall be filled into the test section. After
that the casing tube shall be withdrawn almost up to the upper edge of the filter (Figure 1b).
c) Open end test: the test is performed at the bottom of the casing through the open section of diameter D
(Figure 1c).
© ISO 2007 – All rights reserved 3
DRAFT 2008
---------------------- Page: 13 ----------------------
oSIST prEN ISO 22282-2:2008
ISO/DIS 22282-2
a) with perforated tube b) with filter material
Key Key
1 ground 6 casing 1 ground 5 casing
2 top surface 7 perforated tube 2 top surface 6 filter pack
3 water table L length of the test section 3 water table L length of the test section
4 test section D diameter of the test section 4 test section D diameter of the test section
5 perforated disk
c) open end test
Key
1 ground 2 top surface
3 water table D diameter of the test section
Figure 1 — Test section in non stable soil and rock
6.1.3 Preparation of a test section in stable soil and rock
After drilling of the test section and cleaning of the borehole, the test section shall be prepared according to
one of these following alternatives (see Figure 2):
a) A packer is inflated above the test section (Figure 2a). A perforated tube can be used below the packer.
b) In a temporary open piezometer. A perforated tube shall be used in the test section. An appropriate filter
material shall fill the space between the tube and the borehole wall into the test section. A sealing plug
shall be installed above the filter pack (Figure 2b).
4 © ISO 2007 – All rights reserved
DRAFT 2008
---------------------- Page: 14 ----------------------
oSIST prEN ISO 22282-2:2008
ISO/DIS 22282-2
c) Before withdrawing the casing tube. After that the casing tube shall be withdrawn up to the upper edge of
the test section (Figure 2c).
a) with packer b) temporary piezometer
Key Key
1 ground 6 cavity 1 ground 6 measuring tube
2 measuring tube L length of the test section 2 top surface 7 filter pack
3 water table L length of the test section
3 water table D diameter of the test section
4 test section 4 test section D diameter of the test section
5 packer 5 sealing plug
c) without filter material d) with filter material
Key
1 ground 5 casing
© ISO 2007 – All rights reserved 5
DRAFT 2008
---------------------- Page: 15 ----------------------
oSIST prEN ISO 22282-2:2008
ISO/DIS 22282-2
2 top surface
3 water table L length of the test section
4 test section D diameter of the test section
Figure 2 — Example of test section in stable soil and rock
6.1.4 Preparation of the test section in unsaturated conditions
Measurement of permeability in unsaturated soils is made by injecting water into the test section.
During infiltration created by the permeability test, a field-saturated condition develops around the test section.
Full saturation does not occur due to entrapped air remaining in the soil or provided by the injected fluid. This
may reduce the permeability measured in the field.
When testing unsaturated coarse soils (typically gravels and sands), the flow of water is not spherical or
ellipsoidal as observed below water table in homogeneous soils (Figure 3). The flow net is affected by gravity
and shall be described by specific equations such as those provided in Annex B.5.
a) saturated conditions b) unsaturated conditions
Figure 3 — Water flow in coarse soils
In low permeable soils (silts and clays) the effect of suction at the wetting front can affect the test results,
particularly if the initial saturation of the soil is low. In order to avoid or limit the effect of suction, the soil
around the test section should be pre-saturated previous to proper permeability measurement. This phase
creates a saturated bulb around the test section wall. The equations used to compute the test data in
saturated condition may be used.
The duration of the saturation phase depends on the permeability of the soil in the test section (Figure 4).The
total amount of infiltrated water during the saturation phase shall be recorded.
6 © ISO 2007 – All rights reserved
DRAFT 2008
---------------------- Page: 16 ----------------------
oSIST prEN ISO 22282-2:2008
ISO/DIS 22282-2
0,1 1 10 100 1000 10000
1E-04
1E-05
1E-06
1E-07
1E-08
1E-09
Recommended saturation phase (h)
Figure 4 — Recommended duration of saturation phase
6.2 Conducting the test
6.2.1 General
All test methods can be executed as multi-step tests applying different hydraulic heads or flow rates.
6.2.2 Constant flow rate test method
-6
The constant flow rate test method is generally suitable for k-values greater than 10 m/s.
Before starting the test the water level shall be measured in the annular space or measuring tube in order to
ensure that the water level corresponds to the groundwater surface.
A flow rate shall be chosen which shall allow a variation of the water level in the tube of at least 10 cm during
the first minute in order to obtain a significant change in level between two measuring steps.
NOTE In a usual shape of the test section (D = 10 cm and L = 100 cm), a variation of 10 cm with a flow rate of 100
-3
l/min corresponds to a range of permeability of approximately 10 m/s. If the borehole is emptied (respectively filled) with a
-6
smaller flow rate (approximately 1 l/min), the permeability is less than 10 m/s.
Before beginning the test, the water level shall have reached the initial static water level.
The defined flow rate shall be applied and maintained.
The water level shall be measured at least every minute until 20 minutes have elapsed and then at least every
5 minutes until the end of the flow-phase.
The test can be ended when the water level is stabilised in the measuring tube, i.e. 3 consecutive measures
which not vary more than 1 cm or after 60 minutes.
© ISO 2007 – All rights reserved 7
DRAFT 2008
hydraulic conductivity range (m/s)
---------------------- Page: 17 ----------------------
oSIST prEN ISO 22282-2:2008
ISO/DIS 22282-2
When the water flow is stopped the recovery of the water level shall be measured. The measurement shall
start 30 s after the stop at the latest and shall be continued at least every following minute until the half time of
the flow-phase duration or until the initial water level has been reached.
6.2.3 Variable head test method
-3 –8
Variable head test method is generally suitable for k-values between 10 m/s and 10 m/s.
Before starting the test the water level shall be measured in the annular space or measuring tube in order to
ensure that the water level corresponds to the groundwater surface.
The water level in the borehole or measuring tube shall be changed by injecting or withdrawing water. The
measurements of the water level shall start immediately from the start of the test.
The measurement intervals shall be defined according to the range of permeability. The test can be ended
when 75 % of the water level recovery is obtained.
6.2.4 Constant head test method
-3 –8
The constant head test method is generally suitable for k-values between 10 m/s and 10 m/s.
Before starting the test the water level shall be measured in the annular space or measuring tube in order to
ensure that the water level corresponds to the groundwater surface.
The constant head test requires the supply or withdrawal of water. A constant hydraulic head increment
(injection or withdrawal) shall be applied to test section. The amount of infiltrated water shall be monitored
against time until steady-
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.