document number sans 1936-1 - geotechnical divisiongeotechnicaldivision.co.za/az 96-10 _dss_...

DRAFT SOUTH AFRICAN STANDARD (DSS):

PUBLIC ENQUIRY STAGE

Document number SANS 1936-1

Reference 7135/1936-1/SP

Date of circulation 2011-12-20

Closing date 2012-02-21

Number and title: SANS 1936-1: DEVELOPMENT OF DOLOMITE LAND — PART 1: GENERAL PRINCIPLES AND REQUIREMENTS Remarks:

PLEASE NOTE:

• The technical committee, SABS SC 59P: Construction standards – Geotechnical standards responsible for the preparation of this standard has reached consensus that the attached document should become a South African standard. It is now made available by way of public enquiry to all interested and affected parties for public comment, and to the technical committee members for record purposes. Any comments should be sent by the indicated closing date, either by mail, or by fax, or by e-mail to

SABS Standards Division Attention: Compliance and Development department Private Bag X191 Pretoria 0001 Fax No.: (012) 344-1568 (for attention: dsscomments) E-mail: [email protected]

Any comment on the draft must contain in its heading the number of the clause/subclause to which it refers. A comment shall be well motivated and, where applicable, contain the proposed amended text.

• The public enquiry stage will be repeated if the technical committee agrees to significant technical

changes to the document as a result of public comment. Less urgent technical comments will be considered at the time of the next amendment.

THIS DOCUMENT IS A DRAFT CIRCULATED FOR PUBLIC COMMENT. IT MAY NOT BE REFERRED TO AS A

SOUTH AFRICAN STANDARD UNTIL PUBLISHED AS SUCH.

IN ADDITION TO THEIR EVALUATION AS BEING ACCEPTABLE FOR INDUSTRIAL, TECHNOLOGICAL,

COMMERCIAL AND USER PURPOSES, DRAFT SOUTH AFRICAN STANDARDS MAY ON OCCASION HAVE TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL TO BECOME STANDARDS TO WHICH REFERENCE MAY BE MADE IN LAW.

AZ96.10 2008/08/08 sabs pta

ISBN 978-0-626- SANS 1936-1:2012 Edition 1

SOUTH AFRICAN NATIONAL STANDARD

Development of dolomite land

Part 1: General principles and requirements

Published by SABS Standards Division 1 Dr Lategan Road Groenkloof �� Private Bag X191 Pretoria 0001 Tel: +27 12 428 7911 Fax: +27 12 344 1568 www.sabs.co.za SABS

SANS 1936-1:2012 Edition 1

Table of changes

Change No. Date Scope

Acknowledgement The SABS Standards Division wishes to acknowledge the work of the National Department of Public Works and the National Dolomite Risk Management Working Committee established on instruction of the Cabinet Committee on Governance and Administration in developing this document. Foreword This South African standard was approved by National Committee SABS SC 59P, Construction standards – Geotechnical standards, in accordance with procedures of the SABS Standards Division, in compliance with annex 3 of the WTO/TBT agreement. This document was published in xxxxxx 2012. Reference is made in 3.10 to the "relevant national legislation". In South Africa this means the Engineering Profession Act, 2000 (Act No. 46 of 2000) or the Natural Scientific Professions Act, 2003 (Act No. 27 of 2003). SANS 1936 consists of the following parts, under the general title Development of dolomite land: Part 1: General principles and requirements. Part 2: Geotechnical investigations and determinations. Part 3: Design and construction of buildings, structures and infrastructure. Part 4: Risk management. Annex A forms an integral part of this document. Annexes B and C are for information only.


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Introduction The development of dolomite land continues to present a challenge in South Africa. While opportunities exist in the development of such land, the adverse effects relating to the formation of sinkholes and subsidences, whether naturally or as a result of the development, cannot be ignored. In the absence of risk mitigation measures, sinkhole formation can result in loss of life. In addition, sinkholes and subsidences can cause severe damage to buildings and infrastructure and affect their serviceability. Avoiding the hazard associated with dolomite land by prohibiting development of any kind on such land is not practical as between four and five million South Africans currently reside or work on such land. Twenty-five per cent of Gauteng, the commercial, mining and manufacturing centre of South Africa, is located on dolomite land. At the other end of the spectrum, undue acceptance of risk is not an option given the potential severity of the consequences and the Government’s obligations in terms of the Bill of Rights. Systematic risk mitigation measures are therefore required. South African research shows that 96 % of sinkholes and subsidences that have occurred to date were man-induced, generated by ingress of water from leaking water-bearing infrastructure, poor stormwater management, etc. or due to artificial lowering of the groundwater level. Consequently, intervention through an integrated, comprehensive and pro-active dolomite risk management strategy has the potential to reduce the incidences of ground instability events (sinkhole and subsidence formation) by reducing the likelihood of water gaining entry into the subsurface profile, or controlling de-watering/recharging of the dolomite aquifer. The objective of SANS 1936 is to set requirements for the development of dolomite land in order to ensure that people live and work in an environment that is seen by society to be acceptably safe, where loss of assets is within tolerable limits, and where cost-effective and sustainable land usage is achieved.


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Contents

Page Acknowledgement Foreword Introduction ................................................................................................................................. 1 1 Scope ..................................................................................................................................... 3 2 Normative references ............................................................................................................. 3 3 Definitions .............................................................................................................................. 4 4 Requirements for land safety and usage ................................................................................ 7 4.1 General ........................................................................................................................ 7 4.2 Qualitative performance requirements ......................................................................... 7 4.3 Quantitative performance requirements ....................................................................... 7 4.4 Repair of sinkholes........................................................................................................ 12 5 Compliance with the requirements ........................................................................................ 12 Annex A (normative) Required competence levels for geo-professionals.............................. 13 Annex B (informative) Dolomite land in South Africa................................................................ 15 Annex C (informative) Mechanisms associated with sinkhole and subsidence formation ....... 19 Bibliography .............................................................................................................................. 23


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Development of dolomite land Part 1: General principles and requirements 1 Scope 1.1 This part of SANS 1936 establishes qualitative and quantitative performance requirements for the development of dolomite land with respect to land safety and land usage, to ensure a) a tolerable hazard; and b) that the current land usage does not compromise the future use of such land. NOTE 1 The qualitative requirements are framed around the current body of knowledge and a standard approach to investigating and evaluating dolomite land, as set out in SANS 1936-2, which has enabled a tolerable hazard rating to be established in South Africa. NOTE 2 This part of SANS 1936 provides a performance-based framework within which dolomite land may be developed. It is based on a similar four-level performance-based system that is used in SANS 10400. 1.2 This part of SANS 1936 also provides the means by which compliance with the performance requirements may be established. 1.3 This part of SANS 1936 applies to dolomites of the Malmani and Campbell Rand subgroups. NOTE This part of SANS 1936 may, however, be used for guidance on good practice on other dolomite and limestone formations. 1.4 This part of SANS 1936 does not apply to recent calcretes. 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. Information on currently valid national and international standards can be obtained from the SABS Standards Division. SANS 1936-2, Development of dolomite land – Part 2: Geotechnical investigations and determinations. SANS 1936-3, Development of dolomite land – Part 3: Design and construction of buildings, structures and infrastructure.


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SANS 1936-4, Development of dolomite land – Part 4: Risk management. SANS 2001-BE3, Construction works – Part BE3: Repair of sinkholes and subsidences. 3 Definitions For the purposes of this document, the following definitions apply. 3.1 acceptable acceptable to the authority administering this part of SANS 1936, or to the parties concluding the purchase contract, as relevant 3.2 competence level level of competence measure of proficiency for professionals engaged in work on dolomite land determined in terms of education, category of professional registration, experience, knowledge and recognition by the profession NOTE The competence levels (1 to 4) are defined in annex A. 3.3 competent person person who is qualified by virtue of his experience, qualifications, training and in-depth contextual knowledge of development on dolomite land to a) plan and conduct geotechnical site investigations for the development of dolomite land, evaluate factual data, develop a geological model, derive interpretative data and formulate an opinion relating to the outcomes of such investigations; b) develop and inspect for compliance the necessary precautionary measures required on dolomite

land to enable safe and sustainable developments to take place; c) develop dolomite risk management strategies; or d) investigate the cause of an event and participate in the development of the remedial measures

required 3.4 dolomite area designation classification of dolomite areas in terms of the extent of mitigation required to achieve and maintain a tolerable hazard 3.5 dolomite land land underlain by dolomite or limestone residuum or bedrock (or both), within the Malmani Subgroup and Campbell Rand Subgroup, typically at depths of no more than a) 60 m in areas where no de-watering has taken place and the local authority has jurisdiction, is monitoring and has control over the groundwater levels in the areas under consideration; or b) 100 m in areas where de-watering has taken place or where the local authority has no jurisdiction or control over groundwater levels NOTE For more information on dolomite land in South Africa, see annex B.


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3.6 dolomite risk management strategy process of using scientific, planning, engineering and social processes, procedures and measures to manage an environmental hazard, and encompasses policies and procedures set in place to reduce the likelihood of events (sinkholes and subsidences) occurring on dolomite land 3.7 dwelling house single dwelling unit and any garage and other domestic outbuildings thereto, situated on its own property 3.8 dwelling unit unit containing one or more habitable rooms and provided with sanitation and cooking facilities 3.9 event occurrence of a sinkhole or subsidence 3.10 geo-professional practitioner of geotechnical engineering or engineering geology who is registered in any category of registration provided for in the relevant national legislation (see foreword) 3.11 hazard source of potential harm NOTE A hazard can be a risk source, i.e. an element which alone or in combination has the intrinsic potential to give rise to risk 3.12 hazard rating number of events that can potentially occur per hectare over a 20-year period due to development NOTE A tolerable hazard rating is one that complies with the requirements for a tolerable hazard (see 3.26). 3.13 infrastructure roads, railway lines, runways, liquid-retaining structures, stormwater systems, power lines, pipelines and associated structures; including water, sewer, fuel and gas lines, reservoirs, public swimming pools, attenuation and retention ponds for stormwater management, pump stations, dams, reservoirs and artificial lakes or similar constructed works. 3.14 inherent hazard potential for an event (sinkhole or subsidence) to develop in a particular ground profile on dolomite land 3.15 inherent hazard class IHC classification system whereby a site is characterized in terms of eight standard inherent hazard classes, denoting the likelihood of an event (sinkhole or subsidence) occurring, as well as its predicted size (diameter) NOTE Inherent hazard classes are defined in SANS 1936-2.


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3.16 parcel of land tract of land, comprising one or more farm portions or properties registered in a deeds registry, and identified for the purpose of development 3.17 potential loss of support potential removal of support below the foundation due to a nominal sinkhole or subsidence event 3.18 qualitative performance requirement performance requirement stated in qualitative terms 3.19 quantitative performance requirements performance criteria which enable qualitative requirements for a nominated level or performance to be complied with 3.20 return period recurrence interval estimate of the average interval of time between events of a certain size 3.21 risk effect of uncertainty on objectives NOTE Risk is often expressed in terms of a combination of the consequences of an event and the associated likelihood of occurrence. 3.22 sinkhole feature that occurs suddenly and manifests itself as a hole in the ground 3.23 stand single piece of land registered, or in the process of being registered, in a deeds registry 3.24 subsidence shallow, enclosed depression NOTE Most South African literature previously used the term “doline” when referring to a subsidence as defined above. The use of the term “subsidence” is in line with international literature and practice. 3.25 suitable capable of fulfilling or having fulfilled the intended function, or fit for its intended purpose 3.26 tolerable hazard where the number of events experienced is less than 0,1 events per hectare per 20 years (preferably tending to 0 per hectare), i.e. a return period of an event occurring on 1 ha of more than 200 years. NOTE Mitigating measures might need to be implemented in order to achieve a tolerable hazard.


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4 Requirements for land safety and usage 4.1 General Risk management is commonly understood to be the culture, processes and structures used to effectively manage potential opportunities and adverse effects. In the context of dolomite land, the opportunities include the development potential of the land. The adverse effects include the hazard presented by the formation of sinkholes and subsidences, which result in potential harm or loss (or both). Broadly speaking, risk on dolomite land can be managed by a) placing restrictions on land use; b) ensuring appropriate development is allowed in relation to the inherent hazard; c) establishing requirements for the management and monitoring of surface drainage and de-

watering; d) establishing requirements for the installation of below-ground infrastructure, particularly water-

bearing services; e) establishing requirements for the construction and maintenance of above-ground and below- ground water-bearing structures; and f) establishing design requirements and procedures for buildings and infrastructure to allow, as a

minimum, the safe evacuation of occupants and users in the event of a hazard occurring. 4.2 Qualitative performance requirements The use of dolomite land and land underlain by the Black Reef Formation shall present a tolerable hazard over time. NOTE 1 The Black Reef Formation, where present, is included as part of dolomite land due to the presence of the weathering products of dolomite within this formation and the possibility that the position of the contact between the Black Reef Formation and overlying Malmani Subgroup dolomite is not accurately mapped. NOTE 2 A tolerable hazard is implied in the quantitative requirements contained in 4.3. NOTE 3 For more information on sinkhole and subsidence formation, see annex C. 4.3 Quantitative performance requirements 4.3.1 The inherent hazard class (IHC) of parcels of dolomite land shall be determined by means of a geotechnical site investigation conducted in accordance with the requirements of SANS 1936-2. NOTE Inherent hazards are expressed in terms of three broad categories, namely low, medium and high, typically, but not exclusively, denoting the anticipated number of events per area over time. 4.3.2 Based on the outcomes of such an investigation, the appropriate dolomite area designation (D1 to D4, see table 1) and design level investigation requirements shall be determined in accordance with tables 2 and 3. Parcels of dolomite land shall be described in terms of their land use category, followed in brackets by their dolomite area designation e.g. C3(D2) so that the appropriate precautionary measures can be readily communicated. NOTE 1 The aim of dolomite area designations D2 to D4 is to introduce precautionary and mitigating measures that strive to reduce the frequency of events per hectare to what equates to a tolerable hazard. NOTE 2 Dolomite area designation D1 applies only to those instances where the development of the land presents a tolerable hazard.


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4.3.3 On land categorized as D2 and D3, in terms of table 1, appropriate precautionary measures in accordance with the principles and requirements of SANS 1936-3 shall be implemented to mitigate the risks associated with the development of such land. 4.3.4 On land designated as D4, in terms of table 1, the following site-specific measures shall be implemented: a) site characterization, analysis and design, specification of precautionary measures, supervision

of implementation and formulation of a dolomite risk management plan shall be undertaken by a Competence Level 4 geo-professional (see annex A);

b) the foundation design, design of the structure, precautionary measures and dolomite risk

management plan shall specifically address and effectively mitigate the dolomite risks present on the site;

c) the site characterization, foundation design and design of the structure, precautionary measures

and dolomite risk management requirements shall be reviewed and approved by an independent Competence Level 4 geo-professional (see annex A) and, where relevant, by a structural engineer with a similar level of competence; and

d) all aspects of the development proposal shall be reviewed and approved by the local authority,

who may request a further review by an authority-designated Competence Level 4 peer (see annex A), if required.

4.3.5 The owners of developments located on dolomite land shall establish and implement appropriate dolomite risk management strategies in accordance with the principles and requirements of SANS 1936-4 to mitigate the risks associated with developments on such land. 4.3.6 The local authorities in whose jurisdiction the developments in 4.3.2 to 4.3.5 fall shall establish, implement and maintain a dolomite risk management strategy in accordance with the principles and requirements of SANS 1936-4 to mitigate the risks associated with developments on such land. 4.3.7 Parcels of land underlain by the Black Reef Formation shall comply with the requirements of 4.3.1 to 4.3.6 unless such formation has been assessed as presenting no risk of sinkhole or subsidence formation in accordance with the requirement of SANS 1936-2, and is designated as D1. 4.3.8 In proposing suitable foundation types in D3 and D4 areas, consideration shall be given to the potential loss of support which could be anticipated for the designated inherent hazard class based on expected initial sinkhole size. The philosophy to be applied to the design of the foundations is that there shall be sufficient structural integrity and stability to allow occupants to safely escape in the event of sudden loss of support below the foundations of a structure.

Table 1 — Dolomite area designations

Dolomite area

designation Description

D1 No precautionary measures are required.

D2 General precautionary measures, in accordance with the requirements of SANS 1936-3, that are intended to prevent the concentrated ingress of water into the ground, are required.

D3 Precautionary measures in addition to those pertaining to the prevention of concentrated ingress of water into the ground, in accordance with the relevant requirements of SANS 1936-3, are required.

D4 The precautionary measures required in terms of SANS 1936-3 are unlikely to result in a tolerable hazard. Site-specific precautionary measures are required.

Table 2 — Permissible land usage per inherent hazard class

1 2 3 4 5 6 7 8 9 10

Land usage Inherent hazard class determined in accordance with the requirements of SANS 1936-2

1 2 3 4 5 6 7 8 Designation Description

Dolomite area designation and footprint investigation requirement

Commercial and miscellaneous non-residential usage

C1 Places of detention, police stations, and institutional homes for the handicapped or aged D3 + FPI D4

C2 Hospitals, hostels, hotels D3 + FPI D4

C3 Commercial developments < 3 storeys, including railway stations, shops, wholesale stores, offices, places of worship, theatrical, indoor sports or public assembly venues, other institutional land uses such as universities, schools, colleges, libraries, exhibition halls and museums, light (dry) industrial developments, dry manufacturing, commercial uses such as warehousing, packaging, and electrical sub-stations, filling stations

D2 + FPI D3 + FPI D4

C4 Commercial developments > 3 storeys, including railway stations, shops, wholesale stores, offices, places of worship, theatrical, indoor sports or public assembly venues, other institutional land uses such as universities, schools, colleges, libraries, exhibition halls and museums, light (dry) industrial developments, dry manufacturing, commercial uses such as warehousing, packaging, and electrical sub-stations

D2 + FPI D3 +FPI D4

C5 Fuel depots, processing plants or any other areas for the storage of liquids, waste sites.

D2 + DLI D3 + DLI D4

C6 Outdoor storage facilities, stock yards, container depots D2 + DLI D3 + DLI D4

C7 Parking garages D2 D3 + FPI D4

C8 Parking areas D2 D3 D4

DLI = Design level investigation in accordance with the requirements of SANS 1936-2, as deemed appropriate by the competent person. FPI = Design level investigation specifically below the footprint of the structure.

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SA

NS

1936-1:2012 E

dition 1

SA

NS

1936-1:2012 E

dition 1

Table 2 (continued)

1 2 3 4 5 6 7 8 9 10




High rise dwelling units

RH1 > 10 storeys D4

RH2 > 3 storeys with a population of < 1 500 people per hectare D2 + FPI D4

RH3 > 3 storeys with a residential coverage ratio of < 0,4, no higher than 10 storeys, and a population of < 800 people per hectare

D2 + FPI D3 + FPI D4

Low rise dwelling units

RL1 < 3 storeys with 80 to 120 units per hectare and a population not exceeding 600 people per hectare

D2 + FPI D4

RL2 < 3 storeys with up to 80 units per hectare and a population not exceeding 400 people per hectare D2 + FPI D3 + FPI D4

Dwelling houses

RN1 Up to 60 dwelling houses per hectare with stands larger than 150 m2, and a population of < 300 people per hectare

D2 D3 D4

RN2 Up to 25 dwelling houses per hectare with stands no smaller than 300 m2, and a population of < 200 people per hectare

D2 D3 D4

RN3 Up to 10 dwelling houses per hectare with 1 000 to 4 000 m2 stands, and a population of < 60 people per hectare

D2 D3 D3 + FPI D4

Other

AO Agriculture that does not require irrigation in any form or the storage of water, parkland and public open spaces that are not irrigated and grazing pastures

See SANS 1936-4


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SA

NS

1936-1:2012 E

dition 1

Table 2 (concluded)

1 2 3 4 5 6 7 8 9 10




A1 Agriculture that requires intensive irrigation See SANS 1936-4

A2 Agriculture that requires irrigation, including botanical gardens, sports fields, driving ranges, golf courses, parkland and public open spaces

See SANS 1936-4


NOTE 1 D1, D2, D3 and D4 have the meanings assigned in table 1.

NOTE 2 Residential coverage ratio = footprint area/site area.

Table 3 — Permissible infrastructure and social facilities per inherent hazard class

1 2 3 4 5 6 7 8 9 10

Infrastructure and social facilities Inherent hazard class determined in accordance with the requirements of SANS 1936-2


Dolomite area designation

IN1 Trunk roads (national and regional roads which facilitate intercity travel) and primary distributor roads (major arterial roads forming the primary network for an urban area as a whole), railway lines, power lines, runways, bulk pipelines, including water, sewer, fuel and gas lines, and pump stations

D2 D3 D4

IN2 Reservoirs and public swimming pools, water care works, attenuation and retention ponds for stormwater management and artificial lakes

D2 D3 D4

IN3 Cemeteries D3 D4

IN4 Dams, slimes dams D3 D4

IN5 Solid waste disposal facilities D3 D4

NOTE D1, D2, D3 and D4 have the meanings assigned in table 1

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Table 4 — Inherent hazard categories

1 2

Inherent hazard Anticipated events per hectare per 20 years a

Low

Typically less than 0,1 events but occurrence of events cannot be excluded. Return period of an event occurring in an area of one hectare is greater than 200 years.

Medium Typically between 0,1 and 1,0 events. Return period of an event occurring in an area of one hectare is between 200 and 20 years.

High Typically greater than 1,0 events. Return period of an event occurring in an area of one hectare is less than 20 years.

a Based on events that have occurred per hectare in a 20-year period in the "type" area with no special precautions taken in respect of the design and maintenance of water-borne services

4.4 Repair of sinkholes 4.4.1 Sinkholes on developed land, including recreational areas and publicly accessible spaces, shall be repaired in accordance with the requirements of SANS 2001-BE3. 4.4.2 On agricultural or undeveloped areas other than recreational areas and publicly accessible spaces, an uncontrolled backfill method that provides a positive, domed topography over the sinkhole and a berm upstream from the sinkhole to divert stormwater away from the rehabilitated sinkhole shall be specified. 4.4.3 The uncontrolled backfill method and standard repair methods described in SANS 2001-BE3 shall be implemented under the control of a Competence Level 2 geo-professional. All other repair of sinkholes and subsidences shall be designed and implemented under the control of a Competence Level 3 geo-professional. 5 Compliance with the requirements 5.1 The person responsible for applying any aspect of the various parts of SANS 1936 shall comply with the required levels of competence set out in this part of SANS 1936 and defined in annex A. 5.2 The competent person shall certify compliance with this part of SANS 1936 on all township plans, construction drawings and reports, as relevant, and provide his name, registration particulars and date below such certification.


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Annex A (normative)

Required competence levels for geo-professionals

A.1 Criteria for assessment of competence The following criteria shall be used in assessing the competence of geo-professionals: a) tertiary education; b) category of professional registration; c) experience; d) knowledge; and e) recognition by the profession. A.2 Competence levels A.2.1 Figure A.1 defines the competence levels (levels 1 to 4) in terms of education, professional registration, experience and recognition by the profession. A.2.2 Practitioners in Competence Level 1 shall be in possession of the required tertiary education qualification, shall be registered as candidate geotechnical practitioners, and shall work under the supervision of a registered professional. A.2.3 Geo-professionals in Competence Level 2 shall be registered professionals and possess a level of knowledge and experience in keeping with the norms of the profession. A.2.4 Geo-professionals in Competence Level 3 shall, in addition to the years of experience specified in figure A.1, have in-depth knowledge and experience specifically in the development of dolomite land. They should enjoy recognition by the profession as competent geo-professionals. A.2.5 Geo-professionals in Competence Level 4 shall, in addition to the years of experience specified in figure A.1, enjoy recognition by the profession as specialist geo-professionals, possessing a level of specialist knowledge and experience above that expected of the profession. They should be making a contribution to the state-of-practice of the development of dolomite land by the application of advanced techniques or by means of research, publications or involvement in engineering education. A.2.6 In the context of all the parts of SANS 1936, the experience required for attaining Competence Levels 3 or 4 shall include significant experience in site characterization; analysis and design of foundations, infrastructure and precautionary measures; supervision of implementation and risk management in dolomite areas.


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5 years practice as an experienced

geo-professional

Level 4

Experience geo-professional

Level 3

Expertgeo-professional

Experiencedgeo-professional

7 years experience ingeotechnical engineering

after registration

Recognition

Experience

5 years experience ingeotechnical engineering

after registrationAppropriatetraining and

registration asPr Tech EngGeotechnical

engineeringtechnologist

Geotechnicalengineeringtechnician

Geotechnical engineer

Engineering geologist

Level 2

Registeredgeo-professional

Registration

Geotechnical advisor

Level 1

Candidate

Register as Pr Eng

Register as Pr Sc Nat

Register as Pr Tech Eng

Register as Pr Technic Eng

Education

Under-graduatestudent

Accredited degree incivil engineering

Honours degree in geology or engineering

geology

Accredited B Tech

Accredited National Diploma in civil

engineering

School Learner

Drg.760i NOTE 1 Adapted from Site investigation in construction – Volume 2: Planning, procurement and quality management (see bibliography). NOTE 2 The uppermost block in each level is the requirement for progression to the next level. NOTE 3 The number of years experience is a guideline and assumes that the person is practising predominantly in geotechnical engineering or engineering geology during this period.

Figure A.1 — Competence levels for geo-professionals


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Annex B (informative)

Dolomite land in South Africa

B.1 Distribution of dolomite land in South Africa B.1.1 Although carbonate-related (therefore also dolomite-related) instability can take place in any karstic terrain, most instability features have been recorded in the Chuniespoort Group and the Campbell Rand Subgroup. However, these events could occur in any area underlain by carbonate rocks.

Figure B.1 — Distribution of carbonate rocks in South Africa B.1.2 The main sedimentary carbonate units are the following: a) Malmani Subgroup, Chuniespoort Group; NOTE The requirements of this part of SANS 1936 apply to these groups (see 1.3). b) Campbell Rand Subgroup, Ghaap Group; NOTE The requirements of this part of SANS 1936 apply to these groups (see 1.3). c) Mzimkulu Group; d) Gariep Supergroup;


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e) Gamtoos Group; f) Cango Group; g) Malmesbury Group; h) Nama Group; i) Vanrhynsdorp Group; j) Mzamba Formation; k) Algoa Group; l) Bredasdorp Group; m) Maputaland Group; and n) Sandveld Group. B.1.3 In the Gauteng Province, the carbonate formations comprise the Malmani Subgroup of the Chuniespoort Group (Transvaal Supergroup). The Malmani Subgroup is underlain by the Black Reef Formation. This latter formation represents a laterally continuous unit of conglomerate-quartz arenite-shale with a gradual transition from carbonaceous shale to carbonate, which can be found throughout the Transvaal Supergroup basin. Arising as a weathering product from carbonate, dolomite residuum (wad) can thus be present in the Black Reef Formation. The Malmani Subgroup is subdivided into various formations of which some are chert-poor and some are chert-rich. The dolomite formations are, in places, overlain by younger rocks of the Pretoria Group, Transvaal Supergroup or the Karoo Supergroup (or both), and can be mantled by unconsolidated material of the Cenozoic age. B.1.4 In the Kwazulu-Natal Province, the carbonate rock is hosted in the high-grade metamorphic rocks of the Marble Delta Formation of the Mzimkulu Group, that occurs as an oval-shaped outcrop, 10 km northwest of Port Shepstone. Carbonate rock is also found in the Tugela Group and located near Kranskop and east of Greytown. B.1.5 In the Mpumalanga Province, the carbonate formations comprise the Malmani Subgroup (Chuniespoort Group, Transvaal Supergroup). Alteration of dolomite to limestone (de-dolomitization) has occurred in many places due to the intrusion of the Bushveld Complex. B.1.6 In the North West and Limpopo Provinces, the carbonate formations comprise the Malmani Subgroup of the Chuniespoort Group, Transvaal Supergroup. Alteration of dolomite to limestone due to the intrusion of the Bushveld Complex is particularly evident northeast of Mokopane (formerly known as Potgietersrus). B.1.7 In the Northern Cape Province, carbonate rocks mainly comprise the Campbell Rand Subgroup (Ghaap Group, Transvaal Supergroup). Carbonates can also be found in various groups of the Gariep Supergroup near Eksteenfontein, as well as in the Schwarzrand Formation of the Nama Group, near Vioolsdrif. B.1.8 In the Western Cape Province, carbonate rocks are encountered in the following locations: a) Van Rhynsdorp-Bitterfontein area – Knersvlakte and Kwanous Formations of the Vanrhynsdorp

Group; b) Langebaan area – Langebaan Formation of the Sandveld Group; c) Piketberg area – various formations of the Malmesbury Group;


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d) Hermanus to Mossel Bay area – Bredasdorp Group; and e) Area north of Oudtshoorn – Cango Caves Group. B.1.9 In the Eastern Cape Province, the carbonate rocks form part of the following: a) Gamtoos Group; b) Cenozoic Algoa Group; and c) Mzamba Formation, situated south of Port Edward. B.2 Limestone and dolomite composition B.2.1 Ancient carbonate rocks are predominantly composed of two minerals: calcite (CaCO3) and dolomite (CaMg(CO3)2). When a carbonate rock is dominated by calcite (more than 95 %), it is called limestone. When it is dominated by dolomite (the mineral), it is called dolomite (the rock). Limestone is thus a chemical or biochemical sediment consisting essentially of calcium carbonate (CaCO3), primarily in the form of calcite, and minor constituents such as silica, feldspar, pyrite and siderite. B.2.2 Dolomite, as a rock, contains more than 90 % dolomite and less than 10 % calcite as well as detrital minerals and secondary silica (chert). Very few, if any, sedimentary dolomites are truly only CaMg(CO3)2, and are better represented as: Ca(1+x)Mg(1-x)(CO3)2, encompassing the spectrum from calcian to magnesian dolomites. B.2.3 For the purposes of SANS 1936, any reference to dolomite applies equally to limestone. B.3 Weathering of dolomite and ground instability B.3.1 Rainwater (H2O) takes up carbon dioxide (CO2) in the atmosphere and soil (where the concentration of this gas can be up to 90 times greater than in the atmosphere) to form a weak carbonic acid (H2CO3). The slightly acidic groundwater circulating along tension fractures, faults and joints in the dolomite succession causes leaching of the carbonate minerals. The solubility of dolomite is high in comparison to other rocks, but significant solution cannot be observed over short periods (months and years). B.3.2 The process of dissolution can be represented as follows: CaMg(CO3)2 + 2 H2CO3 → Ca(HCO3)2 + Mg(HCO3)2 B.3.3 The process of dissolution progresses slowly in the slightly acidic groundwater (above and at the groundwater level). The resultant bicarbonate-rich water emerges at springs and is carried away. B.3.4 The process of dissolution results in a vertically zoned succession of residual products which, in turn, are generally overlain by geologically younger formations or soils. Hard, unweathered dolomite bedrock is overlain by slightly weathered jointed bedrock and thereafter, through a sudden, dramatic transition, passes upwards to totally weathered and low strength, insoluble residual material consisting of mainly manganese oxides (wad), chert and iron oxides, that reflect the original insoluble matrix structure (see figure B.2). Depending upon the local subsurface structure, this very low strength, porous and permeable horizon can, in certain locations, be up to several tens of metres thick but is generally less than 10 m thick. With the passage of geological time, together with the downward progression of the intense weathering of the dolomite bedrock, compaction by the mass of the overlying materials results in a


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progressive densification of these low strength materials. Consequently, the vertical succession of the residual products of weathering reflect an upward increase in strength and a decrease in porosity and permeability. This process results in a decrease in overburden quality with depth which, in turn, leads to higher rates of penetration (often observed in drilling investigations) when the dolomite bedrock is approached. Infiltrating water from leaking services or surface accumulations acting on this low density material results in a loss of support through slumping or subsurface erosion. B.3.5 Given sufficient time and the correct triggering mechanisms, instability might occur naturally but it is usually expedited by man’s activities. Instability can occur in the form of sinkholes and subsidences. The primary triggering mechanisms in such instances include a) the ingress of water from leaking water-bearing services, b) poorly managed surface water drainage, and c) groundwater level drawdown. Topography and drainage, the natural thickness and origin of the transported soils and residuum, the nature and topography of the underlying strata, the depth and expected fluctuations of the groundwater level, and the presence of structural features, such as faults, fractures and dykes, are all factors which influence the risk of subsidence taking place.

Figure B.2 — Conceptual diagram of a typical dolomite profile, as historically seen in South Africa


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Annex C (informative)

Mechanisms associated with sinkhole and subsidence formation

C.1 General The geotechnical factors that influence the likelihood of the manifestation of a hazard include ground surface topography, drainage, the nature, thickness and origin of the transported soils and residuum, the nature and morphology of the underlying strata, the depth and expected fluctuations of the groundwater level, and the presence of underlying structural features, such as faults, fracture zones and dykes. Dolomite-related ground movement events are usually induced by human activity. Primary trigger mechanisms include ingress of water from leaking water-bearing structures, poorly managed surface water drainage, and the lowering of the natural groundwater level. C.2 Sinkholes C.2.1 A sinkhole is a feature that occurs suddenly and manifests itself as a hole in the ground. A classification of sinkholes in terms of size is given in table C.1.

Table C.1 — Classification of sinkholes in terms of size

Maximum diameter of surface manifestation

m Sinkhole

< 2 Small size

2 to 5 Medium size

5 to 15 Large size

> 15 Very large size

C.2.2 The mechanism of sinkhole formation is as follows (see figure C.1): a) Cavities, which might be in a metastable state, exist within bedrock or the overburden. b) Active subsurface erosion caused by the concentrated ingress of water results in transportation

(mobilization) of materials downwards into the underlying cavities (receptacles) in or above the dolomite bedrock.

c) Headward erosion leads to successive arch collapse. The final arch might be stable for a

considerable length of time and is sometimes supported by a near-surface stable layer. d) A triggering mechanism leads to the breaching of the final arch causing the void to break through to surface. e) Lowering of the water table may exacerbate these conditions by increasing the potential

development space as a result of lowering the base level for subterranean erosion and by exposing receptacles that were previously submerged.


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C.2.3 A number of independent conditions are necessary before a sinkhole can form: a) Adjacent stable material forms abutments for the roof of the void. b) A condition of arching develops in the residuum. c) A receptacle exists below the arch to accept mobilized material. d) A void develops below the arch in the residuum. e) Some triggering agency is applied to cause the final roof to collapse and ‘daylighting’ of the void.

Groundwater level

Drg.760bGroundwater level

Pipe Leaking pipe

a) Metastable state b) Active subsurface erosion

Sinkhole

Voiddaylights

Groundwater level

Groundwater level

Drg.760c

c) Progressive collapse of roof of void d) Collapse of last arch to produce a sinkhole

Figure C.1 — Typical mechanism for the development of a sinkhole


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C.3 Subsidences C.3.1 Types of subsidences A subsidence is an enclosed depression. In South Africa the term refers to the geomorphological feature that manifests itself in the landscape, and not to the mechanism of formation. A number of subsidence types are identified. De-watering-type subsidences and surface-saturation-type subsidences are two examples. A third type, that may be referred to as an incompletely developed sinkhole, has a similar surface appearance to the former two types, but is caused by downward erosion of subsurface materials. C.3.2 De-watering-type subsidence A de-watering-type subsidence occurs gradually and typically manifests itself as a large enclosed depression, not necessarily circular in shape. The mechanism of this type of subsidence is as follows (see figure C.2): a) An area is underlain by compressible dolomite residuum with the groundwater level within or

above the compressible material. As the groundwater recedes, pore pressures in the residual dolomite soils, typically characterized by high void ratios, gradually dissipate and the effective stress on the soil increases causing consolidation of the compressible material.

b) A surface depression occurs gradually due to the load of the near-surface materials on the

deeper lower density materials that settle into a denser state. c) The size of the features depends on the subsurface profile, i.e. the thickness, nature and depth

of the lower density materials below the OWL (original groundwater level), the configuration and depth of the dolomite bedrock.

C.3.3 Surface-saturation-type subsidence These dolomite-related subsidence features are typically relatively small (i.e. less than 5 m in diameter). The mechanism of this type of subsidence is as follows: a) An area is underlain by compressible dolomite residuum at a relatively shallow depth with the

groundwater level within or below the compressible material. The movement of the groundwater level does not necessarily play a role in ground surface movement.

b) The surface materials are saturated due to poor water management, i.e. poor drainage or a

leaking wet service. c) The wetting front penetrates the surface material and reaches the low density material. d) A surface depression occurs gradually due to the mobilization of the deeper lower density

materials which then settle into a denser state (saturation driven), causing the overlying soils to migrate downward in the subsurface profile to take up the gap left by the material that has settled to the denser state.

e) The rate of movement generally decreases rapidly when the cause of wetting is stopped. f) The size of the features depends on the profile underlying the saturated area, i.e. the thickness,

nature and depth of the near-surface and deeper lower density materials, the configuration and depth of the dolomite bedrock, and the extent of the saturation (e.g. the extent of the area covered by water, the volume of the water, and the length of the period during which saturation occurs).


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C.3.4 Partly developed sinkholes The premature termination of subsurface erosion by ingress of water can also result in a settlement feature at the surface that appears to be similar to a subsidence. The process of sinkhole formation is prematurely terminated due to inadequate receptacle space, choking of the throat, or inadequate energy in the mobilizing agency to continue moving the soil downwards.

Paleo-subsidence not apparent at thesurface but indicated by sagging chert rubble and pebble marker

Shear zone and tension cracks

a) Metastable condition before lowering

of groundwater level b) Metastable condition after lowering

of groundwater level Subsidence developmentcomplete

Drg.760f c) Progressive consolidation of

dolomite residuum (wad) d) Final metastable state

Dolomite rock Pebble marker Chert rubbleDrg.760fa

Figure C.2 — Typical mechanism for the development of a de-watering-type subsidence


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Bibliography

Standards SANS 10400 (SABS 0400), The application of the National Building Regulations. Other publications Buttrick D.B., Van Schalkwyk A., Kleywegt R.J. and Watermeyer, R.B. (2001). Proposed method for dolomite land hazard and risk assessment in South Africa. Journal of the South African Institution of Civil Engineering, Volume 43, No 2. Site Investigation Steering Group. Site investigation in construction – Volume 2: Planning, procurement and quality management. Thomas Telford, London. 1993.

© SABS

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