Department of Civil Engineering and Building Sciences – Vaal University of Technology
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Table of Contents
TOC o “1-3” h z u 1.ABSTRACT PAGEREF _Toc512257220 h 12.INTRODUCTION PAGEREF _Toc512257221 h 12.1 WHAT IS DOLOMITE PAGEREF _Toc512257222 h 22.2 PROBLEMS ENCOUNTERED IN AREAS UNDERLAIN BY DOLOMITE PAGEREF _Toc512257223 h 32.3 THE PROCESS OF DISSOLUTION OF DOLOMITE PAGEREF _Toc512257224 h 32.4 WHAT ARE SINKHOLES PAGEREF _Toc512257225 h 42.5 THE MECHANISM OF SINKHOLE FORMATION PAGEREF _Toc512257226 h 52.6 CLASSIFICATION OF DOLOMITIC AREAS IN SOUTH AFRICA PAGEREF _Toc512257227 h 72.7 PRECAUTINARY MEASURES TAKEN IN AREAS WITH EXISTING RESIDENTS OR INFRASRUCTE AND STRUCTURES PAGEREF _Toc512257228 h 102.8 PRECAUTIONARY MEASURES TAKEN PRIOR CONSTRUCTION IN DOLOMITIC AREAS IN SOUTH AFRICA PAGEREF _Toc512257229 h 112.9 TECHNOLOGY AVAILABLE IN SOUTH AFRICA THAT CAN DETECT SINKHOLES PAGEREF _Toc512257230 h 113.RECOMENDATIONS PAGEREF _Toc512257231 h 124.CONCLUSION PAGEREF _Toc512257232 h 135.REFERENCES PAGEREF _Toc512257233 h 14 ANNEXURE A PAGEREF _Toc512257234 h 16
Dolomite poses a huge risk to construction as it threatens lives of the people who will be utilizing the land due to the risk of sinkholes and land subsidence’s developing. This assignment describes and evaluates areas In South Africa that are underlain by dolomite and limestone, and possible solutions that can assist in monitoring the safety of those affected where areas already dominated by residents and commercial properties by monitoring and placing restrictions as well as monitoring leaking underground water services in those affected areas. There is an increased demand for housing in South Africa, based on that, construction cannot be completely prohibited, however the necessary investigations that confirm the extent of the risk of hazards and the type of dolomite designation needs to be determined by a competent person prior to permitting construction to take place in those designated areas.
Construction in those areas needs a careful design that will allow facilities that can aid residents to vacate the premises when a hazard is detected. Once construction has taken place education of communities, constant monitoring of the underground water services and storm water is major requirement, this will ensure that a minimum risk to dissolution of the dolomite or limestone is reduced.
INTRODUCTION Emphasize on this assignment is based on effects of Dolomite in construction and aspects that needs to be considered for construction of dolomitic areas. It highlights problems encountered in areas underlain by dolomite materials, process of dissolution of dolomite, sinkholes and its mechanisms, classification of dolomitic areas in South Africa, precautionary measures for construction of dolomitic areas and the technology available to detect sinkholes.
2.1 DOLOMITEDolomite is a sedimentary rock similar to limestone, it is commonly known as either dolostone or dolomite rock and it is mainly comprised of dolomite minerals. Dolomite makes up approximately 2 percent of the earth’s crust 4. Ancient carbonate rocks are mostly made of two minerals which are calcite (CaCO3) and dolomite (CaMg (CO3)2).
When a carbonate rock is dominated by more than 95% calcite it is called limestone and when it is dominated by the dolomite mineral, it is referred to as the rock called dolomite (which contains more than 90 % dolomite and less than 10 % calcite and secondary silica called chert 5. Dolomite is thought to form when the calcite (CaCO3) in Limestone is modified by magnesium-rich groundwater through a chemical reaction process called dolomitization. During this process, the available magnesium promotes the conversion of calcite into dolomite (CaMg (CO3)2). Dolomitization can completely either alter the limestone into dolomite or covert it to dolomitic limestone 8.
The figure below shows the areas that are underlain by dolomite in South Africa:
Figure 1 – Distribution of dolomite in northern areas of SA 9
2.2 PROBLEMS ENCOUNTERED IN AREAS UNDERLAIN BY DOLOMITEDolomites and limestone rocks when exposed to rainwater over millions of years’ cause’s a dissolution of the rocks resulting in cave systems, low density materials and voids. The voids formed will eventually result in sink holes/doline and land subsidence. Sinkholes are depressions in the ground and are dangerous to people in areas that are likely to be affected by them as they do not give an alert when they occur. Sinkholes are mostly common in “karst terrain” regions i.e. where the type of rock below the land surface is likely to being dissolved by ground water.
2.3 THE PROCESS OF DISSOLUTION OF DOLOMITE
According to SANS 1936 -1 5, the process of dissolution of dolomite can take a few months or years to occur. With sufficient time and the correct activation of mechanisms, instability might occur naturally but it is usually activated by man’s activities where instability can occur in the form of sinkholes and subsidence. Activities such as when water enters from the leaking services; surface drainage is poorly managed and the ground water level is drawn down. The nature of underlying topographical structure of the land plays a role on determining the possibility of subsidences and sinkholes i.e. topographical strata which have the presence of fractures, faults and dykes are more at risk 5.
SANS 1936 – 1 5 outlines the process of dissolution as follows:
Rainwater (H2O) mixes with carbon dioxide (CO2) in the atmosphere and soil to form a weakcarbonic acid with the chemical composition of H2CO2.
Thereafter the slightly acidic groundwater circulating along tension fractures, faults and joints in the dolomite succession causes the leaching of the carbonate minerals.
The dissolution process can be represented as follows:
CaMg (CO3)2 + 2 H2CO3 ? Ca (HCO3)2 + Mg (HCO3)2
2.4 SINKHOLESRain water and ground water can cause sinkholes, however water entering from leaking services and stormwater can also result in sinkholes 7. Below are pictures that shows the appearance of a sinkhole which were taken from google 8
Figure 2 – Sinkhole in Northern Cape along R31, between Danielkruil and kuruman (14 January 2017) 8 Figure 3 – Sinkhole around the N2 road (10 August 2016) 9
According to SANS 1936:1 5, Sinkholes are classified according to the following categories in table 1 below:
Maximum diameter of surface sinkhole manifestation
2 Small sizes
2 to 5 Medium sizes
5 to 5 Large sizes
15 Very large size
Table 1: Classification of sinkholes
2.5 THE MECHANISM OF SINKHOLE FORMATIONAs mentioned in SANS 1936:1 5, here are a few conditions that need to happen prior to a sinkhole can form: (see figure 5 below):
Flow diagram 1: The mechanism of sinkholes formation
The figures 3 below taken from sciencestruck.com 11, shows the be formed through penetration of water into the soil: mechanism in which sinkhole can
Figure 6: Formation of sinkhole from rain water
As explained in sciencestruck.com 10, the process of sink formation is as follows:
Rain water percolates into the soil and absorbs carbon dioxide from the soil.
With time the carbonated water dissolves the limestone bedrock and starts creating cavities
The layer becomes thin and collapses overtime when a load is applied.
Although rain water can cause dissolution of the rock, fluctuating levels of ground water, excess rainfall and even drought as well as human activity (such as drilling wells, pumping groundwater, mining, construction, faulty drainage or sewage) can also contribute to sinkhole formation 11.
2.6 CLASSIFICATION OF DOLOMITIC AREAS IN SOUTH AFRICAIt has been found that in South Africa, the provinces underlain by dolomite rock formations are Gauteng, North, North-West and Mpumalanga 1. It is therefore important that prior to construction or designing
extensive geological investigations be conducted. The aim is to understand the existing ground conditions and to determine the method of construction that can be adopted should construction be possible in the designated area. As specified in SANS 1936-15, R.B Watermeyer et al 12 and the
PW 344 manual 2 dolomites are classified into 4 categories i.e. D1 –D4. Below is 4 categories and the precautionary measures that can be taken under each dolomite designation class to reduce the hazards per hectare:
D1 No precautions are required to permit construction of housing units due to adequate thickness
D2 The risk of sinkhole or doline is adjudged to be sure that only precautionary measures which are intended to prevent the concentrated ingress of water into the ground, are required to permit the construction of housing units
D3 The risk of sinkhole or doline is adjudged to be sure that only precautionary measures in addition to those pertaining to the concentration of ingress of water into the ground, are required to permit the construction of housing units
D4 The risk of sinkhole or doline formation in such that precautionary measures cannot reduce such risks acceptable limits so as to permit the construction of housing units or the precautionary measures which are required are impartible to implement.
Table 2: Dolomitic area classification
In the above table, Designation D1 applies is only applicable to cases where the development of the land presents a hazard that can be tolerated whereas designation D2 to D4, precautionary measures need to be introduced to counteract the effects of the dolomite per hectare. In designation area D4 precautionary measures taken needs to have the following conditions applicable:
Site characterization, analysis and design, specification of precautionary measures, supervision of implementation and formulation of a dolomite risk management plan should be undertaken by a competence level 4 professional;
The foundation design, design of the structure, precautionary measures and dolomite risk management plan should specifically address and effectively mitigate the dolomite risks present on the site;
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 and where relevant, by a structural engineer with a similar level of competence and
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, if required SANS 1936 -1 5.
PW 344 2 and SANS 1936 – 4 13, indicates that the stability of the area surrounded by dolomite is divided into 3 categories namely low, medium and high. The risks are categorized on the ground movement events anticipated per hectare per 20 year period and are as follows:
Low risk: Meaning that 0 events per hectare are anticipated, but the effects cannot be excluded hence up to 0,1 events are anticipated per hectare,
Medium risk: 0,1 to 1 events are anticipated per hectare and
High Risk: The anticipated events are more between 1,o and more per hectare
Based on the type of risk that the area is subjected to the various design methods for construction can be adopted. Furthermore the current practice requires that the risk class be described in 8 unique classes’ where a lower risk reflects class 1, meaning that the areas are characterized as reflecting a low inherent risk of sinkhole and doline formation.
Class 1 (Low risk) – These are areas characterized as reflecting a low Inherent Risk of sinkhole and doline formation (all sizes) with respect to ingress of water.
Class 2 (Medium risk) – Areas characterized as reflecting a medium Inherent Risk of small sinkhole and doline formation with respect to ingress water.
Class 3 (Medium risk) – Areas characterized as reflecting a medium Inherent Risk of medium sinkhole (2, 0 – 5, 0 m) and doline formation with respect to ingress of water.Class 4 (Medium risk) – Areas characterized as reflecting a medium Inherent Risk of large size sinkhole (5, 0 – 15, 0 m) and doline formation with respect to ingress of water.
Class 5 (High risk) – Areas characterized as reflecting a high Inherent Risk of small sinkhole and doline formation all sizes (<2m – >15m) with respect to ingress of water.
Class 6 (High risk) – Areas characterized as reflecting a high Inherent Risk of medium size sinkhole and doline formation (all sizes) i.e. <2m – >15m with respect to ingress of water.
Class 7(High risk) – Areas characterized as reflecting a high Inherent Risk of Large sinkhole (>15m) and doline formation with respect to ingress of water.
Class 8 (High risk) – Areas characterized as reflecting a high Inherent Risk of a very large size (>15m) sinkhole and doline formation with respect to ingress of water.
PW 344 2 additionally suggest the following recommendations are given for different risk classes:
Residential, light industrial and commercial development can be constructed, provided that appropriate water precautionary measures are applied.
Other factors affecting economic viability such as excavatability, problem soils, etc. must be evaluated.
A residential development with remedial water precautionary measures can be permitted.
No site and services schemes are allowed.
May consider for commercial or light industrial development
Class 3 and 4
Selected residential development with exceptionally stringent precautionary measure and design criteria.
No site and services schemes.
May consider for commercial or light (Dry) industrial development with appropriate precautionary measure.
These areas are usually not recommended for residential development but under certain circumstances selected residential development (including lower density residential development, multi-storied complexes, etc. may be considered,
Commercial and light industrial development may be permitted.
The risk of sinkhole and doline formation is adjudged to be such that precautionary measures, in addition to those pertaining to the prevention of concentrated ingress of water into the ground are required to permit the construction of housing units.
These areas are usually not recommended for residential development but under certain circumstances high-rise structures or gentleman’s estates (4 000 m² with 500m² proven suitable for placing a house) may be considered,
Suitable for commercial or light industrial development.
Expensive foundations designs may be necessary.
Sealing of surfaces, earth mattresses, water in sleeves or in ducts, etc.
No residential development but suitable for parkland development.
Special types of commercial or light industrial (Dry) development can be permitted e.g. bus or trucking depots, coal yards, parking areas.
All surfaces must be sealed.
No development is permitted, it can be nature reserves or parklands.
2.7 PRECAUTINARY MEASURES TAKEN IN AREAS WITH EXISTING RESIDENTS OR INFRASRUCTE AND STRUCTURESSinkhole formation can result in a loss of lives by destroying buildings and infrastructure. There is about 4 million South Africans who currently reside and work in dolomitic land and about 25 % of Gauteng, the commercial mining and manufacturing center of South is lying on a dolomitic land. Due to the number of the people currently residing on such land and the government obligations such as the bill of rights risk mitigation measures need to be taken 10. SANS 1936 – 1 5 and Watermeyer et al 12, further recommends the following risk mitigation measures to manage the dolomite land where people are already residing:
Placement of restrictions on land usage;
Ensuring appropriate developments is made in relation to inherent hazards;
Establishing requirements for management and monitoring of surface drainage and dewatering, installation and maintenance of water bearing services below the infrastructure and a building design that enables a safe evacuation procedure in case a hazard occurs.
2.8 PRECAUTIONARY MEASURES TAKEN PRIOR CONSTRUCTION IN DOLOMITIC AREAS IN SOUTH AFRICAThere is an increased demand of housing in South Africa and therefore construction in dolomitic land cannot be prohibited completely. In order to mitigate the effects of the construction in those areas Watermeyer et al 11 has outlined a four level performance approach which can be adopted as follows:
LEVEL 1 – OBJECTIVE – Development taking place, should ensure that people live and work in a safe environment, damage or loss of assets is within limits and the future usage of such land is not compromised by the current usage.
LEVEL 2 – FUNCTIONAL REQUIREMENTS – The usage of the land should present a minimum risk of dolines and sinkhole formation over time.
LEVEL 3 – PERFORMANCE REQUIREMENTS – The usage of the land should present a minimum risk of dolines and sinkhole formation over time.
LEVEL 4 – EVALUATION -The acceptable development risk needs to be quantified.
Watermeyer et al 12, further states that the four level development risk should be applicable in an area underlain by dolomite or limestone at a directly or at a shallow depth less than:
60m in areas underlain by limestone,
60m in areas underlain by dolomite where no dewatering has taken place and the local authority is monitoring and has control over the ground levels and
100m in areas that are underlain by dolomite where dewatering has taken place or where local authority has no control over ground water levels.
2.9 TECHNOLOGy available IN SOUTH AFRICA THAT CAN DETECT SINKHOLESAs per the reports by CSIR (Council for Scientific and Industrial Research), there is a satellite imaging technology that has been developed by CSIR called the Azimuth technology. This technology is designed to detect centimeter – to millimeter-scale surface deformation. It was used by a student researcher by the name of Andre Theron to detect a sinkhole in Pretoria on an unpopulated region where a subsidence was forming 3.
The subsidence was detected and a total of 6.6 cm of deformation with deformation fractures of about 100mm diameter was recorded between June and August 2015. The subsidence as caused by a leaking underground pipe and it resulted in a sinkhole 3.Therefore such together with the early warning signs of sinkhole such as cracks and subsidences, satellite imaging technologies can be used to detect sinkholes and subsidences 3.
RECOMMENDATIONSBased on the research conducted above, although dolomite has a negative effect in construction, it is possible to construct in low risk areas designated D1 to D3 with the necessary precautionary measures taken. I therefore recommend the following to counteract the effects of dolomite in existing structures (infrastructure and Top Structure):
Before construction, the following can be implemented:
Over and above trial pits and geological investigations, satellite imaging technology can be used to detect the subsidences and sinkhole as it will give a direct indicative of the actual underground conditions. Though it has cost implications, the cost is cheaper in a long term, as it will minimize the time spent of monitoring, managing and conducting remedial work on the damages caused by the sinkholes and subsidence.
Before and during construction, the following can be implemented:99999
That proper and good quality drainage services are provided across all areas affected areas.
Careful monitoring of usage of water should also be monitored to avoid water seepage into the ground.
Trenches should be closed as soon as they are opened, this will minimize the effects of rain water seeping into the trenches and making an easy path into the dolomite or lime stone.
Proper quality control is required to minimize defects that may arise in the stormwater or water and sewer pipes.
Testing of leakages and pressure on pipes should be done effectively prior to handover.
After construction, the following can be implemented:
Community should be educated of the first signs or early hazards of sinkhole and subsidence formation such as cracks, trees that have collapsed, slight collapses near roads etc.
Community utilizing the area should be educated and be made aware or the importance of reporting leaking, blocked and burst pipes and avoiding water wastage.
Community should be addressed on the issue of placing boreholes/wells.
Government should invest on making provisions for constant monitoring and maintenance on Stormwater, sewer and water pipes along the entire life of the infrastructure.
CONCLUSIONThe effect that dolomite has on land usage poses a high risk and threatens lives of those residing and working in the affected areas. There are mitigations that are formulated to minimize the negative effect of dolomite on land usage and construction. Dolomites are classified into categories and there are areas of dolomite which construction can take place such as those designated as designation class D1, however the designs adopted need to allow residents to evacuate the premises once a hazard can be detected. Dissolution of dolomite is not only caused by rain and ground water but by manmade underground water services as well as storm water, careful monitoring and maintenance of the leakage of such services will not eliminate but will minimize the effects of the dolomite on land usage.