ISSUE 13 / ISSN 1793-1665 / MARCH 2009

MACHINERY MAINTENANCE REGIME

SAFETY CONSIDERATIONS DURING THE CONSTRUCTION OF PILED ROAD STRUCTURES ON EXISTING

LANDFILL SITE

SAFETY CERTIFICATION PROCESS FOR RAPID TRANSIT SYSTEMS

PROVISION OF A CRASH CUSHION

TECHNICAL ASPECTS OF EARTH CONTROL

ACCIDENT STATISTICS

EDITORIAL PAGE

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FEATURED ARTICLES...

Safe Operation of Machinery

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MACHINERY MAINTENANCE REGIME

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Introduction

With the advent of globalisation and the accompanying boom in infrastructure and building construction, many kinds of machinery have been developed to increase the efficiency and safety of construction work. Construction machinery covers a wide spectrum of items – ranging from small hand-held powered tools to huge machinery like the crawler crane.

Figure 1: Electric DrillSource:http://www.wpclipart.com/tools/drill/electric_drill.png

Figure 2: 60-tonne Crane

Maintenance Regime

CP79 code of practice for safety management system for construction worksite stipulates the need for an effective maintenance program to be established by the occupier to ensure the safe and efficient operation of hand tools, plant, machinery and equipment used at the worksite. The maintenance program shall include:

(a) listing of hand tools, plant, machinery and equipment;(b) schedule of inspection and maintenance;(c) procedure for breakdown and repair; and(d) record of inspection and maintenance.

The occupier must also ensure that the maintenance personnel are trained and competent.

In addition to CP79, our General Specification, Appendix A (Safety, Health and Environment) also requires the Contractor to assess the Safety, Health and Environmental (SHE) risks especially in terms of age, noise, emissions, condition etc associated with the machinery prior to its entry to site and only deploy it to site if it has minimum SHE risks.

Besides the monthly inspections, our Contractors are also expected to implement a preventive maintenance programme to ensure that all machinery are maintained in a safe and working condition.

Good Practices on LTA Worksites

In this article, we will highlight the good practices that our Contractors have adopted with regards to machinery maintenance regime on their respective sites

Pre-Mobilisation MeetingAll sub-contractors planning to bring heavy machinery on site are to attend a pre-mobilisation meeting with the Contractor where all the necessary documents are submitted. The maintenance schedule and monitoring are then discussed and agreed upon.

Figure 3: Pre-mob Meeting

Machinery Monitoring BoardMonitoring of heavy machinery maintenance schedule on site is carried out on a monitoring board maintained by the Contractor. Servicing status and due dates can be observed at a glance.

Figure 4: Monitoring Board

Critical Parts Inspection TeamAll heavy machinery deployed by sub-contractors on site are subjected to an independent inspection by the Contractor before being allowed to operate. The team conducting the inspection, called the Critical Parts Inspection (CPI) team, consists of 3 qualified mechanics. This additional requirement ensures that all critical parts such as load-bearing joints, gears, limit switches etc are independently inspected before the machinery is being put to service, thus reducing the risk of any accident due to parts failure.

Figure 8: CPI Team Measuring Diameter of Guy Rope

Figure 9: CPI Team Measuring Groove Depth on Sheave

Additional Maintenance Checks by ContractorIn addition to the sub-contractors’ monthly machinery maintenance, the Contractor’s personnel carry out additional checks on the heavy machinery like the long-arm excavators where welding joints are checked for signs of corrosion.

Figure 10: Contractor’s Personnel Carrying out Additional Monthly Checks on Heavy Machinery

Tagging for Heavy MachineryAll heavy machinery are independently checked by Contractor’s safety personnel and tag issued.

Radio-Frequency Identification (RFID) Technology

The robustness of a maintenance regime is only as good as the people maintaining it. Hence, there is a chance that due to human error, a lapse in the maintenance monitoring system can occur, resulting in a machine that had not been inspected or serviced being put into operation or allowed to remain in operation.

In our recent researches into the use of technology for safety on site, RFID came across as a technology that we could harness to improve the robustness of the maintenance regimes on our work sites by removing the reliance on manual monitoring. Imagine having all your machinery installed with active RFID tags that would not only assist in identifying their respective locations on site, would also alert the Contractor when any one of them had missed out on the scheduled maintenance or inspection.

Another possible application of RFID technology in maintenance of machinery would include tagging hand-held or portable electrical machinery with RFID tags containing identifying information such as inspection dates, serviceability status, authorized user, Permit-To-Work due date etc. Such information could be read by a hand-held reader, thus simplifying the competent person’s task of monitoring and tracking these machinery on site.

Figure 13: RFID ChipSource:http://www.wdrode/themen/_images_/images/3/computer/schiebwoche/2004/rfid_cypak_400q.jpg

Figure 14: RFID ReaderSource:http://rfidusa.com/

superstore/image/jett-rfid2.jpg

RFID technology is already widely used and two very well-known local examples are the ERP gantries and automated car park systems. Both make use of RFID technology for the collection of electronic toll and car park charges respectively. With the advancement and lowering cost of this technology, the use of RFID on construction sites may soon become a reality and its applications are only limited by our imagination.

Patrick PhoaSafety & Health Manager

Safety Division

Tagging for Electrical MachineryAll electrical machinery, including electrical hand-held tools are monitored by the Contractor with a dedicated LEW assigned to carry out monthly inspection on them. All machinery/tools that pass the inspection are tagged to indicate that they were inspected and safe for use

Figure 6: Electrical Tag

Figure 5: Electrical Drill with Electrical Tag

Figure 7: Electrical Cutter with Electrical Tag

Figure 11: Crane with Machinery Tag Figure 12: Machinery Tag

SAFETY CONSIDERATIONS DURING THE CONSTRUCTION OF PILED ROAD STRUCTURES ON EXISTING LANDFILL SITE

Introduction

A new 1.3km long road which extends from Buangkok Drive near Upper Serangoon Road/Sengkang East Drive junction to Tampines Road at the north eastern part of Singapore is being constructed under Contract ER158.

The new road starts with a 3-span bridge across Sungei Serangoon that runs eastward through a 0.7km long piled roadway and intersects with the KPE through a flyover and connecting slip ramps before joining Tampines Road.

This article will focus on the safety challenges and the approached taken by the project team and contractor during the construction of piled road structure on the landfill site (see Figure 1 on Typical Cross Section of Piled Road Structures).

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Before Commencement of Work

• Tetanus Inoculations to All Working Personnel Site personnel were protected against tetanus caused by

the toxin of the bacillus clostridium tetani commonly found in landfill site, which typically infects the body through an open wound.

• Engaging Qualified Workplace Safety and Health Officer (WSHO) and Gas Assessor

Competent safety personnel were engaged to implement the requirements as stipulated in the Safety and Health Management System (SHMS). Gas checks were conducted periodically throughout the day by the competent confined space safety assessor to ascertain the presence and concentration of gas during boring work. (Figure 3 shows the safety measures taken in the event that the landfill gases exceeded the threshold limits).

Figure 1- Typical Cross Section of Piled Road Structures

Challenges in Constructing Piled Road Structure on Existing Landfill Site

One of the challenges in this project is to construct a dual 3-lane carriageway that crosses a landfill site at the former Tampines Dumping Ground. It was used for public waste and construction debris between 1983 and 1989.

The works involved extensive excavation of existing landfill surface, installation of bored piles, gas capping & venting system and reinforced concrete slab. The project team and contractor recognized the risks associated with these works, in particular, likely exposure to landfill gas and leachate (is the liquid that drains or ‘leaches’ from a landfill) and thus judiciously followed up with the various contractual requirements on design, safety and risk management.

Safety and Health Hazards of Landfill Gas

Landfill gas is composed of approximately 50% methane and 50% carbon dioxide and is produced by the decomposition of organic waste under anaerobic conditions. It also contains varying amounts of nitrogen, oxygen, water vapor, sulfur and a hundreds of other contaminants - most of which are known as “non-methane organic compounds” or NMOCs. Inorganic contaminants like mercury are also known to be present in landfill. Sometimes, even radioactive contaminants such as tritium (radioactive hydrogen) have been found in landfill gas. Apart from environmental concerns of methane emissions and unpleasant odors associated with landfill gas, uncontrolled landfill gas can present a serious explosion hazard.

Studies have also shown that landfill gas contains toxic organic compounds called Volatile Organic Compounds (VOCs) which can cause numerous health implications such as cardiac defects, retarded ossification (natural process of bone formation), nausea, vomiting and headache.

Following sections highlight some of the initiatives implemented by the project team and contractor.

Environmental Impact Study

Upon award of contract, a specialist consultant was engaged by the contractor to conduct an environmental impact study at the landfill site. Site investigation which include gas measurement was carried out to identify various landfill gases, their pressure/concentration and leachate composition. Soil sampling was also collected at close interval to examine the extent of migration of these landfill gases and leachate.

The investigation revealed that the waste which was dumped at about 4 to 10m below the proposed road level was still in an active state of decomposition and various by-products like gases and leachate were detected within and adjacent to the landfill boundary. These gases comprised mainly methane, carbon dioxide and traces of hydrogen sulphide and carbon monoxide. (Figure 2 shows the characteristics of gases).

Types of Gas

Methane

Carbon Dioxide

Carbon Monoxide

Hydrogen Sulfide

Characteristics

A colourless, odorless gas.

Is not toxic. However, it is highly flammable and may form explosive mixtures with air.

A colourless and odorless gas.

A colourless, odorless yet highly toxic gas.

A colourless, toxic and flammable gas is partially responsible for the foul odor of rotten eggs.

Figure 2: Characteristics of Gases

Safety and Health Plan

A plan was jointly developed among the consultant, project team and contractor to manage safety and health issues during construction. The team adopted a systematic approach in dealing with the potential hazards relating to the project through a comprehensive risk assessment process. Action plans including procedures in dealing with any emergency situations arising from the construction work were formalised before commencement of work

Parameter

Methane

Oxygen

CarbonDioxide

Carbon Monoxide

Oxygen

Threshold Limits

> 5% LEL

>10%LEL

<20.8%

>0.5%

>25ppm

<19.5%

MeasuresTaken

Hot Work prohibited

Evacuate all working personnel

Ventilation to be installed

Evacuate allworking personnel

Figure 3: Safety Measures Taken

• Training on use of Safety Equipment and Respiratory Mask

Site personnel who were exposed to noxious gases were issued with respiratory masks. The supplier of respiratory masks and gas meters was roped in to conduct a series of training to the working personnel including the supervisory staff. They were trained on proper use of respiratory mask and seal check prior to its usage (see Figure 4).

Figure 4: Used of Appropriate Respirator

Training on the Use of Respiratory Mask

Testing tightness of gas mask

• Display of Signages Warning and prohibition signages were erected at strategic

locations within the site premises before commencement of work to remind site personnel including public members of any potential dangers arising from the construction work.

• Permit to Work System A permit to work system was implemented to ensure that

the contractors seek approval prior to commencement of work on a daily basis with submission of all necessary documents such as method statements, site records, checklists including gas monitoring records. The landfill gas was closely monitored on an hourly basis during construction work and test records were displayed prominently outside the barricaded work zone. Site personnel were required to check the limits before entering the work zone.

• First Aid Posts and Washing Points First Aid Posts were set up at strategic locations within

the site premises. Sufficient numbers of first-aiders were trained to render first-aid treatment to site personnel. In addition, washing points were installed on site to provide on-the-spot decontamination. They allow workers to flush away hazardous or toxic waste substances that can cause injury.

During Working at Site

• Toolbox Meeting/ Site Demonstration Daily toolbox meeting was conducted by the WSHO at the

site prior to commencement of work. This served as a useful platform to remind the workforce of the potential hazards pertaining to the day’s work and safety precautions to be taken in mitigating the risks. Site personnel could also take the opportunity to clarify any issues when they were in doubt.

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Figure 5: Briefing & Demonstration

• Barricading of Work Zone A work zone of 20 metres by 20 metres was barricaded to

prevent any personnel from straying or trespassing into the work site.

• Wearing of Gloves To minimize bodily contact with the excavated waste

materials, supervisors and workers were advised to wear appropriate attires such as safety glasses and protective hand gloves when handling them.

• Ventilation Blower was installed to provide fresh air to the borehole.

This would dilute presence of any landfill gases present in the work environment.

Evacuation Route

The evacuation route was identified and the assembly point was located safely away from the landfill area. Drills were conducted to test out the effectiveness of the established emergency procedure. In the event of emergency, the casualty would be sent to the nearby pre-identified medical facility.

Disposal of Excavated Waste Material

The excavated waste material was disposed at a dumping site approved by NEA. The waste material, after excavation from the bored holes was transported to the designated dumping site. A 1m thick backfill capping was placed on top of the disposed waste to prevent the waste material from contaminating the environment.

Conclusion

An effective SHMS was implemented for the construction of piled road structures on existing landfill site at Contract ER158. The project team had ensured that all health and safety measures were strictly complied with and to-date, the project has achieved zero accident.

References 1. Landfill gas and its potential effects on human health at:

http://home.freeuk.net/gerrymandering/Library/potential_human_health_effects.htm

2. The Landfill Gas technical web site, plus news and developments, including topics at http://www.landfill-gas.com/

Koh Hwee YingEngineer, Road Development

Kuek Chin MengHigher Principal Engineering Officer, Road Development

Figure 6: Ventilation System to Dilute the Toxic Gases

SAFETY CONSIDERATIONS DURING THE CONSTRUCTION OF PILED ROAD STRUCTURES ON EXISTING LANDFILL SITE (con’t) SAFETY CERTIFICATION PROCESS FOR RAPID TRANSIT SYSTEMS

Introduction

Singapore has a good reputation for its safety records in developing new rapid transit systems (RTS). Such records are by no means easy achievements. The success hinges on LTA’s commitment to give top priority to safety in developing new systems. In LTA, we adopt the Total Safety Management (TSM) approach in managing safety of roads and RTS projects.

PSR (SafeTo Use)

Roads

PSR (Safe To Use)

RTS

OSHM PSR (Safe To Build)

Total Safety Management

Figure 1: Total Safety Management (TSM) Approach

TSM is a management system developed based on the philosophy of “Safe-To-Build” and “Safe-To-Use”. It comprises of a set of processes.

These processes and their scopes are:

• Project Safety Review (Safe-To-Build) - addresses the demonstration of civil hazard management

for Roads and RTS projects;

• Project Safety Review (Safe-To-Use) for RTS- demonstrates system and operational safety;

• Project Safety Review (Safe-To-Use) for Roads- demonstrates that constructed road system is safe for use

by all; and

• Occupational Safety and Health Management- covers the construction safety and health at worksite.

Background

During the development of RTS in Singapore in the 1980s, comprehensive safety assurance activities were implemented. As was practised at that time, LTA invited Her Majesty of Railway Inspectorate (HMRI) of UK to carry out final audits on the systems before the commencement of passenger service. For the subsequent Woodlands Line that opened in 1996, an external safety consultant was engaged in addition to the service provided by UK Railway Inspectors to conduct the final audits. Subsequently, LTA decided to develop a more structured safety certification approach for all RTS projects and hence in 2000, the “PSR (Safe-To-Use) for RTS” process was established.

Objective of PSR (Safe-To-Use) for RTS

The objective of PSR is to provide a staged and robust check-and-balance process on safety assurance of new RTS projects. The PSR process provides assurance to LTA that the system design developer team and the system Operator have adequate resources to manage safety effectively thereby ensuring that the systems are designed, tested, commissioned and operated with the highest level of safety. The PSR process also facilitates the overall certification of a RTS project before the commencement of passenger service.

PSR Process

The PSR process requires the implementation of a 4-stage safety certification process, namely concept, design, handover and operation. At the end of each stage, a safety submission is required to be prepared and subject to an audit. Figure 2 shows the overview of the PSR process.

Project Initiation

Design stage

Handover stage

Operation stage(before passenger

service)

Passenger Service

Safety submission

Safety submission

Safety submission

Concept stage

Safety submission

Audit

Audit

Audit

Audit

Figure 2: Overview of PSR Process

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Safety submissions are documents or cases made by the system design developer team or the system Operator to demonstrate that adequate commitments, processes and resources are in place to manage safety effectively and that the project is designed, installed, tested and commissioned to achieve a high level of safety. The safety submissions are subjected to audits and endorsement/acceptance.

Roles & Responsibilities

The PSR process involves four main roles, namely reviewer, submitter, auditor and endorser.

LTA’s System Assurance & Integration Division, acting as an internal consultant to the system design developer team, plays a dual-role of reviewer and submitter. They review the contractors’ system assurance submissions, integrate them and make their own safety assertions on the overall level of system safety while producing safety submissions on behalf of the system design developer team. On the other hand, the system Operator is responsible for making their safety submissions in preparation for operation.

LTA’s Safety Division, which is independent of the other divisions involved in the project, is responsible for auditing the safety submissions. It has the duty to express an opinion on the assertions made in the submission and obtain reasonable assurance that the assertions are free of material misstatement and can be substantiated by the various documents and analyses.

Upon the completion of audit, Safety Division will make recommendations to the PSR Committee (RTS) on endorsement/acceptance of the safety submission. The Committee can appoint a technical working group that is independent of the project under deliberation to assist in investigating into any specific safety-related matter, if necessary. If the submission is not endorsed/accepted, the submitter can either resubmit or refer the case to the Corporate Safety Committee for arbitration. Figure 4 shows the endorsement/acceptance process for safety submission.

Safety submission made

Audit conducted

PSR Committeeendorses/

accepts the safety submission?

Safety submission re-submitted

The submitter

accepts PSR Committee’s

decision?

Corporate Safety Committee accepts the Safety

Submission?

No No

Yes No

Technical working group

To implement commitmentsin the earliest possible time

YesYes Advise Advise

Figure 4: Endorsement/Acceptance Process for Safety Submission

Types of Safety Submission

There are four types of safety submissions. They are as follows:i. Concept safety submission;ii. Design safety submission;iii. Handover safety submission; andiv. Operation safety submission.

LTA’s System Assurance & Integration Division prepares the concept, design and handover safety submissions whereas the system Operator makes the operation safety submission.

i. Concept Safety Submission

The concept safety submission is required at the end of the concept definition phase which starts from project initiation to the award of system contracts. The aim of the submission is to demonstrate that major risks along the alignment of the new RTS project have been identified and assessed, and major safety features and design criteria have been defined for each system.

ii. Design Safety Submission

The design safety submission is required at the end of the final system design stage. The aim is to verify that the system has been designed to achieve its safety objectives and requirements. Evidence is collated to demonstrate that safety was considered and incorporated into the design and all design-related safety issues have been addressed to a level of acceptability.

iii. Handover Safety Submission

This is the final stage of submission by the system design developer team and is made for audit prior to the handover of the system to the system Operator. The submission aims to demonstrate that the integrated system has been installed in accordance to good quality control standards and successfully tested and commissioned to achieve the safety requirements specified at the concept and design stages. The system is, therefore, considered to be adequately safe for handing over to the system Operator for them to start trial running.

iv. Operation Safety Submission

The system safety submission is made in three stages; when the Operator has established its safety management system, before the trial running and just before the commencement of passenger service.

The aim of operation safety submission is to demonstrate that the Operator has necessary organisational structure and effective processes in place to operate and maintain the system to an acceptable level of safety.

System fit for use declaration

LTA Chief Executive gives consent to commence

passenger service

CSSAudit

DSSAudit

HSSAudit

Trial running starts

System Design Developer Team

OSS Audit

Operational readiness declaration

CSS – concept safety submissionDSS – design safety submissionHSS – handover safety submissionOSS - operation safety submission

Figure 6: Handshaking Process for Commencement of Passenger Service

Conclusion

The “PSR (Safe–To–Use) for RTS” is a self-certification process customised for Singapore’s railway environment. It is a check-and-balance process to ensure that a new RTS is planned, designed, commissioned and operated with the highest level of safety.

The process strengthens the LTA’s safety assurance framework rendering enhanced confidence to the management with greater visibility and transparency of safety management. The staged audits also provide opportunities to identify any safety issues in the earliest possible time and hence avoiding any costly late-stage design changes.

The debut of the PSR process has inked a new chapter in the Singapore’s RTS safety certification history. The process works well so far. The challenge ahead is to further enhance the process to strive for excellence in ensuring railway safety.

Khoo Shee KangSenior Engineer, RTS Section

Safety Division

SAFETY CERTIFICATION PROCESS FOR RAPID TRANSIT SYSTEMS

System Operator

Figure 3: Safety Audit Interview Meeting

Handshaking Process

To obtain a final consent from LTA for the commencement of passenger service, it is necessary that all the safety submissions at the various stages have been audited and endorsed/accepted. In addition, the system design developer team is required to make a declaration that the system is fit for use and the system Operator to declare their operational readiness before the Chief Executive of LTA gives his consent. The handshaking process is illustrated in Figure 6.

Figure 5: Emergency Exercise as Part of Operational Readiness Audit

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PROVISION OF A CRASH CUSHION

Introduction

A crash cushion is a road safety device that is used to shield roadside hazards in order to reduce the severity of collisions when hit by on-coming vehicles.

Figure1: A Crash Cushion Shielding the Leading Terminal of a Bridge Parapet Wall

Working Principles of a Crash Cushion

A crash cushion reduces the severity of a head-on collision by absorbing the impacting energy to bring the vehicle to a safe and gradual deceleration. The kinetic energy is dissipated either by the deformation of disposable cartridges or by cutting through the steel plates as the device slide backwards along the guidance tracks that are anchored onto the road surface. For side impacts, the crash cushion will contain and redirect the impacting vehicle by the deformation of the side panels similar to a non-rigid road safety barrier.

As for all road safety devices, crash cushions have to be crash-tested to meet the requirements of evaluation standards such as the National Cooperative Highway Research Program (NCHRP) Report 350.

Figure 2: A Crash Cushion that had been Hit Head-On

Locations to install the Crash Cushion

Crash cushions are used to shield rigid objects such as bridge headwalls, viaduct columns and overhead sign supports. They are also used at locations such as at the gore areas or a road bifurcation where there is a likelihood of a vehicle veering off the travelled path and crashing into the rigid objects.

Design Considerations and Proper Use of Crash Cushion

A crash cushion will only reduce the severity of a collision, but it will not reduce its occurrence. Therefore, the first design consideration is to ensure good road geometric design to reduce the likelihood of vehicles veering off the carriageway. The second consideration is to provide a clear zone at the gore area for an errant vehicle to recover if it does veer off the carriageway. A crash cushion should only be considered if the roadside rigid objects cannot be removed or retrofitted to provide a clear zone at the gore area or sidetable.

The various design considerations to provide a crash cushion are summarised in the following page. These design considerations are essential to ensure that the crash cushion will be able to perform as intended and will not create any secondary or undue hazards. The following photograph illustrates some of these issues.

Figure 3: The Installation of the Crash Cushion in Front of the Physical Nosing would Increase the Likelihood of Vehicles Crashing into the

Device. The VIG-Kerb Combination would also Increase the Risk of a Vaulting or Rollover Collision

Figure 4: The Raised Kerb of the Physical Nosing was Removed. This enabled the Crash Cushion to be located further away from the Exit Point to reduce the Likelihood of Vehicles Crashing into the Device.

Conclusion

While the use of a crash cushion is intended to improve the safety of the road system, it is important to ensure that due consideration is given to the proper selection and installation of the device so that it can effectively reduce the severity of a collision, and will not create any undue or secondary hazards.

VIGPhysical Nosing

Kerb

Crash Cushion placed away from Chevron Marking

Raised Kerb Removed

Checklist for Design and Installation of Crash Cushion

A. DESIGN AND INSTALLATION PROCESS Checked?

1a. Has the contractor engaged the manufacturer / manufacturer certified installer to propose the type of crash cushion and to undertake the design of the site modification to install the device?

1b. Has the contractor submitted the checklist for the design and installation of the crash cushion?

B. TECHNICAL CONSIDERATION Checked?

2a. Has the contractor submitted the product acceptance letter to confirm that the device has been fully crash-tested and meets the requirements of the international evaluation standards such as NCHRP Report 350? Is the device appropriate to the speed environment of the carriageway?

2b. Has a minimum length of the chevron-marked nosing been provided in front on the crash cushion (70m for expressways and 50m for non-expressways)?

2c. Is the width of the crash cushion sufficient to shield the hazard at the gore area adequately?

2d. Have the surrounding site features such as kerb, lamp poles, roadside drains, junction boxes etc have been removed or treated to ensure that the installation of the crash cushion and the safety barrier at the transition section will not create any undue or secondary hazards?

2e. Is the crash cushion installed at the correct height relative to the carriageways on both sides of the device?

2f. Is the base of the crash cushion designed and installed in accordance to manufacturer’s specification?

2g. Is the flare rate of the safety barrier transitioning to the crash cushion appropriate based on the speed limit of the main carriageway and in accordance to the manufacturer’s specification for the proposed device?

2h. Is the crash cushion properly transitioned to the adjoining safety barriers, and is the transition section used type-approved?

Lynn Khoh Li Ming Engineer,SafetySafety Division

Level difference between both sides of the carriageway (see Note 2a)

FRONT VIEW

Length of chevron-marked nosing(see Note 2b)

Treatment of exixting site condition (including transition section)(see Note 2d)

ELEVATION VIEW

Transition to adjoining safetybarriers (see Note 2h)

Type of crash cushion(see Note 2a)

Width of crash cushion(see Note 2c)

Safety barrier flare rate(see Note 2g)

PLAN VIEW

Mounting base for crash cushion (seeNote 2f)

TECHNICAL ASPECTS OF EARTH CONTROL

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Introduction

Singapore’s Sewerage and Drainage (Surface Water Drainage) Regulation requires all contractors to limit their total suspended solid (TSS) in water discharge into public drain to less than 50mg/l. The implementation of Earth Control Plans has to be designed and endorsed by Qualified Erosion Control Professionals (QECPs). Given the uncertain events and characteristics of storms in Singapore, how do QECPs estimate the silt discharge and the type of controls required to ensure that legal requirements are met? This article aims to equip readers with fundamental knowledge that QECPs use to estimate TSS discharges when formulating the Earth Control Plans.

Legislation

In the past, Professional Engineers (PEs) are engaged to plan, design, supervise and review earth control plans for construction sites. However, since 1 July 2007, this role can only be taken upon by a Qualified Erosion Control Professional who is a registered PE and has satisfactorily completed specialized professional course in erosion and sediment control.

As stated in the regulations, contractor must engage a QECP to design a system of Earth Control Measures (ECM) to meet the legal requirements. ECM design and installation must be reviewed by QECPs at every phase of construction to ensure the plan is updated and suits the needs of a dynamic site.

Soil Loss Calculation

Typically, QECPs uses one of the following three types of models to calculate soil losses due to erosion:

• Universal Soil Loss Equation (USLE); • Revised Universal Soil Loss Equation (RUSLE) and;• Modified Universal Soil Loss Equation (MUSLE).

RUSLE and MUSLE formulas are derived from USLE which was developed based on regression analyses of data for 10,000 plot-years, hence these 3 models are very similar. For this article, we will focus on MUSLE which is one of the more widely used models for earth control in Singapore’s construction site.

Modified Universal Soil Loss Equation (MUSLE)

Rates of erosion are dependent on rainfall characteristics, soil structure, slope factor, type of cover and management practice

The empirical relationship defined by MUSLE is expressed by the following equation:

T=89.6x(VxQr )0.56 x (K x LS x C x P)

Where:

T = Sediment yield per storm event (tonnes or tons)V = Volume of runoff (m3)Qr = Peak flow (m3/s)K = Soil Erodibility FactorL = Slope Length FactorS = Slope Steepness FactorC = Cover and Management FactorP = Conservation Practice Factor

A brief account of each individual factor and their calculations are described in the following sections.

Calculation of MUSLE Factors

Volume of Runoff (V)

Equation used for calculating runoff volume as follows:

V = ( axItcA ) /1000

Where,

a= Runoff coefficient ( 0.65 can be used for most areas in Singapore)I = Rainfall Intensity (mm/hr)tc = Duration (hr)A = Catchment Area (m3)

I and tc is usually taken to be 60mm/hr and 1hr respectively based on once in two years storm . Rainfall intensity can be derived from the Intensity-Duration-Frequency (IDF) curve by estimating the duration and storm of return period.

Figure 1: IDF Curve

Peak Flow (Qr)

The peak runoff occurs at the point of design when all parts of the catchment receiving steady, uniform rainfall intensity are contributing to the outflow at this point. Peak Flow is computed using the Rational Formula:

a It should be noted that the calculations/formulas described in the article is one of the many ways used in Singapore. QECPs may use other applicable methods in their ECM plans.b For a storm of return period (T) years, the rainfall intensity (I) is the average rate of rainfall from such a storm having a duration equal to the time of concentration (tc).

Qr=1

360000aIA

Soil Erodibility Factor (K)

Soil erodibility refers to the ease of soil being dislodged by the action of rainfall or wind. It is affected by the soil type as shown in figure below:

Qr=1

360tc

V

Soil Type

Low-Plasticity SiltSilty SandClayey SandHigh-Plasticity SiltLow-Plasticity Organic Soil

Erodibility Classification

Soil Structure

Most Erodible Very Fine

The above soils are much more erodible than the following:

Low-Plasticity ClayHigh-Plasticity ClaySilty GravelWell-Graded SandPoorly Graded SandWell-Graded Gravel

Least Erodible Blocky, Platy, Massive

Clay

Slit

Sand

Gravel

Least Permeable

Most Permeable

Figure 2: Hierarchy of Soil Types

Soil data pertaining to silt, fine sand, sand and organic matter composites can be obtained from borelog and its corresponding K factor is to be determined from Soil Erodibility Nomograph Curve.

Figure 3: Soil Erodibility Nomograph Curve

For example, given the following soil data:Silt + very fine sand = 14%Sand = 77%Organic matter = 0.1%Soil Structure = 3 (Medium or course granular)Permeability = 3 (Moderate)

Soil data pertaining to silt, fine sand, sand and organic matter composites can be obtained from borelog and its corresponding K factor is to be determined from Soil Erodibility Nomograph Curve.

Hence, using the Nomograph Curve, K = 0.018

Depending on the extent of earthworks to be carried out, relevant data shall be obtained as soil characteristics varies at different depths and different areas.

Topographic Factor (LS)

LS comprising of both length and steepness factors, can be determined from the LS table given the % slope (height/base length %) and slope length.

Note that a gentler slope with shorter slope length yields lower value, thus the soil loss value is also lower.

Figure 4: Values of Topographic Factor, LS, for High Ration of Rill:Inter-Rill Erosion, such a Highly Disturbed Soil Conditions and freshly

Prepared Construction Sites with Little of No Cover

Land Management Practice (CP)

C and P factors make up the land management practice which depends on the type of cover, controls and slope of the area. Soil covered with erosion control blanket for example has a CP factor of only 0.1 to 0.3 as compared to bare land where the CP factor is 1.0. Refer to the table below for respective values of C and P factors.

Figure 5: C Factor and P Factor Values for Construction Site Rainfall BMPs

13

15

AFR

SR

FSI

*Based on Workplace Safety and Health Act Requirements

14

Figure 6: Silt Fence and Erosion Control Blankets with on-going Re-Vegetation Works at C828

Data Interpretation

Once Sediment Yield, T, has been calculated, estimated TSS can then be predicted:

TSS (mg/l) = T x 106 /V

Using a simple example base on an LTA site, given:A = 1436m2 K = 0.018 LS = 0.04 (relatively flat bare land)CP = 1

Hence, using the formulas mentioned aboveV = 56m3

Qr = 0.16m3/s T = 0.22 tonnes TSS = 3933 mg/l

Given no erosion control administered on a medium plot of flat bare land, TSS is estimated to be 78 times higher than allowable limit of 50mg/l!

From the various factors described above, it can be observed that the higher the values of individual parameters, the larger amount of soil are loss due to erosion. Hence, mathematically, erosion control is about reducing the factors as far as possible. Note that V, Qr and K depends on the natural conditions which cannot be adjusted, thus the factors that can be manipulated are A, LS and CP factors.

Reducing Soil Loss

One of the most effective measures is to reduce catchment area (A), by virtues of proper work scheduling, opening up of the earth only when work is required to be done and turfing backfilled areas as soon as possible.

Reducing presence of slope by levelling, installing effective covers such as erosion control blankets (ECB), turfing and other sediment controls including silt fence, silt traps etc helps to further decrease soil loss significantly.

Water Treatment Plant

Water treatment plant is an essential part of ECM that is usually the last line of defense to treat the water to a TSS of less than 50mg/l. It complements the whole system by treating the silt runoffs that are already reduced to a concentration acceptable to the treatment plant. Many chemical-polymer treatment plants commonly used in Singapore construction sites are only able to treat inlet concentration of less than 10,000 to 20,000 mg/l of TSS with the better but more expensive types able to treat up to about 40,000 mg/l.

Figure 7: Water Treatment Plant at C823

Implementation, Maintenance & Monitoring

Calculations are done base on assumptions that the controls are effectively implemented and maintained in good working condition. Cases where canvas sheets are used instead of ECB or poorly installed/maintained silt fences would render the controls useless and greatly increase the working loads of treatment plants resulting in the inability to meet legal requirements.

Regular monitoring of the final discharge will give a clear indication of the effectiveness of the earth controls. Some of our LTA sites have taken a step further by employing the use of 24-hour Automatic TSS Monitoring System and CCTV Monitoring such as shown in the pictures below.

Figure 8: TSS Monitoring System and CCTV Monitoring at C907

Such system provides real time updates to the project managers when the TSS readings rise above 50mg/l. CCTV monitoring also allows visual checks without having to be physically present on site.

Conclusion

From the example given in this article, it can be seen that Singapore waterways can become easily polluted and silt-ridden given the high numbers of on-going projects and the potential high silt runoffs per area of bare land. Hence, diligent efforts are required to ensure that every necessary control measures are well implemented, maintained and continuously reviewed throughout the whole construction works.

Jernice Kew HuilingEngineer, Environmental

Safety Division

References1. Adapted from Design for Effective Sediment and Erosion

Control on Construction Sites, By Jerald S. Fifield, Ph.D. CPESC, 2004 Forester Press

TECHNICAL ASPECTS OF EARTH CONTROL (con’t) 2008 ACCIDENT STATISTICS*

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257113

0.880.46

Turfing

Erosion Control Blankets

Silt Fence

ASAC Trophy Design

CompetitionLTA Annual Safety Award Convention is now into its 11th year. To inject fresh interest and creativity

on its trophy, LTA is organising a trophy design competition.

Theme: Safety in Construction

3 attractive cash prizes to be wonChampion: $1000;

1st Runner up: $500;2nd Runner up: $250.

Deadline: 15th June 2009

Competition is open to LTA Staff and Contractors

For any enquiries, please email to: asac@lta.gov.sg

EDITORIAL PAGE

20th Safety Workshop & 14th Construction Staff Award (CSA)

Ceremony held on 17th November 2008

The presentations made at the Safety Workshop were:

“Prevention of Compressed Air Illness and Barotrauma in Tunnel Construction” by Dr. Kenneth Choy from Ministry of Manpower.

“Safety Considerations for Traffic Diversion at Woodsville Interchange” by Mr. Ang Pui Boon, Executive Engineer for ER198.

“Safety Performance Scheme – Is It Effective?” by Mr. Yoong Yew Meng, Executive Safety & Health (Safety Division)

For the CSA, a total of 9 recipients were recognised for their exemplary efforts in driving safety and environmental aspects towards excellence at their workplace.

Guidelines on Design for Safety in Buildings and Structures

The guideline was launched by the WSH Council which aims to assist key stakeholders on the process of design safety and the transfer of vital safety and health information along the construction process chain.

Clients, developers, designers (architects and engineers), project managers, quantity surveyors and personnel in the design project team stand to benefit from this new guidelines.

Find more information, please visit the WSH Council website http://app.wshc.gov.sg/cms/.

Editorial Committee

Advisor Corporate Safety Committee

EditorLee Cheng Chuen Patrick

Circulation OfficerTan Chee Lang

WritersKew Huiling JerniceKhoh Li Ming LynnKhoo Shee Kang Koh Hwee YingKuek Chin MengPhua Hock Lye Patrick

Contributions or feedback to:

Land Transport Authority Safety Division251 North Bridge Road, Singapore 179102Tel: (65) 6332 6139 Fax: (65) 63326129Email address: cheng_chuen_lee@lta.gov.sgSafety News is also available online at http://internet-stg.lta.gov.sg/projects/index_proj_safety.htm

The CSSA & CSEA winners with LTA Senior Management, from L-to-R:Lim Lye Hock (C823/828), Tan Thian Huat (CCL 4), Tham Sam Chuan (CCL 5), Soon Lim Seng (CDL E&M), Ravindran s/o Perumal Arunasalam (BLE), CE Yam Ah Mee, Ms. Chiong Yok Cheng (Road Construction Sub-Group), Ko Chee Min Jimmy (CCL 4), GDSC Frederick Wong, Chua Hiang Ping (CCL5).

Not in picture: Chua Chow Chon Benny (BLE).

Safety Tan says………

Every employee must take full responsibilities in adhering to the established Safe Work Practices while operating or maintaining any machinery at the workplaces.