Highway Construction Methods and Equipment

Introduction

Highway construction converts a surveyed corridor into a durable, safe and maintainable transport facility. The work is not simply a sequence of machine operations. It is an integrated production system in which design information, materials, equipment, labour, weather, traffic management, quality control and environmental protection must be coordinated.

A typical highway project progresses through clearing and grubbing, earthworks, drainage and utility works, preparation of the formation, placement of pavement layers, asphalt or concrete paving, finishing, testing and opening to traffic. The construction method selected for each activity affects productivity, cost, safety, pavement performance and the risk of defects. For example, an efficient earthmoving fleet is of little value if unsuitable material is placed in the embankment, and a high-quality asphalt plant cannot compensate for poor temperature control or inadequate compaction.

This chapter presents the principles used to plan and execute highway construction operations. It emphasizes equipment selection, production estimation, sequencing, site logistics, quality control and practical decision-making.

Learning Objectives

After studying this chapter, students should be able to:

1. Explain the principal stages of highway construction.
2. Select appropriate equipment for clearing, excavation, hauling, grading, compaction and paving.
3. Estimate equipment productivity and identify factors that reduce output.
4. Plan the sequence and logistics of earthworks and pavement construction.
5. Describe asphalt and concrete paving methods, including compaction and finishing.
6. Apply quality, safety and environmental controls to construction operations.
7. Analyse a construction scenario and recommend an integrated method and equipment fleet.

1. Construction as a Production System

Highway construction is generally a linear, location-dependent production process. Work fronts move along the alignment, while materials, equipment and personnel must repeatedly enter and leave the work zone. The principal constraints are:

• Geometry: horizontal alignment, grades, cross-fall, formation width and layer thickness.
• Materials: soil classification, moisture, aggregate grading, asphalt binder, concrete constituents and haul distance.
• Resources: equipment capacity, operator skill, labour, fuel, spare parts and finance.
• Time: contractual milestones, weather windows, traffic restrictions and plant availability.
• Quality: density, thickness, smoothness, strength, temperature and surface finish.
• Safety and environment: work-zone traffic, dust, noise, erosion, water pollution and emissions.

The controlling operation is the activity with the lowest effective production rate or the greatest impact on continuity. In a paving operation, the paver may be capable of placing more material than the rollers can compact within the temperature window. The paving rate must then be governed by the rollers, not by the paver's theoretical capacity.

A sound method statement should identify the work sequence, resources, inspection points, hazards, acceptance criteria, contingency arrangements and communication procedures. It should be coordinated with the programme and revised when actual production differs materially from planned production.

2. Site Preparation and Clearing

2.1 Survey, setting out and investigation

Before physical work begins, the contractor confirms the centreline, right-of-way limits, benchmarks, existing utilities, drainage paths, structures, environmental constraints and locations of unsuitable material. Survey control should be protected throughout construction. Typical controls include total stations, GNSS receivers, digital levels, machine-control systems and conventional offset pegs.

The construction team should establish:

• permanent and temporary benchmarks;
• limits of clearing and areas requiring special protection;
• cut and fill boundaries;
• temporary access roads and haul routes;
• stockpile, borrow-pit and spoil-disposal areas; and
• drainage diversions and sediment-control measures.

2.2 Clearing and grubbing

Clearing removes vegetation, rubbish, fences and obstructions. Grubbing removes roots, stumps and organic material that could decay or create weak zones. Equipment may include chainsaws, brush cutters, dozers with root rakes, excavators, stump grinders, loaders, trucks and chippers.

The method depends on vegetation density, stump size, terrain, environmental restrictions and disposal requirements. A dozer is effective for broad clearing on relatively open ground, while an excavator provides better selective removal near utilities, wetlands or retained trees. Burning is often restricted or prohibited; chipping, reuse of timber, controlled disposal and off-site recycling are preferable where permitted.

Topsoil is normally stripped separately and stockpiled for later landscaping. It should not be mixed with structural fill because its organic content and compressibility make it unsuitable for most embankment zones.

2.3 Site logistics and safety

Clearing operations require exclusion zones, felling-direction control, spotters near overhead lines and procedures for discovering hazardous materials or unexploded objects. Trucks must use designated routes, and stockpiles should not obstruct sight distance, drainage or emergency access.

3. Earthworks Methods and Equipment

3.1 Cut, fill and formation

Earthworks establish the subgrade or formation on which the pavement is constructed. Cut sections require excavation, loading, hauling and either placement or disposal. Fill sections require spreading, moisture conditioning, compaction and repeated lift construction. The finished formation must satisfy line, level, crossfall, width, bearing capacity and drainage requirements.

Soil behavior changes during excavation and placement. A material may occupy different volumes in its bank, loose and compacted conditions. These are commonly described as:

• Bank volume: volume before excavation.
• Loose volume: volume after excavation and disturbance.
• Compacted volume: volume after placement and compaction.

The conversion between these states must be based on project test data or reliable material factors. Assuming that one cubic metre in the borrow area always becomes one cubic metre of compacted embankment can cause serious quantity errors.

3.2 Equipment functions

A dozer provides high pushing force and is valuable for short-distance excavation, spreading and rough grading. A scraper excavates, carries and spreads material in one cycle and can be highly productive on suitable soils over moderate haul distances. A hydraulic excavator is versatile in deep cuts, confined areas and mixed materials, but requires separate hauling equipment. A wheel loader is effective for stockpiles and granular materials but is less suitable for precise excavation in narrow cuts.

Motor graders perform fine shaping, camber and crossfall control. Their productivity is strongly affected by the number of passes and the width of the blade cut. Rollers must be selected according to soil behavior: vibratory smooth drums are effective for granular materials, padfoot rollers for cohesive soils, and pneumatic rollers for kneading and sealing certain base and asphalt materials.

3.3 Basic production estimation

For cyclic equipment such as loaders, excavators and trucks, theoretical hourly production may be estimated by:

where Q_t is theoretical production in m³/h, C is volume handled per cycle in m³, and T_c is cycle time in minutes. Actual production is:

where Q_a is actual production in m³/h and E is the overall efficiency factor, expressed as a decimal. It represents delays, manoeuvring, operator performance, weather, access and other losses. The equation assumes that cycle volume is consistently measured and that conditions remain reasonably stable during the estimating period.

For a truck fleet, the number of trucks should be matched to the loading and dumping cycle. If trucks queue at the excavator, hauling capacity is excessive relative to loading capacity. If the excavator waits for trucks, the fleet is too small or the haul route is inefficient.

Worked example. An excavator fills a truck with 8 m³ per loading cycle. Loading takes 4 minutes, travel and return take 18 minutes, and dumping and manoeuvring take 3 minutes. The truck cycle is therefore 25 minutes. The theoretical truck production is:

If site efficiency is 0.75, actual production is:

For an 8-hour shift, assuming the efficiency factor already includes ordinary shift losses, production is 115.2 m³ per truck-shift. If the required output is 460.8 m³ per shift, four equivalent trucks are required. In practice, fleet size should also consider truck availability, loading balance, road conditions and reserve capacity.

3.4 Embankment construction

Embankments are constructed in controlled layers, commonly called lifts. The specified loose lift thickness must be compatible with the material and compaction equipment so that the required density is achieved throughout the layer, not merely at its surface. Each lift is spread uniformly, brought close to the target moisture condition, compacted, tested and approved before the next lift is placed.

The typical sequence is:

1. Prepare and proof-roll the foundation.
2. Place approved fill in a controlled layer.
3. Spread to uniform thickness.
4. Adjust moisture by watering or aeration.
5. Compact using an approved roller pattern.
6. Test density, moisture and level.
7. Correct defects and release the layer.

A proof roll uses a heavily loaded vehicle or roller to reveal yielding or pumping areas. It is a diagnostic operation, not a substitute for formal density and material testing.

3.5 Cut sections and unsuitable material

In cut sections, excavation proceeds with attention to slope stability, groundwater and temporary drainage. Unsuitable material may include highly organic soil, expansive soil, contaminated material or soil that cannot meet specified strength or compaction requirements. Removal and replacement must be authorized because unplanned undercutting can substantially increase cost and disrupt the programme.

4. Placement of Subgrade and Pavement Layers

The pavement system commonly includes prepared subgrade, selected or improved subgrade, subbase, base course and a bound surface course. Each layer has a distinct structural and drainage function. Construction control must verify material source, grading, moisture, thickness, density, level and crossfall.

4.1 Granular layer placement

Aggregates are delivered by trucks, placed by spreaders, graders or controlled dozer operations, and compacted in layers. Segregation occurs when coarse particles roll away from fine particles or when material is dumped in large uncontrolled heaps. It is reduced by correct stockpile management, suitable discharge height, controlled spreading and avoiding unnecessary rehandling.

The compacted thickness is not obtained simply by measuring the loose pile. The contractor must establish a practical loose lift thickness from trial sections. The trial section also determines water requirements, roller type, speed, vibration setting and number of passes.

4.2 Compaction principles

Compaction increases density by reducing air voids. The result depends on soil type, moisture content, layer thickness, contact pressure, vibration, roller speed and number of passes. Excessive roller speed can reduce compactive effort, while excessive vibration near structures can cause damage. Wet material may rut or pump rather than densify; dry material may resist rearrangement.

A field density result is often compared with a laboratory reference density. The compaction ratio is:

where ρ_d,f is the field dry density and ρ_d,max is the specified maximum dry density from the applicable laboratory compaction procedure. The ratio is meaningful only when the laboratory method, material fraction, sampling procedure and field test are compatible. It is not universally appropriate for every treated material or rockfill.

5. Asphalt Pavement Construction

5.1 Production and transport

Asphalt mixture is produced in a batch or drum plant. Aggregate is proportioned, heated and dried; binder is introduced at the specified temperature and dosage; and the mixture is stored or dispatched. Plant control must maintain gradation, binder content, temperature and uniformity.

Transport trucks should be clean, properly covered when required and arranged to prevent segregation and excessive cooling. The haul plan must coordinate plant output, truck turnaround and paver demand. If trucks arrive irregularly, the paver may stop, producing joints and variable texture.

5.2 Paving operation

The paving train generally consists of a delivery truck, paver, breakdown roller, intermediate roller and finish roller. The paver receives the mix, conveys it to the screed, spreads it and provides initial compaction. Screed control establishes thickness and crossfall, but final density depends on the rolling pattern and mix temperature.

Important controls include:

• clean and properly prepared underlying layer;
• tack coat application at the approved rate;
• consistent paver speed;
• adequate material head in front of the screed;
• correct mix temperature range;
• joint location and treatment;
• roller sequence and overlap; and
• avoidance of unnecessary stops.

The compactability window narrows as the mixture cools. Rolling should begin at the appropriate temperature for the mix and ambient conditions, without causing shoving, cracking or pickup. Vibratory rollers should not remain stationary while vibrating on a fresh mat.

5.3 Joints and surface quality

Longitudinal and transverse joints are frequent locations of low density and water entry. Joints should be planned to avoid wheel paths where practicable, cut or formed neatly, cleaned, tack-coated and compacted according to the specification. Surface defects such as segregation, tearing, checking, roller marks and insufficient crossfall should be corrected while correction remains practical.

5.4 Asphalt quantity estimate

For a uniform asphalt layer, the compacted volume is:

where V is volume in m³, L is length in m, W is paved width in m and t is compacted thickness in m. If the target compacted density is ρ, the required mass is:

These equations assume uniform thickness and density and exclude allowances for waste, joints and construction tolerances. Such allowances should be added separately rather than hidden in the basic calculation.

Worked example. A 1,200 m lane is 3.5 m wide and has a compacted asphalt thickness of 0.05 m. The volume is 210 m³. At a compacted density of 2,350 kg/m³:

If an independently justified 2% allowance is made for waste and operational losses, the planned quantity is approximately 503.4 tonnes. The allowance is a planning quantity, not a substitute for measuring actual delivery and placed tonnage.

6. Concrete Pavement Construction

Concrete pavement may be constructed using fixed-form or slipform methods. Fixed forms are suitable for smaller areas, irregular geometry and work requiring edge support. Slipform pavers are efficient for long, continuous alignments and use lateral confinement from the machine to shape the slab without fixed side forms.

6.1 Preparation and concrete supply

Before placing concrete, the foundation must have the specified line, level, moisture condition and cleanliness. Dowel bars, tie bars, reinforcement and embedded items must be accurately positioned and supported. Concrete supply must be continuous enough to avoid cold joints, yet controlled so that delivery does not exceed the placement and finishing capacity.

Batching accuracy, mixing time, slump or workability, air content where specified, temperature and strength development are monitored. Excess water added at site can increase shrinkage and reduce strength; any adjustment must comply with the approved mix design and quality procedure.

6.2 Placement, vibration and finishing

Concrete is discharged, spread, vibrated and struck off. Internal vibrators consolidate the concrete around reinforcement and at edges, while surface vibrators or the paver's vibration system assist uniformity. Under-vibration leaves voids; over-vibration can cause segregation.

Finishing should be limited to achieving the specified profile and texture. Bleed water should not be worked into the surface. Transverse or longitudinal texturing provides skid resistance, and curing compound or wet curing begins as soon as the surface permits. Joints are constructed or saw-cut at planned locations to control cracking. Saw-cut timing must balance the need to avoid random cracking against the risk of raveling or damage from cutting too early.

6.3 Curing and opening

Curing reduces moisture loss and supports hydration. The curing method and duration depend on the specification, cementitious materials, temperature, humidity and wind. Early traffic can damage a slab before adequate strength or maturity is achieved; opening criteria should therefore be based on specified strength, maturity or other approved evidence, not merely elapsed time.

7. Integrated Sequencing and Site Logistics

A highway project should be divided into manageable work zones or production sections. A typical sequence is:

Figure: Typical highway construction sequence with quality release between stages.

Each stage should have a release point. For example, base-course placement should not begin over a subgrade that has failed proof rolling or does not meet level and moisture requirements. Temporary drainage should be maintained continuously; otherwise rainfall can destroy completed earthworks and contaminate granular materials.

Production planning should balance the capacities of linked operations. If a paver places 150 tonnes/h but the plant and trucks deliver only 110 tonnes/h, the paver cannot sustain its nominal output. Conversely, excessive delivery causes truck queues, segregation and unsafe congestion. A logistics plan should show haul routes, turning areas, fuel points, maintenance areas, stockpiles, water supply, emergency access and traffic-control locations.

8. Quality Assurance, Safety and Environmental Management

Quality control is the contractor's operational system; quality assurance includes independent verification that the system and results comply with the contract. Typical tests include soil moisture and density, aggregate grading, asphalt temperature and density, concrete workability and strength, layer thickness, smoothness and geometric survey.

Inspection and test plans should identify the activity, characteristic, test method, frequency, responsible person, acceptance criterion and hold or witness point. Records must be traceable to location, date, material source, equipment and crew.

Major hazards include moving plant, reversing trucks, unstable slopes, overhead services, work-zone traffic, hot asphalt, cement burns, silica dust, noise, vibration and night work. Controls include separation of people and machines, reversing alarms and spotters, high-visibility clothing, lockout procedures, traffic barriers, safe access, training and emergency planning. Personal protective equipment is necessary but should not replace elimination, substitution or engineered controls.

Environmental controls include sediment basins, silt fencing, wheel washing, dust suppression, noise management, spill response, protection of watercourses, controlled waste disposal and rehabilitation of borrow areas. Water used for dust control must not create runoff carrying sediment onto public roads or into streams.

9. Technology and Contemporary Practice

Machine-control systems use GNSS, total stations or three-dimensional models to guide grading and paving. They can reduce rework and improve consistency, but only when the digital terrain model, coordinate system and design surfaces are correct. Survey checks remain necessary.

Telematics provide data on location, engine hours, fuel use, idle time, payload and fault codes. These data can support maintenance and productivity analysis, but monitoring should respect privacy, contractual responsibilities and data governance. Drones may assist in progress measurement, stockpile surveying and safety observation, subject to aviation and site rules.

Artificial intelligence can help classify equipment downtime, predict maintenance needs and compare planned with actual production. It should support, not replace, engineering judgment. Poorly calibrated sensors, incomplete data or an unsuitable algorithm can create false confidence. The responsible engineer must verify automated results against field observations and contract requirements.

Warm-mix asphalt, recycled asphalt pavement, reclaimed aggregates, low-carbon cementitious materials, electric or hybrid equipment and digital quality records can reduce environmental impact. Their use requires evaluation of local specifications, material variability, durability evidence, supply reliability and whole-life performance rather than selection solely on initial cost.

10. Illustrative Case: Rural Highway Upgrade

Background

A contractor must upgrade a 6 km, two-lane rural road. The project includes 180,000 m³ of earthwork, a 200 mm granular subbase, a 150 mm crushed-rock base and two asphalt layers. The route crosses rolling terrain, and rainfall is frequent.

Problem

Early planning proposed one large excavator, six trucks and a grader. The estimate ignored the long return haul, wet-weather delays and the need to maintain public traffic. Trial production showed that trucks queued at the excavator during loading but were unavailable when the grader needed imported material. Rain also softened exposed formation.

Analysis and approach

The revised method divided the route into 500 m production sections. Drainage and temporary outlets were constructed before major excavation. Topsoil was stockpiled separately. The contractor used two excavators, a dozer for short pushes, an articulated-truck fleet for wet and uneven haul roads, and a grader dedicated to layer shaping. A water truck was assigned to moisture conditioning, while a padfoot roller and smooth-drum roller were selected for cohesive fill and granular layers respectively.

A trial embankment established lift thickness, moisture range and roller passes. The asphalt plant, truck fleet and paver were scheduled from a target paving rate rather than maximum plant output. A spare truck and standby pump were included in the risk plan. Traffic was managed with a moving work zone and controlled one-way operation.

Outcome

The sectional approach reduced exposed formation length, improved drainage control and made testing more systematic. Production increased after the truck fleet was matched to haul distance, while rejected layers decreased because moisture and compaction were checked before release. The project still experienced rain delays, but the effect was limited because completed sections were protected and work fronts were not opened unnecessarily.

Learning points

1. Equipment selection must reflect material, terrain and haul conditions, not nominal machine size alone.
2. Drainage is a production requirement as well as an environmental obligation.
3. Trial sections convert general equipment assumptions into verified operating procedures.
4. The production rate of a linked system is governed by its bottleneck.
5. Dividing a linear project into controlled sections improves logistics, quality and risk management.

Challenges and Limitations

Productivity formulas provide estimates, not guarantees. Actual output changes with operator skill, weather, maintenance, traffic, material variability and site geometry. Density specifications may not represent performance adequately where unusual materials, stabilized layers or rockfill are involved. Digital systems can improve control but can also propagate incorrect models rapidly. Finally, a method that minimizes direct construction cost may increase environmental damage, maintenance requirements or public disruption. Decisions should therefore consider safety, quality, time, cost and whole-life consequences together.

Best-Practice Checklist

Before work begins, confirm design information, permits, utilities, survey control, material sources, temporary drainage, traffic arrangements and equipment availability. During work, maintain controlled layer thickness, monitor moisture and temperature, record production and test results, separate people from machines, and inspect work at release points. At completion, verify geometry, smoothness, joints, drainage, as-built information, environmental restoration and safe opening conditions.

Chapter Summary

Highway construction is an integrated sequence of clearing, earthworks, drainage, formation preparation, layer placement, paving, compaction, finishing and verification. Effective execution depends on matching equipment to material and site conditions, balancing the capacities of linked operations and controlling the work through surveyed geometry, trial sections and inspection points.

Cyclic production can be estimated from capacity, cycle time and efficiency, but field conditions must be used to validate the estimate. Earthworks require controlled lifts, moisture conditioning and appropriate rollers. Granular layers must be protected from segregation and compacted to the required density. Asphalt construction depends on plant–truck–paver coordination, temperature control and planned joint treatment. Concrete construction requires accurate support, continuous supply, consolidation, finishing, curing and joint control.

Safety, environmental protection, drainage and quality assurance are continuous responsibilities rather than final checks. Digital machine control, telematics, drones and AI can improve planning and monitoring when their data and limitations are understood. The central professional task is to create a safe, balanced and verifiable production system that delivers the specified highway performance.

Key Terms

Clearing: Removal of vegetation, rubbish and surface obstructions from the construction area.

Grubbing: Removal of roots, stumps and buried organic matter.

Formation: Prepared ground surface supporting the pavement structure.

Borrow material: Soil or aggregate obtained from an approved external source or borrow area.

Cut: Excavation below existing ground to achieve the designed level.

Embankment: Constructed fill raised above existing ground.

Lift: A controlled layer of soil or aggregate placed for compaction.

Proof rolling: Loading a surface to identify weak or yielding areas.

Compaction: Mechanical reduction of air voids to increase material density.

Subgrade: Prepared soil foundation beneath pavement layers.

Subbase: Pavement layer between subgrade and base, often providing structural and drainage functions.

Base course: Strong granular or bound layer directly supporting the surface course.

Paver: Machine that receives, spreads and initially compacts asphalt or concrete.

Tack coat: Bituminous bond coat applied between asphalt layers.

Screed: Paving-machine component that distributes and levels material.

Slipform paving: Concrete paving using a moving machine that shapes the slab without fixed side forms.

Cycle time: Time required to complete one equipment operating cycle.

Production efficiency: Ratio representing productive time relative to available operating time.

Hold point: Specified stage at which work cannot proceed until inspection or approval occurs.

Machine control: Digital guidance of construction equipment using surveyed design data.

Review Questions

Short Answer Questions

1. Distinguish between clearing and grubbing.
2. Why are trial sections useful in earthworks construction?
3. State two factors that influence roller selection.
4. What is the purpose of a tack coat?
5. Why must concrete pavement be cured?

Long Answer Questions

1. Explain the complete sequence for constructing and approving an embankment lift.
2. Compare excavators, dozers, scrapers and wheel loaders for highway earthworks.
3. Describe the production and paving controls required for an asphalt pavement.
4. Explain how site logistics and bottleneck analysis influence equipment fleet size.
5. Discuss quality, safety and environmental controls for a highway construction work zone.

Analytical / Application Questions

1. A haul unit carries 10 m³ and completes a cycle in 30 minutes. If overall efficiency is 0.70, estimate actual hourly production and identify two field conditions that could invalidate the estimate.
2. A paving crew experiences frequent interruptions because trucks arrive in groups and then stop. Analyse the likely causes and propose a revised plant–truck–paver logistics plan.
3. For a rainy rural road project, select an earthmoving and compaction fleet and justify your choice using material behavior, haul conditions, drainage and risk management.

Multiple Choice Questions with Answers

Question 1

Which machine is generally most suitable for fine grading and controlling crossfall?

A. Motor grader

B. Hydraulic breaker

C. Water tanker

D. Crawler crane

Correct Answer: A. Motor grader

Question 2

A machine handles 6 m³ per cycle and completes one cycle in 20 minutes. What is its theoretical production?

A. 12 m³/h

B. 18 m³/h

C. 20 m³/h

D. 36 m³/h

Correct Answer: B. 18 m³/h

Question 3

What is the primary purpose of proof rolling?

A. To apply tack coat

B. To measure asphalt binder content

C. To identify weak or yielding areas

D. To texture fresh concrete

Correct Answer: C. To identify weak or yielding areas

Question 4

Which roller is generally appropriate for cohesive soil?

A. Padfoot roller

B. Steel pipe cutter

C. Concrete pump

D. Scraper

Correct Answer: A. Padfoot roller

Question 5

If a pavement layer is 1,000 m long, 3.5 m wide and 0.04 m thick, what is its volume?

A. 14 m³

B. 40 m³

C. 140 m³

D. 1,400 m³

Correct Answer: C. 140 m³

Question 6

Which practice best reduces asphalt paving interruptions?

A. Allowing trucks to arrive without scheduling

B. Coordinating plant output, truck turnaround and paver demand

C. Stopping the paver whenever the mat is warm

D. Placing all trucks in the work zone at once

Correct Answer: B. Coordinating plant output, truck turnaround and paver demand

Question 7

What is the main function of curing concrete pavement?

A. To increase aggregate size

B. To reduce moisture loss and support hydration

C. To remove all joints

D. To cool the concrete instantly

Correct Answer: B. To reduce moisture loss and support hydration

Question 8

Which material is normally stockpiled separately because it is unsuitable for structural fill?

A. Clean crushed rock

B. Approved granular base

C. Organic topsoil

D. Washed coarse aggregate

Correct Answer: C. Organic topsoil

Question 9

In a linked construction operation, the bottleneck is best described as:

A. The operation with the lowest effective production capacity

B. The newest machine on site

C. The operation with the highest fuel use only

D. The activity performed first each day

Correct Answer: A. The operation with the lowest effective production capacity

Question 10

Why should digital machine-control data be checked against field surveying?

A. Digital systems cannot store design surfaces

B. Incorrect models or coordinates can produce systematic errors

C. Surveying is needed only for concrete curing

D. Machine control operates independently of design information

Correct Answer: B. Incorrect models or coordinates can produce systematic errors