Port Road Pavements Design And Economic Considerations for Case of Appropriateness

M.N.Nagabhushana, Former Senior Principal Scientist, Flexible Pavement Division, CSIR-Central Road Research Institute

Port Road Pavements Design And Economic Considerations for Case of Appropriateness

Ports are among key components of country’s infrastructure whose performance metrics have improved considerably over years. While Indian coastline has 12 major ports handling about 75 percent of India’s port traffic, other(intermediate and minor) ports, about 200 in number, are serving the remaining marine traffic. Recent Policy initiatives of Govt. of India regarding ports have yielded results and there has been an upsurge in terms of total cargo handled at major ports. However, infrastructural facilities of ports within are yet to match the demands due to increase in cargo magnitudes and the existing facilities are falling short and are overstressed. Consequently, the port roads, among others, are deteriorating fast under heavy loads coupled with environmental and climatic factors and need a well conceived developmental approach; both in new constructions as well as in Maintenance and Rehabilitation/upgrading (M&R) phase.

Any comprehensive Port Development Guidelines (PDG) is supposed to assist proponents with the development and construction of new infrastructure including roads and/or the modification of existing infrastructure, within ports held under the jurisdiction of the port trust/authority. The performance requirements stipulated for port land transportation facilities, including roads, within the scope of PDG, shall ensure that the proposed facilities comply with requirements of the land, geotechnical, meteorological, state of sea, environment, tourism and traffic conditions of the port and its hinterland. Port transportation facilities should also have structural stability against self weight, earth & water pressure, waves, water currents. wind, imposed loads, heat, collision of huge vessels, earthquake and ground motions.

Road pavements are the integral and vital part of the land transportation structures and need to be paid the due attention for deriving the optimal benefits under the objectives of port development phase. For this reason, the selection of type, the geometrical and compositional characteristics, the design, construction, quality assurance and maintenance during operation attain significance. The techno-economic considerations constitute a major decisive approach for the development or upgradation of a port road network. Port roads are threatened by tidal conditions, do need a special contemplation as the soil strata remains marshy and weak, or perhaps the weakest. Therefore, a careful consideration involving the pavement structure is to be made, both during new addition of road lengths or M&R of existing roads.

Port Road Pavements
Generally, two basic types of road pavements are constructed in India; either flexible, the most widely used or the rigid pavement, as the next choice and obviously port roads are also constructed with either of these. Though there are further sub groups of pavements; surfaced and unsurfaced in flexible and plain or reinforced in rigid, in simple terms, a flexible pavement can be defined as a pavement layer comprising of a bed of compacted soil, granular layers and a layer or two of bituminous surfacing. Rigid pavements, on the other hand, are made from a well compacted granular layer/a low strength concrete layer (DLC) or both laidover soil and covered by plain or reinforced concrete slabs. Subsequent to these common types, India has been gradually introducing composite pavements/use of treated layers below the bituminous top, short panelled concrete and block pavements.

Port Road Pavements Design And Economic Considerations for Case of Appropriateness

The structural capacity of flexible pavements is attained by the combined action of the different layers of the pavement and hence depend on their ability, in turn. Flexible pavements are designed in such a way that the stress reaching the subgrade is negligible so as to not to exceed the bearing capacity of the subgrade soil. Consequently, the type, thicknesses and quality of the layers above the subgrade vary depending upon strength of soil affecting the cost of the pavement structure. For this reason, port roads may demand higher layer thicknesses and/or basal reinforcement techniques.

Rigid pavements are named so because of the high flexural rigidity of the concrete slab and hence the pavement structure deflects very little under loading due to the high modulus of elasticity of their surface course. The concrete slab is capable of distributing the traffic load into a large area with small depth which minimises the need for a number of layers to help reduce the stress.

Port Road Pavements Design And Economic Considerations for Case of AppropriatenessTypical Flexible and Rigid Pavement Cross-section

During the design of port road pavements with given poor soil and expected heavier traffic load of vehicles and cranes, very often the question of choosing between these two types of pavements; flexible and rigid, comes to the fore. Many factors influence selection of the type of pavement to be constructed. Some of the main factors include; soil characteristics, traffic, material properties, climate, environment including depth of water table, construction considerations, and cost comparison. There are also secondary factors such as: performance of similar pavements in the area, continuity of the cross section, conservation of materials and energy, availability and use of local materials, traffic safety, traffic noise mitigation, experimental features, and agency preferences.

Hence, during the design and construction stages of port road pavement, it is very important to analyse the present situation with all the factors that have an impact on future structure. For such considerations, the basic characteristics of both variants of pavement are listed in the following table 1.

Port Road Pavements Design And Economic Considerations for Case of Appropriateness

While the above philosophy and discussions revolve around the basic pavement types involving flexible and rigid pavements, the container and stock yards are being provided with Interlocking Concrete Pavements (ICP) also and available guidelines indicate about the techniques and methods for design and construction.

Design Considerations and Design Strategy
The design of pavement significantly depends of soil conditions and the amount of traffic loads expected to be carried during its design life. Flexible pavements in India are designed based on California Bearing Ratio (CBR) of subgrade soil and expected number of cumulated axles (measured in million standard axles) during the design life of the pavement or axle load spectrum to arrive at Cumulative Damage Factor (CDF) for rigid pavements. The pavements are normally designed for a period of 15-20 years in case of flexible and 30-40 years in case of rigid pavements. The Indian Roads Congress (IRC) method of design allows use of conventional as well as stabilised and recycling of materials appropriately in flexible pavement layers and thickness of each layer is then computed using the layer theory based algorithm or read from the templates given in the guidelines.

Rigid pavements thicknesses are computed for overcoming the expected damage till the design life, normally for a period of 30-40 years and thickness design of rigid pavements are influenced by traffic loading, subgrade soil, moisture, and temperature differential. First, the thickness of rigid pavements is designed for fatigue failure. The computed thicknesses of the pavements are then checked for the critical combination of load stresses and temperature stresses

Indian Port Authority Recommendations for Pavements
Most of the Indian Port Development Guidelines (PDG) suggests that pavements are designed, constructed and maintained as per the standards and specifications available and applicable for the major land road networks. This implies that the design and specifications comply with Indian Roads Congress(IRC) and Ministry of Road Transport and Highways (MoRT&H) guidelines which are in vogue. Since IRC Guidelines are available for both flexible and rigid pavements, the task has been easier and require only proper and logical data inputs. The design outputs shall be verified to check for the compliance of obtained values with the stipulated limits.

In addition, the PDGs may solicit additional measures for the effective performance requirements. These could demand for suitable geotechnical properties (bearing capacity, settlement limits e.g.) for the intended use of the land for roads as per relevant standards and specifications.

Ground Improvement and Other Requisites
In ports, while roads are constructed over the marshy tidal area, it requires a pre-sub-soil investigation to design suitable ground improvement method before the actual road pavement construction is takenup. The geo-technical investigations normally aim at finding out requisite information, which include but not limited to; depth of soft clay, grain size distribution, clay content, undrained shear strength, initial void ratio, compression index, coefficient of consolidation, unit weight of clayey sub-soil etc.

Deep deposits of soft clay is found all along the coastal and delta areas of the country where Ports come up. Therein, road alignments pass through marine clay sub-soil and conventional construction of roads over such marine clay sub-soil deposits may lead to failures due to very low shear strength and very low permeability of sub-soil. Hence provision of ground improvement measures becomes essential to achieve the required bearing capacity and tolerable total and differential settlements, especially in case of flexible pavements. If the depth of clay layer is moderate (upto 3 m), then it can be removed and replaced by soil having good shear strength. But where the depth of sensitive clays is larger, it becomes essential to adopt ground improvement techniques to achieve the required bearing capacity and tolerable total and differential settlements. Accordingly, different methods of ground improvement may be designed, viz., (1) Use of Geosynthetic Basal Reinforcement below the Road Pavement and (2) Use of Stone Columns(granular material compacted in-situ in long cylindrical boreholes).

The basal reinforcement acts in a manner similar to rigid layer and helps to distribute the embankment load on to the subsoil evenly. Typical Cross section of an embankment is given in the next figure 1.

Port Road Pavements Design And Economic Considerations for Case of AppropriatenessFigure 1: Typical Cross Section of an Embankment with Basal Reinforcement

There are additional requisites also, significant during the development stage and are inclusive within a detailed guidelines. It has to be noted that the primary requirement is that top level for all roads within the Port area shall be the same as that of the terminal yard. Roads within the container yard will generally be a part of the yard, specifications for will be mentioned in the initial guidelines such as ‘the container and stock yards are being provided with Interlocking Concrete Pavements(ICP)’. The other items that may need attention include:
  1. Geometric Design of roads within yard- These are normally done as per IRC and MORTH specifications
  2. Road markings (Lane, Road Edge, Directional Arrows, etc.) - These are normally done as per IRC and MORTH specifications
  3. Road side barriers, fixed type or removable type- These are normally done IRC and MORTH specifications and the choice of type shall depend on the overall yard area design and traffic movement planning that needs to be done
  4. For the drainage in the area, the specifications, if specifically stated under ‘ Utilities’ shall be followed or shall be adequately designed
Comparison of Flexible and Rigid Pavement
Conceptually, when comparing a flexible and a rigid pavement alternate, the same design life should be considered. However, the comparison of pavement alternates with different design lives for the situation is also being followed as an extended comparative process. In such cases, the periodic upkeep adjustments during different stages of the pavement with relatively shorter life to extend its life should also be counted for economic comparison by way of total transportation cost approach. The intention is to determine during the ‘Detailed Project Report’ phase itself, which alternate pavement design life is the most cost effective. Flexible pavements and rigid pavements can be compared on different parameters. Simple analysis uses only two parameters; cost of construction and carbon footprints.

During the pavement design and construction, it is very important to analyse the present situation with all the factors that have an impact on future structure. Firstly, it is necessary to consider the needs of users and the environment which include features of the existing terrain, weather conditions, operating conditions and the traffic load. Based on that, techno-economic analysis can be carried out with the final selection of adequate pavement structure. With significant investments made on port roads, cost economics is to be treated as very essential and with an integral approach.

Comparison Based on Cost Economics
Generally, the major factor in deciding the type of the pavement in design is the initial cost, i.e., investment. Thus, planners often think that the flexible pavement is cheaper than the rigid pavements, which in fact, may be contrary to the results of ‘Life Cycle Cost Analysis’ as many researchers perform cost analysis of pavements. Initial cost of rigid pavement is high but by considering serviceability life of rigid pavement, it is found in many cases that it is economical than the conventional flexible pavement. Construction cost for rigid pavements on weak soils may be cheaper than cost of flexible pavements, in contrast to what is generally thought. However with the strength increase in sub grade the flexible pavement costs and rigid pavement costs get closer. With increasing petroleum product prices, the cost of bituminous pavements will be even higher. Hence, rigid pavement should be openly considered in choosing the pavement types associated with a realistic cost analysis of pavement performed by reasonable estimates. The normally adopted approach, Life Cycle Cost Analysis (LCCA) for economic analysis of pavement investments is deliberated further.

The appropriate solution for economically beneficial pavement type, bituminous or concrete pavement, is calculated by carrying out Life Cycle Cost Analysis (LCCA). Life-Cycle Cost Analysis (LCCA) is an economic analysis used to evaluate the cost-efficiency of alternatives based on the reliable economic analysis methods such as Net Present Value (NPV) concept. It is essential to evaluate the abovementioned cost aspects in order to obtain optimum pavement life-cycle costs. Life cycle cost analysis (LCCA) is a strategy that considers costs all through an advantage’s lifecycle including investment, operation, maintenance and disposal. LCCA adjusts beginning monetary investment with the long term expense of owning and operating the road asset. LCCA methodology involves the following steps:
  1. Estimate the initial construction cost.
  2. Estimate maintenance cost.
  3. Estimate road user costs
  4. Determine life-cycle cost
The procedure involved is simplified in the figure 2.

Port Road Pavements Design And Economic Considerations for Case of AppropriatenessFigure 2: Life cycle Cost Analysis Procedure

Need for Cost Analysis and Inferences from Some Studies
In case of developing countries, like India, there is a shortage of funds required for new infrastructure projects; both for construction and more significantly for their maintenance and repairs. Decision making in planning and design of roads will impact the need of future operation and maintenance activities. Road authorities of all around the world are finding and innovating ways to cope with the high cost of road network maintenance, the increasing demands of road users and the changing traffic type and volume. In such scenario, it is imperative that cost analysis, as above, are carried out and the right option on techno-economic considerations are adopted. While construction costs or initial investment costs are worked out, the possible estimate for maintenance phase is also computed and based on these together, a long term strategy by optimal selection of type of pavement and its design are worked out.

Reviews have shown there are vital inferences from research based studies which can be enumerated as guiding factors in choice of appropriate pavement for a given scenario. The featuristic observations from some studies are given below while further studying the related exercises are recommended for further strengthening of the analysis. Factors that help to chose the pavement suggest:
  • Flexible pavements show wider range of variation in cost with respect to design parameters of traffic and soil CBR
  • The overall variation in cost of rigid pavements is comparatively small
  • The design of a rigid pavement is highly influenced by the occurrence of small number of heavy axle loads
  • The fatigue life of a rigid pavement is prone to small changes in the stress ratio which can happen with a small increase of the loading along the axle load axis
  • It is observed that flexible pavements are more economical for lesser volume of traffic
Combination Pavements
Case studies show that, during operations, the ship cargo loads meant for import or export, are carried by heavy trucks which converge as they go for entry/exit at port gates and result in increased traffic volume. It is imperative that these roads stressed under heavier concentration of loads are meticulously designed to satisfactorily cater to the heavy commercial traffic with higher axle loads. Many times, the functional requirements may demand for a bituminous black topped road pavement, but the flexible pavement may fall short to provide required design life. In such scenario, combination pavements; with cement concrete structural layers, surfaced with bituminous layers might be an option. Design recommendations for the proposed pavement structure primarily may include a Pavement Quality Concrete (PQC) provided over the DLC layer and will preferable have Plain mild steel dowel bars with Deformed steel tie bars. The layer may consider also having bi-directional reinforcement(minimum temp).

In order to functionally complement the above structure with a black top, a bituminous wearing course of Concrete (BC) or Stone Matrix Asphalt (SMA) with mix design using Polymer Modified Bitumen (PMB) and proper interface/interlayer provisions (stress relief/crack retarding layer on the concrete pavement).

A typical cross-section of the recommended combination pavement structure is further Table 2.

Port Road Pavements Design And Economic Considerations for Case of Appropriateness

Concluding Remarks
From the ongoing discussions, it may be inferred that, for port roads, in general, rigid pavement is economical than flexible pavement. This is more conspicuously pronounced as port roads are constructed over the marshy tidal area having deposits of soft clay. The major supporting issues in this respect can be enlisted as follows:
  1. Rigid pavement carries higher flexural strength than flexible pavement i.e. it carries bending and deformation without rupture under wheel/ axle load.
  2. In flexible pavement load is transferred from grain to grain and because of that many failures occurs such as fatigue cracking, rutting and thermal cracking. But in rigid pavement no such phenomenon of grain to grain load transfer exists, hence there is fewer amounts of failure, when properly designed and constructed
  3. Life span of rigid pavement is more than the flexible pavement with low maintenance cost. Studies suggest life cycle cost of flexible pavement will be about 20 % higher than the rigid pavement after 20 years.
  4. Initial cost of rigid pavement is higher but when comparing total cost of pavement through life span rigid pavement is more economical than flexible pavement.
  5. However, in case of roads where stage construction is possible and adopted, then flexible pavement could prove cheaper than rigid pavement. Also, when soil subgrade is of good quality and traffic is also not very heavy, flexible pavements can be more economical. But, these aspects might be helpful only while designing the secondary or tertiary roads of less importance within the port road network.
  6. In case of areas where soil subgrade is weak and/or drainage conditions are also difficult to maintain at desired level of performance, rigid pavement can be a good choice.
  7. Initial cost of rigid pavement (concrete pavement) is reduces by replacing cement by fly ash at some percent, through a proper design. One may explore using other alternatives, as feasible.
However, notwithstanding the above, the use of life-cycle cost analysis is recommended by making realistic inputs. An interest rate based on current financial data should be used. For initial construction costs, the designers should use thicknesses generated for flexible and rigid pavement designs using the recommended/applicable design input values and methodology developed by standards/guidelines by agencies like IRC.

References:
  1. Port Development Guidelines(Revision 1.6), Pilbara Ports Authority, West Perth, WA 6005
  2. The Overseas Coastal Area Development Institute of Japan, Technical Standards and Commentaries, Part III, Facilities, Chapter 6 ‘Port Transportation Facilities’
  3. Manual of Specifications and Standards, Development of Vizhinjam International Multipurpose Seaport Through Public Private Partnership, Govt. of Kerala
  4. Comparative Analysis of Flexible and Rigid Pavement Design(Milan Uljarević and Slobodan Šupić, UDK: 625.021/.022, DOI:10.14415/konferencijaGFS 2016.060)
  5. Comparative Study of Flexible and Rigid Pavements for Different Soil and Traffic Conditions (Atakilti Gidyelew Bezabih & Satish Chandra, Journal of the Indian Roads Congress, July-September 2009)
  6. Flexible Pavement versus Rigid Pavement (Prof.Satish Chandra, NBM&CW September 2017)
  7. Considerations for Rigid Vs. Flexible Pavement Designs When Allowed as Alternate Bids:Technical Report(Report No.FHWA/TX-09/0-6085-1)
  8. A Comparative Study on Rigid and Flexible Pavement: A Review(Milind V. Mohod, Dr.K.N.Kadam, IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE) e-ISSN: 2278-1684,p-ISSN: 2320-334X, Volume 13, Issue 3 Ver. VII (May- Jun. 2016), DOI: 10.9790/1684-1303078488)
  9. Cost Comparison of Pavements (Shrikant M Harle, The Architects International, Vol 1, Issue 1 – 2018)
  10. Guidelines for The Design of Flexible Pavements(Fourth Revision, 2018, IRC)
  11. Port and Industrial Pavement Design with Concrete Pavers - Second Edition,2020, ICPI,USA
  12. David R. Smith ‘Permeable Interlocking Concrete Pavements’, Third Edition, Interlocking Concrete Pavement Institute, Washington, DC
  13. Sustainable Perpetual Asphalt Pavements and Comparative Analysis of Lifecycle Cost to Traditional 20-Year Pavement Design: Technical Report( Technical Report 0-6856-1)

NBM&CW November 2020

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