U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: Date: May/June 1999|
Issue No: Vol. 62 No. 6
Date: May/June 1999
Over the past eight years, the International Engineering Scanning Program, directed by the Federal Highway Administration's (FHWA) Office of International Programs, has been successful in defining the best highway design, construction, and maintenance practices worldwide. Scanning teams, which are made up of representatives from both the public and private sectors, identify state-of-the-art procedures and products that can be adopted and advanced in the United States to improve the quality of the infrastructure. The teams also showcase innovative practices employed in the United States, sharing innovative technology with the host countries.
The most recent geotechnical engineering scan, which was conducted over two weeks in March 1998, added to the success of the International Scanning Program. The participating geotechnical and structural engineers from FHWA, state highway agencies, and industry accomplished their three goals.
The first goal was to discuss practices for implementing load and resistance factor design (LRFD) methods with colleagues in Canada and Europe.
|The U.S. team particpated in round-table discussions at the Danish Geotechnical Institute with geotechnical experts from Denmark, Sweden, and Norway.|
The second goal was to investigate Canadian and European procedures and experiences with innovative contracting practices for subsurface investigations and for the design and construction of geotechnical features, such as earthworks, structural foundations, and earth-retaining structures. Under FHWA Special Experimental Project No. 14, Innovative Contracting Practices, many state departments of transportation (DOTs) are experimenting with contracting practices currently restricted in the public sector. These practices include product warranties, design-build contracting, cost-plus-time bidding, and alternatives to low-bid award contracting. The scanning team sought to identify the best quality-assurance practices that enable state DOTs and other public agencies to reduce construction time and lower the cost of earthworks, walls, and foundations by sharing with contractors the risks in geotechnical design, construction, and investigation. Performance measures for warranted products were also examined to identify relevant and measurable criteria that provide the best measure of long-term performance.
The team's third goal was to identify new or improved mechanically stabilized earth-wall technologies, ground-improvement methods, and in situ testing procedures. These technologies provide rapid and cost-effective solutions to problematic site conditions by supplementing the engineering properties of on-site materials. Many innovative geotechnical technologies developed in Canada and Europe have found their way into everyday practice in the United States. Our international colleagues captivated the team members with tours showcasing new research efforts and products.
International Implementation of LRFD
One of the team's goals was to learn from the experiences of international colleagues who were more familiar with load and resistance factor design. As early as 1983, the Ministry of Transportation in Ontario, Canada, developed a bridge-design code using LRFD, and to provide standardization throughout Europe, the 15-nation European Community adopted a single, uniform bridge-design code, called the Eurocode.
|Precast tunnel segments for the Oresund Link are awaiting transport to sea. The Oresund Link tunnel will provide four travel lanes and two lanes reserved for maintenance and emergency evacuation.|
In the early 1990s, the American Association of State Highway and Transportation Officials (AASHTO) set out to establish a comprehensive new bridge and foundation design code incorporating all available state-of-the-practice design procedures. The result was a limit-states design code using LRFD concepts. The AASHTO LRFD Code was adopted as an optional code by AASHTO in 1994, and it was hailed as a revolutionary advancement in U.S. bridge design.
The code offers many benefits, including more consistent designs that provide a more uniform level of safety throughout the bridge. In addition, the measure of safety of each code design will be a function of the variability of both loads and resistance - an important feature absent in the AASHTO Standard Specifications for Highway Bridges.
However, the initial use of the code by geotechnical engineers indicated that the code was incompatible with many geotechnical needs and many traditional design practices. In academia and in practice, most geotechnical engineers have used only allowable stress design (ASD) methods for the design of geotechnical features. The move to an ultimate strength design approach reflects a significant challenge for geotechnical engineers who have no formal training with this approach, and most engineers are comfortable using the time-honored design approaches embodied in the Standard Specifications.
Nevertheless, AASHTO is convinced that the LRFD approach will provide considerable benefits, and AASHTO will implement the LRFD Code in place of the Standard Specifications in 1999. Geotechnical and structural engineers are being challenged to elevate the quality of bridge- and foundation-design practices in the United States.
The scanning team interviewed Canadian and European counterparts with years of experience in the development and use of LRFD. The objective was to identify the best approach for implementing LRFD in geotechnical engineering and the obstacles that U.S. geotechnical engineers must overcome. Many lessons were learned.
To implement the geotechnical portion of the Eurocode, Part 7, several technical difficulties must be eliminated. The process of "calibrating" load and resistance factors is necessary for the LRFD process to provide superior, consistent designs, but it has been difficult for Canadian and European geotechnical engineers to initially develop load and resistance factors for geotechnical designs. Because the strength of materials such as concrete and steel can be measured with a high level of certainty, structural engineers are able to statistically calibrate and codify these factors for designers. However, in situ soils can be highly variable, and their characteristic strength properties cannot be measured with the same level of certainty as steel and concrete. Also, geotechnical engineers often employ several different techniques and methods to determine characteristic soil strength and design properties; whereas, structural engineers use identical methods for specific materials. The geotechnical engineer must use site-specific knowledge and judgment and must incorporate this into the analytical decision-making process.
|U.S. team members pose in the flood basin.|
Furthermore, geotechnical engineers are unaccustomed to using prescriptive codes that are familiar to their structural counterparts. Therefore, while structural engineers use well-defined statistically calibrated factors, geotechnical engineers use load and resistance factors "calibrated" to past practice, or allowable stress design. This provides continuity in the design process between structural and geotechnical engineers; however, earthwork, wall, and foundation designs are no more consistent or cost-effective than in the past. Thus, all of the geotechnical engineers that are implementing the new process agree that the necessary first steps for geotechnical engineers is to acquire an understanding of the LRFD design approach and to implement it to provide continuity with structural designs.
To provide geotechnical engineers with the flexibility to use their site-specific knowledge and judgment gained through years of experience, the Eurocode will specify load and resistance factor values that are mandatory and will allow each country to calibrate values for non-mandatory load and resistance factors until improved factors can be established. Each country will have the option to develop a National Application Document to accompany the code, allowing the necessary flexibility desired by geotechnical engineers.
In the meantime, European geotechnical and structural engineers are making concerted efforts to improve communications in the design process and to better understand the differences between the geotechnical and structural design processes. In Canada and France, communication and coordination in the design processes have been comprehensively documented.
Based on the information provided by the Canadian and European experts, the team identified several critical needs that are essential to mainstream LRFD methods into geotechnical engineering practices in the U.S. transportation community. For example, geotechnical load and resistance factors need to be comprehensively calibrated to the current ASD design practice in the United States to provide geotechnical engineers with confidence that designs will be as cost-effective and, more importantly, as safe as the current practice provides. Over time, as quality design data and soil data are collected and evaluated, a complete statistical calibration of geotechnical-related resistance factors should be performed.
|This device is being developed in France to evaluate soil-strength parameters. The device measures soil shear strength by twisting the soil sample, rather than using the traditional approach or direct shear.|
Training at the professional and collegiate levels should be provided to geotechnical and structural engineers, including programs to educate the educators. FHWA has provided leadership by developing the two-day National Highway Institute course 13068, "LRFD for Highway Bridge Substructures." In addition, FHWA's Office of Infrastructure Research and Development created national electronic databases to be used interactively by state DOT geotechnical engineers to collect and maintain quality geotechnical design and soil information for future statistical evaluation.
The desire to implement LRFD in geotechnical engineering practices is gaining momentum worldwide. The International Society for Soils Mechanics and Geotechnical Engineering has recently formed a new committee, Technical Committee 23, to investigate the introduction of LRFD in national geotechnical codes and to harmonize efforts for worldwide implementation.
Detailed findings of the FHWA scanning tour have been conveyed to the U.S. geotechnical community, providing impetus and guidance for a complete rewriting of the geotechnical design portions of the AASHTO LRFD Code, currently being completed by the National Cooperative Highway Research Program.
Innovative Contracting: Lessons Learned
In an effort to improve the long-term quality and durability of the highway infrastructure, owners of public facilities around the world are examining the benefits of many innovative contracting practices. There is a desire to establish a more equitable distribution of design and construction risk between the owner and contractor. It is expected that by allowing the contractor to use innovative and proprietary construction techniques while holding the contractor more accountable in the design and construction processes, projects can be delivered more expediently with equal or better cost-effectiveness and long-term performance. Although innovative contracting practices such as design-build contracting and warranties have been used in the private sector, these practices have been restricted in the public sector by laws and regulations, including requirements for low-bid awards.
The Ontario Ministry of Transportation (MOT) has experimented extensively with innovative contracting practices in the past two years. MOT is under an edict to reduce staffing by two-thirds over three years. To maintain the same network of highways with a smaller staff, MOT is experimenting with the concept of Total Project Management. This approach requires that pre-qualified consultants develop complete design packages. In addition to project design, consultants are required to perform work that was traditionally the responsibility of the owner, such as the investigation of subsurface conditions and the identification of land acquisition and utility-relocation needs. Early results of this revolutionary change in practice are mixed, and some Canadian professionals expressed a concern that the quality of subsurface investigations is being adversely impacted. However, MOT is committed to this new process and will continue to evaluate the results and to work with consultants to improve the process.
Design-build contracting practices are slowly being adopted by public agencies around the globe. The Ontario MOT completed eight design-build projects ranging from a large-scale bypass of Toronto to a small-scale bridge replacement in northern Ontario. The Oresund Link, a 16.2-kilometer series of bridges and a cut-and-fill undersea tunnel connecting Denmark and Sweden, offers a significant opportunity for a Danish-Swedish consortium to implement a large-scale design-build project. The French government has perhaps the most experience using design-build contracting for large-scale highway projects. The results of design-build contracting among all the nations visited were mixed, and many best practices for refining the technical details and procedures of the contracting process were captured by the scanning team.
International contractors were often vocal in their opposition to design-build contracting. There are significant costs associated with the development of technical proposals and other submissions for design-build contracts of large-scale works. In some cases, contractors indicated that unjustified liability was placed on them.
However, the team found that properly developed and managed design-build projects could provide the desired results. The following prescribed steps outline the key requirements.
|Dr. George Goble|
Rating the technical qualifications of the potential bidders should be a two-step process. Of paramount importance is the pre-qualification of contractors based on the quality of organizational staffing, experience, and overall quality-control practices. A second phase of pre-qualification based on project-specific, quality-control plans and performance requirements was found to be a very important step and is necessary to limit the number of qualified contractors.
Typically, three to five contractors are selected to develop designs and final plans for bidding. Reasonable stipends should be paid to the contractors who are selected to develop final plans, and all proposals should be reviewed in strict confidence.
Owners should always provide comprehensive and high-quality subsurface investigations to contractors who develop final plans. Owners cannot relinquish the risks of unforseen subsurface anomalies.
Comprehensive quality-control plans should be developed and implemented as intended. The preliminary engineering package provided by the owner must define specific and quantitative design requirements and performance criteria. In some cases, it may be desirable to have confidential design reviews and discussions with the pre-selected contractors to ensure the clear communication of the owner's needs.
Finally, design-build contracting methods have been found to be most effective on major projects that contain substantial engineering content. Significant cost savings were not achieved on design-build projects to widen, resurface, or rehabilitate roads.
In Germany, alternative designs are customarily accepted and evaluated in lieu of designs prepared by the owner at the bidding stage. The Germans have well-documented procedures to evaluate alternative designs. Although contractors bear all financial responsibility for the development of alternative designs, they often submit innovative alternative proposals in an effort to win a contract.
Performance warranties are frequently used for pavements and bridges in Canada and Europe. The team made a special effort to identify performance measures that owners and contractors could mutually accept. Most warranties were limited to one or two years because contractors have difficulty acquiring performance bonds for more than a two-year period. Options for design-build-maintain projects are being considered in some countries.
Perhaps the most important lesson learned is that quality comes with a cost. Marginal levels of quality in construction are often symptomatic of low-bid awards. Contractors desire to provide high quality, but there is a tendency to deliver overly ambitious construction schedules and performance expectations to win the contract. Some countries are adopting award practices that include important technical and quality-related, selection criteria in addition to cost. Past performance, quality-control implementation, timeliness, and overall long-term value should be integral factors in the award process.
The countries that are implementing these innovative contracting practices wish to improve their initial qualification, quality-assurance, and award and warranty procedures. Consistently when these procedures were not well-developed, disputes were chronic and significant.
The scanning team strongly recommended that objective, model pre-qualification criteria for geotechnical work be established for owners. In addition, model performance measures for warranted geotechnical products and model quality-control and quality-assurance plans should be developed for specific features of geotechnical construction. These model documents would provide technical guidance for public agencies that wish to establish their own qualification and quality-control procedures for design-build and warranted projects. The team is optimistic that FHWA, AASHTO, the American Society of Civil Engineers' Geo Institute, and the newly formed Geo Council will provide leadership in developing these needed products for U.S. practices.
Innovative Geotechnical Technologies
Many technologies commonly used throughout the United States by today's geotechnical engineers have roots in product development in Europe and Canada. Notably, mechanically stabilized earth (MSE) retaining walls and slopes were developed in France approximately 35 years ago. Soil-nailed wall technology and in situ soil-testing devices such as the cone penetrometer were also developed in Europe.
The scanning team members were asked to look for new ground improvement and wall technologies. Several new technologies were identified that have significant potential for application in the United States.
Research currently underway in Canada will advance the international geotechnical community's knowledge of MSE wall reinforcement. Full-scale walls are being tested to failure, and communities in earthquake-prone regions will benefit from the testing of MSE walls on large-scale shake tables that model walls subjected to earthquakes.
Known for grand-scale projects to reclaim land lost to the sea, Denmark has developed innovative methods for mitigating beach erosion and new methods to de-water deep soil strata to stabilize land for new construction. With the emergence of market economies in Eastern Europe, European Community nations are setting their sights on improving the infrastructure for moving people and freight between East and West. The Germans and Danes, while building new corridors for carrying goods, are developing new tunneling technologies, including methods to minimize environmental impacts while tunneling below the groundwater table.
The first use of geofoams, or lightweight polystyrene, in highway construction took place in Norway. Geotechnical engineers used the materials, which weigh only one percent of the soils they replace, to construct roadway embankments over soft soils to negate the effects of settlement. The Germans are conducting state-of-the-practice research on geofoams to advance the use of these products.
The French government and FHWA have collaborated recently on several geotechnical research efforts. Most notably, the French are identifying new ground-improvement strategies for de-watering wet clays and bay muds. Research is being conducted on a system that circulates controlled air volumes over pipes pre-drilled into the ground, producing natural suction to remove water from the soils.
These innovative products and practices represent only a fraction of the ideas and products that the host countries shared with the team. Team members prioritized these technologies in accordance with the benefit that can be expected from their implementation. These technologies are being showcased at regional and national geotechnical conferences across the United States.
Table 1 - FHWA Geotechnical Engineering Scanning Team Members and Affiliations
|Jerry DiMaggio||Team Co-Leader
Senior Geotechnical Engineer
|FHWA, Washington, D.C.|
|Tom Saad||Team Co-Leader Division Bridge Engineer||FHWA, Indianapolis, Ind.|
|Tony Allen||State Geotechnical Engineer||Washington State DOT|
|Barry R. Christopher||Team Facilitator||Consulting Engineer|
|Al DiMillio||FHWA Research Engineer||FHWA, Washington, D.C.|
|George Goble||Geo Council (Industry Representative)||Goble Rausche Likins & Associates|
|Paul Passe||State Geotechnical Engineer||Florida DOT|
|Gary Person||State Geotechnical Engineer||Minnesota DOT|
|Terry Shike||State Bridge Engineer (AASHTO Representative)||Oregon DOT|
Marketing and Implementing the Findings
FHWA distributed the Summary Report of the Geotechnical Engineering Study Tour to coincide with the Transportation Research Board (TRB) annual meeting in January 1999. The findings have been shared with the participants at the TRB meeting and with the AASHTO Subcommittee on Highway Bridges and Structures. Team members are also busy marketing the tour findings at regional geotechnical engineering conferences and through key committees of AASHTO and other professional societies. In addition, the members are putting the findings into practice in their agencies.
The FHWA International Engineering Scan Program provided a means for forging a new partnership between FHWA, state transportation agencies, U.S. industry, and the international highway community that will improve communication and collaboration in efforts to advance geotechnical engineering design and contracting practices in the United States. The final outcome will result in more durable and cost-effective roads and bridges throughout the nation.
Table 2 - Countries Visited on the Geotechnical Scanning Tour and the Host Agencies
|Host Country||Host Agency|
|Canada||Ministry of Transportation of Ontario|
|Denmark||Danish Geotechnical Institute|
|Germany||German Federal Geotechnical Institute (BAS)|
|France||Teaching and Research Center in Soil echanics (ENPC-CERMES)|
Geotechnical Engineering Practices in Canada and Europe (1998) and Geotechnology Soil Nailing - European Study Tour (1993) are available on the Web (www.international.fhwa.dot.gov) or by contacting FHWA's Office of International Programs via e-mail: email@example.com; phone: (202) 366-2155; fax: (202) 366-9626; or mail: Office of International Programs, FHWA/USDOT-HPI-10, 400 Seventh Street, SW., Washington, DC 20590.
A Primer on ASD and LRFD Design Methods
The following provides a basic explanation of the differences between allowable stress design (ASD) and load and resistance factor design (LRFD) methods. The common concept behind both design methods is to compare applied forces with available resistances to ensure that a certain level of reserve capacity is available to account for the uncertainty in both the loads and resistances. This reserve capacity provides confidence to the engineer that his/her design is safe against poor performance - or worse, catastrophic failure. The method of defining and quantifying these uncertainties is the fundamental difference between these two methods of design.
ASD, structural elements such as structural foundations, bridge beams and girders, or earth-retaining walls are designed to support, or resist, anticipated service loads, including vehicular live loads, superstructure dead loads, or lateral soil loads. To account for the possibilities that structural elements are overloaded during their service life and that the materials providing resistance to the load are not as strong as expected, engineers apply a global safety factor on the resistance side of the design equation to ensure that the structural elements are large enough to account for all uncertainties in design. In this way, global factors of safety account for the uncertainty in both loads and resistances. The general forms of the equation appear as follows:
General Design Equation: Resistance provided (R) > Loads applied (G L)
ASD: R / F.S. > G L, where the Factor of Safety (F.S.) = 1.5 to 3.5
Although the ASD approach ensures that the supporting design element is sufficient to carry potential overloads, the approach does not supply the designer with two vital pieces of information. The total capacity of the supporting element cannot be ascertained with ASD, and therefore, the mode of failure cannot be predicted with certainty. Often, this means that the global factors of safety are set at overly conservative levels.
In some cases, global factors of safety are not conservative. This may be difficult to imagine since structural elements do not frequently fail. However, rather than attributing this to the quality of the analytical method, this can, in large part, be attributed to the fact that engineers employ judgment and experience in the design process. The ASD method does not provide a rational means to define the level of safety of the design element.
In LRFD, uncertainties in both applied loads and structural and material resistances can be better discerned when they are separated and studied individually. Likewise, if safety factors can be applied in the design equation, both on the load and resistance sides, the designer can better use analytical tools to establish the total capacity of design elements. The designer can more accurately predict dead loads such as the weight of concrete and steel in the superstructure; however, they may apply a more conservative load factor to transient or vehicular live loads.
The general form of the LRFD equation takes on the following simplified appearance:
LRFD: N R > G m L
In this equation, resistance factors (N) are values less than one to account for the uncertainty that the materials providing resistance may not be as strong as anticipated. Load factors (m) are values greater than one to account for the possibility that overloads will be applied to the element during its service life. With the LRFD approach, the designer can better assign margins of safety to each portion of the design equation as suited to the level of confidence with which each load and resistance can be predicted. Therefore, designs can be based on risk and reliability concepts. By calibrating the load and resistance factors to an overall margin of safety, designers can ensure that all designs have prescribed margins of safety against failure.
Thomas K. Saad is the division bridge engineer in FHWA's Indiana Division Office. In this position, he is responsible for marketing new structural, geotechnical, and hydraulic technologies to state, local agency, and consulting engineers. He is a licenced professional engineer and has served FHWA for more than 13 years. He is a graduate of Michigan State University and holds a master's degree in civil engineering from the Georgia Institute of Technology.
Jerry A. DiMaggio is a senior geotechnical engineer with FHWA in Washington, D.C. In this capacity, he is responsible for providing technical assistance on geotechnical engineering features to FHWA and state DOT engineers throughout the nation. He has more than 25 years of experience and specializes in the design and construction of deep foundations and ground-improvement methods. DiMaggio holds a master's degree in civil engineering from Clarkson University in Potsdam, N.Y., and he is a professional engineer and a licensed arbitrator. He is a member of several technical committees and task forces with the American Society of Civil Engineers, TRB, and AASHTO.