Browse Projects > Detailed View

Improving Pavement Resilience by Mitigating Moisture Effects


Pavement surface and foundation distresses caused by shrinking and swelling soils remain a national issue. These distresses are due to changes in soil moisture content caused by three primary sources: evaporation, transpiration, and water table fluctuations. Expansive clay is particularly vulnerable to changes in moisture content, which results in shrinking during drying cycles (desiccation) and swelling during wetting cycles (recharge). Reduced rainfall, higher ambient temperatures, lower vapor pressure, and wind speed influence the rate of soil desiccation during the summer months, generally reaching maximum drying in September. The moisture lost during the summer drying cycle is generally replaced, except in times of drought, during the winter months and reaches its recharge maximum in the spring. When trees, shrubs, and other vegetation are present, an additional moisture demand results from transpiration, which is the transport of water through plants from the roots to the leaves and into the atmosphere. The transpiration period begins during the spring and peaks during the summer months, which adds to the desiccation caused by evaporation. The process of evaporation and transpiration together is called evapotranspiration. When yearly rainfall is insufficient, a zone of permanent desiccation is created, which causes a creep-like effect where the soil does not return to its original volume resulting in permanent subsidence.

Moisture changes result from extreme weather events and seasonal variations and these changes can be worsened by roadway construction procedures such as altering drainage patterns, eliminating surface water retention areas, constructing pavements during the dry season, constructing an impermeable surface alters subsurface evapotranspiration patterns, chemically stabilizing the soil, planting high water demand vegetation, and constructing dense embankments with impermeable clays to seal the slopes. As subsidence continues, a point is reached where the pavement structure and embankment yields and surface distresses such as cracking and faulting occur generally beginning at the edge of the outside wheel path and progressing outward towards the shoulder. While some research has been conducted, there is a great need for assessment guidelines for soil characterization, environmental factors, and the stress state of the pavement system coupled with cost effective methods to mitigate shrink/swell distresses.


This research project will produce a summary of existing work and deliver state of the art guidelines to assess and mitigate damages caused by evapotranspiration to pavement structures. A multi-disciplinary approach is needed in order to identify and understand effective measurement techniques currently available and to mitigate surface and foundation distresses caused by evapotranspiration. This multi-disciplinary approach will include geotechnical engineering, geology, geophysics, hydrology, and arboreal sciences and combine knowledge from all these disciplines to identify the sources of the distress and develop cost effective mitigation methods.


Compacted earthworks are constructed throughout the country and their behavior their stability is often a function of moisture changes. Adverse effects on performance due to moisture changes will be explained. The main audience for this research will include a combination of state and federal geotechnical and design engineers. However, a significant use of state specifications is made by local agencies, private consultants and other practitioners. This research will provide a comprehension understanding of unsaturated soils and further advance the use of unsaturated soil mechanics by state transportation departments. AASHTO would evaluate the draft measurement and mitigation guidelines resulting from this project and advance these through the appropriate balloting process.

Related Research:

Nelson, J., Miller, D. (1992). Expansive Soils, Problems and Practice in Foundation, and Pavement Engineering, John E Wiley and Sons.

Vipulanandan, C., Addison, M., M. Hansen, (2001). Expansive Clay Soils and Vegetative Influence on Shallow Foundations: Proceedings of Geo-Institute Shallow Foundation and Soil Properties Committee Geotechnical Special Publication Number 115, American Society of Civil Engineers, https://doi.org/10.1061/978078445925.

Department of the Army USA. (1983), Foundations in Expansive Soils, Technical Manual TM 5-818-7, September 1983.

Biddle, P.G.. (1998). Tree Root Damage to Buildings, Volumes 1 and 2, Willomead Publishing Ltd.

Roberts, J., Jackson, N, and Smith, M. (2006). Tree Roots in the Built Environment, TSO Publications.

Freeman, T., Driscoll, R., and Littlejohn, G. (2002). Has Your House Got Cracks?, BRE and Thomas Thelford Ltd.

McPherson, E. (2000). “Expenditures Associated with Conflicts between Street Tree Root Growth and Hardscape in California”, Journal of Arboricultural, 26: 289-297.

McPherson, E.G. and Peper, P.J. (1995) Infrastructure repair costs associated with street trees in 15 cities. In Trees and Building Sites. In Proceedings of and International Workshop on Trees and Buildings (G.W. Watson and D. Neely eds.), pp. 49–64. International Society of Arboriculture, Savoy, Ill, USA

McPherson, E.G. and Peper, P.J. (1996) Costs of infrastructure tree damage to infrastructure. Arboricultural Journal 20, 143–160.

McPherson, E., Simpson, J., Peper, P., and Xiao, Q. (1999). “Benefits-Costs Analysis of Modesto’s Municipal Urban Forest”, Journal of Arboriculture, 25: 235-248.

McPherson, E., Costello, L., Perry E., and Peper, P. (2000). Reducing Tree Root Damage to Sidewalks in California Cities, Division of Agriculture and Natural Resources, University of California.

Randrup, T., McPherson, E., and Costello, L. (2001). “A Review of Tree Root Conflicts with Sidewalks, Curbs, and Roads”, Urban Ecosystems, 5:209-225, Kluwer Academic Publishers.

Crow, P. (2005). The Influence of Soils and Species on Tree Root Depth, Forestry Commission.

Kopinga, J., (1994). “Aspects of the Damage to Asphalt Road Pavings caused by Tree Roots”, Proceedings of the International Workshop on Tree Root Development in Urban Soils, Published by the International Society of Arboriculture, 165-178, ISBN/ISSN 1-881956-06-7.

Kristoffersen, P. (1998). “Designing Urban Pavement Subbases to Support Trees”, Journal of Arboriculture.

Kristoffersen, P. (1998). “Growing Trees in Road Foundation Materials”, Arboricultural Journal 23.

MacLeod, R. and Cram W. (1996), “Forces exerted by Tree Roots”, Arboricultural Advisory and Information Service, Surrey, UK.


The following tasks are proposed to be completed in three phases.

Phase 1:

  1. Conduct a literature review on the state of the art of the pavement distresses caused by evapotranspiration across different disciplines.

  2. Conduct surveys of US and State DOTs and arborists experiences to assess the state of the practice.

  3. Identify needs for additional information and select sites for data collection. (Note: Louisiana is currently developing an experimental field program for identifying and mitigating pavement surface and foundation distresses caused by evapotranspiration mechanisms.)

  4. Prepare an Interim report to present Phase 1 findings and clearly delineate the plan of work for Phase 2 and Phase 3 of the project.

Period: 12 months

Phase 2:

  1. Conduct research at selected sites from Phase 1. Monitor sites through the diurnal cycle of desiccation and recharge.Testing at a minimum shall include soil characterization, environmental factors (evapotranspiration), and the in-situ soils state of stress.

  2. Design and construct mitigation strategies for each selected site. Monitor the “new” diurnal cycle of desiccation and recharge for 36 months (3 diurnal cycles) using the testing regime listed above.

Period: 48 months (12 months for project construction and 36 months for monitoring)

Phase 3:

  1. Prepare a final report documenting all aspects of research.

  2. Deliver a manual of design guidelines.

Period: 12 months


The steps necessary for implementation of the research product will be recommended to AASHTO. AASHTO would evaluate the draft measurement and mitigation guidelines resulting from this project and advance these through the appropriate balloting process. The information will be as specific as possible, noting particular documents that may be affected, or techniques or equipment that may be affected. Barriers to implementation will be identified and best practices identified to overcome these barriers to implementation.


Unsaturated soil mechanics plays an important role in the behavior of transportation infrastructure especially when considering that most structures are supported by unsaturated compacted soil. The main audience for these new guidelines will include a combination of state and federal engineers, as well as significant use by local agencies and private consultants. This research need has been identified as a high priority by the TRB Standing Committee AFP60 “Engineering Behavior of Unsaturated Geomaterials.” The committee members and friends represent many sectors of the transportation community, including many state DOTs, FHWA, and Corps of Engineers.

Sponsoring Committee:AKG40, Mechanics and Drainage of Saturated and Unsaturated Geomaterials
Research Period:Longer than 36 months
Research Priority:High
RNS Developer:Louisiana Department of Transportation and Development
Source Info:Louisiana Department of Transportation and Development
Kevin Gaspard, P.E., Louisiana Transportation Research Center, 4101 Gourrier Ave., Baton Rouge, LA. 70808, Tel. 225-767-9104, Fax. 225-767-9108, email kevin.gaspard@la.gov
Zhongjie Zhang, PH.D, P.E., Louisiana Transportation Research Center, 4101 Gourrier Ave., Baton Rouge, LA. 70808, Tel. 225-767-9162, Fax. 225-767-9108, email: zhongjie.zhang@la.gov
Gavin Gautreau, P.E., Louisiana Transportation Research Center, 4101 Gourrier Ave., Baton Rouge, LA. 70808, Tel. 225-767-9110, Fax. 225-767-9108, email: gavin.gautreau@la.gov
AFP60 Committee on Engineering Behavior of Unsaturated Geomaterials
John Siekmeier, P.E. M.ASCE
Committee Research Coordinator
Minnesota Department of Transportation
1400 Gervais Ave., Maplewood, MN 55109-2044
(651) 366-5417
Date Posted:03/27/2018
Date Modified:05/17/2019
Index Terms:Pavement distress, Moisture content, Expansive clays, Swelling,
Cosponsoring Committees: 

Please click here if you wish to share information or are aware of any research underway that addresses issues in this research needs statement. The information may be helpful to the sponsoring committee in keeping the statement up-to-date.