Form Approved
REPORT DOCUMENTATION PAGE
OMB No. 0704-0188
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1. AGENCY USE ONLY (Leave blank)
2. REPORT DATE
3. REPORT TYPE AND DATES COVERED
September 1995
4. TITLE AND SUBTITLE
5. FUNDING NUMBERS
Soil FreezeThaw Effects on Bank Erodibility and Stability
Civil Works Projects
H013 and 32550
6. AUTHORS
Lawrence W. Gatto
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
8. PERFORMING ORGANIZATION
REPORT NUMBER
U.S. Army Cold Regions Research and Engineering Laboratory
72 Lyme Road
Special Report 95-24
Hanover, New Hampshire 03755-1290
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
10. SPONSORING/MONITORING
AGENCY REPORT NUMBER
Office of the Chief of Engineers
Washington, D.C. 20314-1000
11. SUPPLEMENTARY NOTES
12a. DISTRIBUTION/AVAILABILITY STATEMENT
12b. DISTRIBUTION CODE
Approved for public release; distribution is unlimited.
Available from NTIS, Springfield, Virginia 22161
13. ABSTRACT (Maximum 200 words)
When air temperature is below ground temperature, a thermal gradient is established in the soil that causes the soil to
lose heat to the atmosphere. When the soil has lost sufficient heat for soil water to freeze, the newly formed ice
changes soil structure by disaggregating, separating, and reorienting soil particles. The suction set up within the
freezing soil draws water to the freezing zone through the film of unfrozen water surrounding soil particles,
supplying additional water for freezing, so the volume of ice increases. When appropriate thermal and water supply
conditions are in place, disseminated ice lenses can form in the soil. As the ice lenses grow, the soil surface is heaved in
the direction of heat flow from the soil. Soil particles can be displaced down a bank face when surface ice in heaved
soil melts. The amount of ice in a frozen soil by the end of winter can be higher than its water content when unfrozen.
Thus, upon thawing, the previously frozen soil temporarily has an excess of soil water and a disrupted soil structure,
which significantly reduces internal friction and cohesion and reduces the soil's shear strength. In this weakened
state, thawed bank soils are usually more easily eroded by raindrop impacts, overland flows, river and lake ice forces,
currents and waves, and are highly susceptible to mass failures. In some instances newly thawed soils are weaker
than at any other time of the year. Some studies show that processes related to bank soil freezing and thawing cause
more bank recession annually than other processes in areas where seasonal frost forms. However, with time, the
strength of the thawed soil returns as excess water drains from the soil, and soil particle packing and interlocking
increase. Thus, frost-induced reductions in soil strength and soil particle displacements must be included in bank
migration and bank erosion models to be applied in regions with seasonal soil frost.
14. SUBJECT TERMS
15. NUMBER OF PAGES
24
Bank erosion
Ground ice
Thaw-weakened
16. PRICE CODE
FreezeThaw
Soil instability
17. SECURITY CLASSIFICATION
18. SECURITY CLASSIFICATION
19. SECURITY CLASSIFICATION
20. LIMITATION OF ABSTRACT
OF REPORT
OF THIS PAGE
OF ABSTRACT
UNCLASSIFIED
UNCLASSIFIED
UNCLASSIFIED
UL
Standard Form 298 (Rev. 2-89)
NSN 7540-01-280-5500
Prescribed by ANSI Std. Z39-18
298-102
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