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| Settlement, also referred to as ''consolidation'', is a natural [[mechanism]] of soil mechanics that occurs as a result of the dissipation of excess pore pressures and long-term creep of the soil. Due to both primary and secondary consolidation, both embankments as well as foundations comprised of compressible [[soils]] can experience varying degrees of settlement. While some settlement is expected after the [[construction]] of most dams, it is important that the amount of settlement is recorded and tracked in order to help detect any underlying problems in the embankment or foundation that could result from internal erosion or other hard-to-detect problems.
| | "Soil is a nonhomogeneous porous material consisting of three phases: solids, fluid (normally water), and air. Soil deformation may occur by change in stress, water content, soil mass, or temperature."<ref name="EM1110-1-1904">[[Settlement Analysis (EM 1110-1-1904) | Settlement Analysis (EM 1110-1-1904) (USACE, 1990)]]</ref> |
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| ==Foundations==
| | Settlement is a term typically used to describe the vertical movement of soil. The more generalized term is soil displacement which occurs as a result of the following conditions: [[Elastic Deformation]], [[Consolidation]], [[Secondary Compression and Creep]], [[Dynamic Forces]], [[Expansive Soil]], [[Collapsible Soil]]. |
| “Foundation settlement should be considered in selecting a site since minimum foundation settlements are desirable. Overbuilding of the embankment and of the core is necessary to ensure a dependable freeboard. Stage construction or other measures may be required to dissipate high porewater pressures more rapidly. Wick drains should be considered except where installation would be detrimental to seepage characteristics of the structure and foundation. If a compressible foundation is encountered, consolidation tests should be performed on undisturbed samples to provide data from which settlement analyses can be made for use in comparing sites and for final design. Procedures for making settlement and bearing capacity analyses are given in EM 1110-1-1904 and EM 1110-1-1905, respectively."<ref name="EM 1110-2-2300">[[General Design and Construction Considerations for Earth and Rock-Fill Dams (EM 1110-2-2300) | EM 1110-2-2300 General Design and Construction Considerations for Earth and Rock-Fill Dams, USACE, 2004]]</ref>
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| “The shear strength of a soil is affected by its consolidation characteristics. If a foundation consolidates slowly, relative to the rate of construction, a substantial portion of the applied load will be carried by the pore water, which has no shear strength, and the available shearing resistance is limited to the in situ shear strength as determined by undrained ‘Q’ tests. Where the foundation shearing resistance is low, it may be necessary to flatten slopes, lengthen the time of construction, or accelerate consolidation by drainage layers or wick drains. Analyses of foundation porewater pressures are covered by Snyder (1968). Procedures for [[stability]] analyses are discussed in EM 1110-2-1902 and Edris (1992)."<ref name="EM 1110-2-2300"/>
| | ==Calculating Settlement for Static Loads== |
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| “Although excess porewater pressures developed in pervious materials dissipate much more rapidly than those in impervious soils, their effect on stability is similar. Excess pore pressures may temporarily build up, especially under earthquake loadings, and effective stresses contributing to shearing resistance may be reduced to low values. In liquefaction of sand masses, the shearing resistance may temporarily drop to a fraction of its normal value”.<ref name="EM 1110-2-2300"/>
| | The total settlement, which is the response to stress applied to the soil, is comprised of three major components: |
| | #[[Immediate Settlement]] |
| | #[[Primary Consolidation Settlement]] |
| | #[[Secondary Compression Settlement]] |
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| ==Embankments== | | ==Measurement of Settlement== |
| “Factors affecting development of excess porewater pressures in embankments during construction include placement water contents, weight of overlying fill, length of drainage path, rate of construction (including stoppages), characteristics of the core and other fill materials, and drainage features such as inclined and horizontal drainage layers, and pervious shells. Analyses of porewater pressures in embankments are presented by Clough and Snyder (1966). Spaced vertical sand drains within the embankment should not be used in lieu of continuous drainage layers because of the greater danger of clogging by fines during construction”.<ref name="EM 1110-2-2300">[[General Design and Construction Considerations for Earth and Rock-Fill Dams (EM 1110-2-2300) | EM 1110-2-2300 General Design and Construction Considerations for Earth and Rock-Fill Dams, USACE, 2004]]</ref>
| | * [[Settlement Gauges]] |
| | | * [[Survey]] |
| *[[Short-term (Construction)]]
| | * [[Extensometers]] |
| *[[Long-term]]
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| ==Measurement==
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| *[[Settlement Gauges]] | |
| *[[Survey]] | |
| *[[Extensometers]] | |
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| ==Best Practices Resources== | | ==Best Practices Resources== |
"Soil is a nonhomogeneous porous material consisting of three phases: solids, fluid (normally water), and air. Soil deformation may occur by change in stress, water content, soil mass, or temperature."[1]
Settlement is a term typically used to describe the vertical movement of soil. The more generalized term is soil displacement which occurs as a result of the following conditions: Elastic Deformation, Consolidation, Secondary Compression and Creep, Dynamic Forces, Expansive Soil, Collapsible Soil.
Calculating Settlement for Static Loads
The total settlement, which is the response to stress applied to the soil, is comprised of three major components:
- Immediate Settlement
- Primary Consolidation Settlement
- Secondary Compression Settlement
Measurement of Settlement
Best Practices Resources
General Design and Construction Considerations for Earth and Rock-Fill Dams (EM 1110-2-2300) (USACE, 2004)
Settlement Analysis (EM 1110-1-1904) (USACE, 1990)
Instrumentation for Concrete Structures (EM 1110-2-4300) (USACE, 1987)
Citations:
Revision ID: 4120
Revision Date: 11/03/2022
