Settlement
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.
Foundations
“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."[1]
“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)."[1]
“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”.[1]
Embankments
“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”.[1]
Measurement
Best Practices Resources
Settlement Analysis (EM 1110-1-1904) (USACE, 1990)
Instrumentation for Concrete Structures (EM 1110-2-4300) (USACE, 1987)
Citations:
Revision ID: 4119
Revision Date: 11/03/2022