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| | [[Category:Movement Surveillance and Monitoring]] |
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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. |
| “'''(1)''' 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.
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| “'''(2)''' 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).
| | ==Calculating Settlement for Static Loads== |
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| “'''(3)''' 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">[[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>
| | 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</br></br> |
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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>
| | * [[Monuments]] |
| | | * [[Settlement Sensors]] |
| *[[Short-term (Construction)]] | | * [[Survey]] |
| | | * [[Extensometers]] |
| *[[Long-term]] | |
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| ==Measurement Devices==
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| '''Monuments'''
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| *“Stable monuments are required for both horizontal and vertical control. Monuments and their stability are integral parts of the accuracy of each survey project. Erroneous survey control monumentation can contribute to costly errors in all phases of project design and development”.<ref name="EM 1110-1-1002">[[Survey Markers and Monumentation (EM 1110-1-1002) | EM 1110-1-1002 Survey Markers and Monumentation, USACE, 2012]]</ref> | |
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| *“The location of the reference monuments and monolith marker points should be established along the proposed base line as shown in Figure 6-2. The base line should be located so that at completion of construction the line of sight between the reference monuments is unobstructed. Also, consideration should be given to location based on least interference from other operations during alignment measurements. The reference monuments should be located off the structure a sufficient distance not to be influenced by movement of the structure. In instances where this is impractical, the first marker point at each end of the structure may be used as reference points providing the end blocks have low and level foundations. The reference monument elevation should be the least practical vertical height above the structure to maintain the least angle between the precision instrument and monolith marker points. Normally two marker points will be installed on each side of a vertical joint between monoliths. An overall plan and a section between reference monuments and first marker point on the structure should be part of the design submission”.<ref name="EM 1110-2-4300">[[Instrumentation for Concrete Structures (EM 1110-2-4300) | EM 1110-2-4300 Instrumentation for Concrete Structures, USACE, 1987]]</ref> | |
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| '''Settlement Gauges'''
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| *[Paragraph here]
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| '''Survey'''
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| *“Surveys shall be performed at night to avoid troublesome optical distortions due to sunlight and heat radiation. The surveys shall be in accordance with the accuracy with methods established for first-order triangulation measurements as stipulated by the U.S. Coast and Geodetic Survey”.<ref name="EM 1110-2-4300" />
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| '''Extensometers'''
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| *“Extensometers may be divided into two general types, the single-point and the multiple-point. Both of these types are used to measure movement of one portion of a dam relative to another portion. Movement of the dam relative to the foundation, or movement of one portion of the foundation relative to another portion.<ref name="EM 1110-2-4300" />
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| #"Usage. – Internal extensometers are usually installed in uncased drill holes, and are suitable for installation vertically, horizontally, or at any angle. Extensometers used by the Bureau (of Reclamation) include the rod and wire types.<ref name="EM 1110-2-4300" />
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| #"Advantages and limitations. – Extensometers measure relative movement along the length of the installation; therefore, relative movement between any portion of a dam or its foundation may be measured. However, most extensometers have an operating range of 2 to 4 inches (51 to 102 mm), so movements should not exceed this range. If the movement should exceed the operating range of the extensometer, the reading head must be reset and a constant added to all future readings. With proper maintenance and care in reading, extensometers provide excellent information on relative movement.<ref name="EM 1110-2-4300" />
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| #"Description of devices. – Extensometers are designed to measure axial displacement of one or more fixed points along the length of the extensometer. The rod-type extensometer usually consists of multiple anchors installed at different depths in a drill hole. Aluminum rods inside hollow tubes extend from each anchor to a reference head at the collar of the hole. All measurements of movements are made at the reference head. As an anchor moves, the rod attached to that anchor also moves and the amount of movement is measured at the reference head. Theoretically, depending on the size of the drill hole, as many rods and anchors may be installed as desired, but normally, only 5 to 10 anchors are installed per hole.<ref name="EM 1110-2-4300" />
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| #"Installation procedures. – Upon completion and cleaning of the borehole, the anchor string with head packer attached is inserted into the hole and the anchors fixed in place if they are the expandable type. A string that is to be grouted and is not equipped with fixable anchors must have only the bottom portion grouted, or otherwise securely anchored, to prevent ‘floating’ when grout is pumped into the hole. When the bottom anchor is fixed, the rest of the hole is grouted full using a thin sand-cement grout. The measuring head is then installed, ensuring that appropriate rods or wires are attached to the correct transducer or cantilever.<ref name="EM 1110-2-4300" />
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| #"Monitoring procedures. – Extensometers are read on a scheduled periodic basis. Either mechanical readings using depth gauges or electrical readings using portable redout devices are made. Most electrical readout units convert the voltage readings from the electrical sensor to a direct display reading, which is usually in inches. Corrections for spring tension, wire stretch, and temperature are included to provide a final value. Differences between initial and current readings provide the actual movement data.<ref name="EM 1110-2-4300" />
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| #"Maintenance. – The reference heads must be kept free of dust, grit, and moisture to ensure proper, long-term operation. Occasionally, if significant movement has occurred, it is necessary to reset one or more components of the extensometer system. This operation must be conducted precisely according to the manufacturer’s instruction. When the readout components are reset, a reading before and just after resetting is taken to provide the constant to eb added or subtracted from future readings to determine the total movement since installation”.<ref name="EM 1110-2-4300" />
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| ==Best Practices Resources== | | ==Best Practices Resources== |
| {{Document Icon}} [[General Design and Construction Considerations for Earth and Rock-Fill Dams (EM 1110-2-2300)|General Design and Construction Considerations for Earth and Rock-Fill Dams (EM 1110-2-2300) (U.S. Army Corps of Engineers)]] | | {{Document Icon}} [[General Design and Construction Considerations for Earth and Rock-Fill Dams (EM 1110-2-2300) | General Design and Construction Considerations for Earth and Rock-Fill Dams (EM 1110-2-2300), USACE]] |
| {{Document Icon}} [[Instrumentation for Concrete Structures (EM 1110-2-4300)|Instrumentation for Concrete Structures (EM 1110-2-4300) (U.S. Army Corps of Engineers)]] | | {{Document Icon}} [[Instrumentation for Concrete Structures (EM 1110-2-4300) | Instrumentation for Concrete Structures (EM 1110-2-4300), USACE]] |
| | {{Document Icon}} [[Settlement Analysis (EM 1110-1-1904)|Settlement Analysis (EM 1110-1-1904), USACE]] |
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"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
Instrumentation for Concrete Structures (EM 1110-2-4300), USACE
Settlement Analysis (EM 1110-1-1904), USACE
Citations:
Revision ID: 7051
Revision Date: 07/10/2023
