Settlement

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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.

Measurement Devices

Monuments

• “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”.

• “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”.

Settlement Gauges

• [Paragraph here]

Survey

• “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”.

Extensometers

• “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.

1. "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.
2. "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.
3. "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.
4. "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.
5. "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.
6. "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”.

Examples

Best Practices Resources

Survey Markers and Monumentation (EM 1110-1-1002)

Instrumentation for Concrete Structures (EM 1110-2-4300)

Trainings

Citations:

Revision ID: 2870
Revision Date: 09/13/2022