- CLASSIFICATION METHOD OF MEASUREMENT
i) Wall panel, floor/roof slabs----------------------------In square metres
ii) Beams unit and columns, trusses, etc -----------------In running metres or numbers
iii) Channel unit and purlins------------------------------In running metres or numbers
iv) String or lacing courses, copings, bed plates, anchor blocks, plain window
sills, shelves louvers, steps, staircases, etc ---------------In running metres or numbers
v) Kerbs, edgings, etc ----------------------------------In running metres or numbers
vi) Solid blockwork ------------------------------------In cubic metres or square metres
vii) Hollow blockwork ----------------------------------In cubic metres or square metres
viii) Light weight partitions-------------------------------In square metres stating the thickness
ix) Door/window frames --------------------------------In running metres stating the size
x) Waffle units ------------------------------------------In square metres or numbers
xi) Water tank ------------------------------------------In numbers
xii)JALLIES --------------------------------------------In sq.metres of opening filled stating thickness
xiii) Fencing posts---------------------------------------In numbers or cubic metres
xiv) Folded slab-----------------------------------------In cubic metres
Tuesday, June 19, 2007
METHOD OF MEASUREMENT
Failure Mechanisms
Failure Mechanisms:
Possible damaging effects of earthquakes on earth dams and embankments include:
• Slope failure because of inertial loading and/or softening of materials strength or liquefaction.
• Fault displacement under the foundation.
• Crest settlement of dam caused by settlement or by earthquake generated water waves in the reservoir.
• Permanent deformation of foundation soils or dam body.
• Sliding failure of an embankment composed of weak or liquefiable soils.
• Piping and erosion
Major contributors to earth dam failure are overtopping, piping, and structural failure.
Overtopping:
Overtopping is defined as uncontrolled flow of water over the crest of the dam or embankment. Non-overflow (other than spillway) portions of a dam are not usually designed for erosional effect of flowing water, overtopping may lead to failure of the dam due to excessive erosion or saturation of the downstream slope. Adequate spillway capacity should be provided to prevent such damages.
Piping:
Due to the pervious nature of earth dams, the dam body acts as pathways for water seepage. If such seepage is uncontrolled in terms of volume and velocity, and material used in constructing the dam body are not carefully selected, particles of soil with which the dam body is constructed may be taken into suspension by seepage water and carried away. In order to prevent such an occurrence, (a) the seepage gradient is kept well below the critical gradient, (b) the particle size distribution of the filter material used in constructing the dam body is carefully chosen to meet the filter criteria, and (c) hydraulic stability of dam core and potentially dispersive fine-grained soils within foundation is demonstrated with pin-hole tests. Additionally, there should be appropriate seepage control measures at the contact between dam and foundation such as: (a) drainage blanket for soil foundation, (b) removal of rock mass affected by excessive cracking or jointing at dam-foundation interface before dam construction, and (c) ensuring absence of slopes steeper than 10 vertical to 1 horizontal at dam-foundation interface.
Structural Failure:
Structural failure includes failure of upstream or downstream slopes of the dam, as well as cracking, deformation and settlement of the dam body that may lead to overtopping or a piping failure. Earthquake loading may trigger any one of the above failure modes or their combinations.
Defensive Design Measures:
Defensive design features should be incorporated in the foundation and embankment design of new dams regardless of the method of seismic analysis.
These features include (USACE 2004):
1. Additional dam height to accommodate the loss of crest elevation due to deformation, slumping, and fault displacement.
2. Crest details that will minimize erosion in the event of overtopping.
3. Wider transition and filter sections as a defense against cracking.
4. Use of rounded or sub-rounded gravel and sand as filter material.
5. Adequate permeability of the filter layers.
6. Near vertical chimney drain in the center portion of the embankment.
7. Zoning of the embankment to minimize saturation of materials.
8. Wide impervious cores of plastic (non-brittle) cohesive fine-grained soils to accommodate deformation.
9. Well-graded core and uniformly graded filter materials to ensure self healing in the event cracking should occur.
10. Stabilization of reservoir rim slopes to provide safety against large slides into the reservoir.
11. Ground improvement or removal and replacement of foundation material to mitigation liquefaction potential
12. Stabilization of slope adjacent to operating facilities to prevent blockage from slide associated with the earthquake.
13. Flaring embankment sections at the abutment contacts.
Installation of suitable features to prevent piping through earthquake generated seepage cracks.
Earthquake Loads:
For the design of intermediate dams or embankments and dams and embankments whose failure entails unacceptable level of risk, a two-tier seismic design approach is usually adopted so that (a) the dam or embankment remains operational following an earthquake that has a reasonable probability of occurrence during the service life of the facility with distress of a minor nature, and (b) the dam or embankment does not collapse following an earthquake that has a small probability or occurrence during the life of the facility.
The design earthquake for condition (a) usually corresponds to a probability of exceedance of 50% over the operational life of the facility. Such an earthquake is referred to as the Operational Basis Earthquake (OBE).
The design earthquake for condition (b) above is often specified as that having a probability of exceedance of 10% over the operational life of the facility. Such an earthquake is referred to as the Maximum Considered Earthquake (MCE).
Operational life of a dam or an embankment is usually 50 to 100 years. However, a longer operational life should be assumed in situations such as (a) a water-retaining dam that is not appropriately decommissioned at the end of operational life where there is an unacceptable downstream risk in the event of a dam break, and (b) a tailings dam retaining radioactive or other wastes that may pollute groundwater or jeopardize downstream public health and safety in the event of dam break.
Location:
Preference for Location:
Earth dams and embankments should be ideally located away from any potentially active fault or an area underlain by liquefiable or sensitive soils or abutments prone to static or seismic instability.
Design Measures:
If unavoidable, an earth dam or an embankment may be constructed over a fault or at a site underlain by potentially liquefiable or sensitive soils or between potentially unstable abutments only if (a) the dam is designed for the displacements and other dynamic effects of an earthquake that is likely to occur, and (b) potential failure is unlikely to lead to any loss of life and the risk associated with such a selection of site is acceptable area to assess their behavior during earthquake shaking, and how they might affect the ability of a structure to resist earthquake including evaluation of liquefaction potential, if appropriate. The investigation should focus on topics including:
1. Topographic conditions.
2. Description of geology
3. Composition and structure of foundation soils, soils from borrow area and bedrock
4. Principal engineering properties of the rocks and soils including grain characteristics, plasticity, compaction characteristics, shear strength, dispersivity and hydraulic properties
5. Geotechnical investigation would typically include drilling and sampling, in-situ testing (piezocone penetration test, Standard Penetration Test or Field Vane Shear Test, seismic velocity profiling) as appropriate.
Location
Earthquake damage to embankment can arise from two sources, actual ground rupture beneath the embankment and seismic shaking. Failure of a dam due to ground rupture is possible only when the dam is built over an active fault zone or across reactivated or newly activated landslide zone. The location of the dam over the fault zone should be reviewed at the time of site selection and appropriate measures should be taken in the design and construction of a dam over a fault. By far the more common problem in a dam design is to ensure that the dam will be stable under anticipated levels of seismic shaking.
Freeboard:
It is recommended to provide a freeboard of at least 2% to 3% of dam height, but not less than 2m if there is a potential for occurrence of landslides near the dam abutment within the slopes of the reservoir margins or 1m if there is a negligible landslide potential near the dam abutments.
Freeboard:
The freeboard of all embankment dams should be based on most extreme conditions expected for which the dam is designed. The maximum reservoir elevation is determined for the design flood, wind speed, fetch and expected wave run-up conditions. In general, overtopping of the dam is not acceptable. Ample freeboard should be provided to avoid the possibility of overtopping by (a) earthquake-generated water waves, (b) settlement and permanent deformation of crest due to liquefaction which may cause densification or loss of stiffness of the materials or fault rupture. In addition, it may be prudent to use riprap or other crest details that will resist erosion by a succession of overtopping waves.
Possible damaging effects of earthquakes on earth dams and embankments include:
• Slope failure because of inertial loading and/or softening of materials strength or liquefaction.
• Fault displacement under the foundation.
• Crest settlement of dam caused by settlement or by earthquake generated water waves in the reservoir.
• Permanent deformation of foundation soils or dam body.
• Sliding failure of an embankment composed of weak or liquefiable soils.
• Piping and erosion
Major contributors to earth dam failure are overtopping, piping, and structural failure.
Overtopping:
Overtopping is defined as uncontrolled flow of water over the crest of the dam or embankment. Non-overflow (other than spillway) portions of a dam are not usually designed for erosional effect of flowing water, overtopping may lead to failure of the dam due to excessive erosion or saturation of the downstream slope. Adequate spillway capacity should be provided to prevent such damages.
Piping:
Due to the pervious nature of earth dams, the dam body acts as pathways for water seepage. If such seepage is uncontrolled in terms of volume and velocity, and material used in constructing the dam body are not carefully selected, particles of soil with which the dam body is constructed may be taken into suspension by seepage water and carried away. In order to prevent such an occurrence, (a) the seepage gradient is kept well below the critical gradient, (b) the particle size distribution of the filter material used in constructing the dam body is carefully chosen to meet the filter criteria, and (c) hydraulic stability of dam core and potentially dispersive fine-grained soils within foundation is demonstrated with pin-hole tests. Additionally, there should be appropriate seepage control measures at the contact between dam and foundation such as: (a) drainage blanket for soil foundation, (b) removal of rock mass affected by excessive cracking or jointing at dam-foundation interface before dam construction, and (c) ensuring absence of slopes steeper than 10 vertical to 1 horizontal at dam-foundation interface.
Structural Failure:
Structural failure includes failure of upstream or downstream slopes of the dam, as well as cracking, deformation and settlement of the dam body that may lead to overtopping or a piping failure. Earthquake loading may trigger any one of the above failure modes or their combinations.
Defensive Design Measures:
Defensive design features should be incorporated in the foundation and embankment design of new dams regardless of the method of seismic analysis.
These features include (USACE 2004):
1. Additional dam height to accommodate the loss of crest elevation due to deformation, slumping, and fault displacement.
2. Crest details that will minimize erosion in the event of overtopping.
3. Wider transition and filter sections as a defense against cracking.
4. Use of rounded or sub-rounded gravel and sand as filter material.
5. Adequate permeability of the filter layers.
6. Near vertical chimney drain in the center portion of the embankment.
7. Zoning of the embankment to minimize saturation of materials.
8. Wide impervious cores of plastic (non-brittle) cohesive fine-grained soils to accommodate deformation.
9. Well-graded core and uniformly graded filter materials to ensure self healing in the event cracking should occur.
10. Stabilization of reservoir rim slopes to provide safety against large slides into the reservoir.
11. Ground improvement or removal and replacement of foundation material to mitigation liquefaction potential
12. Stabilization of slope adjacent to operating facilities to prevent blockage from slide associated with the earthquake.
13. Flaring embankment sections at the abutment contacts.
Installation of suitable features to prevent piping through earthquake generated seepage cracks.
Earthquake Loads:
For the design of intermediate dams or embankments and dams and embankments whose failure entails unacceptable level of risk, a two-tier seismic design approach is usually adopted so that (a) the dam or embankment remains operational following an earthquake that has a reasonable probability of occurrence during the service life of the facility with distress of a minor nature, and (b) the dam or embankment does not collapse following an earthquake that has a small probability or occurrence during the life of the facility.
The design earthquake for condition (a) usually corresponds to a probability of exceedance of 50% over the operational life of the facility. Such an earthquake is referred to as the Operational Basis Earthquake (OBE).
The design earthquake for condition (b) above is often specified as that having a probability of exceedance of 10% over the operational life of the facility. Such an earthquake is referred to as the Maximum Considered Earthquake (MCE).
Operational life of a dam or an embankment is usually 50 to 100 years. However, a longer operational life should be assumed in situations such as (a) a water-retaining dam that is not appropriately decommissioned at the end of operational life where there is an unacceptable downstream risk in the event of a dam break, and (b) a tailings dam retaining radioactive or other wastes that may pollute groundwater or jeopardize downstream public health and safety in the event of dam break.
Location:
Preference for Location:
Earth dams and embankments should be ideally located away from any potentially active fault or an area underlain by liquefiable or sensitive soils or abutments prone to static or seismic instability.
Design Measures:
If unavoidable, an earth dam or an embankment may be constructed over a fault or at a site underlain by potentially liquefiable or sensitive soils or between potentially unstable abutments only if (a) the dam is designed for the displacements and other dynamic effects of an earthquake that is likely to occur, and (b) potential failure is unlikely to lead to any loss of life and the risk associated with such a selection of site is acceptable area to assess their behavior during earthquake shaking, and how they might affect the ability of a structure to resist earthquake including evaluation of liquefaction potential, if appropriate. The investigation should focus on topics including:
1. Topographic conditions.
2. Description of geology
3. Composition and structure of foundation soils, soils from borrow area and bedrock
4. Principal engineering properties of the rocks and soils including grain characteristics, plasticity, compaction characteristics, shear strength, dispersivity and hydraulic properties
5. Geotechnical investigation would typically include drilling and sampling, in-situ testing (piezocone penetration test, Standard Penetration Test or Field Vane Shear Test, seismic velocity profiling) as appropriate.
Location
Earthquake damage to embankment can arise from two sources, actual ground rupture beneath the embankment and seismic shaking. Failure of a dam due to ground rupture is possible only when the dam is built over an active fault zone or across reactivated or newly activated landslide zone. The location of the dam over the fault zone should be reviewed at the time of site selection and appropriate measures should be taken in the design and construction of a dam over a fault. By far the more common problem in a dam design is to ensure that the dam will be stable under anticipated levels of seismic shaking.
Freeboard:
It is recommended to provide a freeboard of at least 2% to 3% of dam height, but not less than 2m if there is a potential for occurrence of landslides near the dam abutment within the slopes of the reservoir margins or 1m if there is a negligible landslide potential near the dam abutments.
Freeboard:
The freeboard of all embankment dams should be based on most extreme conditions expected for which the dam is designed. The maximum reservoir elevation is determined for the design flood, wind speed, fetch and expected wave run-up conditions. In general, overtopping of the dam is not acceptable. Ample freeboard should be provided to avoid the possibility of overtopping by (a) earthquake-generated water waves, (b) settlement and permanent deformation of crest due to liquefaction which may cause densification or loss of stiffness of the materials or fault rupture. In addition, it may be prudent to use riprap or other crest details that will resist erosion by a succession of overtopping waves.
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