Static structures (e.g. – buildings) – Controlled by condition responsive means
Reexamination Certificate
2002-09-05
2004-09-21
Canfield, Robert (Department: 3635)
Static structures (e.g., buildings)
Controlled by condition responsive means
C052S167100, C052S741300, C052SDIG001, C052S741110, C405S037000, C405S302400, C405S302500, C702S015000
Reexamination Certificate
active
06792720
ABSTRACT:
TECHNICAL FIELD
This invention relates to seismic base isolation of a structure or soil mass from the vibratory motion generated during an earthquake, and more particularly to the preferential inducement of localized soil liquefaction of a particular isolation layer in the soil horizon beneath the structure by applying an electro-osmotic gradient to the saturated soil during the earthquake event and thus raising the pore water pressure to induce localized soil liquefaction within a particular isolation layer in the soil horizon which thus reduces the upward propagation of the earthquake induced shear wave ground motions to the overlying structure.
BACKGROUND OF THE INVENTION
Earthquakes are caused by the resultant relative slippage of the earth crust, generally along or near major tectonic plate boundaries. In certain parts of the world, continuous differential movement occurs between one section of the earth's crust and an adjacent one, causing an accumulation of strain at the boundary. When the stresses caused by this strain accumulation exceed the strength of the earth's materials, a slip occurs between two portions of the earth's crust and tremendous amounts of energy are released. This energy propagates outward from the focus or origin of the earthquake in the form of body and surface elastic stress waves.
The energy released during an earthquake event is transmitted through the earth's crust in the form of body and surface seismic waves. The body waves are composed of P-(compression) waves and S-(shear) waves, with the P-wave traveling significantly faster than the S-wave. The surface waves of most interest are the Rayleigh wave and the Love wave. The Love wave travels faster than the Rayleigh wave. The total energy transported is represented almost entirely by the Rayleigh, the S- and the P-waves, with the Rayleigh wave carrying the largest amount of energy, the S-wave an intermediate amount, and the P-wave the least. The velocity of the P-wave is almost double that of the S-wave, and the velocity of the S-wave is only slightly greater than the Rayleigh wave.
At some distance from an earthquake disturbance a particle at the earth's surface first experiences a displacement in the form of an oscillation at the arrival of the P-wave followed by a relatively quiet period leading up to another oscillation at the arrival of the S- and Rayleigh waves. These events are referred to as the minor tremor and the major tremor at the time of arrival of the Rayleigh wave. The body and surface waves are monitored during an earthquake to gauge the earthquake's intensity.
Earth ground motions experienced during an earthquake are actually quite complex due to the variation in the earth's crust, from strong stiff bedrock to soft weak soils. Considerable energy can be transmitted through the bedrock, and it appears that in many cases the main forces acting on soil elements in the field during earthquakes are those resulting from the upward migration of shear motions from the underlying rock formations. Although the actual wave pattern may be very complex, the resulting ground motion imposed on the soil and the overlying structure are predominantly from the upward propagation of the S-wave components from the underlying bedrock. Deposits of thick soft soils can give rise to amplification of these ground motions in particular in the long period (low frequency) content of the earthquake induced shaking. Such amplification of earthquake induced ground motions can cause extensive damage to buildings, bridges, pipelines, embankments, dams, slopes, and other structures and works constructed on soft soil deposits.
The factors that effect the amplification of earthquake induced ground motions are soil type, grain size distribution, compactness of the soil, thickness of the soil deposit, depth to groundwater, and the magnitude and number of the strain reversals. Deposits of soft soils, such as silts and clays are most likely to amplify ground motions during an earthquake Structures constructed on such soils can be extensively damaged by even a moderate size earthquake. Two recent earthquakes, the 1985 Michoacan (Mexico) and the 1989 Loma Prieta (Calif.), highlight the extensive earthquake induced damage to structures located on soft soil deposits. The 1985 Michoacan earthquake caused only moderate damage in the vicinity of its epicenter but caused extensive damage to structures located on a thick deposit of soft silts and clay some 350 km away in Mexico City. Likewise, the 1989 Loma Prieta earthquake caused minor damage in the vicinity of its epicenter but caused moderate to extensive damage to structures located on the San Francisco Bay mud some 100 km away.
Conventional seismic isolation systems to minimize or prevent damage to a structure by isolating the structure from ground motions during an earthquake consist of the following:
1) sliding bearings with energy absorbing properties to isolate the structure from horizontal earthquake induced ground motions, such as lead rubber, steel neoprene/rubber and fiber reinforced elastomer,
2) sliding bearings with fluid dampers to both isolate the structure from earthquake induced ground motion and modify the structural response to minimize damage,
3) passive mass damping systems consisting of a pendulum suspended weight and associated dampers to absorb vibratory energy and minimize damage to the structure,
4) active mass damping systems consisting of a sensor and computer controlled movement of a mass to minimize vibration and damage to the structure,
5) pneumatic or fluidized foundation isolation system to reduce earthquake induced ground motions being transmitted to the structure.
The above methods have had mixed success in minimizing damage and vibrations to a structure during an earthquake. The passive and active mass damping systems have been shown to be successful during strong winds and minor earthquakes. Bearing isolation systems have in some circumstances, e.g. the 1994 Northridge (Calif.) earthquake, demonstrated to provide poor if any base isolation of the structure from the earthquake induced ground motion. The mass damping systems have demonstrated some protection of a structure due to earthquake vibrations; however, they are expensive, and difficult to implement in existing structures. The energy absorbing sliding bearing systems can be implemented in existing structures; however, their performance during actual earthquake events appear limited in isolating the structure from earthquake induced ground motions and minimizing structural damage.
The main forces acting on soil elements in the field during earthquakes are those resulting from the upward migration of shear motions from the underlying rock formations. Although the actual wave pattern may be very complex, the resulting repeated and reversing shearing deformations, imposed on the soil by the S-wave components are the principal cause of a phenomenon known as liquefaction, which occurs in saturated fine sand, silty sand and silt deposits. When these soil deposits are subjected to repeated shear strain reversals, the volume of the soil decreases with each cycle, i.e. the soil contracts, and due to the lack of drainage of these saturated soils, the soil pore water pressure rises. As the soil pore water pressure rises, the grain to grain contact pressure becomes smaller, until eventually the grain to grain contact pressure drops to zero and the soil loses all of its shear strength and acts like a fluid. Liquefaction can occur in loose saturated fine sands, silty sands and silts as a result of earthquakes, blasting or other shocks.
The factors that effect the occurrence of liquefaction are soil type, grain size distribution, compactness of the soil, soil permeability, and the magnitude and number of the strain reversals. Fine cohesionless soils, fine sand or fine cohesionless soils containing moderate amounts of silt are most susceptible to liquefaction. Uniformly graded soils are more susceptible to liquefaction than well graded soils, and fine sands te
Canfield Robert
GeoSierra LLC
Smith , Gambrell & Russell, LLP
LandOfFree
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