Measuring and testing – Borehole or drilling
Reexamination Certificate
2001-09-27
2003-09-09
Williams, Hezron (Department: 2856)
Measuring and testing
Borehole or drilling
C073S152060, C073S152510, C073S038000, C702S002000, C405S036000, C405S037000, C324S347000, C324S353000
Reexamination Certificate
active
06615653
ABSTRACT:
TECHNICAL FIELD
This invention relates to an in situ method for quantifying the liquefaction tendency of water saturated soils, and for determining the potential of electro-osmosis to prevent soil liquefaction.
BACKGROUND OF THE INVENTION
Soil liquefaction results from an increase in soil pore water pressure induced by transient or repeated ground motions or shocks. Pore water increases may be induced by earthquakes, explosions, impacts, and ocean waves. Soil liquefaction occurs in water saturated, cohesionless soils and causes a loss of soil strength that may result in the settlement and/or failure of buildings, dams, earthworks, embankments, slopes, and pipelines. Liquefaction of sands and silts has been reported in almost all of the major earthquakes around the world. The imposed ground stress waves from earthquakes or other transient or repeated loading induces shaking or vibratory shearing of saturated loose fine sand or silts, causing a phenomenon known as liquefaction. When loose sands and silts are subjected to repeated shear strain reversals, the volume of the soil contracts and results in an immediate rise in the pore water pressure within the soil. If the pore water pressure rises sufficiently high, then the soil grain to grain contact pressure drops to zero, and the soil mass will lose all shear strength and temporarily act like a fluid, i.e. an occurrence of liquefaction occurs. Such temporary loss of shear strength can have a catastrophic effect on earthworks or structures founded on these deposits. Major landslides, settling or tilting of buildings and bridges and instability of dams or tailings ponds and failure of pipelines have all been observed in recent years and efforts have been directed to prevent or reduce such damage.
The factors that effect the occurrence of liquefaction are soil type, grain size distribution, compactness of the soil, soil permeability, magnitude and number of the shear strain reversals. 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 tend to liquefy more easily than coarse sands or gravelly soils. Moderate amounts of silt appear to increase the liquefaction susceptibility of fine sands; however, fine sands with large amounts of silt are less susceptible although liquefaction is still possible. Recent evidence indicates that sands containing moderate amounts of clay may also be liquefiable.
In very coarse sands or gravel, ground water can flow freely enough that pore water pressures never become dangerously high to give rise to liquefaction. Fine sands and silty sands however have moderate to low permeability, which prevents the dissipation of induced pore water pressures and results in liquefaction of the soil. If the soil pore water pressures generated during an earthquake event can be relieved, then the soil will not liquefy and hence will remain stable.
Conventional soil stabilization methods to minimize or prevent liquefaction consist of one of five general methods:
1) remove liquefaction prone soil material and replace with sound material,
2) provide structural support to underlying firm soil strata, e.g. piling,
3) densify the soil to render it less susceptible to liquefaction,
4) strengthen the liquefaction prone soils,
5) provide drainage to prevent build up of soil pore water pressures, e.g. stone or gravel columns or relief wells.
The above methods have proven successful in minimizing liquefaction related damage; however, they are expensive, difficult to implement in existing structures and some of the methods are severely limited in their effectiveness in fine grain soils. An alternative method of preventing soil liquefaction involves activating an electro-osmotic gradient away from the foundation of the structure or towards a series of pressure relief wells, and thus negate the impact of the earthquake shaking on raising the soil pore water pressure and hence maintain the soil shear strength and structural stability.
Electro-osmosis involves the application of a constant d-c current between electrodes inserted in the saturated soil, that gives rise to pore fluid movement from the source electrodes towards the sink electrodes and thus modifies the soil pore water pressures. Electro-osmosis has been used in applications such as 1) improving stability of excavations, 2) decreasing pile driving resistance, 3) increasing pile strength, 4) stabilization of soils by consolidation or grouting, 5) dewatering of sludges, 6) groundwater lowering and barrier systems, 7) increasing petroleum production, 8) removing contaminants from soils, and 9) for preventing liquefaction of soils during earthquake events. Electro-osmosis uses a d-c electrical potential difference applied across the saturated soil mass by electrodes placed in an open or closed flow arrangement. The d-c potential difference sets up a constant d-c current flowing between the source and sink electrodes. In most soils the soil particles have a negative charge. In those negatively charged soils, the source electrode is the anode electrode and the sink electrode is the cathode electrode, and ground water migrates from the anode electrode toward the cathode electrode. In other soils, such as calcareous soils, the soil particles carry a positive charge. In those positively charged soils, the source electrode is the cathode electrode, the sink electrode is the anode electrode, and ground water migrates from the cathode electode toward the anode electrode.
An “open” flow arrangement of the electrodes allows an ingress or egress of the pore fluid. Due to the electrically induced transport of pore water fluid, the soil pore water pressures are modified to enable excavations to be stabilized or pile driving resistance to be lowered. Electro-osmosis is not used extensively due to the high cost of maintaining the d-c potential over long periods of time and the drying out and chemical reactions that occur if the system is activated for long periods of time. For short term stabilization by pore water pressure reduction, electro-osmosis is very effective in fine grained soils, such as fine sands, silty sands, and silts.
For existing or planned structures, the liquefaction tendencies of a site need to be examined and quantified so that preventive measures can be incorporated into the design of the planned structure or the existing structure be appropriately modified. Therefore, there is a need for a definitive method of measuring the liquefaction potential of a soil in situ, quantifying under what loading conditions the soil will liquefy, and also in determining if liquefaction preventative measures such as electro-osmosis are applicable.
Prior methods for evaluating the liquefaction potential of soils consist of two basic approaches, laboratory tests and in situ tests. The laboratory methods require undisturbed soil samples which are difficult to impossible to obtain. The laboratory test methods involve cyclic triaxial, cyclic direct shear, and cyclic torsional triaxial tests. All of these tests apply a cyclic shear stress reversal upon the soil specimen. At the present time, there is not a method for obtaining undisturbed samples, in which the in situ stress state, void ratio, or structure have been preserved in cohesionless soils. Therefore, laboratory methods are considered only qualitative tests in assessing the potential of a soil to liquefy. The in situ methods currently consist of five (5) types, with four (4) of the methods being indirect empirical methods and the fifth (5
th
) method being a direct in situ measurement of a soil's shear strength and an inferred method for quantifying a soil's potential to liquefy. The four (4) indirect empirical methods are; 1) the Standard Penetration Test (SPT); 2) the Cone Penetration Test (CPT); 3) the Piezocone Penetration Test (PCPT) and 4) the Seismic Waves Test (SWT). The fifth direct in situ measurement is the Piezo Vane Test (PVT).
The Standard Penetration Test (
GeoSierra LLC
Smith , Gambrell & Russell, LLP
Wiggins David J.
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