ELECTROKINETIC CONDITIONING OF FOUNDATION PILES
A method for increasing load capacity of a foundation pile installed in a ground material is described. At least one electrode is embedded in the ground material at a predetermined distance from the installed pile. A direct current power source is provided, with the power source including at least one cathode connection and at least one anode connection. One of the at least one cathode connection is secured to the foundation pile. One of the at least one anode connections is secured to each of the at least one electrodes. The direct current power source is activated to generate an electric gradient between the foundation pile and the at least one electrode, with the electric gradient being selected to direct charged soil particles in the ground material toward the foundation pile.
Latest CLEVELAND STATE UNIVERSITY Patents:
- PROCESS FOR REMOVING CARBON DIOXIDE FROM ACETYLENE USING CHA-TYPE ZEOLITE MEMBRANE
- Recombinant prothrombin analogues and uses thereof
- TUMOR RESECTION PROCESS AND COMPOSITION CONTAINING IRIDIUM-CONTAINING NUCLEOSIDE
- Compositions and methods for modulating HEXIM1 expression
- CORONAVIRUS TREATMENT COMPOSITION AND METHOD
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/417,980, entitled “INCREASING CAPACITY AND ENHANCEMENT SET-UP OF STEEL PILES IN CLAY SOIL BY ELECTROKINETIC METHOD” and filed Nov. 30, 2010, the entire disclosure of which is incorporated herein by reference, to the extent that it is not conflicting with the present application.
BACKGROUNDPile foundations are used for the support of heavy structures, such as bridges, tall buildings, and the like. A significant increase in the load capacity of these piles is often observed over time after initial installation, and is generally attributed to consolidation of cohesive ground materials, such as clay and cohesive soils, around the pile after pile installation, and dissipation of water pressure around the piles. This time-dependent strength gain is commonly referred to as “set-up,” and may be tested by “re-strikes” of the piles, measuring the displacement of the pile as the result of application of a known force. While formulas have been developed to estimate pile capacity as a function of elapsed time after installation, these formulas may not be relied upon in subjecting the piles to loads during construction. As such, the construction of pile foundations often requires a waiting period after installation of the piles for a sufficient increase in load capacity. Further, the pile capacities are often not utilized to their full potential as construction often relies upon capacities that are measured, for practical reasons and safety considerations, after several days to a few weeks from installation when only a fraction of set-up has occurred, for example, to minimize construction delays.
SUMMARYThe present application describes systems and methods for accelerating or increasing set-up of an embedded structure (e.g., a steel foundation pile) by generating an electrical polarity at the embedded member that is opposite an inherent electrical polarity of a cohesive ground material into which the structure is embedded, such that the cohesive ground material is attracted to or adhered to the embedded structure. In one such exemplary embodiment, a positive electrical polarity is generated at a steel foundation pile to attract cohesive clay soil particles having a net negative surface charge.
The present application also describes systems and methods for facilitating removal of an embedded structure (e.g., a steel foundation pile) by generating an electric polarity at the embedded member that is the same as an inherent electrical polarity of a cohesive ground material into which the structure is embedded, such that the cohesive ground material is dispersed or repelled from the conductive member, thereby reducing ground material adhesion to the embedded conductive member for reduced resistance to pull-out.
Still other advantages, aspects, and features of the present application will become readily apparent to those skilled in the art from the following description, wherein there is shown and described an exemplary embodiment of the present application. As it will be realized, the present application is capable of other different embodiments, and its several details are capable of modifications in various aspects, all without departing from the scope of the present application. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.
Further features and advantages will become apparent from the following detailed description made with reference to the accompanying drawings, wherein:
The Detailed Description merely describes exemplary embodiments of the invention and is not intended to limit the scope of the claims in any way. Indeed, the invention is broader than and unlimited by the exemplary embodiments, and the terms used in the claims have their full ordinary meaning
Also, while the exemplary embodiments described in the specification and illustrated in the drawings relate to installation and removal of steel foundation piles, it should be understood that many of the inventive features described herein may be applied to installation and removal of other ground embedded structures, such as posts, spikes, and plates.
Installation or driving of foundation piles into cohesive soil, such as clay-based soil, generally causes the soil around the driven pile to undergo plastic deformation and to develop increased pore water pressure around the driven pile, both of which may contribute to an initial reduced or limited load capacity of the pile. After the pile driving or installation is completed, excess pore pressure around the driven pile tends to dissipate, and the cohesive soil tends to re-consolidate around the driven pile, resulting in increases in the load capacity of the driven pile over time. However, due to variations in soil conditions, both between the locations of multiple piles employed in a single construction product, and across multiple soil strata into which a single pile is driven, set-up of a conventionally installed pile may vary, and in some cases, little or no set-up may occur, or even a relaxation or loss of load capacity may occur over time. This variability in set-up has conventionally required significant waiting periods to verify sufficient load capacity of the driven piles, and to determine the number of piles required to support the eventual load. Further, due to the long periods of time over which natural set-up may occur, and the prohibitive costs and inconveniences of substantial construction delays, construction projects are often unable to take advantage of this long-term strength gain, and redundant piles are driven during construction to satisfy load capacity requirements after only a minimal waiting period.
The present application contemplates methods and systems of improving load capacity or set-up of a ground embedded structure, such as a steel foundation pile, by applying an electric gradient between the embedded structure and the surrounding soil. Without being bound by theory, applicant believes that this application of an electrical gradient between the embedded structure and a clay-based surrounding soil generates an electrochemical attraction between charged soil particles surrounding the embedded structure and the oppositely charged embedded structure. In the case of clay-based soils, the inherent negative surface charge of the clay particles facilitates electrochemical attraction to a positively charged structure. In some applications, moisture collected in the soil around the embedded structure allows for a phenomenon known as electrophoresis, in which an electrical field applied to the water creates an electric gradient between the soil and the embedded structure, causing charged clay particles to migrate through the water and toward the embedded structure. Adherence of these clay particles to the embedded foundation pile creates a clay plug surrounding the pile, increasing the effective diameter of the pile, resulting in an increased shaft resistance of the pile.
While many different arrangements may be utilized to apply an electric gradient between an embedded structure (e.g., an installed foundation pile) and the surrounding soil, in one embodiment, as shown schematically in
While not intending to be bound by theory, applicant believes that the application of a low-intensity direct current through the water-saturated soil surrounding an installed foundation pile mobilizes negatively charged clay particles or colloids C (e.g., kaolinite, illite, montmorillonite, and chlorite clays) to cause the clay colloids to move toward the positively charged pile (or toward positively charged electrodes proximate the pile), a phenomenon referred to as electrophoresis. At the same time, neutral or net positive charged water molecules W are mobilized to disperse from the surface of the pile and flow toward the surrounding negative electrodes, a phenomenon referred to as electroosmosis. The effects of these two simultaneous phenomena include an increase in soil density around the pile, a decrease in water content and pore pressure around the pile, the formation of a clay plug around the pile, effectively increasing the effective diameter of the pile, and an increase in shaft resistance due to this increased effective diameter. All of these effects are believed to increase or accelerate set-up of the installed pile, as compared to the natural expected set-up of the pile in the absence of an applied electrokinetic field.
Many factors involving an installed foundation pile and the conditions of the surrounding soil may affect the amount of set-up experienced by the pile, including, for example, pile length, pile diameter, soil conditions (including materials, cohesiveness, and moisture content), elapsed time, and vibrations during pile driving (which may produce capacity impairing cracks in clay or other cohesive soil materials)
In an exemplary steel foundation pile installation and conditioning application, a stainless steel foundation pile having a diameter of approximately 12-24 inches and a length of approximately 40-150 ft. is embedded in clay based ground soil. Three electrodes (e.g., stainless steel rods) having a diameter of approximately 1-2 inches and a length of approximately 25 ft. (or at least about half the length of the pile) are embedded in the surrounding soil at a distance from the pile of approximately 3-6 feet and evenly spaced from each other. An adjustable direct current power source is provided, with a positive terminal connected to the pile, and branched negative terminals connected to each of the electrodes. The power source is operated to deliver a voltage of approximately 100-300 V DC, to provide a gradient of approximately 1-30 V/ft between the pile and the electrodes. This voltage is maintained for a predetermined period, for example, at least 24-48 hours, before a static or dynamic load capacity test is employed to verify that sufficient set-up has occurred to support the structure to be built on the pile or piles.
According to another aspect of the present application, electrokinetic conditioning of an embedded structure, such as a foundation pile, may be utilized to loosen the embedded structure, for example, to remove or reposition the embedded structure. In such an application, an electric gradient opposite of the electric gradient described above may be applied, with a negative charge applied to the embedded structure and a positive charge applied to one or more embedded electrodes surrounding the embedded structure. While not intending to be bound by theory, applicant believes that the application of a low-intensity direct current through the water saturated soil surrounding an installed steel foundation pile, with a negative charge applied to the pile, mobilizes negatively charged clay colloids to cause the clay colloids to move away from the negatively charged pile and toward the positively charged electrodes. At the same time, neutral or net positive charged water molecules are mobilized to migrate toward the surface of the pile further reducing the shaft resistance of the pile. The effects of these two simultaneous phenomena are believed to include a reduction in soil density around the pile, an increase in water content and pore pressure around the pile, and a resulting decrease in pile shaft resistance. All of these effects are believed to loosen the installed pile, thereby facilitating removal or repositioning of the pile within the ground soil.
EXAMPLE I Preliminary TestingPreliminary bench scale tests were conducted on two pile-clay specimens. The material specifications for the preliminary tests are given in Table 1.
The procedure followed in preparing the kaolinite sample was similar to the one for Standard Proctor Test, ASTM D-698. The soil was compacted in a mold with a diameter of 4 inches and a height of 6.5 inches (including the collar). The soil was mixed with 34% by weight water and then compacted in four equal layers by a hammer that delivered 25 blows to each layer. The hammer had a mass of 2.5 kg (5.5 lb) and a drop of 12 inches.
Care was taken during pile driving to avoid the formation of vibration cracks in clay. The mold was then covered with a rubber pad to prevent moisture loss due to evaporation. The specimen was then kept in a covered tub partially filled with water. This arrangement was helpful in maintaining the moisture content of the soil sample throughout the testing period.
The two specimen to be tested were statically loaded using a tri-axial testing machine (model #, supplier) on Days 0, 1, 3, 8, 16, 22, 31 and 46 after the end of initial driving. A rapid moisture content test (e.g., OHAUS, MB 200) was conducted prior to each static load test. An electric gradient of 30 Volts DC/ft was applied to Specimen A, between static load tests, Day 3 onwards.
Two kaolinite clay-steel pile specimen were prepared in accordance with the method explained above. Laboratory testing was then performed using a preliminary test program. The two specimens to be tested were statically loaded using the tri-axial testing machine on Days 0, 1, 3, 8, 16, 22, 31 and 46 after the end of initial driving. A rapid moisture content test was conducted prior to each static load test. An electric gradient of 30 Volts DC/ft was applied to Specimen A, between static load tests, from Day 3 onwards. Static load test results are presented in tabular and graphical form below.
Load versus penetration data taken on all test days is presented in Table 2. Specimen A was subjected to an electric gradient of the order of 30 Volts DC/ft between test days, starting from Day 3. Because specimen B was not connected to any voltage source, it was allowed to achieve set-up as a natural process.
As shown in Table 2, at day 3, pile capacities of both specimens were almost identical (239 N for Spec A and 244 N for Spec B). It can be seen from these graphs that from Day 3 onwards, there were substantial increases in the pile static capacity values of Specimen A as compared to Specimen B. An electric gradient of 30 Volts DC/ft was applied to Specimen A for a duration of 100 hours between Day 3 and Day 8. The resulting pile capacity values at the end of Day 8 were 693 N and 293 N for Specimens A and B, respectively. Further application of the electric gradient to Specimen A for a cumulative duration of 443 hours resulted in capacities of 1937 N for Specimen A (compared to 419 N for Specimen B) at the end of Day 31.
The moisture contents of both specimens were approximately 34% at the beginning of the testing sequence, and showed a variation of approximately ±5% during the course of the testing period.
Results obtained from preliminary tests confirmed the feasibility of the research proposal and paved the way for more extensive research. Specimen A, after being subjected to an electric gradient of 30 V DC/ft for a duration of 100 hours, gained pile capacity 9 times greater than the pile capacity of Specimen B, which was allowed to achieve natural set-up. Moisture contents of Specimens A and B did not alter substantially during the course of the testing period.
Further, expanded bench scale testing was also performed, using twenty soil samples, designated P1 through P20, of varying clay content, moisture content, and electrokinetic field conditions, as shown in Table 3 below:
Static load tests were scheduled to be conducted on days 0, 1, 3, 7, 14, 21, 35, 42, 49, 56 and 63. Specimens P1-P8 constituted soil samples containing a mixture of sand and kaolinite clay having the clay content identified in Table 3, while specimens P9-P20 were formed entirely from kaolinite clay. DC voltages applied to the respective samples between static load tests, as identified in Table 3, began on day 3 of the test. The test program was developed to compare the pile capacity values between: (a) specimens subjected to electric gradient versus those that are not; (b) specimens subjected to different DC potential values; (c) specimens with different clay content; and (d) specimens with different moisture content.
The specifications for all other materials used in the testing are given in Table 4.
The aspect ratio, or the ratio of length of the pile to the diameter of the pile, was increased by increasing the test length of the pile from 3 inches to 6 inches and reducing its diameter from 1 in. to ¾ in. A solid stainless steel pipe section with a solid conical bottom as was preferred over the earlier hollow pipe to eliminate plugging effect or end face resistance of the soil sample. The conical shape of the base of the pile was selected to minimize the effect of the end face resistance of the pile. Additionally, the use of a solid pile in place of a hollow pile was intended to eliminate the effects of resistance against an inner surface of a hollow pile. Twenty such pile segments were manufactured at Spee-D-Metals in Cleveland, Ohio.
The procedure followed to prepare the soil sample was similar to the one for Standard Proctor Test, ASTM D-698. The soil was compacted in a PVC mold with a diameter of 6 inches and a height of 12 inches. Due to the additional height of the mold, the specimens could be tested over a longer period of time resulting in more experimental data.
Twenty soil samples with different sand-clay proportions and moisture contents were prepared on the basis of the test plan, as discussed above and shown in Table 3. The soil was mixed with the appropriate percentage by weight of water and then compacted in four equal layers by a hammer that delivered 25 blows to each layer. The hammer had a mass of 2.5 kg (5.5 lb) and a drop of 12 inches.
During preliminary testing, piles were driven into the soil sample by lightly tapping the pile head. To make the driving sequence more uniform for the extended testing, a pile driving arrangement was constructed.
During the design and construction of the pile driving arrangement, an appropriate weight, size and length for the hammer ram were determined by a trial and error method. The design of the collar was dependant on the size and weight of the ram. The purpose of the collar was to avoid simultaneous movement of the ram and the pile, while the ram was raised to a known drop height. So it had to act like a separator between the ram and the pile. At the same time, the collar had to be always in contact with the pile and move with it during the entire event of driving. The soil specimen, the pile, and the hammer and collar arrangement had to maintain individual as well as relative positions during the driving sequence to obtain an accurate pile alignment and uniform driving. This was achieved by constructing a support frame structure fabricated from solid steel rods and metal plates.
In installing the piles, the hammer ram was manually raised to a fixed drop height using a rope and pulley assembly, and released so that it fell under the influence of gravity. Several blows were needed to drive the pile six inches into the ground. The amount of energy needed to drive the pile was recorded, in the form of the hammer weight, number of blows and the drop height. The weight of the hammer ram was constant and approximately equal to 1.7 kg (3.75 lbs). Accordingly, a drop height of 3 inches induced an energy in the amount of 0.94 lb-ft for each hammer blow.
In preparing for the test, twenty ¾ inch diameter stainless steel piles 110 (as shown in
These steel pile-clay specimens were covered with plastic secured by a rubber band at all times to minimize the effect of loss of moisture content due to evaporation. The specimens were stored in an aluminum water tank between testing events to reduce moisture losses due to evaporation. The tank was partially filled with water and covered with a thick polythene cover.
A rapid moisture content test was conducted on every specimen on all test days before performing a static load test, using an OHAUS MB 200 moisture analyzer. The rapid moisture content detector oven dried a soil sample in approximately 15 minutes (as compared to a 16 to 24 hour drying time in case of a regular moisture content determination test), and displayed the absolute value of the moisture content in terms of a percentage on a LCD screen. The moisture content test was performed in accordance with ASTM D4959.
The specimens to be tested were statically loaded using a tri-axial testing machine (e.g., an ELE Tri-axial Testing System). The tri-axial machine includes a hydraulic loading platform to which a load cell and penetrometer are attached. The load cell and penetrometer are connected to a computer which is programmed to dynamically read, tabulate and plot the load versus penetration graph for each specimen.
The static load test was performed in accordance with ASTM D-1143 with a minor variations to account for use of a tri-axial testing machine instead of a conventional axial compressive loading device.
To generate an electrokinetic field between the piles and the surrounding electrodes, a positive terminal of a direct current voltage source was connected to the pile, and negative terminals of the voltage source were connected to each of the three electrodes. The DC voltage source applied a constant voltage across the specimen for the duration of the test period. The test program requirement made it mandatory that at least three voltage sources, which could apply 1V, 10 V and 30 V on different specimens, be used simultaneously. A log was maintained to record the durations of application of electric gradient.
The temperature of each of the specimens was monitored on a regular basis, including during the application of electric gradient. A lab thermometer was used to record temperatures.
The twenty soil-pile specimens of varying clay content, moisture content, and eletrokinetic field exposure were subjected to static load testing on days 0, 1, 3, 7, 14, 21, 28, 252, and 256, as measured from the end of initial driving. (Static load testing was terminated during testing on Day 42 due to technical problems with the load cell used in the tri-axial testing machine). The application of electrokinetic fields to the specimens was carried out from Day 3 to Day 42, with only brief termination of the electrokinetic fields during static load testing.
Test data taken on Days 0, 1, 3, 7, 14, 21, 28, 42 (partial), 252 and 256 are presented in Tables 6-15 respectively. Static load tests were terminated at the point when the specimens shows no further signs of increase in load for two or three consecutive increments of pile penetration. The maximum stable load attained by each specimen on all test days is tabulated in Table 16.
A rapid moisture content test was conducted on every specimen on all test days before performing a static load test. Table 17 shows a moisture content log at the beginning and the end of the testing period, in which slight reductions in the moisture contents of these specimen during the course of the testing period are noted.
The temperature of the specimens was monitored on a regular basis, especially during the period over which the electric gradient was applied. Table 18 shows a log of temperatures that was maintained during the testing period, in which the temperatures of the specimens were observed to be close to the ambient temperatures on the respective test days, and not substantially affected by the application of the electric gradient.
While various inventive aspects, concepts and features of the inventions may be described and illustrated herein as embodied in combination in the exemplary embodiments, these various aspects, concepts and features may be used in many alternative embodiments, either individually or in various combinations and sub-combinations thereof. Unless expressly excluded herein all such combinations and sub-combinations are intended to be within the scope of the present inventions. Still further, while various alternative embodiments as to the various aspects, concepts and features of the inventions--such as alternative materials, structures, configurations, methods, circuits, devices and components, software, hardware, control logic, alternatives as to form, fit and function, and so on--may be described herein, such descriptions are not intended to be a complete or exhaustive list of available alternative embodiments, whether presently known or later developed. Those skilled in the art may readily adopt one or more of the inventive aspects, concepts or features into additional embodiments and uses within the scope of the present inventions even if such embodiments are not expressly disclosed herein. Additionally, even though some features, concepts or aspects of the inventions may be described herein as being a preferred arrangement or method, such description is not intended to suggest that such feature is required or necessary unless expressly so stated. Still further, exemplary or representative values and ranges may be included to assist in understanding the present disclosure; however, such values and ranges are not to be construed in a limiting sense and are intended to be critical values or ranges only if so expressly stated. Moreover, while various aspects, features and concepts may be expressly identified herein as being inventive or forming part of an invention, such identification is not intended to be exclusive, but rather there may be inventive aspects, concepts and features that are fully described herein without being expressly identified as such or as part of a specific invention. Descriptions of exemplary methods or processes are not limited to inclusion of all steps as being required in all cases, nor is the order that the steps are presented to be construed as required or necessary unless expressly so stated.
Claims
1. A method for increasing load capacity of a foundation pile installed in a ground material, the method comprising:
- embedding at least one electrode in the ground material at a predetermined distance from the installed pile;
- providing a direct current power source including at least one cathode connection and at least one anode connection;
- securing one of the at least one cathode connections to the foundation pile;
- securing one of the at least one anode connections to each of the at least one electrodes; and
- activating the direct current power source to generate an electric gradient between the foundation pile and the at least one electrode, the electric gradient being selected to direct charged soil particles in the ground material toward the foundation pile.
2. The method of claim 1, wherein embedding at least one electrode in the ground material comprises embedding a plurality of electrodes in the ground material at substantially equal predetermined distances around the foundation pile.
3. The method of claim 1, wherein activating the direct current power source to generate the electric gradient comprises supplying an electrical potential from approximately 1 volt DC to approximately 100 volts DC.
4. The method of claim 1, wherein activating the direct current power source to generate the electric gradient comprises generating an electric gradient of approximately 30 volts DC/ft.
5. The method of claim 1, wherein the charged soil particles comprise clay particles.
6. The method of claim 1, further comprising maintaining the electric gradient between the foundation pile and the at least one electrode for a period of at least approximately 48 hours.
7. The method of claim 1, further comprising measuring a moisture content of the ground material and adjusting one of an electrical potential supplied by the direct current power source and a duration during which the electric gradient is maintained based on the measured moisture content.
8. The method of claim 1, further comprising measuring a clay content of a soil sample obtained from the ground material and adjusting one of an electrical potential supplied by the direct current power source and a duration during which the electric gradient is maintained based on the measured clay content.
9. The method of claim 1, wherein embedding the at least one electrode in the ground material at a predetermined distance from the installed pile comprises embedding the at least one electrode at a distance of at least approximately 3 feet from the installed pile.
10. The method of claim 1, wherein embedding the at least one electrode in the ground material comprises embedding the at least one electrode to a depth of at least approximately one half of the pile length.
11. A system for electrokinetic conditioning of an installed steel foundation pile, the system comprising:
- a foundation pile embedded in a cohesive ground material containing negatively charged particles;
- at least one electrode embedded in the ground material at a predetermined distance from the foundation pile;
- a direct current power source having a positive terminal connected to the foundation pile, and at least one negative terminal connected to each of the at least one electrodes.
12. The system of claim 11, wherein the at least one electrode comprises a plurality of electrodes embedded in the ground material at substantially equal predetermined distances around the foundation pile.
13. The system of claim 12, wherein the plurality of electrodes are embedded in the ground material at a distance of at least approximately 3 feet from the surface of the foundation pile.
14. The system of claim 11, wherein the at least one electrode in the ground material is embedded to a depth of at least approximately one half of the pile length.
15. The system of claim 11, wherein the direct current power source comprises an adjustable direct current power source having a maximum voltage of at least approximately 100 V DC.
16. The system of claim 14, wherein the negatively charged particles comprise clay particles.
17. The system of claim 11, wherein the foundation pile comprises steel.
18. A method of loosening a conductive structure embedded in a ground material, the method comprising:
- embedding at least one electrode in the ground material at a predetermined distance from the embedded structure;
- providing a direct current power source including at least one cathode connection and at least one anode connection;
- securing one of the at least one anode connections to the foundation pile;
- securing one of the at least one cathode connections to each of the at least one electrodes; and
- activating the direct current power source to generate an electric gradient between the foundation pile and the at least one electrode, the electric gradient being selected to direct charged soil particles in the ground material away from the foundation pile.
19. The method of claim 18, wherein activating the direct current power source to generate the electric gradient comprises supplying an electrical potential from approximately 1 volt DC to approximately 100 volts DC.
20. The method of claim 18, wherein the conductive structure comprises a steel foundation pile.
Type: Application
Filed: Nov 30, 2011
Publication Date: Nov 8, 2012
Applicant: CLEVELAND STATE UNIVERSITY (Cleveland, OH)
Inventor: Lutful I. Khan (Strongsville, OH)
Application Number: 13/307,794
International Classification: E02D 13/00 (20060101); E02D 11/00 (20060101);