Method and Apparatus for Making an Expanded Base Pier

Abstract

A system for constructing a support column includes a mandrel with an upper portion and a tamper head. A feed tube extends through the mandrel for feeding flowable material to the head. The tamper head includes a lower enlarged chamber with a reducing surface at an upper portion for compacting material and restricting upward flow of aggregate. The tamper head is of a size providing an enclosed region for allowing cementitious materials to be placed therein. The system allows a support column including a cementitious inclusion on top of an expanded base to be built.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the priority of U.S. Provisional Patent Application Ser. No. 61/239,649, filed Sep. 3, 2009; the disclosure of which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to construction of a structural support column. More specifically, the present invention relates to a method and apparatus for building an expanded base pier to bypass weak soils and transfer structural loads to underlying strong soils.

BACKGROUND OF INVENTION

Heavy or settlement-sensitive facilities that are located in areas containing soft or weak soils are often supported on deep foundations, consisting of driven piles or drilled concrete columns. The deep foundations are designed to transfer the structure loads through the soft soils to more competent soil strata.

In recent years, aggregate columns have been increasingly used to support structures located in areas containing soft soils. The columns are designed to reinforce and strengthen the soft layer and minimize resulting settlements. The columns are constructed using a variety of methods including the drilling and tamping method described in U.S. Pat. Nos. 5,249,892 and 6,354,766; the driven mandrel method described in U.S. Pat. No. 6,425,713; the tamper head driven mandrel method described in U.S. Pat. No. 7,226,246; and the driven tapered mandrel method described in U.S. Pat. No. 7,326,004; the disclosures of which are incorporated by reference in their entirety.

The short aggregate column method (U.S. Pat. Nos. 5,249,892 and 6,354,766), which includes drilling or excavating a cavity, is an effective foundation solution when installed in cohesive soils where the sidewall stability of the hole is easily maintained. The method generally consists of: a) drilling a generally cylindrical cavity or hole in the foundation soil (typically around 30 inches); b) compacting the soil at the bottom of the cavity; c) installing a relatively thin lift of aggregate into the cavity (typically around 12-18 inches); d) tamping the aggregate lift with a specially designed beveled tamper head; and e) repeating the process to form an aggregate column generally extending to the ground surface. Fundamental to the process is the application of sufficient energy to the beveled tamper head such that the process builds up lateral stresses within the matrix soil up along the sides of the cavity during the sequential tamping. This lateral stress build up is important because it decreases the compressibility of the matrix soils and allows applied loads to be efficiently transferred to the matrix soils during column loading.

The tamper head driven mandrel method (U.S. Pat. No. 7,226,246) is a displacement form of the short aggregate column method. This method generally consists of driving a hollow pipe (mandrel) into the ground without the need for drilling. The pipe is fitted with a tamper head at the bottom which has a greater diameter than the pipe and which has a flat bottom and beveled sides. The mandrel is driven to the design bottom of column elevation, filled with aggregate and then lifted, allowing the aggregate to flow out of the pipe and into the cavity created by withdrawing the mandrel. The tamper head is then driven back down into the aggregate to compact the aggregate. The flat bottom shape of the tamper head compacts the aggregate; the beveled sides force the aggregate into the sidewalls of the hole thereby increasing the lateral stresses in the surrounding ground.

The driven tapered mandrel method (U.S. Pat. No. 7,326,004) is another means of creating an aggregate column with a displacement mandrel. In this case, the shape of the mandrel is a truncated cone, larger at the top than at the bottom, with a taper angle of about 1 to about 5 degrees from vertical. The mandrel is driven into the ground, causing the matrix soil to displace downwardly and laterally during driving. After reaching the design bottom of the column elevation, the mandrel is withdrawn, leaving a cone shaped cavity in the ground. The conical shape of the mandrel allows for temporarily stabilizing of the sidewalls of the hole such that aggregate may be introduced into the cavity from the ground surface. After placing a lift of aggregate, the mandrel is re-driven downward into the aggregate to compact the aggregate and force it sideways into the sidewalls of the hole. Sometimes, a larger mandrel is used to compact the aggregate near the top of the column.

U.S. Pat. No. 7,604,437 is related to a mandrel for making aggregate support columns wherein flow restrictors are provided to prevent upward movement of aggregate through the mandrel during driving of the mandrel. The mandrel contemplated in this art relates to formation of an aggregate support column such as described in U.S. Pat. Nos. 6,425,713 and 7,226,246 discussed above.

U.S. Pat. Nos. 4,992,002 and 6,773,208 relate to methods for casting a partially reinforced concrete pier in the ground. One method involves the use of an elongate mandrel with a cupped foot having a larger cross-sectional area than the mandrel, wherein flowable grout that is placed in the mandrel flows through openings located near the bottom of the mandrel into the space between the mandrel and the foot. The other method involves the installation of an elongate hollow tubular casing that is then filled with fluid concrete that is allowed to set while the casing remains in the ground. Each of these references is merely to concrete hardened inclusions and does not allow for the additional stability and strength provided by a pier that has an expanded base.

In the area of soil improvement, it is often desirable to install a stiff inclusion into the ground to transfer loads through a soft or weak soil layer. Although these soil layers may also be treated by non-cementitious aggregate columns, non-cementitious columns are typically confining-stress dependent (i.e., they rely on the strength of the sidewall soils to prevent bulging). Occasionally, it is desirable to utilize cementitious inclusions to bypass weak soils and transfer loads to underlying strong soils. The object of the present invention is to efficiently form a strong and stiff expanded base (either cementitious or non-cementitious) at the bottom of the column and to provide an efficient means for the introduction of grout, concrete, post-grouted aggregate, or other cementitious material through the upper portions of the column to form a cementitious inclusion.

BRIEF DESCRIPTION OF INVENTION

The present invention relates to a system for constructing a support column. A mandrel has an upper portion and a tamper head. A feed tube extends through the mandrel for feeding aggregate, concrete, grout, or other flowable materials to the tamper head. The tamper head includes a lower enlarged chamber with a reducing surface at an upper portion thereof for compacting aggregate or concrete and restricting upward flow of aggregate or concrete. The tamper head is of a size providing an enclosed region for allowing cementitious materials to be placed therein.

A method of constructing such support columns with the system is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following detailed description made with reference to the drawings, wherein:

FIG. 1 is a side cross-section view of a first embodiment of a mandrel;

FIG. 2 is a side cross-section view of a second embodiment of the mandrel with a valve;

FIG. 3 is a side cross-section view of a third embodiment of the mandrel with internal upward flow restrictors;

FIG. 4 is a side cross-section view of a fourth embodiment of the mandrel with a grout return pipe;

FIG. 5 illustrates a method of constructing a pier with the mandrel of FIG. 1;

FIG. 6 illustrates an alternate method of constructing a pier with one embodiment of the mandrel of the invention;

FIG. 7 is a side cross-section view of an alternative embodiment of the mandrel using a closed top system to allow air pressure to build;

FIG. 8 is a more detailed view of the operation of a closed top system including use of an external air source; and

FIG. 9 is a graph showing results of load tests performed on columns made according to Example I as compared to reference piers.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the attached figures, various embodiments of a new and novel mandrel for forming an expanded base pier, as part of a hardened inclusion, is provided.

FIG. 1 illustrates an embodiment of a base mandrel contemplated herein. In this embodiment, a tamper head (2) is formed as a unitary structure attached to one end of a feed tube or pipe (4) to form the mandrel assembly (1). The feed pipe (4) can typically be 4″ to 12″ in diameter and has an upper end (not shown) opposite the tamper head (2) in which aggregate, concrete, grout, and other flowable material can be fed. The tamper head (2) typically comprises an enlarged lower chamber (3), typically 10″ to 24″ in diameter. The reducing surface (5) from the lower chamber walls to the feed pipe walls serves the function as a compaction plate for compacting aggregate or concrete as described hereinbelow, as well as serving as an upward flow restrictor while the initial aggregate is being driven such that the aggregate or concrete forms a “plug” within the chamber (3) and does not flow back up into the feed pipe (4). The lower chamber (3) at the bottom of the head allows for formation of a densified bottom expanded base and provides an enclosed area for the placement of grout or concrete. Stiffeners (6) can also be placed between the feed pipe (4) and lower chamber (3) to assist in load transfer during driving.

FIG. 2 illustrates an embodiment of a base mandrel similar to FIG. 1, but includes a special valve mechanism (7) that may be used to further block the flow of aggregate or concrete from the lower chamber (3) into the feed pipe (4). The valve mechanism (7) seats against the reducing surface (5) of the feed pipe (4) and physically restricts the flow of aggregate, or concrete, back up into the feed pipe during downward driving (as opposed to the “plug” formed as described above with reference to FIG. 1). When the feed pipe (4) is lifted, the valve mechanism (7) opens to allow the downward flow of grout, concrete, or other flowable material through the feed pipe (4) and into the lower chamber (3). The valve mechanism (7) may be manipulated by a pipe extending to the top of the mandrel or by a mechanism that pins the valve mechanism (7) to the sidewalls of the feed pipe (4).

The purpose of the valve mechanism (7) envisioned with reference to FIG. 2 is to allow subsequent compaction of the bottom aggregate or concrete expanded base initially placed and formed. For instance, the mandrel would first be driven in the ground with the lower chamber (3) charged with aggregate or concrete. The feed pipe (4) would then be lifted, and the valve mechanism (7) would open. Grout or concrete would then be added through the feed pipe (4). The mandrel assembly (1) would then be driven back down, thereby allowing for further compaction of the aggregate or concrete at the bottom to form an expanded base.

FIG. 3 illustrates another variation of the embodiment of FIG. 1. More specifically, chain links (8) are attached within the tamper head (2) so that upon tamping, the chain links (8) move inward to constrict the aggregate or concrete in the lower chamber (3) and restrict aggregate or concrete from flowing upward into the feed tube (4). It is also envisioned that internal notches may be provided in lieu of chains in order to provide non-mechanical (or passive) upward flow restriction.

FIG. 4 illustrates a further embodiment of a mandrel similar to that shown in FIG. 1 but which includes a special provision for ensuring grout placement. Instead of having a single chute feed pipe or tube (4) as shown in FIG. 1, the embodiment contemplated with reference to FIG. 4 has a feed pipe including a primary feed pipe (4) and a grout return pipe (9) that is used to ensure that a continuous column of grout is installed. Positive flow of grout from the top of the grout return pipe (9) demonstrates that the mandrel is full of grout before or during mandrel extraction (lifting) operations.

A method of use is shown with reference to FIG. 5, which shows an installation sequence with the base mandrel depicted in FIG. 1. Step A shows placing a mound (10) of the aggregate on the ground surface. Step B shows driving the mandrel assembly (1) through the mound (10) of aggregate (to form an initial charge of aggregate) and to the final driving elevation. During the driving process, the aggregate in the lower chamber (3) forms a plug (11) in the neck of the feed pipe (4) at the bottom of the tamper head (2). Step C shows lifting of the mandrel assembly (1) wherein the aggregate plug (11) or initial charge remains in place at the bottom of the hole (it is understood that the initial charge may also be added after driving of a closed tamper head, such as with a sacrificial cap covering the bottom opening of the tamper head). Step D shows re-driving the mandrel assembly (1) one or more times to compact the aggregate at the bottom of the hole and to form an expanded base (12). Grout or concrete (13) may then be pumped through the feed pipe as shown. Step E shows placing grout or concrete (13) from the element up from the bottom while removing the mandrel. When the grout return pipe (9) as shown in FIG. 4 is used in conjunction with Step E of the construction process shown in FIG. 5, grout continuity within the mandrel shaft is determined if grout continues to flow out of the grout return pipe (9) during extraction. The finished support column comprises an expanded base with a cementitious inclusion located thereon.

An alternative method of use can also be shown with reference to FIG. 5. Step A consists of filling the lower chamber (3) of the tamper head (2) with concrete (13). This may be achieved by driving the tamper head (2) through a mound (10) of concrete (13) as shown on FIG. 5 or by pumping concrete (13) through the feed tube (4) while the tamper head (2) is resting on the ground surface. In this case, the ground surface seals the concrete (13) from flowing out of the bottom of the lower chamber (3). As shown in FIG. 5, the tamper head (2) is then driven to design elevation with the concrete (13) at the bottom of the tamper head (2) forming a plug at the bottom of the assembly mandrel (1). The valve mechanism (7) shown in FIG. 2 or the chain links (8) shown in FIG. 3 may be used within the tamper head (2) to facilitate plugging. Step C shows the retraction (lifting) of the assembly (2) to allow the concrete to flow out of the bottom of the tamper head (2). Step D shows the placement of concrete (13) through the feed pipe (4) and the subsequent or simultaneous lowering of the mandrel assembly (1) onto the previously placed concrete (13) to force the concrete (13) outward thus forming an expanded base (12). Step E shows the simultaneous placement of grout or concrete (13) through the feed tube (4) while extracting the mandrel assembly (1) to the ground surface. This technique forms an expanded base pier comprised of concrete (13) at the expanded base (12) and concrete (13) within the pier shaft (or inclusion) on top of the expanded base (12).

The benefits of the system contemplated herein are the efficient formation of an expanded base (12) that allows load to be transferred to the bottom of the pier and the very quick and efficient formation of the grouted inclusion by rapidly raising the mandrel while placing grout or concrete (13). While the method sequence of FIG. 5 depicts the use of the base mandrel shown in FIG. 1, it is envisioned that the method could principally be used with any of the mandrels shown in FIGS. 1-4.

FIG. 6 shows an alternative construction sequence where Steps A through C are as described above. In Step D, the mandrel assembly (1) is lowered to compact the aggregate and a secondary disposable pipe (14) is inserted into the mandrel assembly (1) to rest on the expanded base (12). In Step E the mandrel assembly (1) is raised and additional aggregate (15) is allowed to fill the annular space between the disposable pipe (14) and the sidewall of the cavity (16). The aggregate (15) placed in this step is not compacted. In Step F the disposable pipe (14) is then used as a conduit to place grout into the inclusion by filling the voids in the loose aggregate (15) around the disposable pipe (14). Typically, the disposable pipe (14) is not removed but can be cut at ground level or just below ground level and made part of the permanent inclusion. A hopper (17) can be used to place the aggregate (15) within the feed pipe (4).

FIG. 7 illustrates a further embodiment of a mandrel similar to that shown in FIG. 1 but which includes a closed system for the placement of concrete, grout, or other flowable materials. The mandrel of this embodiment includes an external feed tube (18) that enters the mandrel feed tube (4) near the top of the mandrel to allow for the passage of a flowable material (19). The external feed tube (18) is used to pump concrete, grout, or other flowable materials into the primary feed tube (4). The top of the mandrel is sealed with a top plate (21) making this a closed system. An air pressure gage (20) may optionally be installed to measure the internal air pressure within the mandrel and allow for the use of a pressure release valve (22) to facilitate removal of excess internal pressure during pumping. The mandrel system of FIG. 7 may be used in conjunction with the construction sequences shown in FIG. 5.

FIG. 8 illustrates yet another embodiment of the mandrel similar to that shown in FIG. 7. In this embodiment, an air source, such as compressor (24), may optionally be used to apply elevated air pressure to trapped air (23) within the mandrel feed pipe (4) to evacuate concrete (13) from the mandrel.

The following examples illustrate further aspects of the invention.

Example I

As an example, an embodiment of the system of the present invention was used to install a support column, also described herein as an Expanded Base Pier (“EBP”), at a test site in Iowa. The test site was characterized by 4 feet of sandy lean clay underlain by sand. This testing program was designed to compare the load versus deflection characteristics of this embodiment of the EBP to reference piers constructed in successive lifts, such as a pier constructed by the tamper head driven mandrel method. The reference piers of this example had a nominal diameter of 20 inches and an installed length of 23 feet. One reference pier was constructed of aggregate only to a diameter of 20 inches. Another reference pier was constructed with a grout additive, commonly referred to as grouted pier, to a diameter of 14 inches.

In this embodiment of the invention, the EBP was formed by filling the extractable mandrel (FIG. 1) with a combination of open graded aggregate and fluid grout. The mandrel had a lower chamber (3) outside diameter of 14 inches and a feed pipe outside diameter of 12 inches. The mandrel of this embodiment was connected at its open end (opposite the tamper head) to an open hopper for filling. The full mandrel was then advanced to a depth of 23 feet below the ground surface. The mandrel assembly was then raised 3 feet and lowered 3 feet a total of 3 times to form a bottom expanded base. Each raising and lowering of the mandrel is referred to as a “stroke.” The mandrel was then raised 3 feet, lowered 2 feet, and then slowly extracted to the ground surface allowing a column of grout and aggregate to be placed in the cavity created during mandrel installation. The EBP was constructed with a base diameter of 20 inches, and a shaft diameter of 14 inches. Once the mandrel was fully extracted, a 1 inch diameter reinforcing steel rod was inserted the full length of the EBP. A concrete cap was then poured above the EBP to facilitate load testing.

The extractable mandrel of this embodiment was attached to a high frequency hammer which is often associated with driving sheet piles. The hammer is capable of providing both downward force and vibratory energy.

The reference pier and the EBP were load tested using a hydraulic jack pushing against a test frame. FIG. 9 shows the results of the load test compared with the reference piers. At a top of pier deflection of 0.5 inches, the reference pier with aggregate supported a load of about 23,300 pounds, the reference pier with grout supported a load of about 50,000 pounds, and the EBP supported a load of about 70,300 pounds. At a top of pier deflection of 1 inch, the reference pier with aggregate supported a load of about 38,800 pounds, the reference pier with grout supported a load of about 62,700 pounds, and the EBP supported a load of about 97,000 pounds. The load carrying capacity of the pier constructed in accordance with this embodiment of the present invention showed a 2.5 to 3 fold improvement when compared to a reference pier with aggregate, and a 1.4 to 1.5 fold improvement when compared to a reference pier with grout. The difference in the behavior relative to the grouted pier is caused by the formation of the bottom expanded base during the construction of the EBP.

Example II

As another example, the system of another embodiment of the present invention was used to install five EBP elements at a test site in Virginia. The test site is characterized by hard clay. Prior to installation of the EBP, 30 inch diameter drill holes were excavated to a depth of 8 feet below the ground surface. The voids were then loosely backfilled with sand. The EBP elements of this example were formed within the backfilled holes.

In this embodiment of the invention, the EBP was formed by filling the mandrel described in FIG. 7 with concrete. The mandrel of this embodiment featured a “closed top” as opposed to the “open hopper” configuration as described with reference to Example I. The full mandrel was then advanced to a depth of 8 feet below the ground surface. The mandrel was then raised 3 feet, and then lowered 2 feet for three repetitions to create the expanded base. A process of raising the mandrel 3 feet, and then lowering 1 foot was then used to complete the full length of the pier. Once the concrete had cured, each of the piers was excavated and the pier base and shaft diameters were measured.

The lower chamber in this embodiment had a nominal 12 inch diameter outer dimension. The excavated and measured piers had an average nominal diameter of 18 inches. Expanded bases at the bottoms of the piers exceeded 24 inches demonstrating the effectiveness of this construction technique.

The mandrel in this embodiment was attached to a similar hammer as in the embodiment of Example I.

The foregoing detailed description of embodiments refers to the accompanying drawings, which illustrate specific embodiments of the invention. Other embodiments having different structures and operations do not depart from the scope of the invention. The term “the invention” or the like is used with reference to certain specific examples of the many alternative aspects or embodiments of the applicant's invention set forth in this specification, and neither its use nor its absence is intended to limit the scope of the applicant's invention or the scope of the claims. This specification is divided into sections for the convenience of the reader only. Headings should not be construed as limiting of the scope of the invention. The definitions are intended as a part of the description of the invention. It will be understood that various details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation.

Claims

1. A system for constructing a support column, comprising:

(a) a mandrel having an upper portion and a tamper head, and a feed tube extending therethrough for feeding aggregate, concrete, grout, or other flowable materials through the mandrel to the tamper head; and

(b) the tamper head defining a lower enlarged chamber having a reducing surface at an upper portion thereof for compacting aggregate or concrete and for restricting upward flow of aggregate or concrete into the mandrel during compacting and the temper head being of sufficient size for providing an enclosed region for allowing cementitious material to be placed therein.

2. The system of claim 1, further comprising a valve mechanism movable between an open position and a closed position for closing off the feed tube from communication with the tamper head during tamping operations.

3. The system of claim 1, further comprising stiffening members secured between the reducing surface and the mandrel for providing load support during tamping operations.

4. The system of claim 1, further comprising chains attached within the interior of the tamper head for restricting upward flow of material into the feed tube during downward movement of the mandrel.

5. The system of claim 1 further comprising notches within the interior of the tamper head for restricting upward flow of material into the feed tube during downward movement of the mandrel.

6. The system of claim 1, further comprising a second tube extending through the mandrel on the side of the feed tube for allowing cementitious material to flow upward through the second tube for inspection of the cementitious material during pumping.

7. The system of claim 1, further comprising a hopper located at the top of the mandrel for feeding aggregate into the feed tube of the mandrel.

8. The system of claim 1, further comprising a closure cap on an end of the feed tube opposite the tamper head and a concrete supply tube connected to the feed tube.

9. The system of claim 8 further comprising an air pressure source connected to the feed tube for evacuating concrete from the feed tube through air pressure supplied thereto.

10. A method of constructing a support column comprising use of a mandrel assembly having a feed tube connected to a tamper head at an opening thereof for allowing aggregate, concrete, grout, or other flowable material to flow into the tamper head, the method comprising:

(a) providing the tamper head of a shape with a defined lower enlarged chamber having a reducing surface at an upper portion thereof for compaction and for restricting upward flow of material into the feed tube during tamping, the tamper head further sized to provide an enclosed region for allowing cementitious material to be placed therein;

(b) driving the mandrel assembly into a ground surface to a given depth thereby forming a cavity;

(c) lifting the mandrel assembly to release an initial charge of aggregate or concrete from the tamper head into a bottom of the cavity;

(d) re-driving the mandrel assembly to compact the aggregate or concrete at a bottom of the cavity and to form an expanded base, the expanded base having a width greater than the tamper head; and

(e) withdrawing the mandrel assembly while continuously feeding cementitious material or aggregate to be subsequently fully or partially treated with grout through the feed tube, thereby forming a cementitious inclusion at least partially within the cavity, the cementitious inclusion having a width of the cavity and being formed on top of the expanded base.

11. The method of claim 10 wherein the tamper head is filled with the initial charge of aggregate or concrete before driving.

12. The method of claim 10 wherein the cementitious material is one selected from the group consisting of concrete, grout, or aggregate that is subsequently fully or partially treated with grout.

13. The method of claim 10, further comprising providing a valve mechanism movable between an open position and a closed position at the opening between the feed tube and the tamper head, for restricting the passage from flow connection into the tamper head, and moving the closure member to an open position for introducing cementitious material into the tamper head, and into a closed position during downward compaction.

14. The method of claim 10, further comprising introducing cementitious material into the enclosed region.

15. The method of claim 10, further comprising introducing a pipe through the feed tube and tamper head after formation of the expanded base, placing aggregate during the withdrawing step to partially surround the pipe, and introducing cementitious material into the pipe following aggregate placement to treat the aggregate.

16. The method of claim 10, further comprising chains attached within the interior of the tamper head for restricting upward flow of material into the feed tube during downward movement of the mandrel.

17. The method of claim 10, further comprising notches within the interior of the tamper head for restricting upward flow of material into the feed tube during downward movement of the mandrel.

18. The method of claim 10, further comprising providing the mandrel with a second tube adjacent the feed tube to allow for the inspection of cementitious material during pumping.

19. The method of claim 10, further comprising a hopper located at the top of the mandrel for feeding aggregate into the feed tube of the mandrel.

20. The method of claim 10, further comprising a closure cap on an end of the feed tube opposite the tamper head and a concrete supply tube connected to the feed tube.

21. The method of claim 20 further comprising an air pressure source connected to the feed tube for evacuating concrete from the feed tube through air pressure supplied thereto.