Method of providing a support column
View Patent ↗A primary earth penetrating mandrel formed of a hollow shell steel plate octagonal in cross-section has an upper end and a blunt lower end joined by an upwardly and outwardly tapered wall. The mandrel is driven downwardly in the earth to simultaneously form a vertical tapered cavity while compacting the sidewall of the cavity to provide structural integrity. The mandrel is then moved upwardly from the bottom of the cavity and aggregate is deposited in the bottom of the cavity following which the mandrel is lowered so that its blunt lower end engages the deposited aggregate and densifies the aggregate by vertical vibratory action and static force with these steps being repeated until the pier top is near the surface of the earth at which time the upper aggregate portions are densified by either the primary mandrel or a secondary mandrel having a substantially larger lower end surface than the lower end surface of the primary mandrel. A second embodiment includes a conduit in the primary mandrel for injecting concrete or grout into aggregate previously deposited in the cavity.
1. A method of forming an aggregate pier comprising the steps of:
(a) driving a downwardly tapered mandrel having a flat lower end surface and a non-tapered edge around the periphery of the flat lower end surface, with the width of the flat lower end surface and non-tapered edge being no greater than the width of the mandrel at a lower end thereof, into the ground by a power driven apparatus to form a downwardly tapered cavity to a desired depth for said aggregate pier while outwardly compacting the sidewalls of the cavity as the cavity is being formed; (b) moving the tapered mandrel upwardly a sufficient distance to permit access to the lower end of the cavity; (c) depositing a layer of aggregate in the cavity; (d) lowering the tapered mandrel downwardly in the cavity so that the flat lower end of the mandrel engages the aggregate in the cavity and densifies the aggregate in the cavity, and displaces a portion of the aggregate laterally into the adjacent wall of the cavity by force applied by the flat lower end and non-tapered edge of the mandrel; and (e) repeating steps (b), (c) and (d) until a pier component of a desired height is formed.
2. The method as described in claim 1 , wherein the compacting of the sidewall of the cavity is sufficient to maintain structural integrity of the cavity sidewalls during steps (b), (c), (d) and (e).
3. The method as described in claim 1 , wherein step (a) is effected by application of vertical vibration energy and vertical static force to the tapered mandrel.
4. The method as described in claim 3 , wherein the vertical vibration energy is provided by a vibratory hammer.
5. The method as described in claim 1 , wherein the tapered mandrel has a plurality of rigid panels flaring upwardly and outwardly above the flat lower end and defining the downward taper of the tapered mandrel.
6. The method of claim 1 , wherein step (d) is effected by application of static force and vertical vibration to the tapered mandrel while in contact with the aggregate.
7. The method of claim 1 , wherein the mandrel is moved a distance in step (b) sufficient to position the flat lower end of the mandrel at or near the top of the cavity.
8. The method of as described in claim 1 , including forming the upper end of the pier subsequent to step (e) by compacting aggregate near the top end of the cavity with a secondary tamping mandrel having a flat lower end surface of greater area than the area of the area of the blunt lower end surface of the downwardly tapered mandrel.
9. The method of claim 8 , wherein the secondary tamping mandrel is a hollow shell including a smaller diameter bottom guide portion and a top cylindrical portion having a diameter exceeding the diameter of the upper end of the downwardly tapered mandrel and wherein the smaller diameter portion is connected to the top portion by an outwardly flared canted portion and the small diameter lower portion has a transverse smaller lower end surface with the diameter of the lower portion being approximately the same as the diameter of the top of the cavity formed by the upper end of the downwardly tapered mandrel.
10. The method of claim 8 , wherein the secondary tamping mandrel has a conical surface facing downwardly to engage the upper end of the previously formed cavity.
11. The method as described in claim 8 , wherein the secondary tamping mandrel is vibrated vertically by a vibratory hammer while concurrently applying static force to the aggregate near the top of the pier.
12. The method as described in claim 8 , wherein the secondary tamping mandrel is a beveled mandrel that is vibrated vertically by a hydraulic hammer while being concurrently urged downwardly by static force.
13. The method as described in claim 1 , wherein the tapered mandrel is a unitary hollow steel shell structure including a plurality of planar panels flaring upwardly and outwardly from and above the flat lower end surface of the mandrel.
14. The method of claim 1 , wherein the tapered mandrel includes a peripheral circular flange at its upper end which extends completely around the top of the mandrel to inhibit upward movement of surficial soil during mandrel penetration to an embedded position.
15. The method of claim 1 , wherein the tapered mandrel includes a tapered lower section and a straight-sided untapered upper section.