Each pile anchor includes an elongated pile anchor post-tensioning element, preferably a bolt or tendon 26 , that extends through a pile anchor base plate 32 on the top surface of or preferably grouted into the concrete cap 10 , then through the concrete cap 10 , and finally into a drilled pile hole 34 that is filled with pile anchor cementious material 36 to secure the pile anchors in the ground or soil The embedded portion of the tendon or bolt 26 includes a lower end 38 and an upper end, generally designated by the reference numeral The lower end 38 of the bolt is bare, i.
The cementious material 36 preferably fills the pile holes 34 to the bottom 82 of the excavation area. An end nut 42 may be provided on the lower end of the bolt 26 see, for example, FIGS. The upper end 40 of the embedded portion of the bolt 26 is encased in an elongated hollow tube, preferably in a plastic sleeve 44 or the like, and most preferably by PVC tubing, along a major upper portion of its length, to prevent bonding with the cementious material 46 of the concrete foundation cap 10 and the pile anchor cementious material 36 and to allow for post-tension stretching.
A centralizer 84 is preferably mounted around the lower portion 38 of the anchors 26 so as to position the pile anchor bolt 26 centrally within the pile hole As stated previously, the hollow tubes 15 and plastic sleeve 44 for encircling or encasing the anchor bolts 14 and the elongated pile anchor bolts 26 , respectively, are preferably made of PVC tubing.
The plastic sleeves or tubing shield the bolts and prevent them from adhering to the cementious material. As such, the bolts can be tensioned after the cementious material has hardened and cured in order to post-tension the pile anchors and the foundation cap of the present invention.
Alternately, the bolts can be wrapped in plastic tape, or otherwise sheathed, to prevent the bolts from adhering to the cementious material during curing and allow the bolts to stretch freely under tension over the entire sheathed length of the bolts.
After the cementious material 36 has been poured into the drilled pile holes 34 to fix the pile anchor tendons or bolts 26 in the ground , a void or highly compressible area 54 is formed between the top of the pile anchor cementious material 36 and the adjacent lower surface 52 of the cementious material 46 of the concrete foundation cap The void 54 is preferably formed using a compressible including crushable spacer or void forming element generally designated by the reference numeral 50 , which is inserted between the top of each filled pile hole 34 and the adjacent lower surface 52 of the cap 10 to be formed.
One embodiment of the void forming element 50 is representatively shown in FIGS. The void forming element defines a void or hollow area 54 above each pile anchor 12 and is provided with a generally circular aperture 56 through which the sleeved tendon or bolt 26 extends before passing through the cap The void forming element 50 is made to slide down the bolt 26 to sit on the bottom of the excavation area over the top of each filled pile anchor cementious material Alternatively, the void forming element 50 can be constructed as a hollow disc or as a compressible disc, such as a disc made of expanded polyurethane or of styrofoam.
The element 50 can be virtually any natural or man-made material that is highly compressible or crushable under 10 psi pressure or greater and which allows the concrete cap foundation 10 to be pulled downwardly compressing and consolidating the underlying soils to the required bearing strengths and allowing the pile anchors 12 to be pulled upwardly to develop the skin friction resistance equal to the pile anchor bolt or tendon post-tension.
The void forming element 50 may also be constructed as an inflatable or pressurized bladder which will allow the pile anchor 12 to be pulled upwardly and the foundation cap 10 be pulled downwardly by tensioning the anchor bolts As a further construction, the void forming element 50 can be made of a material that will develop great compressive strength when contacted with a catalyst after tensioning the anchor bolt or tendon.
This embodiment includes materials in which the development of such compressive strength can be retarded for days. As further shown in FIGS. According to one preferred embodiment, this coupling component 64 may be embodied as a piece of PVC pipe approximately four inches in diameter and two inches in length.
According to the embodiment shown in FIGS. These smaller tube couplers 66 communicate with the hollow space 54 created by the void forming element 50 and are each attached to a grout tube 68 , one tube acting as an inlet and the other tube acting as an outlet.
The grout tubes 68 extend upwardly from the tube couplers 66 along the length of the sleeved bolt or tendon 26 to its uppermost end. Following post-tensioning, grout or other cementious material may be forced into the inlet grout tube to fill any remaining void space not eliminated by the crushing of the void form.
When grout is forced through the inlet tube to the void space and begins to exit from the grout outlet tube, this indicates that any remaining void space has been filled. This grout tube construction is optional, however, and is not necessary to the effectiveness of the present invention. The uppermost end of the tendon or bolt 26 which protrudes from the top of the cap 10 is fitted with a pile anchor base plate 32 and a post-tensioning nut 70 is threaded onto the tendon or bolt to post-tension the pile anchor 12 and the concrete cap 10 after the cementious material 46 of the cap has hardened.
The void created by the void forming element 50 is compressed and element 50 is crushed by the post-tensioning, allowing the pile anchor 12 to pull upwardly until skin friction resistance with the surrounding soils equaling the required tendon tension is achieved. The required bolt or tendon tension exceeds the maximum structure uplift load determined for each pile anchor. The steps undertaken to form the completed foundation of FIG.
As shown in FIG. Representatively, this area 80 has a depth of about 4 feet. Within the excavation area 80 and starting from the bottom 82 thereof, a plurality of spaced pile holes 34 are drilled or driven. These pile holes 34 typically have a diameter of about inches and a depth from about 30 feet to about 50 feet. In the representation illustrated in the drawings, the pile holes are 24 inches in diameter and 40 feet deep, and twenty pile holes 34 are formed.
After the pile holes 34 are formed, pile anchor bolts or tendons 26 are inserted therein. The pile anchor bolts or tendons 26 are preferably fitted with centralizers 84 to maintain their position in the center of the pile holes in preparation for the pouring of the cementious grout or material therein.
When the tendons or bolts 26 have been centered in the pile holes 34 , cementious material 36 is poured or pumped therein up to the bottom 82 of the excavation area Alternatively, the bolts 26 and centralizers 84 can be inserted after the cementious material 36 is in the pile holes After the cementious material 36 for the pile anchors has hardened, the void forming element or plastic void form 50 is inserted over the top of the bolt 26 and positioned over the top of the anchor pile cementious material 36 at the bottom 82 of the excavation area The next step in forming the foundation is illustrated in FIG.
With the void forming elements 50 in place, a concrete leveling course 86 is laid on the bottom 82 of the excavation area In the representative embodiment illustrated, the leveling course 86 is approximately four inches in depth to correspond with the height of the void forming element 50 as shown. Of course, different thicknesses of the leveling course 86 can be used to accommodate void forming elements of different thicknesses.
Preferably, the top surface of the void forming elements 50 should be substantially flush with the upper surface 88 of the leveling course In the representative embodiment illustrated, the cap is 5 feet thick. Following concrete pour and cure of the concrete 46 , the pile anchor base plates 32 are installed over the pile anchor bolts 26 atop or preferably grouted into the concrete foundation cap 10 and the post-tensioning nuts 70 are lifted by jacking, or torqued by threading snugly against the pile anchor base plates 32 , during the post-tensioning of the pile anchor bolts 26 , as illustrated in FIG.
Finally, if a void forming element 50 with the tube coupler 66 and grout tube 68 construction has been used, pressurized grout can be forced through the inlet grout tube 68 and into any remaining void areas, as at in FIG. Turning now to the embodiment of the present invention illustrated in FIGS. Since the components of pile anchor foundation of the second embodiment are identical or very similar to corresponding embodiments of the first embodiment illustrated in FIGS.
There are several differences as described below First, rather than utilizing hoops for the reinforcing steel rebar 24 as shown in FIG. The lower rebar bolts or headed rebar rest on top of the embedment ring and are secured to the sleeve of anchor bolts by rebar hoops, as will be readily understood by those skilled in the art. Similarly, the upper reinforcing steel rebar bolts or headed rebar are secured to the tower anchor bolts and , and to sleeve of pile anchor bolt by similar rebar hoops.
Second, it has been found that the central portion of the foundation cap does not have to be the full cap thickness as shown at According to clause 7. Static load test is the best way of verifying the load-carrying capacity of piles, however, it is not very attractive because it is expensive and time-consuming.
Traditionally, engineers have designed pile foundations based on calculations from theoretical soil mechanics. The commonest approach is to divide the soil into layers and assign soil properties to each layer. These two properties will enable the quick determination of the bearing capacity factors for evaluation of the load-carrying capacity of the pile. From the soil profile, the shaft friction on the pile from different layers is summed up to obtain the total shaft friction resistance of the pile.
The base resistance of the pile is also obtained based on the soil properties of the layer receiving the tip of the pile. A b is the cross-sectional area of the base of the pile while q b is the base resistance. On the other hand, the typical equations for obtaining the base resistance of a single pile are given below;. The factor of safety usually varies between 2.
EN allows the resistance of individual piles to be determined from;. Where F c,d is the design axial load on the pile, while R c,d is the compressive resistance of the pile. F c,d should include the weight of the pile itself, and Rc,d should include the overburden pressure of the soil at the foundation base. However, these two items may be disregarded if they cancel approximately. They need not cancel if the downdrag is significant, or when the soil is very light, or when the pile extends above the ground surface.
For piles in group, the design resistance shall be taken as the lesser of the compressive resistance of the piles acting individually, and the compressive resistance of the piles acting as a group block capacity. Methods for assessing the compressive resistance of a pile foundation from ground test results shall have been established from pile load tests and from comparable experience.
Generally, the compressive resistance of the pile shall be derived from;. The values of the partial factors may be set by the National annex. The values of the correlation factors may be set by the National annex. The recommended values are given in Table A10 of EN To estimate pile shaft friction and end bearing from ground parameters, the following relationships may be applied;. For piles in clay, N c is usually taken as 9. The procedure for determining the compressive resistance of a pile from static load tests is based on analysing the compressive resistance, R c,m , values measured in static load tests on one or several trial piles.
The trial piles must be of the same type as the piles of the foundation, and must be founded in the same stratum. An important requirement stated in Eurocode 7 is that the interpretation of the results of the pile load tests must take into account the variability of the ground over the site and the variability due to deviation from the normal method of pile installation.
In other words, there must be a careful examination of the results of the ground investigation and of the pile load test results. These are also called as uplift piles or anchor piles. In those areas where there are chances of extraction of piles from the ground, the uplift piles will work well. Foundations are broadly classified as deep and shallow foundations based on the load carrying capacity and the properties of the soil at the site. The most widely used type — Pile foundations, come under the category of deep foundations.
Any foundation structure that have a depth greater than three times the breadth of the structure can be categorized as pile foundations. Pile foundations form slender and columnar structures intended to transfer mainly compressive loads from large superstructures.
The below-transferring medium may be either weak soil or a stratum that is compressible or a strong rock stratum like structure. Lings, U. Bristol, and U. Soil Mech. SM2, — Lehane, R. Jardine, A. Bond, and R. Jardine, R. Overy, and F. Alawneh, Tension piles in sand: a method including degradation of shaft friction during driving, Transportation Research Board No.
Randolph and C. GT12, — Armaleh and C. Mansur and A. SM5, — Download references.
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