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Section 1808 Pier and Pile Foundations
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The following words and terms shall, for the purposes of this section, have the meanings shown herein.
FLEXURAL LENGTH. Flexural length is the length of the pile from the first point of zero lateral deflection to the underside of the pile cap or grade beam.
PIER FOUNDATIONS. Pier foundations consist of isolated masonry or cast-in-place concrete structural elements extending into firm materials. Piers are relatively short in comparison to their width, with lengths less than or equal to 12 times the least horizontal dimension of the pier. Piers derive their load-carrying capacity through skin friction, through end bearing, or a combination of both.
Belled piers. Belled piers are cast-in-place concrete piers constructed with a base that is larger than the diameter of the remainder of the pier. The belled base is designed to increase the load-bearing area of the pier in end bearing.
PILE FOUNDATIONS. Pile foundations consist of concrete, wood or steel structural elements either driven into the ground or cast in place. Piles are relatively slender in comparison to their length, with lengths exceeding 12 times the least horizontal dimension. Piles derive their load-carrying capacity through skin friction, through end bearing, or a combination of both.
Augered-cast-in-place piles. Augered-cast-in-place piles are constructed by pumping grout into an augered hole during the withdrawal of the auger. The pile is reinforced with a single reinforcing bar, a reinforcing steel cage or a structural steel section.
Caisson piles. Steel cased piles are constructed by driving a steel shell to a watertight seal at the top of rock and drilling of an uncased socket within the rock. The shell and socket is filled with a steel core section and concrete or grout.
Compacted Concrete Piles. Compacted concrete piles are constructed by filling a shaft with low strength concrete as the casing is withdrawn.
Concrete-filled steel pipe and tube piles. Concrete-filled steel pipe and tube piles are constructed by driving a steel pipe or tube section into the soil and filling the pipe or tube section with concrete. The steel pipe or tube section is left in place during and after the deposition of the concrete.
Driven uncased piles. Driven uncased piles are constructed by driving a steel shell into the soil to shore an unexcavated hole that is later filled with concrete. The steel casing is lifted out of the hole during the deposition of the concrete. These piles are allowed only if the concrete is placed under pressure.
Enlarged base piles. Enlarged base piles are cast-in-place concrete piles constructed with a base that is larger than the diameter of the remainder of the pile. The enlarged base is designed to increase the load-bearing area of the pile in end bearing. Enlarged base piles include piles installed by driving a pre-cast concrete tip or by compacting concrete into the base of the pile to form an enlarged base.
Steel-cased piles. Steel-cased piles are constructed by driving a steel shell into the soil to shore an unexcavated hole. The steel c asing is left permanently in place and filled with concrete.
Jacked Piles. Steel pipe piles installed by hydraulically jacking the pile into the ground against a dead weight reaction. Piles installed by other static forces shall be considered in this category.
Micro-piles/mini-piles. Small diameter drill ed steel cased piles, driven uncased piles or caisson piles.
Open End Pipe Pile. Steel pipe driven open ended that may or may not be filled with concrete or soil.
Piles are permitted to be designed in accordance with provisions for piers in Section 1808 and Sections 1812.3 through 1812.11 where either of the following conditions exists, subject to the approval of the commissioner:
- Group R-3 and U occupancies not exceeding two stories of light-frame construction, or
- Where the surrounding foundation materials furnish adequate lateral support for the pile.
Pier and pile foundations shall be designed and installed on the basis of a foundation investigation as defined in Section 1802.
The investigation and report provisions of Section 1802 shall be expanded to include, but not be limited to, the following:
1. Recommended pier or pile types and installed capacities.
2. Recommended center-to-center spacing of piers or piles.
3. Driving criteria.
4. Installation procedures.
5. Field inspection and reporting procedures (to include procedures for verification of the installed bearing capacity where required).
6. Pier or pile load test requirements.
7. Durability of pier or pile materials.
8. Designation of bearing stratum or strata.
9. Reductions for group action, where necessary.
Pier and pile foundations shall be designed and installed under the direct control of an engineer knowledgeable in the field of geotechnical engineering and pier and pile foundations and shall be subject to special inspection performed under the direct control of such engineer. The engineer shall certify to the commissioner that the piers or piles as installed satisfy the design criteria.
The use of types of piles not specifically mentioned herein is permitted, subject to the approval of the commissioner, upon the submission of acceptable test data, calculations and other information relating to the structural properties and load capacity of such piles. The allowable stresses shall not in any case exceed the limitations specified herein.
Pile caps shall be of reinforced concrete, and shall include all elements to which piles are connected, including grade beams and mats. The soil immediately below the pile cap shall not be considered as carrying any vertical load. The tops of piles shall be embedded not less than 3 inches (76 mm) into pile caps and the caps shall extend at least 4 inches (102 mm) beyond the edges of piles. The tops of piles shall be cut back to sound material before capping.
Piers or piles shall be braced to provide lateral stability in all directions. Three or more piles connected by a rigid cap shall be considered braced, provided that the piles are located in radial directions from the centroid of the group not less than 60 degrees (1 rad) apart. A two-pile group in a rigid cap shall be considered to be braced along the axis connecting the two piles. Methods used to brace piers or piles shall be subject to the approval of the commissioner.
Piles supporting walls shall be driven alternately in lines spaced at least 1 foot (305 mm) apart and located symmetrically under the center of gravity of the wall load carried, unless effective measures are taken to provide for eccentricity and lateral forces, or the wall piles are adequately braced to provide for lateral stability. A single row of piles without lateral bracing is permitted for one- and two-family dwellings and lightweight construction not exceeding two stories or 35 feet (10 668 mm) in height, provided the centers of the piles are located within the width of the foundation wall.
Piles supporting walls shall be driven alternately in lines spaced at least 1 foot (305 mm) apart and located symmetrically under the center of gravity of the wall load carried, unless effective measures are taken to provide for eccentricity and lateral forces, or the wall piles are adequately braced to provide for lateral stability. A single row of piles without lateral bracing is permitted for one- and two-family dwellings and lightweight construction not exceeding two stories or 35 feet (10 668 mm) in height, provided the centers of the piles are located within the width of the foundation wall.
Piers or piles shall be installed in such a manner and sequence as to prevent distortion or damage to piles being installed or already in place to the extent that such distortion or damage affects the structural integrity of the piles.
Piles shall be spaced to meet the following requirements:
- Spacing of piles shall provide for adequate distribution of the load on the pile group to the supporting soil. In no case shall the minimum center-to-center spacing of piles be less than 24 inches (610 mm) nor less than the following for the specific types of piling indicated in this chapter.
- Minimum spacing between enlarged base concrete piles shall be 4 feet 6 inches (1372 mm), center to center except that where the shafts of such piles are cased for their full length, this spacing may be reduced to 3 feet 6 inches (1067 mm). Where a question exists as to possible damage to adjacent previously driven piles, these minimums shall be increased. Minimum center-to-center spacing of piles at the bearing level shall be at least two and one half times the outside diameter of the shell.
- Unless special measures are taken to assure that piles will penetrate sufficiently to meet the requirements of 1808.2.8 without interfering with or intersecting each other, the minimum center to center spacing of piles shall be twice the average diameter of the butt for round piles, one and three quarters times the diagonal for rectangular piles; or, for taper piles, twice the diameter at a level two thirds of the pile length measured up from the tip.
- In cases of practical difficulty, the spacing of new piles from existing piles under an adjacent building may be less than the above values provided that the requirements relating to minimum embedment and pile interference are satisfied and that the soil under the proposed and existing buildings is not overloaded by the closer pile grouping.
Piles located near a lot line shall be designed on the assumption that the adjacent lot will be excavated to a depth of 10 feet (3048 mm) below the nearest legally established curb level. Where such excavation would reduce the embedded length of the pile, the portion of the pile exposed shall be deemed to provide no lateral or vertical support, and the load carrying determination shall be made the resistance offered by the soil that is subject to potential excavation has been discounted.
Splices shall be constructed so as to provide and maintain true alignment and position of the component parts of the pile during installation and subsequent thereto and shall be of adequate strength to transmit the vertical and lateral loads (including tensions) and the moments occurring in the pile section at the location of the splice without exceeding the allowable stresses for such materials as established in Section 1809. In all cases splices shall develop at least 50 percent of the capacity of the pile in bending. In all cases pile splices situated in the upper 10 feet (3048 mm) of the pile shall be capable of resisting (at allowable working stresses) the applied moments and shears or the moment and shear that would result from an assumed eccentricity of the pile load of 3 inches (76 mm), whichever is greater. For piles located near a lot line, the embedded length of such piles shall be determined on the basis that the adjacent site will be excavated to a depth of 10 feet (3048 mm) below the nearest established curb level as required in Section 1808.2.6.2.
Allowable pier or pile loads shall be determined in accordance with Section 1808.2.8.1 through 1808.2.8.9.
The allowable axial and lateral loads on piers or piles shall be determined by load tests or a recognized method of analysis. The allowable load shall be determined by a licensed engineer experienced in geotechnical engineering and shall be approved by the commissioner.
The allowable axial load on a pile shall be the least value permitted by consideration of the following factors (for battered piles, the axial load shall be computed from the resultant of all vertical loads and lateral forces occurring simultaneously):
- The capacity of the pile as a structural member.
- The allowable bearing pressure on soil strata underlying the pile tips.
- The resistance to penetration of the piles, including resistance to driving, resistance to jacking, the rate of penetration, or other equivalent criteria.
- The capacity as indicated by load test, where load tests are required.
- The maximum loads prescribed in Section 1808.2.8.1.3.
The allowable pile load shall be limited by the provision that the pressures in materials at and below the pile tips, produced by the loads on individual piles and by the aggregate of all piles in a group or foundation, shall not exceed the allowable bearing values established in Table 1804.1. The transfer of load from piles to soil shall be determined by a recognized method of analysis. As an alternative, for purposes of this section, piles or pile groups may be assumed to transfer their loads to the underlying materials by spreading the load uniformly at an angle of sixty degrees with the horizontal, starting at a polygon circumscribing the piles, located as follows:
- For piles embedded entirely in decomposed rock or granular soils, or in controlled fill materials, the polygon shall be circumscribed at a level located two-thirds of the embedded length of the pile, measured up from the tip.
- For piles penetrating through silts and clays into bearing in decomposed rock or granular soils, the polygon shall be circumscribed at the bottom of the strata of silts or clays.
The plans for the proposed work shall establish, in accordance with the requirements relating to allowable bearing pressure, the bearing stratum to which the piles in the various sections of the building must penetrate and the approximate elevations of the top of such bearing stratum. Where penetration of a given distance into the bearing strata is required for adequate distribution of the loads, such penetration shall be shown on the plans. The indicated elevations of the top of the bearing strata shall be modified by such additional data as may be obtained during construction. All piles shall penetrate to or into the designated bearing stratum.
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