1811.5 PILE FOUNDATIONS

1811.5.1 Design

1811.5.1.1 All pile foundations shall be designed by a Louisiana Registered Civil Engineer. Piles shall be designed to include all applied weights and forces including the weight of mats or caps and the weight of earth or fill on top of the mats or caps considering the effect of buoyancy; the weight of the piles shall be given due consideration in the foundation design. In cases where piles are loaded eccentrically, provisions shall be made for such eccentricities. Further analyses should be performed for laterally loaded piles or piles subjected to dynamic loading (machinery, etc.). Consideration also should be given in design to the influence of negative skin friction (downdrag) on foundation performance.

1811.5.1.2 Except for lightly loaded piles (as defined elsewhere in Section 1811) where the minimum spacing is 2 ft., the minimum pile spacing for all other piles shall be either 3 ft., or three pile diameters (widths), or as determined by the expression: 

SPAC =0.05 (L1 ) + 0.025 (L2) + 0.0125 (L3), in which: 
SPAC = Center-to center spacing of piles, ft. 
L1 = Pile penetration up to 100 ft. 
L2 = Pile penetration from 101 to 200 ft. 
L3 = Pile penetration beyond 201 ft. 

1811.5.1.3 Greater spacing than the minimum value may be required to satisfy group perimeter shear as provided in 1811.5.3.1 and to assure that piles will not interfere with or intersect each other during installation.

1811.5.1.4 Piles extending above the ground surface, surrounded by relatively weak soil, or standing in free water, that are used as load-carrying members, shall be designed structurally in their unrestrained length in accordance with applicable column formulas as referenced in the IBC; due consideration to effective unsupported length must be given. Location of pile fixity shall be determined with due regard to soil properties. 

1811.5.1.5 Where subsurface investigation records or site conditions indicate possible deleterious action of pile materials because of soil constituents, changing water levels, electrolysis, corrosion, or other factors, such materials shall be adequately protected. The effectiveness of such methods of processes for the particular purpose shall have been thoroughly established by satisfactory service records or other evidence that demonstrates the effectiveness of such protective measures. Cutoff of untreated timber piles shall not be higher than the lowest groundwater level anticipated for the life of the structure as determined by an engineering investigation based on established data, but in no case shall the cutoff be less than 7 ft. below the natural ground surface. Structural steel piles driven below the permanent water table and into natural soil need not be protected against electrolysis and/or corrosion unless previous site history or field conditions warrant otherwise. 

1811.5.1.6 The design and installation of pile foundations shall be under the direct supervision of a Louisiana Registered Civil Engineer who shall certify to the Director that the piles, as installed, satisfy the design criteria.

1811.5.1.7 Pile foundations shall be designed and installed on the basis of a geotechnical investigation and report which shall include soil borings and laboratory tests or other subsurface exploration at locations and depths sufficient to determine the position and adequacy of the bearing soils except where sufficient data upon which to base the design and installation is available. The investigation and report shall include, but not be limited to, the following:

1. Descriptions of the soil and groundwater conditions.
2. Recommended pile types and predicted capacities.
3. Pile installation criteria.
4. Field observation procedures.
5. Probe piles and pile load test requirements.
6. Designation of bearing stratum or strata.
7. Estimated movements (settlement, heave, etc.) 
8. Other applicable installation and performance considerations such as: lateral capacity, dynamic loads, vibrations, downdrag, fill placement, groundwater lowering, etc. 

1811.5.1.8 Allowable stresses greater than those specified for each pile type in this Code may be permitted when supporting data justifying such higher stresses are filed with the Director.

1811.5.2 Pile Load Capacity for a Single Pile

1811.5.2.1 Pile Load Test Case. The pile load testing procedure for a single pile shall be conducted in general accordance with ASTM D 1143 under the direction of a Louisiana Registered Civil Engineer. The load-carrying capacity for a single pile in compression shall be established by any of the following methods:

Method 1: When the total test load applied equals twice the proposed load capacity for a single pile, it shall be left in place at least 48 hr., the last 24 hr. of which shall be free from settlement. If after deducting rebound following the total release of this load, the net settlement at the top of the pile does not exceed 0.01 in. per ton of total test load, the pile shall be considered adequate to support the proposed load capacity for a single pile.

Method 2: Where the pile is loaded to failure, the loading value where the load settlement curve begins to show an accelerated deviation from its previous trend may be considered the “yield point” for the given soil and pile. One-half of this value or one-half of the value at which the net settlement is estimated as ½ in., whichever is smaller, shall be considered as the allowable load capacity for a single pile.

Method 3: The failure load of a pile, one-half of which shall be used as the allowable load capacity for a single pile, shall be defined as that load which produces a settlement or movement of the tip equal to ¼ inch. The movement of the tip can be determined by actual measurement or by either of the following two load distribution cases, both involving the measurement of the butt movement. In each case, the failure load shall be defined as that load producing a movement of the butt equal to the elastic deformation of the pile under the failure load plus ¼ inch.

1811.5.2.2 Arbitrary Load Distribution Case. In determining the elastic deformation of the pile, the assumption may be made that the soil-pile friction value is constant from tip to butt and that tip bearing is ignored. For a prismatic pile of one material, the butt movement shall be calculated by the expression: 

Df = (PfL /AE) + 0.25 in., in which: 
Df = Butt movement, in. 
Pf = Failure load, kips 
L = Pile length, in. 
A = Pile cross-sectional area, sq. in. 
E = Modulus of elasticity of pile material, ksi.

1811.5.2.3 Actual Load Distribution Case. In determining the elastic deformation of the pile, the actual distribution of stress into the soil may be used provided that sufficient soil data have been obtained.

1811.5.2.4 A variation of the load capacity of a load-tested pile shall be allowed without additional load tests provided that the load capacity is derived primarily from skin friction. The change in load capacity for a single pile may be increased by not more than 20% with an increase in length of 20%. A decrease in length of not more that 10% may be accompanied with a decreased in capacity of 20%. Such stipulation shall be made in writing by a Louisiana Registered Civil Engineer experienced in geotechnical engineering if the pile foundation will be subject to downdrag forces, lateral loads, vibration loads, etc.

1811.5.2.5 Where the number of piles required for a structure is 100 or less, the load per pile does not exceed 150 psf of embedded pile area, and the actual pile penetration into the supporting soil is not less than 40 ft., the Director may waive, upon specific request, the load test requirements. The basis for such a waiver shall be data developed by the geotechnical investigation and geotechnical engineering analysis (see 1811.2 and 1811.5.1.7). The allowable load on a single pile shall not exceed the value determined by the geotechnical engineering analysis.

1811.5.2.6 The Director may, upon specific request, accept as adequate proof of the load capacity of a single pile, load test results from other piles subject to all of the following conditions.

1. The previously load-tested pile must be located within 300 ft. (measured in any direction) from the new pile in question, but not to exceed 500 ft. to the farthest footprint of the building or structure.
2. The load test(s) must have been performed on the same type and length of piles that are to be used at the new location.
3. Adequate soil information (see 1813.2 and 1811.5.1.7) establishes the geotechnical similarity of the soils at each location. The variations described in 1615 1811.5.2.4 shall not be permitted in conjunction with this provision. 

1811.5.3 Pile Group Capacity

1811.5.3.1. The supporting value of piles depending primarily on friction when driven in clusters or groups may be investigated on the basis of group perimeter shear by the expression:

Qa = (PLc / FSF)+ \ 2.6qu (1 + 0.2 w/b) ] / FSB, in which:
Qa = Allowable load-carrying capacity of pile group, lb.
P = Perimeter distance of pile group, ft.
L = Length of pile, ft.
C = Average (weighted) cohesion or shear strength of material between 1624 the surface and the depth of the pile tip, psf.
u = Average unconfined compressive strength zone below pile tips, psf; the thickness of the zone shall consider the pile group size and shape, as well as the soil stratigraphy and soil properties.
W = Width of base of pile group , ft.
B = Length of base of pile group, ft.
A = Base area of pile group, sq. ft.
FSF = Factor of safety for the group friction area = 2 
FSB = Factor of safety for the group base area = 3 

1811.5.3.2 The soil properties (c and qu ) used in the above formula shall be based on data obtained from the geotechnical investigation as described in 1811.2 and 1811.5.1.7. In the application of this formula, the weight of the piles, pile caps and mats, considering the effect of buoyancy, shall be included.