BEARING AND FRICTION DESIGN CAPACITY FOR HELICAL ANCHORS AND PILES


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1 BEARING AND FRICTION DESIGN CAPACITY FOR HELICAL ANCHORS AND PILES Original Author: Editor: Gary L. Seider, P.E., Hubbell Power Systems, Inc./CHANCE, Centralia, MO, USA Wayne G. Thompson, PE, CTL Thompson, Fort Collins, CO BEARING CAPACITY METHODOGOLY General The capacity of a helical anchor/pile is dependent on the strength of the soil, the projected area of the helix plate(s), and the depth of the helix plate(s) below grade. The soil strength can be evaluated by use of various techniques and theories (Clemence et al, 1985). The projected area is controlled by the size and number of helix plates. For a given helix depth, two modes of soil failure may occur: shallow and deep. The terms shallow and deep refer to the location of the bearing plate with respect to the earth s surface. By definition, shallow foundations in tension exhibit a brittle failure mode with general eruption of soil all the way to the surface and a sudden drop in load resistance to almost zero. With deep foundations in tension, the soil fails progressively, maintaining significant postultimate load resistance, and exhibits little or no surface deformation. The dividing line between shallow and deep foundations has been reported by various researchers to be between three and eight times the foundation diameter. It is generally assumed that the soil failure mechanism will follow the theory of a general bearing capacity failure. This theory suggests that the capacity of a helical anchor/pile is equal to the sum of the capacities of the individual helix plates. The helix capacity is determined by calculating the unit bearing capacity of the soil at each helix and then multiplying the result by the individual helix s projected area. Friction along the central shaft is typically not used to determine capacity, but may be included when the central shaft is round pipe shaft (Type RS) and at least 31/2 inch (89 mm) in diameter. See figure 1 for a graphical representation of a compression application. A necessary condition for this method to work is that the helix plates be spaced far enough apart to avoid overlapping of their individual pressure bulbs, i.e., stress zones in the soil. This will prevent one helix from significantly influencing the performance of another. A preferred spacing between any two helical plates on a helical anchor/pile is three times the diameter of the lower helix. This is consistent with the findings of others (Bassett, 1977) for multibelled concrete piers. For example, the distance between a 10 inch (254 mm) and 12 inch (305 mm) helix is three times the diameter of the lower helix, or 10 x 3 = 30 inches (762 mm). The following is Terzaghi s general bearing capacity equation, which allows determination of the ultimate capacity of the soil. This equation and its use form the basis of determining helix capacity by the software. Q ult = A h ( CN c + q'n q 'B w N ) Q ult = Ultimate capacity of the soil C = Soil cohesion q' = Effective overburden pressure Bw = Footing width (base width) Nc, N q and N are bearing capacity factors. The following is quoted from (Bowles, 1988) where the various terms of the bearing equation are discussed:
2 1) The cohesion term (CN c ) predominates in cohesive soils. 2) The depth term (q'n q ) predominates in cohesionless soils. Only a small D (vertical depth to footing or helix plate) increases Qult substantially. 3) The base width term (0.5 'BwN ) provides some increase in bearing capacity for both cohesive and cohesionless soils. In cases where B w < 9.8 ft. to 13.1 ft. (3 m to 4 m), this term could be neglected with little error. The base width term of the bearing equation is not used since Bw for helical anchors/piles is small (< 16 (0.4 m)) and the resulting bearing capacity of that term would also be quite small. The effective overburden pressure (q', of consequence for cohesionless soils) is the product of depth and the effective unit weight of the soil. The water table location may cause a reduction in the soil bearing capacity. The effective unit weight of the soil is its insitu unit weight when it is above the water table. However, the effective unit weight of soil below the water table is its insitu unit weight less the unit weight of water. The case of a fluctuating water table when a helical anchor/pile installation is terminated above the water table with the likelihood that the water table will rise with time to be above the helix plates requires special considerations. In this situation, the helical plate configuration and depth should be determined with the water at its highest anticipated level. Then the capacity of the same helical anchor/pile should be determined in the same soil with the water level below the plate configuration, which will typically produce higher load capacities and a more difficult installation, i.e., it will require more installation torque. It is sometimes the case that a larger helical anchor/pile product series, i.e., one with greater torque capacity, must be used in order to facilitate installation into the dry conditions. This method sums the individual bearing capacity of each helix on a multihelix anchor/pile to determine the total ultimate multihelix anchor/pile capacity (Q t ) provided that the helix spacing along the shaft is 3 helix diameters. Q t = Q h Q t = Total ultimate multihelix anchor/pile capacity Q h = Individual helix capacity The ultimate capacity of an individual helix is evaluated using the following equation. An upper limit for this capacity is based on the helix mechanical strength. Q h = A h (CN c + q'n q ) Q s Q s = Capacity upper limit, determined by the helix mechanical strength Cohesive Soil Determination of helix ultimate bearing capacity in a cohesive or finegrained soil is accomplished by using the following equation. Q h = A h CN c = A h C9 C = Cohesion N c = Bearing capacity factor for cohesive component of soil = 9 The N c bearing capacity factor when applied to helical anchors/piles is often taken as equal to 9, as it is in other deep foundation applications, The design engineer has the option of overriding the default clay bearing capacity factor of 9. NonCohesive Soil Determination of helix ultimate bearing capacity in a noncohesive or granular soil is accomplished by using the following equation. Q h = A h q'n q = A h 'dn q d = Vertical depth to helix plate N q = Bearing capacity factor for noncohesive component of soil ' = Effective unit weight of the soil
3 The bearing capacity factor N q is dependent on the angle of internal friction ( ) of the cohesionless soil. When a value for the friction angle is provided the graph (N q vs ) in Figure 2 can be used to determine the value of N q. = Angle of internal friction N = Blow count per ASTM D 1586 Standard Penetration Test The estimated value of the angle of internal friction from the above relationship and Figure 2 can be used to estimate the bearing capacity factor N q. Mixed or c  Soil Determination of helix ultimate bearing capacity in a mixed soil is accomplished by using the following equation. Q h = = A h (CN c + q'n q ) A h (CN c + 'dn q ) Angle of Internal Friction Figure 2 The curve given in Figure 2 is adapted from work by Meyerhof (1976). It is based on the following equation, which is Meyerhof s N q values divided by two for long term applications. N q = 0.5 ( 12 x ) /54 N q = Bearing capacity factor for noncohesive component of soil = Angle of internal friction When the angle of internal friction is not known, but blow counts from ASTM D 1586 Standard Penetration Tests are available, then the following relationship can be used to estimate the angle of internal friction. This relationship is based on empirical data provided by Bowles (1968). Again, it is suggested that results be used with caution. The terms in this equation are as defined in above sections. The use of this equation is fairly uncomplicated when accurate values are available for both the cohesion and friction terms of the equation. However, it is recommended that another approach be taken when accurate values are not available for both terms of the equation. One suggestion is to first consider the soil as cohesive and determine capacity. Then consider the same soil as cohesionless and determine capacity. Finally, take the lower of the two results and use that as the soil bearing capacity. APPLICATIONS Compression The Total Ultimate Helix Compression Capacity (Q bc ) for a helical pile is calculated as the sum of the individual helix capacities (Q h ). Individual helix capacity in soil is determined by calculating the bearing capacity at each helix. The Recommended Total Ultimate Compression Capacity (Q hc ) is the total ultimate compression capacity of the helical pile such that no individual helix exceeds the mechanical strength (Q s ) of the helix plate. In soft or loose soil conditions (Nvalue of less than 4), columnar buckling of the shaft may control the total ultimate compression capacity. = 0.28 N Tension The Total Ultimate Helix Tension Capacity (Q bt ) for a helical anchor is calculated the same as for
4 a helical pile, except the individual helix capacity (Q h ) is determined by calculating the bearing capacity above the helix. The Recommended Total Ultimate Tension Capacity (Q ht ) is the total ultimate tension capacity of the helical anchor such that no individual helix exceeds the mechanical strength (Q s ) of the helix plate. Alternate Approach Another common design theory is to take the bearing area of one helix, and then add the shear capacity of a cylinder of soil between top and bottom helix plates. [Editor s note: This section is a late addition and will need to be expanded upon depending on group feedback] INSTALLATION TORQUE A simple, industry accepted and reasonably accurate method to estimate the ultimate capacity of helical anchors and piles is called installation torque vs. capacity correlation, an empirical method originally developed by the A.B. Chance Company over 40 years ago. Precise definition of the relationship for all possible variables remains to be achieved. However, simple empirical relationships have been used for a number of years. The principle is: As a helical anchor/pile is installed (screwed) into increasingly denser soil, the resistance to penetration, called installation torque, is measured. The higher the resistance encountered, the higher the torque required, and therefore, the higher the axial capacity. The following torque/capacity relationship is cited by Hoyt and Clemence (1989): Qu = Kt x T Qu = Ultimate uplift capacity (lb. (kn)) Kt = Empirical torque factor (ft 1 (m 1 )) T = Average installation torque (lb.ft. (knm)) The value of Kt may range from 3 to 20 ft 1 (10 to 66 m 1 ), depending on soil conditions and anchor/pile design (principally the shaft size and shape). FRICTION CAPACITY METHODOLOGY The following section describes the theories generally used to calculate the theoretical capacity of piles due to friction along the shaft. In the case of slender shaft (less than 3 inch [76 mm] helical anchor/piles, friction capacity along the shaft is generally not calculated. However, friction capacity is significant enough to calculate when the shaft size exceeds 31/2 inch (89 mm). The general equation is: Q f Where: B f s L f = [ Bf s L f ] = Diameter of steel, or concrete pile column = Sum of friction and adhesion between soil and pile = Incremental pile length over which B and f s are taken as constant There are several empirical methods available to calculate skin friction along a pile shaft. Two design methodologies are explained below: Gouvenot Method Gouvenot reported a range of values for skin friction of concrete/grout anchors and micropiles both under pressure and without pressure based on a number of field load tests. Helical piles with grouted shafts are installed without pressure. The soil conditions are divided into three categories based on friction angle ( ) and cohesion (C). The equations used to calculate f s are: Type I: Sands and gravels with 35 < < 45 and C = 0: f s = o tan Where: o = Mean normal stress for the concrete/grout column Type II: Mixed soils; fine loose silty sands with 20 < < 30 and sandy clays with 205 lb/ft 2 < C < 1024 lb/ft 2 (9.8 kpa < C < 49 kpa): f s = o sin + C(cos ) Type III: Clays and marls with 1024 lb/ft 2 < C < 4096 lb/ft 2 (49 kpa < C < 196 kpa) f s = C Where: 1024 lb/ft 2 < C < 2048 lb/ft 2 (49 kpa < C < 98 kpa) And f s = 2048 lb/ft 2 (98 kpa) Where: 2048 lb/ft 2 < C < 4096 lb/ft 2 (98 kpa < C
5 < 196 kpa) The Gouvenot Method assumes a uniform shaft diameter for each soil layer. Department of the Navy Design Manual 7 Method The Navy method is well known in the foundation industry. It provides a simplified, straightforward approach to pile foundation design. For cohesive soils ( Method): Table 3. Skin Friction Resistance Values for Concrete Piles Noncohesive Soils P o Angle of Internal Friction (degrees) (psf) S = Average Friction Resistance on Pile Surface (psf) f s = C a Where: C a = Adhesion factor Navy Design Manual 7 provides the following table as recommended values of adhesion to determine ultimate load capacity of piles for cohesive soils. Table 2. Navy Method Recommended Values for Adhesion Cohesive Soils PILE TYPE Soil Consistency Cohesion, C (psf) Adhesion, C a (psf) Timber or Very Soft Concrete Soft Medium Stiff Stiff Very Stiff Steel Very Soft Soft Medium Stiff Stiff Very Stiff For Noncohesive soils: The engineer can use the Alternate Method from the 1974 Edition of DM 7 per the following equation. Q f = [ B(S) L f ] Where: S = Average friction resistance on pile surface area = K(P o tan ) where K is assumed to equal 1 P o = Average overburden pressure on element L f K = Coefficient of lateral earth pressure = Angle of internal friction The tables below provide the average friction resistance (S) on straight concrete and steel piles in noncohesive soils. Table 4. Skin Friction Resistance Values for Steel Piles Noncohesive Soils P o Angle of Internal Friction (degrees) (psf) S = Average Friction Resistance on Pile Surface (psf) Values shown are 75% of the values given for straight concrete piles in Table 3 Tables 3 and 4 are derived from graphs in the Dept. of Navy Design Manual 7 (1974). A later edition of DM 7 (1986) limits the depth to 20B at which the average overburden pressure is assumed to increase, where B is the diameter of the pile. This limit was based on experimental and field evidence as cited in a previous section of this paper. In the case of slender shaft helical anchor/piles, the 20B limit is probably conservative, but it is not clear what a more practical limit should be. REASONABILITY CHECKS General Consideration should be given to the validity of the values obtained when determining the bearing capacity of the soil. The calculated theoretical ultimate capacity is no better than the data used to obtain that value. Data from boring logs, the water table depth, and load information may not accurately represent actual conditions where the helical anchor/pile must function. Empirical values that are used and estimates of strength parameters, etc. that must be made
6 because of lack of data affect the calculated bearing capacity value. In those situations where soil data is insufficient or not available, a helical trial anchor/pile probe can help determine such items as, location of bearing strata, location of soft/loose soil, and the presence of obstructions, such as, cobbles, boulders, and debris. Helical Anchor/Pile Spacing Once the capacity of the helical anchor/pile is determined, it is important to review the location of the foundation element with respect to the structure and to other helical anchors/piles. It is recommended that the centertocenter spacing between adjacent anchors/piles be no less than five times the diameter of the largest helix. The minimum spacing is three feet (0.91 m). It is suggested that this latter spacing be used only when the job can not be accomplished in any other way and should involve special care during installation to ensure that the spacing does not decrease with depth. Minimum spacing requirements apply only to the helix bearing plate(s), i.e., the anchor/pile shaft can be battered to achieve minimum spacing. Spacing between the helical anchors/piles and other foundation elements, either existing or future, requires special. Group effect, or the reduction of capacity due to close spacing, has never been accurately measured with helical piles. However, bearing capacity theory would indicate that capacity reduction due to group effect is possible. Factor of Safety The equations discussed above are used to obtain the ultimate capacity of a helical anchor/pile. An appropriate Factor of Safety must be applied to reduce the ultimate capacity to an acceptable design (or working) capacity. It is the responsibility of the user to determine the Factor of Safety to be used. In general, a minimum Factor of Safety of 2 is recommended. For tieback applications, the Factor of Safety typically ranges between 1.25 and 2. Project specific conditions may warrant the use of higher factors of safety. DISPLACEMENT Lateral and vertical displacement of helical piles is an area that is constantly being studied. Some pile manufacturers have test data indicating vertical and horizontal pile deflection in various soil types. In some case, field load testing is still the best way to get an accurate representation of a pile s response to load. [Editor s note: This section is also a late addition and will need to be expanded upon depending on group feedback] REFERENCES BASSETT, 1977, Reference details were not available at the time of this writing. CLEMENCE, S. P., Editor, Contributing Authors ADAMCZAK, Jr., S., CLEMENCE, S. P., DAS, B. M., FINN, M., HOWARD, R. E., KULHAWY, F. H., MITSCH, M. P., MOONEY, J. S., RAPOPORT, V., VALDES, J. A., YOUNG, A. G., 1985, Uplift Behavior of Anchor Foundations in Soil, American Society of Civil Engineers, New York City, New York. BOWLES, J. E., 1968, Foundation Analysis and Design, First Edition, McGrawHill, New York City, New York. BOWLES, J. E., 1988, Foundation Analysis and Design, Fourth Edition, McGrawHill, New York City, New York. MEYERHOF, G. G., 1976, Bearing Capacity and Settlement of Pile Foundations, Journal of the Geotechnical Engineering Division, Proceedings of the American Society of Civil Engineers, Volume 102, No. GT3, New York City, New York. US NAVY DESIGN MANUAL DM7, NAVFAC, 1974, Foundations and Earth Structures, Government Printing Office, Washington, DC. US NAVY DESIGN MANUAL DM7, NAVFAC, 1986, Foundations and Earth Structures, Government Printing Office, Washington, DC. CHANCE CIVIL CONSTRUCTION TECHNICAL DESIGN MANUAL, 2007, published by the Chance Division of Hubbell Power Systems, Inc. (Bulletin # ), Centralia, MO. HOYT, R. L. and CLEMENCE, S. P., 1989, Uplift Capacity of Helical Anchors in Soil, Proceedings of the 12 th International Conference on Soil Mechanics and Foundation Engineering, Rio de Janeiro, Brazil.
7 CADDEN, ALLEN, GOMEZ, JESUS, 2002, "Buckling of Micropiles", ADSCIAF Micropile Committee, Dallas, TX., ADSC, Dallas, TX. GOUVENOT, D., (1973), Essais En France et a L Etranger sur le Frottement Lateral en Fondation: Amelioration par Injection, Travaux, 464,Nov, Paris, France. VICKARS, R. A., VICKARS, J. C. T., TOEBOSCH, GARY, United States Patent 5,707,180, Method and Apparatus for Forming Piles InSitu, US Patent Office, Washington, DC.
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