For simplicity sake, we will assume that the design professional provided an anchor or pier that is sufficient to support the applied load. The mechanical properties of the foundation are also adequate to fully develop the strength of the soil for which they are to be installed. The designer uses the soil strength parameters above or below a helix, depending on whether the load is in tension or compression.
There are two modes of soil failure -shallow failure mode or deep failure mode. The mode depends upon helix depth. The definition of “shallow” or “deep” foundation refers to the depth of the bearing plate vs. the earth’s surface. Shallow foundation will exhibit a brittle failure with an eruption of soil or instant drop in load resistance to zero. In deep foundations, the soil fails in a progressive manner, exhibiting little surface disruption. Chance Company utilizes 5 diameters as the break between shallow and deep foundations which is typically 1.5 m/ 5 ft. Foundation anchors should always be applied as a deep foundation because they provide increased ultimate capacity and failure is progressive and not sudden.
The bearing capacity of the pier is taken as the sum of the capacities of each individual helix. The helix capacity is determined by calculating the unit bearing capacity of the soil and applying it to the individual soil areas. The friction along the central shaft is disregarded for any capacity it may or may not attribute. In certain cases, the friction can be included (shaft is round and a minimum 90 mm/3.5 in. in diameter). To develop the maximum capacity out of each helix, it is necessary to space the helices such that their stress zones are not overlapping. Research by Chance Company has shown that three diameter spacing is sufficient to meet this requirement.
To determine the capacity of each helix, the following equation is used:
Qh= Ah (9c + q Nq ) < Qs
Where:
Qh = individual helix bearing capacity
c = soil cohesion
q = effective overburden pressure
Nq = bearing capacity factor
Qs = upper limit determined by helix strength
The shear strength of a soil is most often characterized by cohesion (c) measured in kPa (psf) and the angle of internal friction “phi” (Φ) which is stated in degrees. Soils are classified according to their cohesiveness or non-cohesiveness. Cohesive soils derive their shear strength from cohesion and are fine grained soils (clay) and non-cohesive soils derive their shear strength from friction (sands and gravels).
The product “9c” from the above equation is the shear strength provided through cohesion in fine grained soils, 9 being the bearing capacity factor for cohesive soils. The product “q Nq“ is the shear strength due to friction between the granular particles in a granular, non-cohesive soil. The bearing capacity factor is determined from the chart to the side. The bearing capacity factor for non-cohesive soils is dependent on the angle of internal friction (Φ). The curve is based on Meyerhoff bearing capacity factors for deep foundations and has been empirically modified to reflect the performance of foundation anchors. The overburden pressure (q) is determined by multiplying the density of the soil times the depth of the soil to the helix. When multiple soil layers exist, the pressure is calculated for each layer then added together.
There are times when a soil has both c and Φ value. Typically soil reports do not contain enough data to determine values for both c and Φ. It is the design professional responsibility to determine whether the soil is cohesive or cohesiveless and which is more likely to control the ultimate capacity of the soil. If there is any concern about which properties will be most reliable, calculate for both behaviors and choose the smaller capacity.
Tension anchor capabilities are calculated by using average parameters for the soil above the helices, whereas compression capacities may be calculated similarly however, averaging the soil strength parameters below the helices. The use of field testing is recommended to verify the accuracy of theoretically predicted foundation anchor capacities.
Installation Torque vs. Anchor Capacity
The Chance Company has long promoted that the amount of torsion force required to install a foundation anchor relates directly to the ultimate capacity of the foundation in tension and compression. Precise definition of the relationship for all possible variables remains to be achieved. Further recommended reading on the subject is in the paper “Uplift Capacity of Helical Anchors in Soil” by R.M. Hoyt and S.P. Clemence (Bulletin 2-9001). It states the formula for torque /anchor capacity as:
Qu = Kt x T
Where:
Qu = ultimate uplift capacity
Kt = empirical torque value (ft-1 or m-1)
T = average installation torque (ft-lb or kN-m)
The value of Kt may range from 3 to 20 ft-1 (10 to 66 m-1), depending on soil conditions and anchor design (primarily shaft size). The value of Kt value must be determined on a site specific basis depending on the variables listed above. Typically the same values of Kt can be used for both tension and compression loading. Monitoring of installation torques is mandatory in order to provide the designer with acc
urate ultimate bearing capacities of the installed anchors /piers.
This information is provided in summary for basic understanding of the helical pier design theories. For a more detailed summary please refer to the Chance Civil Construction Technical Design Manual.
Resources:
Please contact EBS Engineering and Construction for a copy of the Chance Civil Construction Technical Design Manual.
