Transmission And Substation Foundations - Technical Design Manual

SECTION 4: DESIGN METHODOLOGY

HeliCAP Helical Capacity Design Software Hubbell Power Systems, Inc., engineers developed HeliCAP® helical capacity design software to determine the bearing capacity of helical piles and anchors in soil. It has been revised

assistance, refer to the Help screen or contact Hubbell application engineers. The image below is from HeliCAP v3.0 helical capacity design software. It shows a typical work page with the soil profile on the left and helical pile capacity on the right.

several times to provide additional features such as side resistance for steel pipe piles and grouted-shaft helical piles. HeliCAP software is available to engineers and designers upon request. The software uses the same theory of general bearing capacity as presented in Section 4.2 for deep foundations (depth ≥ 5B). A key feature of HeliCAP software is that it is designed to work with the information commonly available from soils reports. In North America, soil investigation usually includes a soil boring as described in Section 2 of this manual. The most common information available from the soil boring is the soil profile, groundwater location, and SPT blow count data per ASTM D1586. To utilize this data, the software includes blow count correlations for shear strength, angle of internal friction, and unit weight. These correlations are generally accepted as reasonable approximations given the available blow count data. The equations, factors, empirical values, etc., presented in this section are used in the HeliCAP v3.0 helical capacity design software. Using this software makes the selection of a helical pile/anchor much quicker versus using hand calculations. It allows calculations to be made quickly while varying the different parameters to arrive at the most appropriate solution. As with any calculations, the results from this software are no better than the input data used to generate them. The software will assist in determining an appropriate helical lead configuration and overall pile/anchor length. It also provides an estimate of the installation torque. The helical lead configuration can vary by the number and sizes of helix plates required to develop adequate capacity. Helical pile/anchor length may vary due to the combined effects of the lead configuration and soil strength. Generally speaking, the shorter the pile length for a given load, the better the performance will be in regard to deflection under load. HeliCAP® design software calculates ultimate capacity and must have an appropriate Factor of Safety applied to the results. The software has additional features that allow it to be used for other applications, but it is beyond the scope of this manual to present all facets of the software. For additional

4.5.1 Helicap Software Bearing Capacity Methodology

As detailed earlier in this section, the individual plate bearing method states that the capacity of a single or multi-helix pile/anchor is determined by summing the bearing capacities of the individual helix plates. Thus: Q t = ΣQ h where Q t = Total ultimate multi-helix pile/anchor capacity Q h = Individual helix ultimate capacity HeliCAP design software determines the ultimate bearing capacity of an individual helix with the following equation. An upper limit for this capacity is based on helix strength that can be obtained from the manufacturer. See Section 6 of this manual for the mechanical strengths of helix plates.

EQUATION 4-25

Q h = A h (cN c + q’N q ) ≤ Q s

where A h = Projected helix area Q s = Capacity upper limit

= helix mechanical strength

4.5.1.1 Sands ( ’ > 0; c = 0) HeliCAP design software determines the ultimate bearing capacity of a helix (Q h ) in a non-cohesive sand or gravel soil with Equation 4-26, which is derived from Equation 4-25 with the fine-grain (clay) term eliminated (also see Equation 4-13). The bearing capacity factor (Nq) is dependent on the angle of

4-22 | www.hubbell.com/hubbellpowersystems