Transmission And Substation Foundations - Technical Design Manual (TD06088E)

For example, a Type SS175 helical pile in firm soil with a remedial repair bracket connected to an existing 2500 psi concrete footing has an ASD allowable compression strength of 36.8 kip based on the strength of the repair bracket (P1). The P4 geotechnical allowable capacity of 52.5 kip is greater, which means the bracket strength controls the design. This is also true if the soil is soft; the ASD allowable compression strength is 27.7 kip based on the bracket strength. The allowable load for remedial repair brackets is less because of the eccentric compressive load. Note from the table above the allowable strength can increase with stronger concrete. Note from Table C-4 above that the allowable shaft (P2) compressive strength for SS175 in soft soils is sig- nificantly less than the torque correlated (P4) capacity. That is one reason why pipe shaft or grouted shaft helical piles are used. The table below from Section 7 of the TDM lists the P2 (shaft) and P3 (helix) ASD allowable strengths for Type RS3500 3-1/2” OD pipe shaft helical piles. It is used to easily determine P2 and P3, which can then be compared to P4 to see which will control the design. The table is broken down by soil type, end condi- tion, and number/diameter of the helix plates.

Table C-7 – RS3500 - P2 Shaft Compression Strength and P3 Helix Strength in Firm or Soft Soil

HELICAL PILES AND ANCHORS

For example, a Type RS3500 helical pile in firm soil & fixed end condition with multi-helix plates (3 or more plates) has ASD allowable compression strength of 76.6 kip. But the ASD allowable compression strength in soft soil is either 65.9 kip with a fixed end condition, or 54.3 with a pinned end condition. These ASD allowable strengths are much higher than for SS175 in soft soil, which are 30.2 kip and 15.4 kip with fixed and pinned end conditions respectively. The P4 torque based geotechnical allowable capacity for RS3500 is 45.5 kip, which is less than the structural strength of RS3500 for any combination of soil type and end condition and thus controls the design. This is why SS/RS Combo piles are a good choice in soft overbur- den soil conditions. Another way to increase structural strength is with grouted shaft Helical PULLDOWN Micropiles per Table C-7 below. The grout column increases the section modulus, which in turn increases the axial compression strength. Another benefit of the grout column is increased axial capacity (P4) base and shaft resistance, due to the soil-grout bond [Method 1] .

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