Transmission And Substation Foundations - Technical Design Manual

SECTION 4: DESIGN METHODOLOGY

Chance Helical Pile/Anchor Ultimate Bearing Capacity

Q C COMPRESSION

Q T TENSION

c

T

GROUND SURFACE

GROUND SURFACE GROUND SURFACE

5 ft (1.5 m) OR 5B (WHICHEVER IS 5 ft (1.5 m) OR 5B

5B MIN DEPTH 5B MIN

GREATER) MIN DEPTH MIN DEPTH

SIDE RESISTANCE CAPACITY ≈ 0 SIDE RESISTANCE CAPACITY

SIDE RESISTANCE CAPACITY ≈ 0 SIDE RESISTANCE CAPACITY

B

B

Single-Helix Plate Bearing Capacity Model—Helical Piles with Slender Shafts Figure 4-4

4.2.2.1.b Sands ( ’ > 0; c = 0) For clean, saturated sands, the cohesion is normally considered to be zero, and Equation 4-5 is used to calculate the ultimate capacity. Q ult = A h (q’N q + 0.5 ’BN ) Even in sands with moisture or a small amount of fines that may give some cohesion, this is usually ignored. Because the area of the plate is small, the contribution of the width term to ultimate capacity is very small and the width term is often ignored, leaving: Q ultU = A h q’N q For deep installations, the bearing capacity factor (N q ) is usually obtained from values used for determining the end-bearing capacity for deep pile foundations, which are different than the values used for shallow foundations. There are a number of recommendations for N q available in foundation engineering textbooks as shown in Figure 4-5. The difference in N q values shown in Figure 4-5 is largely related to the assumptions used in the failure mechanism. Figure 4-6 gives a reasonable chart of N q values as a function of the friction angle of the soil ( ’) that may be used for helical piles and anchors in cohesionless soils. The value of N q in Figure 4-6 is obtained from: EQUATION 4-13

overburden pressure, will not increase with depth. Critical depth is not specifically defined for helical piles and anchors, but engineers often use it with deep installation in saturated sands. 4.2.2.1.c Mixed Soils ( ’ > 0; c > 0) The ultimate capacity of a deep single-helix helical pile in mixed-grain soils can be calculated from traditional bearing capacity theory using Equation 4-6: Q ult = A h (cN c + q’N q + 0.5 ’BN ) Note: The term 0.5 ’BN is typically ignored for helical piles because the helix plate is small.

4.2.2.2 Tension Loading—Axial Uplift (Deep Single Helix) 4.2.2.2.a Saturated Clays ( ’ = 0; c > 0)

Under tension loading, the ultimate uplift capacity (Q ultU ) of a single-helix helical anchor in clay is calculated using the same approach given in Section 4.2.2.1.a. In some cases, a reduction may be made in the undrained shear strength to account for soil disturbance above the helical plate as a result of installation, depending on the sensitivity of the clay. As previously noted in Section 4.2.1.2.a, for a deep installation (D/B > 7.5) the breakout factor (F c ) has a default value of 9. The bearing capacity equation becomes: Q ultU = A h (9S u + ’D) 4.2.2.2.b Sands ( ’ > 0; c = 0) In sands, the tension capacity of a helical anchor is generally assumed to be equal to the compression capacity provided that the soil above the helix is the same as the soil below the helix in a zone of about 3 helix diameters. Again, for clean, saturated sands, the cohesion is normally considered to be zero, reducing the ultimate uplift capacity to:

EQUATION 4-14

N q = 0.5(12 ’)

’/54

Note: In some sands, the unit end-bearing capacity of deep foundations may reach a limiting value. The point at which this occurs is generally termed the critical depth. Critical depth is defined as the depth at which effective vertical stress, a.k.a.

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