Chance Technical Design Manual

STEP 6 - DETERMINE THE ALLOWABLE HELICAL ANCHOR STRENGTH

EQUATION 8-36

F 1 = 1/2 K a g H 2 F 1 = 1/2 (0.33) (120) 23

2 = 10474 lb/lf of wall The horizontal force from the surcharge load (F 2 ) is deter mined using Equation 8-37. EQUATION 8-37 F 2 = K a qH = 0.33 (100) 23 = 759 lb/lf of wall Using 19’ Soil Screw® anchors installed at a 15° angle, the hori zontal length (L X ) of the Soil Screw anchor is determined using Equation 8-38. EQUATION 8-38 L x = L cos(15°) L x = 19 cos(15°) = 18.4 ft The Factor of Safety against sliding is determined using Equation 8-39. EQUATION 8-39 STEP 5 - CHECK REQUIRED BEARING CAPACITY AT THE BASE OF THE WALL Determine the eccentricity (e) of the resultant vertical force using Equation 8-40. EQUATION 8-40 e = [F 1 (H/3)] + [F 2 (H/2)] g HL x = [10474 (23/3)] + [759 (23/2)] 120 (23) 18.4 = 1.75 < (L x /6) = (18.4/6) = 3.06 The vertical stress ( s v ) of the bottom of the wall is determined using Equation 8-41. EQUATION 8-41 s v = g HL x + qL x L x - 2e = 120 (23) 18.4 + 100 (18.4) 18.4 - 2 (1.75) = 3532 psf Given the allowable bearing capacity (Q allow ) is 4000 psf: EQUATION 8-42 Q allow = 4000 psf > s v = 3532 psf FS = g HL x tan( j ) F 1 + F 2 = 120 (23) 18.4 tan(30) 10474 + 759 FS = 2.61

ALLOWABLE DESIGN STRENGTH OF TYPE SS5 HELICAL ANCHOR (SERVICE LIFE = 75 YEARS), TABLE 8-5

ALLOWABLE DESIGN STRENGTH (TEMPORARY STRUCTURES) (kip)

ALLOWABLE DESIGN STRENGTH 75 yrs (kip)

Ta 75 yrs (kip)

V 75 yrs (kip)

50

37

45

37

The Soil Screw Anchor wall is a temporary structure with a design life of one year. From Table 8-5, the allowable design strength of the Chance® SS5 anchor is 45 kip. This table is based on the following electrochemical properties of soil: Resistivity: >3000 W -cm pH: >5 <10 Chlorides: 100 ppm Sulfates: 200 ppm Organic content: 1% max

DESIGN EXAMPLES

100

90

80

70

60

50

40 N q Values

30

20

10

0

10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44

Angle of Internal Friction φ BEARING CAPACITY FACTOR N q VS SOIL FRICTION ANGLE φ FIGURE 8-12

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