The following typical details and design procedure are based primarily on Caltrans’ method for use on highway construction, but the method is very similar to other methods presently in practice. Design facing pressures are based on the French Clouterre empirical method. The cast-in-place portion of the facing is designed for this pressure for permanent
FIGURE 8.57 Section through facing of soil nailed wall showing concrete reinforcement and soil nail connection. (From J. W. Keeley, Soil Nail Wall Facing: Sample Design Calculations, Federal Highway Administration, 1993, with permission)
walls only. The strength of the shotcrete construction facing is ignored. Only the ultimate limit state is addressed; no serviceability calculations are made for cracking or deflections. Sample design calculations are illustrated following the presentation of the procedure.
Typical Details. (See Figs. 8.57 to 8.59.)
1. Use a shotcrete layer with a 4-in (100-mm) minimum thickness.
2. Include a single layer of welded wire fabric at mid-thickness. Common options are:
6 X 6-W4.0 X W4.0 (4 gauge wire; diameter, 0.225 in; cross-section area = 0.080 in2/ft or 0.17 mm2/mm)
4 X 4-W2.9 X W2.9 (6 gauge wire; diameter, 0.192 in; cross-section area = 0.087 in2/ft or 0.18 mm2/mm)
3. Use two continuous horizontal no. 4 grade 60 reinforcing steel bars at each nail.
4. Use 1-ft-wide (300-mm) vertical geocomposite drain between nails; connect the geocomposite drain to a 2-in-round (600-mm) plastic weep hole outlet drain just above finished grade near the bottom of the wall.
5. Place the ASTM A36 steel nail bearing plate, 1 in X 9 in X 9 in (25 X 225 X 225 mm) with wedge washer and nut on the outside face of the shotcrete; set into place before the shotcrete hardens. Add studs to this plate to engage permanent facings that are placed over this initial shotcrete layer.
Step 1: 4-in (100-mm) Shotcrete Construction Facing. This is the only facing required for temporary walls (service life less than 18 months) and the first portion required for permanent walls. It is placed immediately after each stage of excavation and nail placement. Current AASHTO and American Concrete Institute (ACI) codes do not address the loadings or the structural capacities for this facing. Therefore, many current designs rely on details that have shown good performance on previous projects rather than design calculations.
Step 2: 8-in (200-mm) CIP Permanent Facing—Compute Design Nail Load and Pressure at Facing. The design nail load at the facing is computed for the given nail size, steel grade, and nail spacing according to the French Clouterre empirical method. The French determined through field tests that the nail load at the facing (T0) did not exceed about one-half the maximum nail load (T ) near the soil failure surface. They
v max
established a design nail load for the facing that varies from 0.6 times T for closely spaced nails to 1.0 times T for larger nail spacings. T is the ultimate limit state established for the nail tension (Caltrans procedure) at 0.75 X f (T = A. X 0.75 X
у v max nail
fy), where fy is the yield strength of the nail. The design pressure for the facing is then simply the design nail load at the facing (T0) divided by the nail tributary facing area (i. e., horizontal nail spacing times vertical nail spacing).
Step 3: 8-in CIP Permanent Facing—Design for Flexure. The cast-in-place facing is designed so that its ultimate strength is greater than the moments in the facing computed by simple continuous beam equations for the facing pressure from T0. Only one layer of grade 60 reinforcing steel placed near the middle of the section is used. (See Fig. 8.57.) The controlling d is used for the ultimate strength computation.
Step 4: 8-in CIP Permanent Facing—Nail Connection Design. The connection of the nail to the cast-in-place facing is designed to carry the nail’s ultimate limit state in tension, T. The nail bearing plate is sized to limit the bearing pressure from T to the
max max
ultimate value allowed by AASHTO (0.6f’c). The plate thickness is determined to provide sufficient bending strength for the moment from the bearing pressure about the nail nut. Studs are welded to the bearing plate to carry Tmax entirely by the 8-in (200-mm) CIP permanent facing. The ultimate punching shear capacity of the steel embedment is computed according to American Concrete Institute specifications.
Sample Design Calculations for Soil Nail Wall Facing (Based on Caltrans Methods)
Step 1: 4-in Shotcrete Construction Facing. Details as previously described may be used.
Step 2: 8-in CIP Permanent Facing—Concrete Design Nail Load and Pressure at Facing. Given No. 8 nails; f = 60 kips/in2; Anail = 0.79 in2; horizontal nail spacing Sh = 6 ft; and vertical nail spacing Sv = 6 ft.
Begin by calculating the design nail loads.
T = design nail load at soil failure surface at ultimate limit state
max
= 0.75f A = 0.75 X 60 X 0.79 = 35.6 kips
у nail
T0 = design nail load at facing at ultimate limit state
(Note: This is the French Clouterre equation.)
The design nail load T0 is then used to calculate the facing design pressure.
Wu = facing design pressure at ultimate limit state
27.0
——- = 0.75 kip/ft2
6 X 6
Step 3: 8-in CIP Permanent Facing—Design for Flexure. Note: Try this wall section with No. 6 reinforcing bars spaced at 12 in horizontally and vertically; f c = 3250 lb/in2. c
Required ultimate moment per foot (horizontal and vertical; positive and negative)
Ultimate design capacity for trial section
8-in CIP permanent facing
depth of concrete compression
фмп=Фм(^ — _
= 0.9 X 0.44 X 60 X ^3.38 — __ j X _ = 5.9 ft • kips/ft > 2.7 OK
Check of minimum steel requirements. According to AASHTO, the tension reinforcement must be equal to or greater than the lesser of that required to develop a moment (1) 1.2 times the cracking moment (based on the gross section modulus S ) and (2) 1.33 times that required by analysis for the specified loading conditions. This leads to the following equations for the cross-section area of the reinforcement:
1.2 X S X 7.5V2
_______ g_______ _
ф__ X 0.9d
The strength reduction factor ф is 0.90. Substitution gives the following results:
Thus, As must be at least 0.26 in2/ft to meet these requirements. For temperature and shrinkage, minimum As is 0.13 in2/ft (No. 4 @18 in). The final selection is No. 6 reinforcing bars spaced at 12 in, which provides As of 0.44 in2/ft, a nominal level of reinforcing. For complete designs, also check cantilever sections at the top and bottom of the wall and any other special facing sections, such as at expansion or contraction joints.
Step 4: 8-in CIP Permanent Facing—Nail Connection Design
Design bearing plate at ultimate limit state for Tmax
T = 0.75f A = 0.75 X 60 X 0.79 = 35.6 kips
max y nail
Calculate the ultimate concrete bearing strength under the plate using a strength reduction factor of ф = 0.70. Therefore,
Ultimate concrete bearing strength = 0.85фf’c
= 0.60f’ .
c min
where w = ultimate bearing pressure.
Required steel area for studs to resist Tmax. The design strength of the stud fds is determined as the lesser of (1) the stud yield strength (f = 50 kips/in2) multiplied by a strength reduction factor of ф = 0.90 and (2) 80 percent of the stud tensile strength (fu = 60 kips/in2). Therefore,
fds = min Щ; 0.8fJ ф^ = 0.9 X 50 = 45 kips/in2
0. 8fu = 0.8 X 60 = 48 kips/in2
Therefore
fds = 45 kips/in2 T
Required stud area As = —
fds 35.6
Try 4/2-in ф studs.
As = 4 X 0.196 = 0.79 in2 OK
Check of anchor head bearing for ‘A-in ф stud. ds = 0.5 in. Determine head area Ah.
dh^2
Ah 0.79
A 0.196
T. = 0.312 in > 0.25 in
Head thickness is OK.
Ultimate connection embedment design capacity Pd Pd = 4ф VfA
d c cp
f’ = 3250 lb/in2
c
ф = 0.65 = strength reduction factor in this case
Acp = 291.7 in2 = effective stress area (see Fig. 8.59)
Pd = 4 X 0.65 X V3250 X 291.7 X = 43.2 kips
P, > T = 35.6 kips OK
d max
