The capacity of the foundation soil should be determined using accepted engineering principles and measurement of material parameters such as cohesion and angle of friction, or on the basis of field data such as the standard penetration test or the shear vane test. (See Chap. 8 for pertinent information.) One agency uses the following for default values:
1. Use angle of friction ф = 30° for granular soils and a cohesion value of c = 1000 lb/ft2 (48 kPa) for plastic soils to determine post embedment. Water encountered in soils above embedment depths will require special designs.
2. Use 2000 lb/ft2 (96 kPa) for allowable bearing capacity unless higher values are approved by the soils engineer.
3. A maximum of 2 ft (600 mm) of unbalanced fill on one side of the noise wall will be allowed. Good compaction must be achieved on the low side of the wall prior to placing unbalanced fill.
The AASHTO Guide Specifications recommend the following safety factors for the design of spread footings that support noise walls:
|
Group |
Overturning |
Sliding |
|
I |
2.0 |
1.5 |
|
II |
1.5 |
1.2 |
|
III |
1.5 |
1.2 |
|
IV |
1.5 |
1.2 |
For walls supported on two or more rows of piles, the design should follow procedures in Standard Specifications for Highway Bridges (AASHTO, Washington, D. C., 2004). For walls supported on a single row of piles, the pile must be designed as a column, considering both axial loads and bending. Also, the pile must be designed for the shear from the lateral loads.
For panel-and-post type walls, the embedment depth of the post can be determined using Rankine or Coulomb earth pressure theories. The following equation follows from static equilibrium analysis and applies for a pile or post on level ground:
applied ultimate lateral load, lb (N)
vertical distance from lateral load to top of embedment, ft (mm) (disregard upper 6 in (150 mm) of soil at ground surface)
net horizontal ultimate lateral soil pressure limit, lb/ft2 (Pa) per ft (mm) of depth required depth of embedment, ft (mm)
Note that both P and R are ultimate values. The design load must be increased by an appropriate load factor, and the resisting soil pressure decreased by an appropriate load factor.
Example—U. S. Customary units. P = 200 lb, h = 6 ft, and R = 600 (lb/ft2)/ft. Determine d.
By trial and error, it is found that d = 3.2 ft satisfies Eq. (9.3). The final trial gives
0 = 1638 — 427 — 7 — 1200 0 « 4 (close enough; OK)
The post should be embedded a distance of 3.2 + 0.5 = 3.7 ft below the ground surface.
The maximum moment in the pile or post can be expected to occur at a depth of
0. 25d. In this case, the maximum moment is
M = P(h + 0.25d)
= 200(6 + 0.25 X 3.2)
= 1360 ft • lb
Example—SI units. P = 890 N, h = 1830 mm, and R = 0.0287 Pa. Determine d. By trial and error, it is found that d = 975 mm satisfies Eq. (9.3). The final trial
gives
0. 0287(975)3 _ 2(890)(975) _ (890)2
12 3 3(0.0287)(975)
2,216,739 _ 578,500 _ 9,436 _ 1,628,700
103 (close enough; OK)
