Shear Strength Applications in Plastic and Limit Equilibrium - Unsaturated Soil Mechanics in Engineering Practice - page 631

450

400

Square footing

q f 0 = 342 kPa

350

Strip footing

300

q f 0 = 285 kPa

250

200

150 c u 0 = 50 kPa

φ b = 15°
N c = 5.70 for strip footing
N c = 6.84 for square footing

100

50

-50 0 50 100

Decrease Increase

Matric suction change [ Δ ( u a - u w )], kPa

Figure 12.68 Variations in ultimate bearing capacity as a result of matric suction changes (from
Rahardjo and Fredlund, 1992).

b

φ

50
40
30
20
10
0

25 °

20 °

15 °

Decrease

10 °

5 °

− 100 − 80 − 60 − 40 − 20

0 20 40 60 80 100

− 10

− 20

− 30
− 40
−

Increase

50

Percent change in matric suction, Δ ( u a - u w )/ c u 0 (%)

Figure 12.69 Variation in ultimate bearing capacity with variation in matric suction (from
Rahardjo and Fredlund, 1992).

changes as much as the initial undrained shear strength [i.e.,
(u a −

in the matric suction in the subsoil. “Road bans” may be
placed on highways in the spring of the year because of
the low matric suction in the subgrade (i.e., the low bearing
capacity of the road).
Highway, railway, and airport designs have not commonly
been viewed as bearing capacity designs. There appears to
have been a lack of understanding of how to assess the shear
strength of a compacted subgrade soil with matric suction.
More recently there has been renewed interest in a bearing
capacity approach in transportation applications.
The wheel loads applied to a highway surface are trans-
mitted through the base and subbase onto the underlying
subgrade. The theory of elasticity can be used to compute the
stresses transmitted to the top of the subgrade. The subgrade
may undergo a bearing capacity failure if the transmitted

100% of c u 0 ]. The percent change in the ulti-
mate bearing capacity will be higher for higher φ b values.

u w ) =

12.4.5 Bearing Capacity of Layered Systems
The concepts of bearing capacity are also relevant for same
man-made earth structures. These applications involve the
design of highways, railways, and airport systems. High-
ways and airport runways commonly consist of an asphalt
layer, a base, and a subbase layer overlying the subgrade
soil. The railway system consists of the track structure (i.e.,
rails and ties) resting on ballast and subballast layers. In
both cases, it is the bearing capacity of the subgrade which
is of primary importance.
The environmental conditions change throughout the year
for the highway, railway, or airport, resulting in a change