Design Process of Deep Soil Mixed Walls for Excavation Support

Simplified Methods for Design of Case History
Figure 9. Islais Creek Transport/Storage Project Assumed Section..
Selecting a pressure diagram for the design is very difficult in the case of a multiple layer stratigraphy. Simplified profiles
must be used to facilitate the calculations required in the simplified method. Since the least competent soil controls the
lateral earth pressure, the Bay Mud was combined with other layers into one design soil unit.
Two different earth pressure diagrams were constructed (Fig. 10). First, a trapezoid-shaped diagram with a maximum
pressure of 51.25 kN/m 2 was calculated using an apparent earth pressure of (Weatherby, 1998):
p AEP
= 0.3γH (5)
where p AEP is the apparent earth pressure, γ is the total unit weight of the soil and H is the height of the excavation. Using
the tributary area method, the strut loads were determined as S 1 =95.8 kN, S 2 =288.1 kN and S 3 =196.4 kN.
The second diagram follows suggestions by Weatherby (1998) for multiple strut walls. A maximum pressure of 45.97
kN/m 2 was calculated using the “25H” trapezoid diagram. Using the tributary area method, the strut loads were determined
as S 1 =79.29 kN, S 2 =189.14 kN and S 3 =147.46 kN.
Once the pressure diagram and the anchor loads are known, the shear and bending moment diagrams can be drawn. The
maximum moment for each diagram was calculated, as is typical in practice, at the depth of the first strut (3.14 kN·m and
2.82 kN·m, respectively). Both of these moments can be safely resisted by the W30x108 wide flange beam, which was
selected as reinforcement for the soil-cement columns. A major limitation of this design method is the lack of wall
deflection predictions and the assumption that the maximum bending moment occurs at the first strut. The validity of this
assumption will be discussed later.
The lateral displacement of the wall can be predicted using the deformation control method as described by Clough and
O’Rourke (1990) and Briaud and Lim (1999). Based on the Briaud and Lim method (Fig. 3 and 4), the estimated mean
deflection could range from 11.7 mm to 47.6 mm, and the estimated deflection at the top of the wall could range from 29
mm to 46.8 mm using k equal to 0.3 and H equal to 11.7 m.
Based on Clough and O’Rourke (1990), the shoring system stiffness (S) from Equation 1 was 128 given wall stiffness (EI)
of 304,565 kN·m 2 /m (neglecting the soil-cement) and average vertical spacing between the strut supports (h) of 4.1 m. The
factor of safety against basal heave was estimated using the software program, XSTABLE, as 1.5. The estimated maximum
horizontal movement of the system was 0.5% of the depth of the excavation, or 58 mm from Fig 5.
International Journal of Geoengineering Case Histories ©, Vol. 1, Issue 2, p. 65
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