Cantilever soldier pile design requires that we determine the soldier pile embedment, size, and estimate lateral wall displacements. Cantilever soldier piles are feasible only if we do not encounter any groundwater above the final excavation subgrade. Dewatering might be required if groundwater is encountered above the excavation subgrade.
Cantilever soldier pile walls are commonly used when excavation depths are limited to 5m or less (15ft). In most cases, cantilever depths are kept to under 3.5m (10ft) because controlling wall displacements requires an increasingly stiffer steel section. In rare cases cantilever excavations can reach up to 6.5m (20ft) when good soils are encountered, and stiffer soldier piles are used.
The most common way of designing cantilever walls is by using the free earth method. In this approach, we need to calculate the point where moment equilibrium is achieved below the excavation. Typically, engineers assume active lateral earth pressures on the driving side and passive lateral earth pressures on the resisting side. With soldier pile walls, we can account for 3-dimensional pile spacing effects by adjusting lateral soil pressures with the flange or drilled pile diameter size below the excavation.
Figure 1: Cantilever soldier pile wall analysis with DeepEX deep excavation software
Because the free earth method balances out only moments and not wall shears there are general recommendations for increasing the calculated wall embedment by 20 to 40% to achieve a real safety factor of 1.0. Additional safety factors must be applied beyond by increasing the provided wall embedment by at least 25%.
The free earth method can produce excessive lateral wall displacements. For this reason, certain transportation authorities recommend adjusting displacements by assuming a virtual fixity point at 25% of the required embedment for achieving moment equilibrium. While there is little theoretical justification, such an adjustment appears to produce much more reasonable wall displacement results that are close to actual measured wall displacements.
When using steel beams, the structural section modulus will be determined by assuming an allowable yield stress of 0.6 Fy (where Fy is the yield strength of the steel). In a simplified approach the steel section modulus can then determined as Sxx = Mmax/0.6 Fy. With reinforced concrete walls a safety factor of 1.5 is commonly used when a service design is performed.
Figure 2: Arching effects for cantilever soldier pile walls in DeepEX deep excavation software
Figure 3: With DeepEX and HoloDeepEX you can see soldier pile walls in virtual reality
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