Engineering and Construction

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Steel sheet piling is widely used for the construction of a cofferdam, which is defined as a temporary structure designed to support the sides of an excavation and to exclude water from the excavation. Tomlinson (1986) presents an in-depth discussion of the different types of cofferdams and construction techniques. For example, the first step in the construction of a bridge foundation could be the installation of a large circular cofferdam. Then the river water would be pumped out from inside the cofferdam and the river bottom would be excavated to the desired bearing strata for the bridge foundation.

In general, the following loads could be exerted on the cofferdam:

1. Hydrostatic groundwater pressures located outside the cofferdam. If the cofferdam is constructed in a river or other body of water, then the hydrostatic water pressure should be based on the anticipated high water level.
2. Hydrostatic pressure of water inside the cofferdam if the water level outside falls below the interior level during initial pumping of water from the cofferdam. This condition could cause bursting of the cofferdam by interlock tension.
3. Earth pressure outside the cofferdam.
4. Other loads, such as surcharge loads, wave pressures, and earthquake loading.


Based on these loading conditions, the cofferdam must have an adequate factor of safety for the following conditions:

1. Heave of the soil located at the bottom of the cofferdam.
2. Sliding of the cofferdam along its base.
3. Overturning and tilting of the cofferdam.
4. The inward or outward yielding of the cofferdam (i.e., maximum moment and shear in the sheet piling and interlock tension).
5. Piping of soil that could undermine the cofferdam or lead to sudden flooding of the interior of the cofferdam.

An important consideration in the design of cofferdams is the ability of the sheet pile wall to deflect. If struts are used that brace the opposite sides of the excavation (i.e., braced excavation), the deflection of the sheet pile wall will be restricted. The next section will discuss the earth pressures exerted on braced excavations. If the sheet pile wall is able to deform, such as by using raking braces, then the deflections of the wall during construction will permit mobilization of active pressures in accordance with the previous discussion. The USS Steel Sheet Piling Design Manual (1984) and NAVFAC DM-7.2 (1982) present a further discussion of the design of cofferdams.

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In the case of cantilevered sheet pile walls, the sheet piling is driven to an adequate depth to become fixed as a vertical cantilever in resisting the lateral active earth pressure. Cantilevered walls usually undergo large lateral deflections and are readily affected by scour and erosion in front to the wall. Because of these factors, penetration depths can be quite high, resulting in excess stresses and severe yield. According to the USS Steel Sheet Piling Design Manual (1984), cantilevered walls using steel sheet piling are restricted to a maximum height H of approximately 15 ft (4.6 m).

Design charts have been developed for the analysis of cantilevered sheet pile walls. For example, Fig. 11.16 is a design chart reproduced from the USS Steel Sheet Piling Design Manual (1984). The value of a is based on the location of the water level as indicated in Fig. 11.16. The chart is entered with the ratio of the passive earth pressure coefficient kp divided by the active earth pressure coefficient kA. By intersecting the depth ratio curves or moment ratio curves, the depth of the sheet pile wall (D) or maximum moment Mmax can be calculated. Note that the depth D from Fig. 11.16 corresponds to a factor of safety of 1.0. The USS Steel Sheet Piling Design Manual (1984) states that increasing the depth of embedment (D) by 20 to 40 percent will provide for a factor of safety of toe failure of approximately 1.5 to 2.0.

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The design of sheet pile walls requires the following analyses:

1. Evaluation of the earth pressures that act on the wall, such as shown in Fig. 11.14
2. Determination of the required depth D of piling penetration
3. Calculation of the maximum bending moment Mmax, which is used to determine the maximum stress in the sheet pile
4. Selection of the appropriate piling type, size, and construction details

A typical design process is to assume a depth D (Fig. 11.14) and then calculate the factor of safety for toe failure (i.e., toe kick-out) by the summation of moments at the tieback anchor (Point D). The factor of safety is defined as the moment due to the passive force divided by the moment due to the active force. Values of acceptable factor of safety for toe failure are 2 to 3. An alternative solution is to first select the factor of safety and then develop the active and passive resultant forces and moment arms in terms of D. By solving the equation, the value of D for a specific factor of safety can be directly calculated.

Once the depth D of the sheet pile wall is known, the anchor pull Ap must be calculated. The anchor pull is determined by the summation of forces in the horizontal direction, or:


Based on the earth pressure diagram (Fig. 11.14) and the calculated value of Ap, elementary structural mechanics can be used to determine the maximum moment in the sheet pile wall. The maximum moment divided by the section modulus can then be compared with the allowable design stresses of a particular type of sheet pile.

For cohesive soil, the long-term condition often governs the design. In this case, an effective stress analysis can be performed (c′ and Φ′) using the estimated location of the groundwater table.

For convenience, the effective cohesion is neglected and the analysis is performed by using only Φ′.

Therefore, the long-term effective stress condition for cohesive soil is analyzed as described in the preceding discussion for sheet pile walls having granular soil. Since Φ′ for a cohesive soil is usually less than Φ′ for granular soil, the active earth pressure will be higher and the passive resistance lower for cohesive soil.

Some other important design considerations for sheet pile walls include the following:

1. Soil layers. The active and passive earth pressures should be adjusted for soil layers having different engineering properties.

2. Penetration depth. The penetration depth D of the sheet pile wall should be increased by at least an additional 20 percent to allow for the possibility of dredging and scour. Deeper penetration depths may be required based on a scour analysis.

3. Surcharge loads. The ground surface behind the sheet pile wall is often subjected to surcharge loads. Equation 11.3 can be used to determine the active earth pressure resultant force due to a uniform surcharge pressure applied to the ground surface behind the wall. Note in Eq.11.3 that the entire height of the sheet pile wall (i.e., H + D, see Fig. 11.14) must be used in place of H. Typical surcharge pressures exerted on sheet pile walls are caused by adjacent foundation loads, railroads, highways, dock loading facilities and merchandise, ore piles, and cranes.




4. Unbalanced hydrostatic and seepage forces. The previous discussion has assumed that the water levels on both sides of the sheet pile wall are at the same elevation. Depending on factors such as the water tightness of the sheet pile wall and the backfill permeability, it is possible that the groundwater level could be higher than the water level in front of the wall, in which case the wall would be subjected to water pressures.

This condition could develop when there is a receding tide or a heavy rainstorm that causes a high groundwater table. A flow net can be used to determine the unbalanced hydrostatic and upward seepage forces in the soil in front of the sheet pile wall. Adjustments to the active and passive resultant forces for seepage underneath the sheet pile wall can be determined by using Fig. 11.15.

5. Other loading conditions. The sheet pile wall may have to be designed to resist the lateral loads
due to ice thrust, wave forces, ship impact, mooring pull, and earthquake forces. If granular soil
behind or in front of the sheet pile wall is in a loose state, it could be susceptible to liquefaction
during an earthquake.

6. Factors increasing the stability. A factor usually not considered in the design analysis is the densification of loose sand during driving of the sheet piles. However, since sheet piles are relatively thin, the densification effect would be less for a sheet pile than for a comparable round pile, because the sheet pile displaces less soil.

Another beneficial effect in the design analysis is that many sheet pile walls used for waterfront construction or for foundation excavations are relatively long and hence the soil is in a plane strain condition. As previously discussed, the plane strain shear strength is higher than the shear strength determined from conventional shear strength tests.