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Earth Pressure against Sheet Pile

Substantial experimental research and field measurements were carried out regarding active, passive and compaction-induced pressures developing against rigid retaining walls. In contrast, relatively few experimental studies and field measurements were conducted for earth pressures acting on sheet piles, especially in soft soils. Sheet piling is the most common type of flexible earth retaining systems used as waterfront structures. In contrast to the construction of other types of retaining walls, the building of sheet pile walls does not usually require dewatering of the site. Sheet piling is ideally suited to sites with high ground water tables or low bearing capacity soils. Many authors had developed theoretical solutions for cantilever sheet pile or anchored or propped sheet pile design. The conventional method for analyzing cantilever sheet piling in homogeneous foundation soils is based on the assumption (Clayton and Milititsky, 1983) that the piling rotates about some pivot point, causing active state stress to develop in the back of the wall above the pivot point and in front of wall below the pivot point as shown in figure 2.49. The wall derives its stability from the passive pressures that develop in front of the wall above the pivot point and in back of the wall below the pivot point.

Only limited number of research studies has been conducted to examine the current sheet pile design procedures via field measurements. The following section presents studies that conducted field measurements monitoring the active earth pressures against the supporting side of a strutted sheet pile wall in excavation in cohesive soils.

DiBiagio (1977) reported the measured total lateral active earth pressure against a propped sheet pile sizes wall during excavation. DiBiagio indicated that because of the corrugations in the wall, the cells must be placed either on the protruding or indented corrugations, as shown in figure 2.50. In this case, total pressure cells were placed at both the protruding and indented locations at the same depth in soft clay (figure 2.50). DiBiagio observed that measurements (at locations P1 and P2) were almost equal immediately after the sheet pile supplier driving (at the same depth at different locations), but later, the measured total pressures (at the same depth) were different and changed with the excavation progress. DiBiagio pointed out that the different measured pressures P1 and P2, presumably were due to sheet pile movements or clay consolidation or arching. DiBiagio (1977) suggested that a best estimate of the average total stress per unit length of the wall can be made by averaging P1 and P2, but clearly this average may be significantly in error.

Stille (1979) measured anchor loads and deflection of anchored sheet pile wall during all excavation stages in clay. Stille observed that measured anchor loads were in good agreement with that calculated with new Swedish design rules. Stille indicated that the timedependent properties of clay (the creep effect), must be considered to get a good agreement between the calculated and measured deflection. Tamano (1983) measured the lateral active earth pressure in the backfill behind a multi-tied anchored sheet pile wall in excavation. Tamano observed that the distribution of lateral earth pressure exerting on the anchored sheet pile wall was transformed by redistribution of earth pressure from triangular shape to trapezoidal distribution as the wall displace during excavation. Gigan (1984) measured the active soil pressure in a backfill behind an anchored wall with active tie-rods, as well as the sheet pile deformation in order to verify the validity of the most widely used calculation method.

Finno (1989) measured pore water pressure, sheet pile deformation, and strut loads in a 40 ft deep braced excavation in soft to medium stiff, saturated clays in Chicago. Finno observed that relatively high excess pore pressure developed during the sheet pile installation and strut loading; but these initial pore pressures dissipated rapidly as shown in figure 2.51. 18 Pore pressures due to excavation unloading were rather small. Little net change in pore pressure was observed at the end of construction. The maximum measured loads in each strut were approximately equal to the magnitudes specified by the design earth pressure envelop.

Kort and Van Tol (1999) conducted a full scale field test on two steel sheet pile walls in soft clay with one thin layer of peat (in Pernis, near Rotterdam) to model the braced excavation. The earth pressure and water pressure on the retained side and excavated side were measured. The earth pressure distribution at eight points on the retained side and four points on the excavated side did not show any significant changes during the various excavation stages. Kort et al. (2000) pointed out that the change of total pressure was due to the change of water pressure. Kort (2002) indicated that increase of lateral displacement and bending moment in the long-term was caused by a relatively small change in the earth pressure.

Endley et al. (2000) measured and analyzed the lateral earth pressure and deflection of an anchored sheet pile wall constructed as a dock. Endley et al. (2000) indicated that high lateral earth pressures resulted from the backfill construction methods, which in turn, produced large sheet pile deflection and stresses. Endley et al. proposed that “manufacturers’ values of section modulus and moment of inertia are not correct for the Larssen section. Crimping (or even welding) the two sections do not alter the properties. The section modulus and inertia moment of the specified sheet pile sections should be independently verified. The values of section modulus and moment of inertia, in conjunction with lateral earth pressure, play an important role in wall deflection.”

Peck (2002) reported the measurements of strut loads (shown in figure 2.54) based on field observations of open-cut in clay with propped-sheeting taken place during the construction of the Chicago subway. Peck indicated that the magnitude of the total lateral earth pressures was in satisfactory agreement with either the plane or general wedge theories for purely cohesive soils with no effective internal friction, but that the distribution of the pressure was non-hydrostatic.

To date, only several field measurements had been conducted to monitor the earth pressure against sheet piles. However most of the studies are related to anchored or strutted sheet piles in excavation or do not include measurements of earth pressures directly in place, i.e. include measurements of strut or anchor loads. Even fewer studies are known to investigate the passive earth pressure against the supporting side of sheet piles. No studies had been found to directly measure the passive earth pressure against cantilever sheet pile wall, especially in a peat deposit, and also no recorded work is known to measure the compaction-induced passive earth pressure against a cantilever sheet pile wall in peat.

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