ch10-sheet piles (571-607)

CHAPTER

10

DESGIN AND ANALYSIS OF SHEET PILE WALLS

10.1 INTRODUCTION A sheet pile wall is a flexible structure which depends for stability upon the passive resistance of the soil in front of and behind the lower part of the wall. Stability also depends on the anchors when incorporated. Retaining walls of this type differ from other walls in that their weight is negligible compared with the remaining forces involved. Several methods are available for design and analysis of these structures. Usually, most of them neglect the effect of friction  between the soil and the wall, but this omission is fairly satisfactory when determining active  pressure values. However, the effect of wall friction can almost double the Rankine value of K  p .

10.2 TYPES OF SHEET SHEET PILE WALLS In general, two types of sheet sheet pile walls are common. As shown in Fig.(10.1), these are:

10.2.1 CANTILEVER SHEET PILE WALLS These walls are used mainly for temporary works where the free height is generally less than 6m. They are constructed by driving the sheet piling to a depth sufficient for developing a cantilever beam-type reaction to resist the active pressures on the wall. That is, the embedment length must be adequate to resist both lateral forces as well as a bending moment.

10.2.2 ANCHORED SHEET PILE WALLS When the height of the backfill material behind a cantilever sheet pile wall exceeds about 6m, tying the wall near the top to anchor plates, anchor walls, or anchor piles becomes more economical. This type of sheet pile walls is also known as anchored bulkheads. bulkheads. These walls are used in waterfront construction to form wharves or piers for loading and unloading ships or  barges.

H

Ground Surface

Ground Surface

Granular Backfill

Anchor tie rod H

W.T.

W.T.

Granular backfill

Derdge line

Derd Derd e Line Line D

Granular subsoil

(a) Cantilever Sheet pile

D

Cohesive subsoil

(b) Anchored Sheet pile

Fig.(10.1): Types of common used sheet pile walls.

F ounda undation for C i vil E ngine ngi nee er s

C hap hapter ter 10: D esgin sgi n of Sheet Sheet Pi le Walls

10.3 DESIGN METHODS OF CANTILEVER SHEET PILE WALLS For the sheet pile wall geometry shown in Fig.(10.2a), several methods of design are a re available; these are as follows:

10.3.1 LIMIT STATE OR “BS 8002: 1994” 1994” METHOD According to this method, the following design procedure is recommended: 1. A minimum surcharge load of 10 kN/m2 must be applied to the surface for the design of sheet pile walls. 2.  Not less than 0.5m deep or 10% of the clear height retained should be considered for unplanned future excavation in front of the sheet pile wall. 3. The sheet pile wall displacement should not exceed 0.5% of the wall height. 4. The shear strength parameters should be reduced by a mobilization factor, M, the value of which depends on whether the design is for undrained or drained conditions. Re presen  p resentat tatiive..c u  for undrained conditions ( conditions (  u  0 ): Design c u  M  1.5  for drained conditions, use the lesser of the following two values: Re presen  p resentati tative.. tan max. Re presentati  p resentative..c (a)  Design tan   ; and Design c  1.2 1.2 (b) The representative critical state strength which would be mobilized at the collapse of significant ground movement. 5. The design values of wall friction,  , and undrained wall adhesion, c w , be the lesser of the representative values determined by tests or: Design tan   (0.75).Design. tan  ; and Design c w



(0.75).Design .c u

6. The depth of penetration of pile, d, is obtained by balancing the disturbing and restoring

moments about c, together with the horizontal forces established using pressure distribution shown in Fig.(10.2b). The method generates two equations containing two unknowns d and d o , which are solved by iteration until the correct values are obtained.

10.3.2 TRADITIONAL METHODS These methods involve the determination of the safety factor for passive resistance, F p ,  based on different lateral earth pressure distributions. They are described described as follows:

(1) Gross Pressure Method This method is also referred to as the CP2 method; after the Institution of Structural  Engineers' Original Code of Practice published in 1951,  1951,  in which, the total theoretical value of passive resistance is divided by a safety factor ( F p = 2.0). It is based on the  pressure distribution shown in Fig.(10.2c) from which

do  

can be obtained by taking

moments of thrusts about the sheet pile base, then, d o   is increased by 30 % to give the required depth of penetration (d).

572

F ounda undation for C i vil E ngine ngi nee er s

C hap hapter ter 10: D esgin sgi n of Sheet Sheet Pi le Walls

10.3 DESIGN METHODS OF CANTILEVER SHEET PILE WALLS For the sheet pile wall geometry shown in Fig.(10.2a), several methods of design are a re available; these are as follows:

10.3.1 LIMIT STATE OR “BS 8002: 1994” 1994” METHOD According to this method, the following design procedure is recommended: 1. A minimum surcharge load of 10 kN/m2 must be applied to the surface for the design of sheet pile walls. 2.  Not less than 0.5m deep or 10% of the clear height retained should be considered for unplanned future excavation in front of the sheet pile wall. 3. The sheet pile wall displacement should not exceed 0.5% of the wall height. 4. The shear strength parameters should be reduced by a mobilization factor, M, the value of which depends on whether the design is for undrained or drained conditions. Re presen  p resentat tatiive..c u  for undrained conditions ( conditions (  u  0 ): Design c u  M  1.5  for drained conditions, use the lesser of the following two values: Re presen  p resentati tative.. tan max. Re presentati  p resentative..c (a)  Design tan   ; and Design c  1.2 1.2 (b) The representative critical state strength which would be mobilized at the collapse of significant ground movement. 5. The design values of wall friction,  , and undrained wall adhesion, c w , be the lesser of the representative values determined by tests or: Design tan   (0.75).Design. tan  ; and Design c w



(0.75).Design .c u

6. The depth of penetration of pile, d, is obtained by balancing the disturbing and restoring

moments about c, together with the horizontal forces established using pressure distribution shown in Fig.(10.2b). The method generates two equations containing two unknowns d and d o , which are solved by iteration until the correct values are obtained.

10.3.2 TRADITIONAL METHODS These methods involve the determination of the safety factor for passive resistance, F p ,  based on different lateral earth pressure distributions. They are described described as follows:

(1) Gross Pressure Method This method is also referred to as the CP2 method; after the Institution of Structural  Engineers' Original Code of Practice published in 1951,  1951,  in which, the total theoretical value of passive resistance is divided by a safety factor ( F p = 2.0). It is based on the  pressure distribution shown in Fig.(10.2c) from which

do  

can be obtained by taking

moments of thrusts about the sheet pile base, then, d o   is increased by 30 % to give the required depth of penetration (d).

572