BUILDER'S ENGINEER

Sheet pile walls are many times tied to some kind of anchors through tie rods to give them greater stability as shown in Fig. 20.21. The types of anchorage that are normally used are also shown in the same figure.

Anchors such as anchor walls and anchor plates which depend for their resistance entirely on passive earth pressure must be given such dimensions that the anchor pull does not exceed a certain fraction of the pull required to produce failure. The ratio between the tension in the anchor Ta and the maximum pull which the anchor can stand is called the factor of safety of the anchor.

The types of anchorages given in Fig. 20.21 are:

1. Deadmen, anchor plates, anchor beams etc.: Deadmen are short concrete blocks or continuous concrete beams deriving their resistance from passive earth pressure. This type is suitable when it can be installed below the level of the original ground surface.

2. Anchor block supported by battered piles: Fig (20.21b) shows an anchor block supported
by two battered piles. The force Ta exerted by the tie rod tends to induce compression in
pile Pj and tension in pile P2. This type is employed where firm soil is at great depth.

3. Sheet piles: Short sheet piles are driven to form a continuous wall which derives its resistance from passive earth pressure in the same manner as deadmen.

4. Existing structures: The rods can be connected to heavy foundations such as buildings, crane foundations etc.

Location of Anchorage
The minimum distance between the sheet pile wall and the anchor blocks is determined by the failure wedges of the sheet pile (under free-earth support condition) and deadmen. The anchorage does not serve any purpose if it is located within the failure wedge ABC shown in Fig. 20.22a.

If the failure wedges of the sheet pile and the anchor interfere with each other, the location of the anchor as shown in Fig. 20.22b reduces its capacity. Full capacity of the anchorage will be available if it is located in the shaded area shown in Fig. 20.22c. In this case



1. The active sliding wedge of the backfill does not interfere with the passive sliding wedge of the deadman.
2. The deadman is located below the slope line starting from the bottom of the sheet pile and making an angle Ø with the horizontal, 0 being the angle of internal friction of the soil.

Hagerty and Nofal ( 1 992) provided a set of design charts for determining

1. The depth of embedment
2. The tensile force in the anchor rod and
3. The maximum moment in the sheet piling

The charts are applicable to sheet piling in sand and the analysis is based on the free-earth support method. The assumptions made for the preparation of the design charts are:

1. For active earth pressure, Coulomb's theory is valid
2. Logarithmic failure surface below the dredge line for the analysis of passive earth pressure.
3. The angle of friction remains the same above and below the dredge line
4. The angle of wall friction between the pile and the soil is 0/2

The various symbols used in the charts are the same as given in Fig. 20.15 where,

ha = the depth of the anchor rod below the backfill surface
hl =the depth of the water table from the backfill surface
h2 - depth of the water above dredge line
H = height of the sheet pile wall above the dredge line
D = the minimum depth of embedment required by the free-earth support method
Ta = tensile force in the anchor rod per unit length of wall

Hagerty and Nofal developed the curves given in Fig. 20.18 on the assumption that the water table is at the ground level, that is h1 = 0. Then they applied correction factors for h1, > 0. These correction factors are given in Fig. 20.19. The equations for determining D, Ta and M(max) are


The theoretical depth D as calculated by the use of design charts has to be increased by 20 to 40% to give a factor of safety of 1.5 to 2.0 respectively.