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 T 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 P1 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.
|Figure 20.21 Types of anchorage: (a) deadman; (b) braced piles; (c) sheet piles; (d)|
large structure (after Teng, 1969)
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 0 with the horizontal, 0 being the angle of internal friction of the soil.
|Figure 20.22 Location of deadmen: (a) offers no resistance; (b) efficiency greatly|
impaired; (c) full capacity, (after Teng, 1969)