Engineering and Construction

Because piles are usually rather slender members, they are not efficient in resisting lateral loads. Hence the pile cap is often equipped with additional components, such as batter piles, lateral tieback anchors, or deadman, which are used to resist the lateral load. The lateral resistance can also be increased by placing and compacting sand or gravel near ground surface to provide a surface zone of high lateral resistance material and by increasing the diameter of the pile near ground surface by using a concrete collar, pile wings, deepened concrete cap, or constructing short piers adjacent to and in contact with the laterally loaded pile (Broms, 1972).

Batter Piles. A cost-effective method to resist lateral loads on pile foundations is to use batter piles.

If the lateral load were transmitted to the pile cap in only one direction, then batter piles would be installed in line with the lateral load. For lateral loads in both directions, batter piles would need to be installed on all sides of the pile cap. Inclinations of batter piles typically range from 1:12 to 5:12 (horizontal:vertical).

Early analyses of batter piles were based on the assumption that the vertical piles would only carry vertical loads and the batter piles would only carry lateral loads. This made the analyses easy in the sense that standard pile design from Secs. 6.3.2 and 6.3.3 could be used to determine the allowable load capacity of the vertical piles. The batter piles were designed as compression or tension members with their resultant horizontal component carrying the lateral loads. However, studies have shown that when a pile group with batter piles is subjected to lateral load, the batter piles do indeed resist most of the lateral load, but the vertical piles also carry some lateral load. This results in bending moments being introduced into the vertical piles, which were only designed to resist compressive stresses. The bending moment led to failure of the vertical piles due to lack of tensile reinforcement.

Since the simple analysis is not appropriate for lateral loading of pile groups having batter piles, more complicated methods have been developed. These methods often use complex computer programs that consider the relative stiffness of the entire pile system (i.e., vertical and batter piles) as well as the soil lateral resistance.

Pressuremeter Test. Section 2.4.4 has presented a discussion of the pressuremeter test. In order to determine the stress-strain curve for horizontal loading, the equipment is either inserted in a predrilled borehole or a self-boring pressuremeter is utilized. Since this field test measures the in situ horizontal response to lateral pressure, it is ideally suited for the design of piles subjected to lateral loads. For information on the design methods, see “Laterally Loaded Piles and the Pressuremeter:

Comparison of Existing Methods,” (Briaud et. al., 1984).

Lateral Load Tests. Lateral load tests can be performed on an individual vertical pile, an individual batter pile, pile groups having only vertical piles, and pile groups with both vertical and batter piles. Lateral load tests are considered to be the most accurate method of determining the lateral load capacity of a pile or pile group. Unfortunately, a lateral load test can be very expensive and time consuming. Such a test measures the lateral load versus lateral deflection of the pile or pile group. For lateral load tests on pile groups, a pile cap must be constructed or the piles must be interconnected so that they act together. In a general sense, the type of piles, depth of embedment, and pile cap conditions should be constructed so that they simulate in-service conditions.

Often the pile cap is designed so that it also resists the lateral load due to passive soil pressure along its side. To simulate this condition, the cap can be cast against the soil or fill can be compacted against the side of the pile cap.

The usual method of applying the lateral load is to construct a rigid reaction system and then install hydraulic jacks between the reaction system and the test piles. As shown in Fig. 6.18, rigid reaction systems can consist of the following:

1. Reaction piles. A series of vertical and batter piles are anchored to a pile cap. The lateral resistance of the reaction piles must be greater than the test piles.
2. Deadman. This can consist of timbers or steel supports bearing against the sides of an embankment or slope so as to provide the necessary solid end support for the hydraulic jacks.
3. Weighted platform. A platform can be constructed and then weights added to the platform in order to provide the necessary resistance against the maximum lateral load to be applied to the test piles.

Once the test setup is constructed, the test pile or group is subjected to an incremental increase in lateral load. Depending on the type of lateral load that will be exerted on the pile during its actual use, different types of loading schedules have been developed. For example, ASTM D 3966-95 (2004) provides loading schedules for standard loading, excess loading, cyclic loading, and surge loading. The ideal situation would be to model anticipated field lateral-loading conditions. Thus, for example, if the pile is subjected to cyclic lateral loading for in-service conditions, then a cyclic loading sequence should be used. The pile is usually loaded to a value that is at least twice the lateral design load.

During the loading schedule, the horizontal deflection of the pile or pile group is measured. As shown in Fig. 6.19, the lateral load versus pile head deflection can be plotted and used for the design of the pile or to check that the design load will not cause lateral movement in excess of the allowable value.

The allowable lateral load is usually based on the maximum allowable lateral defection of the pile head. For some projects, the structural engineer may specific this value. In other cases, the building code may dictate the allowable lateral load capacity from load tests. For example, the International

Building Code (2006) in Sec. 1807.2.9.3 states: “The resulting allowable load shall not be more than one-half of that test load that produces a gross lateral movement of 1 inch (25 mm) at the ground surface.”

For further information on the lateral load testing of a pile or pile group, see ASTM D 3966-95 (2004), “Standard Test Method for Piles Under Lateral Load.”

p-y Curves. This method predicts lateral pile response by using a finite-difference model along with horizontal nonlinear springs, with each spring representing the lateral soil resistance as defined by its p-y curve. Each p-y curve represents the relationship between p, which is the horizontal soil resistance (expressed in units of force per length) and y, which is the horizontal displacement. The p-y curves depend on many factors, such as the soil type, type of loading (i.e., periodic or constant), pile characteristics (e.g., diameter), depth below ground surface, and group interaction effects. Generic  p-y curves, based on lateral load tests of piles, have been developed for soft clays, stiff clays, and sands and have been incorporated into computer programs that are used to obtain the pile defections as well as the shear and bending moments in the piles (Matlock, 1970; Reese et al., 1974, 1975). For closely spaced piles in a group, it has been proposed that the p-y curves be reduced by using “p-multipliers” to reduce all the p-values on a given p-y curve (Brown et al., 1987). The use of reduction factors for pile groups subjected to lateral loads will be discussed in Secs. 6.4.3 and 6.4.4. For more information on p-y curves, see Coduto (1994).

Passive Earth Pressure Theory. Since the pile or pier deforms laterally into the soil, passive earth pressure theory can be used to obtain the lateral bearing capacity. Passive earth pressure theory is also used for the design of retaining walls, where for example, the retaining wall footing moves laterally into the soil due to the lateral load induced by the wall backfill soil.

Table 11.1 presents magnitudes of rotation needed to develop passive pressure. Assuming uniform rotation of the upper 5 ft (1.5 m) of the pile, the amount of lateral movement for the soil to reach passive pressure is about 0.4 in. (1 cm) for a pile embedded in loose sand (i.e., 0.006 times 60 in. = 0.4 in.) and about 21/2 in. (6 cm) for a pile embedded in soft cohesive soil (i.e., 0.04 times 60 in. = 21/2 in.). Thus for a loose sand deposit, a pile head that has a maximum allowable lateral deflection of 0.4 in. (1 cm) would only develop passive resistance in approximately the upper 5 ft (1.5 m) of soil.

This limited depth of passive resistance has been recognized by the International Building Code (2006), which states: “piles driven into firm ground can be considered fixed and laterally supported at 5 ft (1.5 m) below the ground surface and in soft material at 10 ft (3 m) below the ground surface” (Sec. 1807.2.9.2).

The next two sections will present passive earth pressure theory for cohesionless and cohesive soil.