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Practical Advice for Foundation Design – Micropiles for Structural Support - page 12 / 25





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GSP 131 Contemporary Issues in Foundation Engineering

Analytical results that discount these connections may be unconservative in the upper 1.5 to 3 m. The simple practical solution is therefore to either place additional steel section in the upper length, or to use a longer casing in the top section which would therefore develop acceptable bending resistance. Where a second concentric casing is used, joints should be staggered by 1 m vertically.

5.2.6 Lateral Resistance of Micropile Groups. micropile behavior in a group configuration is individual elements. The deflection of a pile group

Similar to other types influenced by spacing under a lateral load may

of piles, between be 2 to 3

times larger than the deflection Holloway et al. (1981) and Brown pile groups have significantly less

of a single pile loaded to the same intensity. et al. (1988) reported that piles in trailing rows of resistance to a lateral load than piles in the lead

row, and therefore exhibit greater deflections.

This is due to the












pile-soil-pile results in the

lateral capacity of a pile group being less than the sum of individual piles comprising the group. Hence, laterally

the lateral capacities of the loaded pile groups have a

group efficiency of less than 1. Based on FOREVER National Project on horizontally conclusions were drawn:

experiments conducted as part of the loaded micropile groups, the following

  • For in-line micropiles, group effects are negligible for micropile spacing between 6 to 7 diameters; and

  • For micropiles arranged in a row (i.e., perpendicular to the direction of loading), group effects are negligible for micropile spacing just greater than 3 diameters.

The lateral capacity of an individual pile in a pile group is a function of its position in the group and the center to center pile spacing. Brown et al. (1998) proposed a p- multiplier, Pm, be used to modify the p-y curve of an individual pile based upon the piles row position. For piles in a given row, the same Pm value is applied to all p-y curves along the length of the pile. In a lateral load test of a 3 by 3 pile group in very dense sand with a center to center pile spacing of 3b, Brown found the leading row of piles had a Pm of 0.8 times that of an individual pile. The Pm values for the middle and back row of the group were 0.4 and 0.3, respectively.

A summary of additional laterally loaded driven pile group studies is provided in FHWA-HI-97-013 (1996). Also, preliminary results from the FOREVER project indicate that Pm multipliers for micropiles may be slightly higher than those recommended herein, however, only limited data from centrifuge testing is available at this time.

Micropiles can easily be inclined to provide additional resistance to lateral loading. Pile groups that contain inclined piles are relatively stiff and will undergo less lateral movement for a given load than for a system with the same number of vertical piles. However, this increased system stiffness also results in greater bending moments in the pile cap which is a concern, especially for highly seismic regions. Moreover,


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