A Model Study of Micropile Groups Subjected to Lateral Loading under Different Relative Density-Paper

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This model study examined the lateral-load behaviour of micropile groups in loose sand (30% relative density) and dense sand (80%). Tests used single piles and four-pile square groups with length-to-diameter (L/D) ratios of 12–80; group spacings were 2D, 4D, and 6D. The piles were installed in a model sand tank and loaded horizontally until their lateral resistance was exceeded.<br /><br />Sand density, pile L/D ratio, and group spacing all influenced group capacity and efficiency. For loose sand, both ultimate lateral capacity and efficiency were greatest at 2D spacing, followed by 4D and 6D. In dense sand, they were greatest at 6D spacing, followed by 4D and 2D. Increasing L/D generally raised capacity and efficiency, although gains became small at higher ratios. Single-pile capacity was higher in dense than in loose sand. Group capacity generally increased rapidly with L/D up to about 36, with limited gains at larger ratios.<br /><br />Positive group effects—where group capacity exceeded the summed capacity of equivalent individual piles—occurred at 2D spacing in loose sand and 6D spacing in dense sand. A few high-L/D groups at 4D spacing in dense sand also showed positive effects; the remaining groups showed negative effects.<br /><br />Failure mode depended primarily on L/D ratio and sand density, not group spacing. Shorter groups failed through soil resistance being exceeded, while longer groups generally experienced structural failure, marked by cracking in the pile body. The transition from soil to structural failure occurred at higher L/D ratios in loose sand than in dense sand.<br /><br />The results indicate that micropile group design under lateral loading should account for both soil density and pile spacing; the spacing that maximizes performance differs between loose and dense sand.

Keywords

micropile groups

lateral load behaviour

loose sand

dense sand

pile spacing

group capacity

group efficiency

length-to-diameter ratio

positive group effect

structural failure