Reinforcement of Existing Deep Foundations with Micropiles-Paper

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The paper examines the use of self-drilling micropiles to reinforce an existing bored-pile foundation so a two-story parking structure could support an extension of an adjacent five-story building. The original 0.9 m diameter, 9 m long piles were designed for a service load of 1,500 kN; the new structural requirements doubled this to 3,000 kN.<br /><br />Four micropiles were proposed around each existing pile and connected to it by a rigid pile cap. The authors first estimated the original pile’s load-settlement behavior using German piling recommendations, then checked it against a three-dimensional PLAXIS model. The two approaches agreed well over the settlement range relevant to design. The numerical model was then used to assess how the existing pile and micropiles would share load, including the effect of micropile inclination.<br /><br />With the reinforcement, the system could carry the 3,000 kN service load at about 1.5 cm settlement, compared with approximately 4 cm for the original pile alone. At this settlement, the existing pile carried about 1,800 kN and the four micropiles together carried about 1,200 kN. Each micropile was designed for a characteristic vertical load of approximately 300 kN. A 15-degree inclination was selected to allow installation close to the existing pile while limiting the size of the pile cap.<br /><br />The study concludes that micropiles can effectively increase the capacity of existing deep foundations while maintaining acceptable settlements. Their flexible installation and low-vibration equipment make them especially suitable for retrofit work in confined spaces and occupied facilities.

Keywords

self-drilling micropiles

foundation reinforcement

bored-pile foundation

load-settlement behavior

PLAXIS model

pile cap

micropile inclination

structural retrofit

settlement analysis

deep foundation capacity