Micropiles are small in diameter (less than 300 mm) cast-in-place replacement piles, composed of steel reinforcement and placed or injected grout. They can withstand axial (compressive/tensile) and lateral loads, and work as components in a composite soil/pile mass or as small-diameter substitutes for conventional piles. Micropiles are used as underpinning elements to enhance bearing capacity of existing foundations and prevent excessive settlements.
They can also be used as foundations for new structures and land stabilization.
Micropiles are distinguished in providing innovative drilling and grouting techniques, which in return enhance load resistance. Reinforcement steel elements comprise to about 50 percent of the pile’s volume and is considered to be the main load bearing element, while the grout serves
to transfer the load to the surrounding soils.
To evaluate the skin friction of micropiles embedded in gravelly soils, a numerical model was developed using the geotechnical software program GEO5 for piles. The model was validated using existing field tests. Upon achieving satisfactory results, the model was used to generate 30
load tests that were performed on micropiles in three types of gravelly soils, ranging from Silty Gravel (GM) to Well graded Gravel (GW). Load-Displacement curves were then developed and carefully examined. The parametric factors and geotechnical properties that could affect the skin
friction were examined in a detailed manner. Present capacities measured from the Load-Displacement curves were then compared with existing design equations proposed by Reese and O’Neill, Hassan and O’Neill and Kulhawy. Based on all the information gathered, modifications to existing design equations were developed when needed to predict the skin friction for micropiles embedded in Gravelly Soils, while providing a marginal factor of safety.