Sand state and performance analysis of micropiles

The incomplete description of the state of sand in current micropile design and past full scale tests have resulted in a number of anomalous results. A state dependent bounding surface sand constitutive model within the framework of critical state soil mechanics is formulated here. The constitutive model is coded with C++ and implemented into the finite difference geotechnical software FLAC3D. The model and its implementation are verified by using laboratory triaxial tests on Toyoura, Ottawa, and Fontainbleau sands.; The performance of single, vertical group, and inclined network piles installed in sands with different initial state is studied systematically using the state dependent constitutive model. The finite difference grid in FLAC3D for pile arrangement with the state dependent sand model was first validated using a full scale test on a micropile group on Fontainbleau sand. The FLAC3D model was then used to study the effect of key variables that control the performance of micropiles.; The results show that the side resistance of micropiles is influenced by the initial normal stress applied to the pile and its positive increment by shear induced dilation of dilative sand and negative increment due to contraction of contractive sand. The load transfer in a group of piles is controlled by two mechanisms: one due to overlap of stresses induced by adjacent piles, and the other due to dilatancy. Overlap of shear stresses as determined by pile spacing might lead to a positive group effect on side resistance. Shear induced dilation in dilative sands contributes to a positive group effect on side resistance in bored piles with well developed interfaces; however its effect is negative for contractive sand. Both mechanisms contribute positively to the group effect on tip resistance. The studies also show that the effect of inclination on pile resistance is affected by the sand state. When considered as structure elements, outward inclined network pile groups have higher resistance than inward inclined pile groups.