The internal capacity of a micropile frequently governs its overall design, because of its small cross sectional area and the large resistance provided by the grout/ground bond due to the construction techniques.
The reinforcement steel is the element that carries most of the load. However the load is resisted by both the steel and the grout. It is important to take into account this composite action to optimize the internal pile design. The use of steel pipe or casing as reinforcement elements has become more popular, especially when requiring minimal deflections or supporting lateral loads.
In practice, the design compressive stress in the steel reinforcements is limited to 50% of the yield strength. The pile capacity is normally derived from the allowable structural capacity of the reinforcements in the preliminary design.
Other components, such as the grout and additional reinforcement bars can be included to enhance the allowable structural capacity. However extra care is needed to ensure its effectiveness during construction.
The grout commonly consists of cement and water, with water-cement ratios between 0.40 and 0.55. The minimum ratio is set by the requirement that the grout should be fluid enough to allow efficient pumping and injection. The maximum ratio is imposed results because an excessive amount of water would cause bleeding, low strength, increased shrinkage and poor durability. Fine sands can be added to the mix to reduce costs. Sand cement ratios are limited to 3, but they rarely exceed 1.5. Admixtures are added to modify grout properties: prevent shrinkage, reduce water content, maintain pumpability, accelerate or retard setting, and, prevent bleeding.