Unlike most pile designs, minipiles are primarily designed based on their internal capacity rather than external carrying capacity due to their small cross-sectional area. This distinction arises from two key reasons:
High Slenderness Ratio: Minipiles exhibit a high slenderness ratio, meaning their length is significantly greater than their diameter. For instance, a 5-meter long minipile with a 200mm diameter has a shaft area 100 times larger than its cross-sectional area. This disproportionately large shaft area leads to a greater mobilization of shaft friction compared to end bearing resistance, making shaft friction the dominant load-carrying mechanism.
Settlement Requirements: To fully mobilize the end bearing capacity of minipiles, settlements of 10-20% of the pile diameter are necessary. In contrast, maximum shaft resistance can be developed with settlements as small as 0.5-1% of the pile diameter. This implies that achieving full end bearing capacity requires substantially larger settlements than those needed for maximum shaft resistance.
Given these factors, minipiles are often designed as friction piles, where the load-carrying capacity primarily relies on shaft friction. This design approach capitalizes on the large shaft area and the ease of mobilizing shaft resistance.
Another crucial aspect of minipile design is the use of left-in casings. These casings offer several advantages:
- Corrosion Protection: Left-in casings protect the main reinforcement bars from corrosion, ensuring the long-term structural integrity of the minipile.
- Lateral Buckling Resistance: The casings provide additional restraint against lateral buckling, a potential failure mode for slender elements like minipiles.
- Improved Grout Quality: The casings prevent groundwater intrusion during concreting, guaranteeing the quality and strength of the grout.
- Necking Prevention: During the lifting of casings after concreting, the presence of the casing prevents the formation of necking, a localized reduction in the cross-sectional area that can weaken the minipile.
In conclusion, the design of minipiles is unique due to their small cross-sectional area and high slenderness ratio. These factors necessitate a focus on internal capacity and shaft friction as the primary load-carrying mechanism. The use of left-in casings further enhances the performance and durability of minipiles, making them a versatile and reliable foundation solution for various applications.