Horizontal loading on suction piles depends on attachment point, soils
Where to connect mooring lines?
The results of resistance testing of suction piles to horizontal loading conducted for the US Navy agree with analytical solutions developed earlier, and produced the following findings:
- The ultimate horizontal loading capacity of the piles increases significantly as the loading point is moved toward the tip of the pile, but decreases beyond the point of translation (equidistant lateral movement).
- For clay seabeds, the loading point associated with the maximum ultimate horizontal loading capacity is slightly below mid-length of the buried distance (0.55 of the length).
- For sand, the ultimate horizontal loading capacity is greater (about 0.8 of the length of burial).
- The longer the pile is for a given contact area with the soil, the larger the ultimate horizontal loading capacity, and that ratio effect is more pronounced in sand.
These conclusions were cited in the technical paper entitled "Ultimate Horizontal Loading Capacity of Suction Piles," authored by Sangchul Bang and Yeongki Cho of the South Dakota School of Mines and Technology. The paper was presented at the Eleventh International Offshore and Polar Engineering Conference in Stavanger earlier this year.
The authors conducted the tests for US Navy studies of the behavior of suction piles in anchoring self propelled mobile offshore air bases. The proposed design is 150 meters in width by 1,500 meters in length.
The graph above shows the ultimate horizontal loading capacity of a pile in clay under various loading points.
In order to determine the maximum horizontal loading for mooring purposes, pile and mooring designers must know where that mooring line-to-pile connection point must be, and also the ideal dimensions and aspect (width-to-length) ratio of the pile.
When the mooring line is attached to the middle of a buried suction pile, as opposed to the top or bottom, the ultimate horizontal pile loading capacity increases significantly. Suction piles, when horizontally loaded, can rotate about a point within the pile length or outside, or translate (lateral yielding motion). Unless the line connection is ideally positioned to induce translation, rotation will reduce the loading capability.
The graph above shows the ultimate horizontal loading capacity of a pile of sand under various loading points.
During testing, the study used the soil failure wedge in the shape of a tetrahedron, which has been used previously. A suction pile having a diameter of three meters and a length of six meters was selected as the control pile. Developed lateral earth pressures between the pile and the soil were calculated. The study assumed that an equal percentage of soil friction and cohesion developed at all times, which the authors stated was not necessarily true.
Thirteen cases were developed to describe possible modes of pile movement. Maximum displacement occurred at the top or bottom of the pile and no lateral movement occurs at the point of rotation. The major cases developed were:
- Rotation point is above the top of the pile, which occurs when the load was applied below the point of translation and maximum displacement was at the bottom of the pile.
- Rotation point is between the top of pile and mid-point, which occurs when the load is applied at the bottom of the pile and maximum displacement takes place at the bottom.
- Rotation point is between the bottom of pile and mid-point, which occurs when the load is applied at the top of the pile, and that is where maximum displacement occurs.
- Rotation point takes place below the bottom of the pile, which occurs when the load is applied above the translation point and maximum displacement occurs ata the top of the pile.
- Pure translation takes place when deformation of soil takes place along the entire length of the pile simultaneously.
For suction piles embedded in clay, the pile horizontally loaded at the depth of 0.55L, which means 0.55 of the distance between the pile top (0.00) and the pile bottom 1.00) providing a translational movement rather than any rotation.
The maximum horizontal loading capacity for translation is about 2.8 and 2.3 times the ultimate horizontal loading for loads applied respectively at the top and bottom of the pile.
For suction piles embedded in clay, the translation point is much closer to the pile tip than when positioned in clay. That pile connection point in clay is at a depth of 0.8L, which means 0.8 of the distance between the pile top (0.00) and the pile bottom 1.00) providing a translational movement rather than any rotation. The maximum horizontal loading capacity is about five times greater than minimum values at the top or bottom.
The team also investigated the width-to-length ratio (aspect ratio) in clay and sand. Pile ratios tested were 2:1, 1:1, 1:2, 1:3, and 1:4. For suction piles in clay, the loading point for maximum horizontal loading shifted toward the tip of the pile with the increase in the aspect ratio. The aspect ratio of 1:4 performed best.
For section piles in sand, the best aspect ratio was again 1:4. The longer the pile is for a given contact area between the pile and sand, the more effective it becomes. According to the authors, the effect of the pile aspect ratio is more pronounced for sand due to the effect of the soil strength increase with depth.
Bank, S., Cho, Y., "Ultimate Horizontal Loading Capacity of Suction Piles," Proceedings of the Eleventh International Offshore and Polar Engineering Conference, Stavanger, Norway, June 17-22, 2001.