Experimental and numerical investigation of the effect of vertical loading on the lateral behaviour of monopiles in sand

Abstract

The influence of combined loading on the response of monopiles used to support offshore wind turbines (OWTs) is investigated in this paper. In current practice, resistance of monopiles to vertical and lateral loading is considered separately. As OWT size has increased, the slenderness ratio (pile length, L, normalised by diameter, D) has decreased, and foundations are tending towards intermediate footings with geometries between those of piles and shallow foundations. Whilst load interaction effects are not significant for slender piles, they are critical for shallow footings. Previous research on pile load interaction has resulted in conflicting findings, potentially arising from variations in boundary conditions and pile slenderness. In this study, monotonic lateral load tests were conducted in a geotechnical centrifuge on vertically loaded monopiles in dense sand. Results indicate that for piles with L/D = 5, increasing vertical loading improved pile initial stiffness and lateral capacity. A similar trend was observed for piles with L/D = 3, when vertical loading was below ≈ 45% of the pile’s ultimate vertical capacity. For higher vertical loads considered, results tended towards the behaviour observed for shallow footings. Numerical analyses conducted show that changes in mean effective stress are potentially responsible for the observed behaviour.

Influence of Scour Protection on the Vertical Bearing Behaviour of Monopiles in Sand

“…The free length of the pile body can be expressed as e = 8D, calculated from the original seabed surface. It is worth noting that the prototype dimensions of the pile were smaller than those used for OWTs in practical construction [16]. Large piles could not be used in this centrifuge due to several boundary effects, e.g., limitation of the centrifuge basket and of the strong box holding the sand specimens.…”

“…The ratio of outer pile diameter to mean particle size of the sand (D/D 50 ) was approximately 164, which was considered large enough to avoid the effect of particle size [16]. The ratio of pile wall thickness to mean particle size of the sand (t/D 50 ) was 9.1, very close to the suggested limiting value of 10 [20,21], allowing the full interaction between the pile annulus and the soil [16]. Influence of water was excluded in the experimental trials conducted in this article.…”

Influence of Vertical Load on Lateral-Loaded Monopiles by Numerical Simulation

“…To reveal its underlying mechanism, the variations of mean effective stresses and lateral effective stresses along the depth of the piles were extracted, as presented in Fig. 7 [25]. The locations of selected stress points are illustrated in Fig.…”