Search

Helical pile foundations are a new foundation type for offshore wind power applications with high bearing capacity and good recovery that can be quickly and easily constructed. In this study, the finite element method was used to simulate the bearing characteristics of helical pile foundations after installation. For simulations, a blade, deeply buried in a single layer of sand, was selected. Through numerical simulations, the vertical bearing characteristics of a single helical screw pile and an ordinary pile without blades were compared, and the compression and uplift characteristics of the helical pile were revealed. In addition, the effects of pitch, blade diameter, inclination angle, number of blades, and blade spacing on the bearing characteristics of a single helical pile were analyzed. The results show that the single helical pile has the highest bearing capacity and bearing efficiency when the pitch is 0.02 times the blade buried depth, the blade diameter is 2.5 times the pile diameter, the multi-blade spacing is more than two times the blade diameter, and the number of blades is less than or equal to three. However, compared with the straight pile, the vertical bearing capacity of the single inclined helical pile did not improve significantly.

Helical foundations used in transmission towers, wind turbines, solar panel systems and other similar structures must exhibit adequate performance under cyclic loading. Although a number of studies on the cyclic behaviour of helical anchors has increased, the available observations are still incomplete in terms of providing conclusive recommendations for practical applicability. To address this need, cyclic and monotonic axial tensile load tests were conducted on a single-helix model anchor in very dense dry sand in a centrifuge. The cyclic tests were carried out with different load amplitudes and with the number of cycles varying between 1000 and 3000. After the cyclic tests, the post-cyclic tensile capacity was evaluated by way of monotonic loading tests. Two different displacement regimes were observed during cycling, with a noticeable development of cumulative permanent displacements in the first approximately 100 cycles. In most of the tests, a stable cyclic response and a slight reduction in the post-cyclic capacity were observed. Additionally, the effect of the cyclic loading sequence was evaluated by way of two additional tests in which the cyclic loading was applied with different amplitudes in different orders. The results indicated that the order of the cyclic sequences influences the displacement response and the losses in the post-cyclic capacity.

Helical piles are environmentally friendly and economical deep foundations that, due to environmental considerations, are excellent additions to a variety of deep foundation alternatives available to the practitioner. Helical piles performance depends on soil properties, the pile geometry and soil-pile interaction. Helical piles can be a proper alternative in sensitive environmental sites if their bearing capacity is sufficient to support applied loads. The failure capacity of helical piles in this study was measured via an experimental research program that was carried out by Frustum Confining Vessel (FCV). FCV is a frustum chamber by approximately linear increase in vertical and lateral stresses along depth from top to bottom. Due to special geometry and applied bottom pressure, this apparatus is a proper choice to test small model piles which can simulate field stress conditions. Small scale helical piles are made with either single helix or more helixes and installed in fine grained sand with three various densities. Axial loading tests including compression and tension tests were performed to achieve pile ultimate capacity. The results indicate the helical piles behavior depends essentially on pile geometric characteristics, i.e. helix configuration and soil properties. According to the achievements, axial uplift capacity of helical model piles is about equal to usual steel model piles that have the helixes diameter. Helical pile compression bearing capacity is too sufficient to act as a medium pile, thus it can be substituted other piles in special geoenvironmental conditions. The bearing capacity also depends on spacing ratio, S/D, and helixes diameter.

The helical piles have been being treated as a kind of novel foundation for offshore wind turbines recently, due their fast installation, high uplift capacity, convenience for recycling, and other advantages. The recycling of the helical pile especially will reduce the cost significantly and protect the environment as much as possible. However, the research for this area is basically in infancy and there is no reference for predicting the recycling torque of a helical pile in sand. In order to predict the recycling torque of single-plate helical piles in dense sand: a theoretical model, which was inspired by the way to predict the installation torque of single-plate helical pile in sand, was developed, and a series of single gravity model tests were conducted to verify that theoretical model. The theoretical model can predict the recycling torque of single-plate helical pile considering the influences of the size of helix and the vertical force on the shaft. This model fills in the blank of predicting the recycling torque of a single-plate helical pile in sand and it is also useful guidance for the choice of suitable recycling equipment.

Izpētīta skrūvpāļu pamatu sistēma un to darbības princips spiedē. Apskatīts slodzes nodošanas mehānisms uz grunti. Apskatītas trīs aprēķina metodes: Latvijas būvnormatīvs LBN 214-03 (СНиП 2.02-03-85), ASV kompānijas “CHANCE” un Kanādas būvinženieru izstrādātā lielas nestspējas skrūvpāļu aprēķinu metodes, kas ir paredzētas skrūvpāļu nestspējas aprēķiniem. Veikts metožu salīdzinājums. Izklāstīti skrūvpāļu pamatu izveidošanas principi, izvērtētas to priekšrocības un trūkumi. Skrūvpālis analizēts četrās dažādās gruntīs – smiltī, cietā un plūstošā smilšmālā un mālsmiltī ar vienu līdz sešām nesošajām lāpstiņām. Skrūvpāļu aprēķini spiedē veikti pēc ASV kompānijas “A.B.CHANCE” aprēķinu metodikas. Izvērtēti nespēju ietekmējošie faktori. Skrūvpāļu darbības analīzes laikā rastas sakarības, kuras var tikt izmantotas nosakot ekonomiski izdevīgāko un racionālāko skrūvpāli. Tā kā neviena no aprēķinu metodēm nenosaka skrūvpāļu deformācijas noteikšanas un izvērtēšanas principus, izveidots galīgo elementu aprēķina modelis datorprogrammā „Lira 9.2”. Ar tā palīdzību apskatīta skrūvpāļa un grunts darbība dažādās gruntīs pie noteikta slodžu lieluma ar vienu un divām nesošajām lāpstiņām. Tāpat aprēķinātas skrūvpāļu deformācijas un spriegumi grunts masīvā. Pārvietojumu aprēķināšanas metodikas izstrādāšana, un galīgo elementu aprēķina modeļa verifikācija un modifikācija grunts plastiskās darbības ievērtēšanai - ir virziens tālākiem pētījumiem.