A simplified approach for negative skin friction calculation of special-shaped pile considering pile-soil interaction under surcharge

A simplified approach was proposed to analyze the negative skin friction calculation of special-shaped pile considering pile-soil interaction under surcharge. Based on the concentric cylinder shearing theory, considering the changes of pile shape(such as, taper angle and diameters of pile base, etc.), the load-transfer of special-shaped pile was built. The accuracy of the developed simplified approach was verified by numerical simulation model with the same condition. Then, the influence factors, such as, taper angles, the diameter of pile base, surcharge, and pile-soil interface parameters were analyzed and discussed. The results show that the developed simplified approach can calculate NSF of special-shaped pile under surcharge effectively. A limited parametric study indicates that in many practical situations special-shaped piles(such as belled wedge pile shown in this work) offer a design option that is more economical than traditional uniform cross-section piles.

Theoretical Formula for Calculating Negative Skin Friction of Pile in Layered Soil and Its Application

Based on Zeevaert＇s method, a theoretical formula was developed to calculate the negative skin friction of pile in layered soil. For practical purpose,a cut-and-try method was proposed to determine neutral point. Case studies indicate that the total calculated negative skin friction was in agreement with the measured one, which verifies the feasibility and practicability of theoretical formula. Furthermore, the methods for calculating efficiency factor of drag load and settlement were also given.

Analysis of Negative Skin Friction on Single Pile above a Tunnel

A finite element program with two different soil models is adopted to study the negative skin friction of an unloaded single pile embedded in the settlement trough of a tunnel. The results show that it is necessary to consider the nonlinear property of soil and to set interface elements between piles and soil in analysis of the negative skin friction of piles. Among a pile group, the pile located above the tunnel centerline bears larger downdrag than the ones far away from tunnel centerline.

Dragload and Downdrag Performances of Inclined Pile Group Embedded in Consolidating Soil

The performance of inclined pile group embedded in consolidating soil under surcharge load was investigated by experiment in comparison with vertical pile group and single pile under the same conditions; dragload, downdrag, and layered soil settlement were measured. A three-dimensional numerical model was built via FLAC3D software, and verified by the experimental results. Influence factors, such as consolidation time, pile spacing, and pile tilt angle were analyzed. The results show that dragload of inclined pile group increases with the increase of consolidation time and pile spacing or the decrease of pile tilt angle. Downdrag of inclined pile group increases with the increase of consolidation time, pile spacing and pile tilt angle.

Loading Sequence Effects on Dragload and Downdrag for Pile Foundation

Negative skin friction (NSF) is one of the important problems when designing a pile foundation. However, the influence of loading sequence on the dragload and downdrag for pile foundation is seldom studied. In this paper, a three-dimensional numerical model was established using FLAC3D. Compared with the results of model test, the established model could be used to study the NSF of pile foundation. The influencing factors were discussed including the length-diameter ratio of pile and the loading sequence of pile head load and surcharge. A case history was analyzed using FLAC3D. The calculated results are in good agreement with the measured results. It is concluded that the dragload and downdrag are remarkably influenced by the loading sequence of pile head load and surcharge. The dragload and downdrag reach the maximum values under the condition of surcharge after pile head load.

Negative differential friction coefficients of two-dimensional commensurate contacts dominated by electronic phase transition

Friction force(f)usually increases with the normal load(N)macroscopically,according to the classic law of Da Vinci–Amontons(f=μN),with a positive and finite friction coefficient(μ).Herein near-zero and negative differential friction(ZNDF)coefficients are discovered in two-dimensional(2D)van der Waals(vdW)magnetic CrI_(3)commensurate contacts.It is identified that the ferromagnetic–antiferromagnetic phase transition of the interlayer couplings of the bilayer CrI_(3)can significantly reduce the interfacial sliding energy barriers and thus contribute to ZNDF.Moreover,phase transition between the in-plane(p_(x)and p_(y))and out-of-plane(p_(z))wave-functions dominates the sliding barrier evolutions,which is attributed to the delicate interplays among the interlayer vdW,electrostatic interactions,and the intralayer deformation of the CrI_(3)layers under external load.The present findings may motivate a new concept of slide-spintronics and are expected to play an instrumental role in design of novel magnetic solid lubricants applied in various spintronic nano-devices.

An improved method for evaluating the rotational speed stability of a hydro-viscous clutch in mixed lubrication

Rotational speed stability is an important evaluation indicator of the performance of a hydro-viscous clutch(HVC).To improve the rotational speed stability of HVCs in mixed lubrication and the running condition of the friction pairs,the speed stability of an HVC in mixed lubrication was studied.To this end,the friction coefficients of both copper-based and paper-based friction pairs were experimentally tested using an MM1000-III wet friction machine.Theoretically,a torsional vibration model of the system is presented.The phase plane analysis method is applied to evaluate the stability of the torsional vibration model,where a critical negative gradient(CNG)is defined.The results show that the friction coefficient in mixed lubrication is an important parameter for the stability of the rotational speed.The system will be unstable when the negative gradient of the friction coefficient-slip speed is larger than the CNG.According to the definition of the CNG,suggestions regarding choice of friction pairs are made to improve the rotational speed stability of an HVC in mixed lubrication.