As a kind of old construction form, pile foundation still plays a tremendous role nowadays. The fast developing and innovation of technology promote new types of pile constantly emerging, which has been a huge success. DX pile is one of those piles and has been widely used. The author illustrates the development, technological characters, advantages, research progress both at home and abroad, and engineering applications of DX pile.
This paper introduced the developing process of DX pile technology in theory research, equipment and engineering methods and practices. From the invention of the DX extruding-expanding rig, the drilling-extruding-cleaning rig, to the rotary extruding-expanding rig, DX pile technology achieved great progress and development.
In this paper, considering the special field with hydraulic fill sand in upper soil layer and fine sand in the lower soil, we adopt straight piles and DX piles in foundation of LNG storage tanks; a series of self-balanced load tests for piles are conducted. The purpose is to determine the ultimate bearing capacities and settlement of piles, analyze its load transfer mechanism, and study the distribution and development of the skin friction resistances and tip resistances. Comparing DX piles with straight piles, we find that DX piles are superior in ultimate bearing capacities and settlement; at the same time, the length of DX piles can be reduced apparently.
Based on numerical simulation, bearing capacity and settlement control for DX piles are studied. The property differences of conventional straight piles, DX piles with 2 bells and DX piles with 3 bells are presented in condition of identical length, diameter, and bearing capacity. Under the same load, DX piles with 3 bells have smaller settlement than that of DX piles with 2 bells and straight piles. As to bearing capacity, DX piles with 2 bells are 1.5 times greater than that of the straight piles, while DX piles with 3 bells are 1.2 times greater than that of DX piles with 2 bells. The paper can provide theoretical references for the design of bearing capacity of DX piles during the engineering practice．
This paper analyzes DX single pile with 2 bearing plates on its resistance of uplift load by numerical analysis method. The parameters for analysis are drawn from lab test and verified by large scale loading test of DX piles. The Q-s curve and axial force of DX pile are obtained,and the conclusions based on the analysis are drawn.The uplift bearing capacity of DX single pile is about more than twice larger than that of the normal single pile and its bearing plates work out from the upper to the lower order, while when the bearing capacity reaches certain level, the lower plate will play a more important role.
DX pile which is a new-type of variable cross-section pile has the characteristics of easy, convenient and fast construction. When constructing the DX pile, only one extra procedure of extruding and expanding the bell cavities is added, after drilling (impacting) hole. So that the form of pile shaft, bells and pile tip bearing the load together can be achieved. DX pile has not only high bearing capacity, but also effectively controlled settlement of pile deformation. The static load test of a highway bridge foundation shows that the bearing capacity of DX pile is more than twice of the straight pile with the same parameters, while the settlement is only a half. Meanwhile, the ascending displacements of anchor piles are less than 1 mm, which means even in the uplift performance, DX pile also has a great advantage.
DX pile which is typical varied cross-section pile has a different load transfer mechanism compared to straight pile. The bearing capacity of the pile is increased, while the settlement is significantly reduced at the same time. The characteristic of multi-pivot bearing makes the settlement mechanism very complicated and hard to calculate. This paper compares the settlement characteristics of DX pile and straight pile on the basis of lots of projects to verify the DX pile settlement formula and the reliability of correction factor.
DX pile group is simulated by numerical analysis software FLAC3D to study the mechanics behavior and the pilesoil settlement rules. Analysis of the influence of pile spacing and plate arrangement to ultimate bearing capacity and settlement control, and comparison of the ultimate bearing capacity of straight pile group with 8 piles with the DX pile group with 6 piles are done. The result indicates that the pile of DX pile group has the same bearing mechanism with the DX single pile. The displacement of the soil below the bearing plate is greater than that above the bearing plate, which leads to the free surface of the upper cavity. 3D (D is the diameter of the bell) pile spacing can perform the best bearing capacity and settlement control. Within the same stratum, bell arrangement (staggered arrangement and parallel arrangement) has little effect to the bearing capacity of DX pile group. The ultimate bearing capacity of DX pile group with 6 piles is as 1.27 times as the straight pile group with 8 piles. With DX pile group, we can not only effectively control the settlement, but also reduce the number of piles and save the project cost. These research results provide valuable basis for the rational design of pile groups of this type.
This paper analyzes 4 types of group-pile which contains 4 piles with spacing 1.5D, 2D, 2.5D and 3D（D is diameter of the bearing plate）by numerical analysis, and draws some conclusions. The main conclusions are: group pile effect of DX pile increases sharply when pile spacing decreases；the bearing capacity of 3D spacing group-pile is 1.32 times larger than that of 1.5D spacing group-pile；effect coefficient of group-pile increases from 0.59 to 0.78 when spacing increases from 1.5D to 3D；the maximum value of settlement of soil among group-pile occurs between two bearing plates, and it decreases when the spacing of pile increases.
Different industry has different calculation method for bearing capacity of pile. Industry of railway, high way and building treat the tip bearing capacity of pile in different ways. DX pile is a new kind of varied cross-section pile. Applying calculation methods in the specification of these three industries on DX pile will induce significant different results. Based on 18 case of DX pile, the difference of calculation methods were discussed and a new design method for DX pile of bridge was proposed.
DX pile is a new type of variable cross-section pile developed from last decades. This pile has been applied in many foundations of buildings for its great advantage of high bearing capacity and low settlement. Nowadays some foundations of bridges of highway road have adopted DX pile. A case of DX pile in Beijing was analyzed in this paper. Two DX piles and one straight pile were tested by static loading method. The vertical bearing capacity and Q-s curves were obtained. The settlements of different piles were also compared. The axial force along the shaft of DX pile was listed and the effects of bearing capacity of bells were analyzed.
When the DX-pile is under uplift load, the pile-soil interaction consists of pile shaft resistance and uplift resistance provided by the bells which effectively raises the uplift-pile ultimate bearing capacity. The friction resistance along pile shaft friction has been study well and has lots of results, but the problem of calculating the resistance of bell is still under studied which is the key to DX-pile uplift capacity. This paper calculates the bearing capacity of DX-pile and compares it with the measured value on the basis of a project to discuss and study these calculation methods, and a new method of calculation is also proposed.
As the existence of load-carrying plate DX pile changes the load transfer mechanism of piles, improves capacity and also significantly reduces the settlement deformation. In this paper, on the basis of field trials, the distribution characteristics of load of DX piles and equal diameter piles are analyzed, and thus according to Geddes solution the soil pile superimposed stress is solved by using a program developed by authors. The results show that in the same load conditions, the settlement of DX pile is 0.55 to 0.80 times as compared with equal diameter pile. The changes of the settlement caused by load-carrying plate are analyzed.
The in-situ vertical resisting compression static test of DX pile is made and it is compared with post-grouting bored pile. The results indicate that DX pile and post-grouting bored pile can enhance capacity and control settlement effectively;effect of four test piles is not quite different, but the DX pile is of smaller diameter and shorter length which can meet large capacity; its concrete bearing capacity of per cubic meter is 1.5 times larger than that of post-grouting bored pile; DX pile can reduce engineering materials to lower project cost and speed up the construction progress.
DX pile is a new kind of variable cross-section pile invented recent years, its unique load-carrying bells will reduce the settlement. Through small scale model test to study the theory of the DX pile. This experiment uses strain gage for studying of axial force analysis, dial indicator for studying of settlement, lever instrument for grading loading. It bases on the comparative tests with the straight pile and DX parallel experiment. The results show that the DX pile is superior to straight pile both on bearing capacity and settlement; the upper bell bears more than that of the below, but with the total load increasing, the below bell’s bearing capacity proportion increase.
Aiming at the asymmetric double-arch tunnel engineering that lies at Jiaozhou Bay in Qingdao, the section located in the intersection between main tunnel and ramb tunnel is chosen as a typical cross section and a three-dimensional model for numerical analysis. Four construction schemes are constituted according to different construction sequences and different digging lengths. Then these schemes are simulated with FLAC3D program. By analyzing ground surface settlement, vault settlement values, displacement of supporting structure, principle stress in surrounding rock, as well as principle stress in supporting structure, the most disadvantage position of structural deformation and stress during construction is determined. It concludes that the surrounding rock deformation could be better controlled by using of the construction sequence that first middle drift and then right hole of ramb tunnel, and the reasonable construction scheme is suggested.
The geology of the region which Baojianshan tunnel crossed through was very complex. The layer was consisted of broken phyllite which was easily softened with water. The rock mass had collapsed several times while excavation by the construction methods which designers offered. In this article, the sub-classification of phyllite was established according to State Standard Classification of Rock and Designing Standard of Railway Tunnel. Based on this sub-classification, a series of construction methods in phyllite were set up eventually. And Baojiashan tunnel was successfully built using the construction methods mentioned above.
Through small scale model test of DX piles in lab, the test results can provide necessary basis for computing formula of settlement. A series of tests about DX piles with 22 mm diameter in sand are carried out in a small test chamber with the loading frame;the Q-s curves are derived, and then the bearing capacities are determined and compared with the straight piles in the same condition. At the meantime, miniature earth pressure cells are installed to test the stress in the surrounding soil of DX piles, so that the load transfer law in the soil can be researched. The test results show that the capacity and settlement characteristics of DX pile are much superior to the straight pile with the same parameters. For the single-bell DX pile, the location of the bell has barely effect on the capacity. However, when the bell locates on the lower part of pile shaft, the stress of soil around the pile tip increases a lot. For two-bell DX pile, the stresses around the two bells are similar, and the range of soil stress induced by bells in vertical is around two times of bell diameter. For the DX pile with three bells, the soil stress around the bells spread increasingly, while the amplitude is small.