Papers by Keyword: Soft Soil

Abstract: Wuhan-Guangzhou passenger special line is the most important backbone of China high speed railway net. The line span is 968 km, among of them; roadbed is 388 km, occupied 40.1% of the total line. Due to the rigorous residual settlement of the roadbed, the CFG (Cement Flayash Gravel) pile is used as the composite foundation to enforce the intensity of roadbed and reduce the post-settlement. The paper studies on CFG pile composite foundation at the Wuhan experimental section, using the finite element numerical simulate the interaction of the pile and pile surrounding soil under the permanent load conditions. The results show that the stimulation model has the more accordance with the actual observation results. The residual settlement comes from the soil layer is bigger than the pile length.

Abstract: Based on a typical projects, the horizontal displacement in depth, horizontal displacement and vertical subsidence of pile top, and settlements of surrounding buildings are monitored with the process on digging of deep foundation pit. The study on digging process of foundation pit is analyzed by using finite element software. Using mapped meshing method, from mixing the pile near the semi-circular area (radius = 50m), the meshing appropriate encryption in order to improve the accuracy of the external semi-circular area (radius = 65m) mesh is less appropriate sparse.Layer by layer to kill the layers of the soil unit and activate the soil nails (spring element), the simulated excavation and synchronization of soil nails construction.Verify the arc form of failure surface in side of deep foundation pit in soft soil area. Which is valuable for reference to similar structure engineering of foundation pit.

Abstract: Present research results have verified the significant soil reinforcement effect of dewatering. But the reinforcement effect of dewatering suffers ignorance in current excavation design process due to lack of systematic experimental study on the strengthening mechanism, causing a certain amount of waste. This paper first theoretically describes the relationship between the increase of soil shear strength and that of compression modulus owing to pumping, then investigates the influence of well-point pumping on Shanghai soil’s shear strength and deformation properties through indoor test. The results may provide useful suggestions for future excavation support design.

Abstract: Fast drained and consolidated mechanism of soft soil under excessive pore water pressure generate excessive pore water pressure in the soft soil through the addition of vertical and horizontal drainage channel as well as the applied load, because the drainage path is significantly reduced, the seepage velocity speed up and water excreted rapidly under the excessive pore water pressure, also, the effective stress increase rapidly between the soil particles and accelerate consolidation of soft soil so as to improve the strength of soft soil.

Abstract: The change of Shanghai soft soil’s density with drainage time during foundation pit dewatering was investigated, through taking samples in each soil layer to be laboratory tested after the groundwater level of observation hole in project site had been lowered below the position soil sampled for at least 5 days. Experimental results show that groundwater included in drainage zone couldn’t be discharged immediately with the drop of groundwater table, which may be named as “lag draining” phenomenon; soil’s water-holding density can also be deduced from soil saturated density and specific field except directly measured from laboratory test. The effect of engineering pumping will reduce the discrete of soil density distribution; a centralized distribution of soil density emerges after dewatering.

Abstract: It is difficult to investigate the geological condition entirely, and the physical and mechanical indexes are inaccurate. All these uncertain factors make both the theoretical and numerical calculation results of foundation settlement different with actual situation. Based on summarizing research results about settlement prediction methods for soft subgrade, actual example analysis is provided with using the measured settlement data to predict the settlement of the soft ground treated by the preloading drainage consolidation method after construction. The analysis method of soft ground settlement during secondary consolidation stage is discussed, and a new calculation model for secondary consolidation settlement is proposed. Considering the different characteristics of the settlement curves during primary and secondary consolidation stages, an improved method is provided to calculate settlement by piecewise curve fitting, and the rationality of the method is verified by analyzing the monitoring data of typical settlement plate in Shenzhen Western Corridor Project. To sum up, in actual engineering practice the settlement developing trends and final settlement can be well predicted with using part of the measured settlement data obtained. Comparing to other methods, settlement prediction with using actual measured data is relatively accurate and reliable. It does not need geological data and the calculation process is simple for engineering and technical personnel to grasp and accept.

Abstract: For the stratum distribution around Taipei Basin, it is typical that the top part of stratum formation is covered in soft or very soft soil with uneven thickness and sits on bedrock with extremely high bearing capacity. For the tall building located at the basin rim, pile foundations are often required due to lack of bearing capacity, as these buildings are sitting on soft or extremely soft soil. When it comes to calculating the bearing or pulling capacity, there are two major ways of calculation, equation of static mechanics and estimation with “N” value from in-situ penetration tests, as specified in the current “Design Code for Building Foundations.” However, the methods mentioned above apply only to general soil, and there is no specific recommendation for the friction and pile tip resistance. Therefore, in foundation pile design, the calculation of friction and pile tip resistance on the part of piles in bedrock is always bothering engineers. Yet, the contribution from the part of piles in bedrock, in fact is significant. As a result, the characteristics of the part of piles in bedrock are a topic worth studying. In this study, pile loading tests at 7 locations at Nangang, Taipei City are taken as examples. Finite element method (FEM) is introduced for simulation of in-situ drilled foundation piles mentioned above that are commonly used. The result of simulation is compared with that of actual static pile loading tests, and it is found that there is consistency between the simulation result and that of loading tests when the bedrock penetration depth of foundation piles reaches 1.6~2.0 times of pile diameter, as FEM is introduced for the simulation of the relationship of axial loading of pile versus pile-head top settlement.

Abstract: Generally, it is difficult to control the trench stability and panel joint permeability in the construction of a diaphragm wall. The high construction cost is inevitable because of the environmental pollution due to the slurry, waste of water consumption, and debris disposal treatment. A new technique, called the dry trenching diaphragm wall with self-guided double-rectangular-casing, is thereafter developed in the current study to avoid these problems. In this technique, two pieces of rectangular steel casing are driven in sequence and guided each other. The trench is made by squeezing into the soft soil. The panel alignment and straightness are assured without any joints due to the dry concrete pouring instead of the influences of the slurry. Compared to the regular diaphragm wall, the technique is an environment-friendly, less emissive and energy-conserved one by eliminating the slurry pollution with the low cost. It shows that the quality of panel connection is reliable, and the permeation-proof performance is superior with high efficiency in wall-forming due to non-joint interlocking.

Abstract: Based on Newmark-β gradual integration method and elastic-plastic mechanical theory, numerical analyses of the effects of soft soil depth and thickness on the characteristics of seismic response of free bank field with soft soil layers are conducted by using finite element program Midas/GTS. The numerical results show that the high frequency components of seismic excitations can be filtered and the low frequency components are amplified correspondingly when seismic waves are transmitted through the free bank field from the bedrock.

Abstract: Widening of existing highway embankments can improve their capacities and ease the high traffic pressures, but it may cause a series of engineering problems including longitudinal pavement cracks, drop off or even local or global instability of embankments especially those on soft soils. How to prevent severe cracks occurring on pavement induced by widening of the existing embankments has been a great concern of geotechnical engineers. A numerical analysis by using a commercial FEM program was conducted to investigate the influence of the thickness of foundation soils on the development of transverse stress in the pavement of the existing and widened embankments. The effect of widening time on the stress was also discussed. The results indicate that compressive stress will be caused in the existing pavement prior to the widening, while considerately lager tensile stress will be induced in the existing pavement after widening. In the view of reducing pavement tensile stress, a most suitable time to begin widening was proposed for engineering practice.