Table of Contents
Analysis of long term deflection for soft soil
Review of literature.
Soft soil is identified as a type of soil with low shear strength and high water content and it possesses large time-dependent deformations when high structures are constructed. The settlement is continued even after a long time of construction. The heavier ground deformation can cause damages to pavements and related structures and geotechnical engineers have to consider the design and maintenance features. The necessity of construction of transportation elements and other road embankments are necessary to facilitate design and construction. The multiple behaviour and characteristic studies are accurate to be conducted and the development of the elastic viscoelastic model has to be developed for addressing the issues related to. To describe the nature and behaviour of soils numerous EVP models are to be proposed and constant creep studies are to be conducted. Researches are to be done regarding void ration and vertical strain matters. Creep tests are to be conducted with a duration of more than three months. The permeability of foundation soils may be studied to perform the analysis and predict the plain strain.
The soft soils are considered to be problematic which is concerned with the contraction of transportation elements and road structures. The low shear strength parameters and water content with high compressibility are the main factors that are to be considered for the evaluation of mechanical and index properties of soft soils. The soft soils exhibit significant creep and creep on soil deposits and there are many challenges related to design and construction on such soil deposited sites. The studies on embankments and other structures indicate that the soil suffers from excessive damages and it continues even after the construction is completed. If the rate of creep is higher it may lead to a heavy deflection of the structure and practices are to be done on-site to get rid of adverse effects. There are some environmental considerations and conventional analysis for the determination and valuation of consolidation effects of soil that neglects the time-dependent deformation and long term performance of the structures.
It is necessary to study the properties of soils by establishing numerous EVP models that account on time and rate behaviour of the soil. The EVP model and analysis are not familiar to many practising engineers and soil consultants. This is mainly due to the complexity of the method and cost and time consumed for the methods accordingly. The time-dependent model may be effectively studied to determine the long term deflection and monitoring of the field cases. The main part of the study is to model the foundation of soft soil and propose some of the EVP models. The EVP models are different from the ordinary elastoplastic models because there is an incorporation of time and rate behaviour of soil along with. There are many EVO model and the models could be divided as No stationary flow surface models, overstress EVP models, and other models
The concept of a non-stationary flow surface model was developed (Hashigushiet.al,2000). The main difference between elastic theories and no stationary theories is that the definition of yield condition. The time-dependent relation and surface condition involves flow charge changes and the viscoelastic elastic strain may remain constant. The model is selected in such a way that the viscoelastic deformation may be proceeded with stress state exist and strain can be considered into elastic and viscoelastic parts. Here the associated flow rule is to be applied and consistency condition shall be checked with viscoelastic stretching. The viscoelastic strain rate for any stress state soil in the yielding condition must be zero. If the soil is considered to yield along the surface when totals elastic stress state may reach the yield surface and stress state of the soil along with viscoelastic triggers on creep characteristics and soil appear to be overconsolidated due to high effective stress. The next mode that is to be discussed is the overstress type EVP model .here the overstress theory is considered as a 3d expansion of the Bingham model and its three-parameter model which consists of slider and dashpot. It was assumed the existence of a plastic yield surface and the main difference between dynamic and state behaviour is expected to take when the stress state is below the yield surface within the nucleus and the viscoelastic stress state along the dynamic surface was not possible (Perunho et.al, 2018).
The rate formulated a constitutive model for viscoelastic continuum and experimental results prove that the formulation on fully saturated clay with a mixture of solids and fluids which was vicious and no viscous was studied (Madhav et.al, 2017). The concept of the study was to determine the properties of the soil-structure system which consisted of absorbed water and soil skeleton for effective stress determination and EVP parameter ad overall hardening in soil skeleton. The static equilibrium is assumed that when clay reached that state and end of primary consolidation are anticipated. It was noted that the assumptions of the authors were not strictly valid. From the model, it was also noted that the consolidated clay has never reached static equilibrium and the consolidation ends and it could lead to an explanation of creep and stress relaxation and secondary consolidation due to strain effects. The study was confronted and helped in idealization that could help in the prediction of yield in unrealistic and softening behaviour is applied in the softening of consolidated soils by (Hashiguchi et al., 2002). The main limitation of the EVP model was that they could not describe creep nature and acceleration creep as when mathematical structure helps in determining the stress-strain relationship. While considering the creep formulate d models in suggested that the effects of time on the compressibility of soft soils are to considered with high importance (Liu et.al, 2018). There is no single stress-strain curve for one-dimensional compression of clay (Liu et al., 2019). This can better be represented by a family of curves in different consideration of pre-consolidation states and represented by a group of curves with reconsolidation stress and it represents the equilibrium void ratio of different burden pressure at a specified time under sustained loading. the concept of delayed compression and a series of models was proposed for one-dimensional behaviour fo pressure for overconsolidated clay and creep time with the value of effective stress and strain for time-dependent (Santhanathan et.al, 2018). It's necessary to calculate the equivalent time along with the reference timeline is considered as positive and negative value is considered as a reference timeline.
However, these models tended to underestimate the effect of time and strain rate with changed drain strength. The model developed such that it could stimulate the accelerating creep and deviator stress was considered close to the strength envelope. The realistic modelling technique is needed to be addressed and the model shall be accessible for consolidated and overconsolidated clays. Determination of logarithmic creep was the main limitation of the establishment of the model. When time is considered as an infinity parameter, creep strain is also related as an infinity parameter and the settlement of clay particles was characterized as instant and delayed parts and overstress function was defined for all values of excess functions and the mathematical model was developed with seven parameters with particular stress-strain behaviour. the creep coefficient was considered as constant and the result of long term creep shown that the coefficient may not be considered as constant. The dependency of the creep coefficient on nonlinear creep function can be related to the coefficient and it is evident.
While dealing with other types of the model there is numerous EVP model ad the concept may be fit for stationary flow surface. A model was described to evaluate the time-dependent elastic-plastic strain and this was initially considered to develop the components of strain. the plasticity and classical theory was necessary to characterize the time-dependent stress-strain behaviour of cohesive soils. For estimating the creep deformation it was necessary to study the formulation of the volumetric part. In the model in which the consideration was given for von misses stress and the concept of the immediate and delayed settlement was accounted (Hashiguishi et.al,200). The plastic shear distortion without a volumetric change in the boundary surface is considered. It is necessary to determine the accurate production for the models concerning clay models and creep and stress relaxation is treated as the main phenomenon for physical consideration for the delayed response of soil stress relaxation compared to high creep. The time-dependent model was proposed for cohesive soil and end chronic, model deal with the plastic response of materials integrate the essence of material behaviour. The sub loading surface is necessary for normal yield and the surrounding material corresponds to the elastic region.
While comparing various models and numerical implementation it was studied that their area several recommendations such that many models tend to the same behaviour characteristics with accuracy and behaviour. Many models discussed normally consolidated soil and formulated isotropic conditions. Most of the soil models had to be critically verified and reviewed within boundary conditions for triaxial or plane strain stress conditions. The laboratory condition may always don’t comply with the actual field conditions and very few models are to be tested for long term deflection and stress state formation and verification od model requires data for long term field monitoring. The long term performance and verification with data of real cases are to be done with high importance. Many models are established to be complex and with numerous soil parameters that are difficult within physical significance and real field conditions. The anisotropy studies are to be numerically modelled and tested within actual site data. The complexity of the model and deficiency of field data is to be considered for validating the performance of the models.
The theoretical values of EVP models are complex in nature and complicated when coup[ed under the influence of diffusion theory. There are various FE methods available for predicting the excess deformation and pore properties of the soil strata, The time effects of foundation and behaviour of soil shall be predicted for different EVP formulation. As per the cases discussed the numerical implementation for predicting the geological structures is to be considered with great importance and predictions of short period restriction to particular soil aspects and fewer accuracy requirements for predicting the behavioural characteristics. Conclusions
There were numerous models considered in the design. Most of the models in the study were found to be mathematically complex and large material parameters and determination of these values are not easy. The creep and time-dependent parameters are to be considered with obvious deformation and validation against long term deflection. The mathematical complication in analyzing the EVP mode makes it difficult for practical application and modelling involves different parameters like complex laboratory experiments. Some of the parameters like physical significance and practical implementation of the model are expected to be more difficult and the complexity of the model increased with an increase in prediction ability. The complexity in modelling the soil structure is difficult to be implemented and more knowledge on material parameters is to be considered and resources may be computed to implement the analysis in field problems.
For conducting the numerical analysis its necessary that some characteristics and factors are to be considered with importance like the model should be relatively simpler compared to mathematical computations, the material parameters should be less, all material parameters may have physical meaning and should be determined effectively from conventional laboratory testing and the model shall be able to produce good results in less time. There are many applications like modelling the site establishment with linearly elastic a plastic 1-dimensional bar elements and the analysis be such that slip is occurred to be above or below the establishment. The modelling of the interface is to be such that material considered may be rigid plastic or compatible with joint elements.
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