Reliability assessment for serviceability limit states of stiffened deep cement mixing column-supported embankments

The reliability and deterministic analyses of wood-cored stiffened deep cement mixing and deep cement mixing column-supported embankments(referred to as WSCSE and DCSE,respectively)considering serviceability limit state requirements are presented in this paper.Random field theory was used to simulate the spatial variability of soil
cement mixing(SCM)material in which the adaptive Kriging Monte Carlo simulation was adopted to estimate the failure probability of a column
supported embankment(CSE)system.A new method for stochastically generating random values of unconfined compressive strength(qu)and the ratio(Ru)between the undrained elastic modulus and qu of SCM material based on statistical correlation data is proposed.Reliability performance of CSEs concerning changes in the mean(μ),coefficient of variation(CoV),and vertical spatial correlation length(θv)of qu and Ru are presented and discussed.The obtained results indicate that WSCSE can provide a significantly higher reliability level and can tolerate more SCM material spatial variability than DCSE.Some performance of DCSE and WSCSE,which can be considered satisfactory in a deterministic framework,cannot guarantee an acceptable reliability level from a probabilistic viewpoint.This highlights the importance and necessity of employing reliability analyses for the design of CSEs.Moreover,consideration of only μ and CoV of qu seems to be sufficient for reliability analysis of WSCSE while for DCSE,uncertainties regarding the Ru(i.e.both μ and CoV)and θv of qu cannot be ignored.

Field testing of stiffened deep cement mixing piles under lateral cyclic loading

Construction of seaside and underground wall bracing often uses stiffened deep cement mixed columns （SDCM）. This research investigates methods used to improve the level of bearing capacity of these SDCM when subjected to cyclic lateral loading via various types of stiffer cores. Eight piles, two deep cement mixed piles and six stiffened deep cement mixing piles with three different types of cores, H shape cross section prestressed concrete, steel pipe, and H-beam steel, were embedded though soft clay into medium-hard clay on site in Thailand. Cyclic horizontal loading was gradually applied until pile failure and the hysteresis loops of lateral load vs. lateral deformation were recorded. The lateral carrying capacities of the SDCM piles with an H-beam steel core increased by 3-4 times that of the DCM piles. This field research clearly shows that using H-beam steel as a stiffer core for SDCM piles is the best method to improve its lateral carrying capacity, ductility and energy dissipation capacity.

Engineering behaviour of in situ cored deep cement mixed marine deposits subjected to undrained and drained shearing

The deep cement mixing(DCM)is used to improve the capacity and reduce the settlement of the soft ground by forming cemented clay columns.The investigation on the mechanical behaviour of the DCM samples is limited to either laboratory-prepared samples or in-situ samples under unconfined compression.In this study,a series of drained and undrained triaxial shearing tests was performed on the in-situ cored DCM samples with high cement content to assess their mechanical behaviours.It is found that the drainage condition affects significantly the stiffness,peak and residual strengths of the DCM samples,which is mainly due to the state of excess pore water pressure at different strain levels,i.e.being positive before the peak deviatoric stress and negative after the peak deviatoric stress,in the undrained tests.The slope of the failure envelope changes obviously with the confining pressures,being steeper at lower stress levels and flatter at higher stress levels.The strength parameters,effective cohesion and friction angle obtained from lower stress levels(c′0 andφ′0)are 400 kPa and 58°,respectively,which are deemed to be true for design in most DCM applications where the in-situ stress levels are normally at lower values of 50-200 kPa.Additionally,the computed tomography(CT)scanning system was adopted to visualize the internal structures of DCM samples.It is found that the clay pockets existing inside the DCM samples due to uneven mixing affect markedly their stress-strain behaviour,which is one of the main reasons for the high variability of the DCM samples.

Reliability-based settlement analysis of embankments over soft soils reinforced with T-shaped deep cement mixing piles

This paper presents a reliability-based settlement analysis of T-shaped deep cement mixing(TDM)pile-supported embankments over soft soils.The uncertainties of the mechanical properties of the in-situ soil,pile,and embankment,and the effect of the pile shape are considered simultaneously.The analyses are performed using Monte Carlo Simulations in combination with an adaptive Kriging(using adaptive sampling algorithm).Individual and system failure probabilities,in terms of the differential and maximum settlements(serviceability limit state(SLS)requirements),are considered.The reliability results for the embankments supported by TDM piles,with various shapes,are compared and discussed together with the results for conventional deep cement mixing pile-supported embankments with equivalent pile volumes.The influences of the inherent variabilities in the material properties(mean and coefficient of variation values)on the reliability of the piled embankments,are also investigated.This study shows that large TDM piles,particularly those with a shape factor of greater than 3,can enhance the reliability of the embankment in terms of SLS requirements,and even avoid unacceptable reliability levels caused by variability in the material properties.

Experimental study on geosynthetic-reinforced sand fill over marine clay with or without deep cement mixed soil columns under different loadings

Geosynthetics and deep cement mixed(DCM)soil columns have been widely used to improve soft soil grounds in many countries and regions.This paper presents an experimental study on a geosynthetic-reinforced sand fill over marine clay with or without DCM columns under different loadings.Two tests were conducted on the sand fill reinforced with fixed-end and free-end geosynthetics over marine clay under three-stage local loading to investigate the effects of the boundary conditions of geosynthetic reinforcement on reducing settlements.It is observed that the fixed-end geosynthetic sheet is more effective in reducing settlements than the free-end condition under identical local loading.Another test was conducted on the fixed-end geosynthetic-reinforced sand fill over the marine clay improved by DCM columns under single-stage uniform loading.The vertical stresses on the marine clay and on the DCM columns,as well as the tensile strains of the geosynthetic sheet in the overlying sand fill,were measured.The results revealed that the stress concentration ratio increases with an increase in consolidation settlements,and the maximum tensile strain of the geosynthetic sheet occurs near the edge rather than at the center of the top surface of the DCM columns.