Richard B Kelly

Numerical Modeling of Geotextile-Reinforced Embankments over Deep Cement Mixed Columns Incorporating Strain-Softening Behavior of Columns

Geotextile-reinforced embankments over deep cement mixed (DCM) columns are widely used for the construction of highway embankments over soft clay with low shear strength and high compressibility. Numerical modeling based on the finite element method (FEM) is widely used to investigate the behavior of these embankments during construction and serviceability, incorporating consolidation of the foundation soil over time. However, not much attention has been paid to the strain-softening behavior of DCM columns beyond yield, which is essential in ultimate limit-state computations to determine the stability of embankments during the failure of columns. This paper presents a constitutive model, which is an extension of the Mohr-Coulomb model, for the simulations of strain-softening behavior of cement admixed clays. The model is validated using triaxial test data found in the literature for cement admixed Singapore and Hong Kong marine clays and Ariake clay. A two-dimensional (2D) coupled mechanical and hydraulic numerical implementation of a geotextile-reinforced DCM column-supported (GRCS) embankment constructed over a very soft soil in Finland is carried out incorporating strain-softening behavior of DCM columns. Even though the isolated columns and overlapped column walls used in this embankment do not yield significantly under the service loads, the model simulations show good agreement with field data, confirming the capability of the 2D plane strain finite-element model in predicting the GRCS embankment behavior. Finally, the finite-element model with strain-softening DCM columns is used to investigate the progressive failure of a typical hypothetical GRCS embankment with isolated columns in a square pattern. Results clearly illustrate the bending failure mode caused by progressive softening of the DCM columns, including the plastic hinge development within the columns.

Numerical Modeling of an Embankment over Soft Ground Improved with Deep Cement Mixed Columns: Case History

AbstractThis paper describes a case history of a deep cement mixed (DCM) column–supported embankment that is part of the Ballina Bypass section of the Pacific Highway Upgrade project in Australia. Measured settlements during and after construction of the embankment were significantly greater than the predicted settlements and suggested that the DCM columns were yielded. The case history was analyzed using a finite-element model based on the coupled theory of nonlinear porous media. Two cases were analyzed: with and without the strain-softening behavior of DCM columns caused by breakage of the soil-cement structure. The computed settlements, excess pore-water pressures, and lateral deformations were compared with field measurements. Results show that there was good agreement between the measured and the computed parameters when the strain-softening behavior of the columns was included. These results clearly show that consideration of the strain softening of DCM columns in the analysis is important if yield...

Interpretation of the lime column penetration test

Dry soil mix (DSM) columns are used to reduce the settlement and to improve the stability of embankments constructed on soft clays. During construction the shear strength of the columns needs to be confirmed for compliance with technical assumptions. A specialized blade shaped penetrometer known as the lime column probe, has been developed for testing DSM columns. This test can be carried out as a pull out resistance test (PORT) or a push in resistance test (PIRT). The test is considered to be more representative of average column shear strength than methods that test only a limited area of the column. Both PORT and PIRT tests require empirical correlations of measured resistance to an absolute measure of shear strength, in a similar manner to the cone penetration test. In this paper, finite element method is used to assess the probe factor, N, for the PORT test. Due to the large soil deformations around the probe, an Arbitrary Lagrangian Eulerian (ALE) based finite element formulation has been used. Variation of N with rigidity index and the friction at the probe-soil interface are investigated to establish a range for the probe factor.

Case studies of geophysical imaging for road foundation design on soft soils and embankment risk assessment

Population growth along the coast of eastern Australia has increased demand for new and upgraded transport infrastructure within intervening coastal floodplains and steeper hinterland areas. This has created additional challenges for road foundation design. The floodplain areas in this region are underlain by considerable thicknesses of recently deposited alluvial and clayey marine sediments. If characterisation of these deposits is inadequate they can increase road construction costs and affect long-term road stability and serviceability. Case studies from a major coastal highway upgrade demonstrate how combining surface wave seismic and electrical geophysical imaging with conventional geotechnical testing enhances characterisation of these very soft and soft soils. The geophysical results also provide initial foundation design parameters such as void ratio and pre-consolidation pressure. A further significant risk issue for roads is potential embankment instability. This can occur during new road construction or when upgrades of existing embankments are required. Assessing the causes of instability of existing steeper embankments with drilling and probing is often difficult and costly due to access and safety problems. In these situations combinations of electrical, ground penetrating radar and P-wave seismic imaging technologies can rapidly provide information on the likely conditions below both the roadway and embankment. Case studies show the application of these technologies on two unstable road embankments. It is concluded that the application of both geophysical imaging and geotechnical testing is a cost-effective enhancement for site characterisation of soft soils and for risk assessment of potentially unstable embankments. This approach overcomes many of the current limitations of conventional methods of site investigation that provide point location data only. The incorporation of geophysics into a well crafted site investigation allows concentration on fewer but higher quality soil probings and geotechnical boreholes. The application of both geophysical imaging and geotechnical testing in road foundation design is a cost-effective enhancement for site characterisation of soft soils and for risk assessment of potentially unstable embankments. The incorporation of geophysics allows concentration on fewer but higher quality soil probings and geotechnical boreholes.