Bryan A. McCabe

An Improved Experimental Test Set-up to Study the Performance of Granular Columns

This paper describes an innovative design of a newly developed large test setup for testing the performance of footings supported on soft clay reinforced with granular columns. This advanced testing method is used to examine the settlement of footings supported on granular columns. Two important features of the equipment are (a) the axial loading system which allows samples to be consolidated under Ko condition while the load is applied onto a small foundation area of the sample, and (b) a relatively large sample size of 300-mm diameter and 400-mm high. The system is also equipped with pressure cells located beneath the footing and top cap to measure the pressure distribution with respect to foundation displacement and a lateral strain gage to monitor boundary effects. This paper reports on some of the early findings from the preliminary tests carried out using this equipment. Samples for testing were prepared by consolidating kaolin slurry in a large one-dimensional consolidation chamber. The granular columns were installed using the replacement method by compacting crushed basalt (uniformly graded with 90 % between 1.5–2-mm particle sizes) into a preformed hole. The preliminary tests have yielded promising results, validating the functionality of the equipment and support the prospect of increasing the knowledge with respect to settlement response and design of a footing supported on granular columns.

Stone column effectiveness in soils with creep: a numerical study

ABSTRACT While it is well established that vibro stone columns reduce primary settlement and improve bearing capacity, their effect on creep compression has largely been overlooked to date. However, with increasing pressure to develop marginal sites underlain by soft organic soils, the effect of ground treatment on creep is an important emerging issue in geotechnical engineering. In this paper, a series of axisymmetric unit cell analyses have been carried out using the PLAXIS 2D finite element program in conjunction with the Soft Soil Creep (SSC) model. Examination of the evolution of settlement improvement factor with time has indicated that the presence of creep leads to a lower ‘total’ improvement factor than would be obtained for primary consolidation settlement alone. Separate ‘primary’ and ‘creep’ improvement factors have also been derived; the latter are much lower than the former, but are nevertheless greater than unity. Creep results in a stress transfer process; as the soil creeps, vertical stress is transferred from the soil to the stone column. The additional load carried by the column induces additional yielding and shear-plane formation in closely-spaced columns. The additional increment of stress transferred to the already yielded column reduces its efficacy.

Modeling stone column installation in an elasto-viscoplastic soil

The majority of numerical studies investigating stone column performance have used “wished-in-place” columns (no installation effects) in conjunction with elastoplastic soil models (no viscous effects). In the first part of this paper, cylindrical cavity expansion (CCE) is used in conjunction with the PLAXIS 2D elasto-viscoplastic soft soil creep (SSC) model to evaluate the effect of creep on column installation. Creep leads to lower post-installation lateral earth pressure coefficients (K) than if primary consolidation was considered alone, although the values of K/K0, which decay with distance from the column center, are nevertheless greater than 1·0. In the second part of this paper, two sets of unit cell analyses have been carried out to investigate the effect of accounting for column installation on subsequent settlement performance. The results indicate that “primary” settlement improvement factors are larger than “total” settlement improvement factors and that the relative differences between them increase when installation is taken into account.

Pile Group Settlement Estimation: Suitability of Nonlinear Interaction Factors

In this paper, predictions of pile settlement determined by appropriate superposition of two-pile interaction factors are compared with those computed from continuum analysis. A finite-element package is used in conjunction with the nonlinear Hardening Soil model and the pile group sizes studied are larger than in previous research. The study is presented in two phases: (1) examination of the modulus variation within the group allowing the elimination of some variables, and (2) comparison of pile settlement predictions in the context of the significant variables. The research shows that predictions using the former approach match the continuum analyses very closely for friction piles and reasonably well for end-bearing piles, and show potential for reducing computational time and effort in such problems. The paper also distinguishes between alternative definitions of interaction factor and shows that most accurate predictions arise from interaction factors calculated when only one of the two piles is loaded.

A practical approach for the consideration of single pile and pile group installation effects in clay: Numerical modelling

The first author gratefully acknowledges the College of Engineering and Informatics, NUI Galway, and the University of California Education Abroad Program for funding this research. In addition, the authors gratefully acknowledge the GeoEngineering department at Massachusetts Institute of Technology who compiled and provided the authors with the MITS1 dll for this research.

Small Stone-Column Groups: Mechanisms of Deformation at Serviceability Limit State

AbstractStone columns are a popular form of ground improvement that can be applied to a variety of foundation solutions involving soft soil deposits. Many design methods are based on an assumption that columns with a length-to-diameter ratio greater than 4 bulge uniformly along their length. This mode of deformation is not valid for small groups of columns in which interaction between individual columns and the footing gives rise to more complex behavior. The deformational behavior of stone columns at serviceability limit state is of utmost importance, because settlement rather than bearing capacity criteria are generally more onerous in soft soils. The results of some high-quality laboratory studies have been informative in this respect, but the extrapolation of results from small-scale tests comes with the caveats associated with single-gravity laboratory testing. The FEM, in combination with advanced constitutive models, can provide a link between the deformational behavior and settlement performance o...

Pyritiferous mudstone-siltstone: expansion rate measurement and prediction

The expansion rates of a pyritiferous Irish mudstone–siltstone fill material have been measured over a period of 19 months in an apparatus devised to replicate underfloor conditions. The testing, performed in a temperature-controlled environment, has shown that both fill density and depth submerged in water have significant influences on the progress of the expansion. It appears that whereas thermal expansion or contraction has an effect on expansion rates immediately after a temperature change, there is no long-term temperature effect on the rate of expansion. In addition, an examination of chemical test results for 60 houses in a housing development in the Dublin area has confirmed that pyrite content is the dominant control over the degree of expansion. A molecular–molar analysis of the pyrite chemical process equations, in addition to a knowledge of the original pyrite content and rate of oxidation, has been used to give a lower-bound estimate of the amount of heave in the laboratory experiments.

A review of the settlement of stone columns in compressible soils.

The behaviour of Vibro Replacement stone columns has yet to be captured fully by analytical or numerical means, and predicting stone column performance in soft cohesive soils brings specific challenges. In this paper, a new database of settlement improvement factors is developed, drawn from both published and unpublished data. The database illustrates that the improvement is generally predicted quite well by Priebe’s (1995) basic improvement factor. Moreover, the extent of improvement reflects the construction method, with the preferred dry bottom feed system performing consistently better than other column construction systems.

Biaxial loading of offshore monopiles: numerical modelling

AbstractIn this study, finite-element (FE) modeling was used to examine the influence of biaxial lateral loading of a monopile foundation. Two different soil models were adopted to investigate the influence of state-dependent and anisotropic behavior of soil on monopile response. A series of analyses was carried out in which both cyclic and static lateral loads were applied to the pile. Results show that biaxial loading significantly influenced the pile-head load-displacement response in addition to the development of significant accumulated displacements and coexistent soil loosening and densification. Moreover, predictions determined using a state-independent soil model were shown to be significantly nonconservative; these findings have important implications for environmental loading of offshore monopiles.

The Use of Carbonation Depth Techniques on Stabilized Peat

The phenolphthalein indicator method, the pH of slurries method, X-ray powder diffraction analysis (XRD), and Fourier transform infrared spectroscopy (FTIR) are all commonly associated with evaluating the carbonation front depth in concrete. Recent research (using a closed chamber method) has indicated that stabilized peat is a net sink of carbon dioxide (CO2) (at least in the short term), as the binder takes in CO2 both from the atmosphere and any CO2 released by oxidized peat. Therefore, the application of carbonation depth techniques to stabilized peat are of interest and are explored in this paper for the first time as a precursor to developing a greater understanding of the carbonation process in this material. The carbonation fronts obtained by XRD, loss on ignition, and FTIR all arose at similar depths, as these methods all identify the depth at which the calcium carbonate concentration is elevated with respect to the baseline (i.e., the maximum advancement of the carbonation front). In contrast, the phenolphthalein method underestimated the carbonation front considerably. The pH of the stabilized peat slurries method gave the depth at which leaching of calcium hydroxide and calcium ions occurred, rather than a carbonation depth, and exceeds the depths determined from the other techniques. Advantages and disadvantages of all techniques are discussed.