In deep excavation construction, improved soil layers consisting of overlapping cement-admixed columns formed by deep mixing method or jet grouting are often used to stabilise an excavation in soft soils. The purpose of such soil layers is to resist lateral compression generated by movement of the retaining wall. Cement-admixed soils are well known to have high heterogeneity in strength. In this paper, the heterogeneity in strength and Young's modulus are studied using random finite-element analyses, considering three sources of variation: namely, a deterministic radial trend in strength and Young's modulus; a stochastic fluctuation component due to non-uniform mixing; and positioning errors arising from off-verticality of the mixing shafts. The results show that positioning errors have the largest effect on the strength of the slab as a whole, whereas the radial trend has the smallest effect, when normalised by the volume-average strength. Based on the results obtained, methods are proposed which allow equivalent homogeneous mass strength and modulus of the improved slab to be determined for a chosen percentile of exceedance or reliability index, which can be used in deterministic finite-element analyses.
This paper examines the interaction between the spatial variations in binder concentration (i.e. cement slurry concentration) and in situ water content, in cement-mixed soil, using field and model data as well as statistical analysis and random field simulation. The field data are first analysed to shed light on the spatial variation in the in situ water content, including its scale of fluctuation. A statistical model is then developed which takes into account the variation in binder concentration and in situ water content. This leads to a two-parameter model for the prediction of the mean, variance and probability distribution function of the strength of the cement-treated soil. The scale of fluctuation for the variation in binder concentration arising from imperfect mixing within a cement-mixed column is then examined using centrifuge model data. This indicates that the scale of fluctuation in binder concentration is much shorter in range than that of the in situ water content. The combined effect of these two scales of fluctuation is then studied by simulating the resulting random field using Monte-Carlo simulations. This indicates that the size of the sampling region has a significant effect on the scale of fluctuation that is captured. If the sampling region is of a similar size to the column diameter, the measured scale of fluctuation reflects that of the binder concentration. As the size of the sampling region increases, so does the measured scale of fluctuation. This explains the wide range of scales of fluctuation that have been reported for cement-treated soil. To capture both scales of fluctuation in core sampling, some boreholes should be sunk at close spacings of less than a column diameter, in order to capture short-range variation.
The strength of soil improved by deep mixing processes often exhibits significant spatial variability. This paper presents a statistical model for strength distribution in deep-mixed columns, based on the assumption that spatial variation in strength arises from spatial variation in binder mass fraction. The statistical inputs of the model are the probability density function and coefficient of variation of the binder distribution, both of which were obtained from centrifuge model data and were assumed to be independent of the amount of the binder introduced into the ground. The theoretical framework of the model is first developed, followed by an assessment of the probability density function for the binder distribution. The variation of binder mass fraction within a deep mixing column is fitted by a truncated normal distribution. The model is benchmarked against core strength data from three phases of deep mixing operation in a construction project. The simplicity of the model precludes consideration of the effects of detailed mixing blade configuration, in situ variation in soil properties as well as workmanship. In spite of this, the computed histograms of strength distribution gave a reasonable reflection of the measured strength distribution. This suggests that despite the variety and complexity of deep mixing processes, one may still be able to develop a simple and workable model of the strength distribution taking into account just a few important factors.
Basalt Enhanced Weathering as a Carbon Dioxide Removal (CDR) technology accelerates natural weathering, enhancing the CO2 removal from the atmosphere. The main objective of the ongoing field trials in Scotland and the UK is to combine geochemistry modelling with in-field measurement to most accurately quantify CO2 sequestration. To measure the weathering signal in the field, we track changes in indicators such as soil inorganic carbon (SIC), soil organic carbon (SOC), exchangeable cations, trace/immobile elements, and soil biomass. Pore water analysis is critical for directly quantifying CO2 sequestration. Bicarbonate in soil pore water is a  CO2 removal indicator, as it forms through the reaction of silicate minerals with dissolved CO2 during the initial weathering process. We analyze pore water for pH, alkalinity, Electrical Conductivity (EC), major cations, and anions. This task can be challenging due to sampling issues, the absence of rainfall, and the time-sensitive nature of alkalinity measurements. Analyses of pore water chemistry rely on the ability to separate water from solids with minimal modification of its chemistry. Rhizon samplers and ceramic lysimeters are commonly used for pore water extraction. They may not be ideal for parameters like pH and alkalinity due to certain limitations, such as degassing of dissolved gases, and biases in molecule diffusion through the membrane. In response, we are testing a centrifuge method for pore water sampling from basalt amended fields. In the initial trial, statistical significance tests were conducted to compare the pH and total alkalinity between control plot and Treatment 126 t/ha in both centrifuge and rhizon samples, revealing a statistically significant difference (p < 0.05) in values within the centrifuge samples. However, no significance was observed in the rhizon samples. We present the results of ongoing tests from different treatments and soil types conducted to investigate whether centrifuge would be a suitable method for pore water sampling and alkalinity measurement for the enhanced weathering field trials.
This study examines the effect of geometric imperfections arising from variations in both the diameter and orientation of jet-grouted columns on the water-tightness of a jet-grouted cut-off wall. A three-dimensional discretised algorithm is used to detect and measure the discontinuities in the wall. A statistical evaluation of the gross flow rate through the cut-off wall with random flow-through pathways is carried out using Monte Carlo simulations. Based on the statistical results, a dimensionless design chart is proposed which can help engineers to devise safe and economical wall designs through trade-offs among various parameters such as target diameter, column spacing and number of rows.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='translate(288.889 -214.211)'/%3e%3cuse xlink:href='%23a' transform='translate(108 342.749)'/%3e%3cuse xlink:href='%23a' transform='translate(469.189 342.749)'/%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
