Spatial Variability in Field Unconfined Compressive Strength and Hydraulic Conductivity of Jet Grout Cut-Off Wall

Jet grouting technique is often adopted to construct cut-off walls and seepage barriers for water retaining structures such as dams. This involves construction of single or multiple rows of overlapping cement-admixed soil columns, by in-situ injection of cement-based grouts under high pressure along with or without compressed air and/or water. Such improved grounds are often characterized by high spatial variability in properties. Although there are several case studies available in literature on the spatial variability of field unconfined compressive strength (UCS), however such studies are scarce for field hydraulic conductivity (HC) which is a critical design parameter for cut-off walls. Moreover, studies on the spatial correlation of properties for JG improved grounds are also limited. The paper evaluates the statistical parameters of the field UCS and HC for the jet grout cut-off walls under two cofferdams, namely upstream and downstream cofferdams of a major multipurpose irrigation project in India. The dataset consists of over 1000 samples for UCS and more than 500 samples for HC for both the cofferdams for curing period of 28 days and 56 days. The study herein employs the Kolmogorov-Smirnov test and different goodness of fit tests to assess the suitability of normal, log-normal, and gamma distributions. Due to insufficient dataset, bootstrapping (with replacement) technique is applied by re-sampling the insufficient data subsets of UCS values corresponding to curing period of 56 days for upstream coffer dam, and all the datasets of HC for JG columns. The spatial parameters, including mean, standard deviation, and coefficient of variation in both UCS and HC are evaluated. In addition, the characteristic strength and confidence level were also evaluated for field UCS. The values of horizontal autocorrelation length in field UCS and HC are also investigated. The findings of the study contribute significantly in establishing the mixing parameters for future jet grouting projects. In addition, the knowledge of the evaluated spatial parameters is also useful for the geotechnical engineers to conduct the reliability-based numerical analysis taking in to account of spatial variability and heterogeneity of jet grouted improved grounds.