Development and application of a multi-needle system for determining anisotropy and local spatial variability in soil thermal conductivity

Conventional single thermal needle configurations are limited in capturing spatial, directional, and time-dependent variabilities in soil thermal conductivity. This paper presents the development, calibration, and application of a laboratory-based Multi-Needle System (MNS) that enables programmable, multi-location thermal conductivity measurements within a single specimen. System performance was validated using agar-water gel and reference sand specimens, demonstrating stable and repeatable measurements. The capability of the MNS was then evaluated using two probe configurations across three application scenarios. Measurable spatial and directional variability was observed in the nominally homogeneous Case 1 samples and was found particularly pronounced in near-saturated fine silt, with orientational sensitivity dependent on soil type and state. Heterogeneity associated with non-uniform water distribution was noted in the Case 2 specimens subjected to wetting-draining cycles. Jetting substantially altered initial states of the Case 3 specimens and led to systematic post-jetting variations in thermal conductivity during re-sedimentation and reconsolidation. Finer and well graded soils appeared to exhibit greater variability than coarser sands. The findings demonstrate the potential of multi-location thermal measurements as a diagnostic tool for detecting soil heterogeneity and manually-induced disturbance, providing more representative thermal characterisation for submarine cable design and other thermal related structures.