Soil Creep Effect on Time-Dependent Deformation of Deep Braced Excavation

This paper describes recent advances in the effect of soil creep on the time-dependent deformation of deep braced excavation. The effect of soil creep is generally investigated using the observational method and the plain-strain numerical simulation method. The observational method is more applicable for deep braced excavations in soft clays constructed using the top-down method. The plain-strain numerical simulation method can be conveniently used for parametric analysis, but it is unable to capture the spatial characteristics of soil creep effect on lateral wall deflections and ground movements. The additional lateral wall deflections and ground movements that are generated due to the soil creep effect can account for as large as 30% of the total displacements, which highlights the importance of considering the effect of soil creep in deep braced excavations through soft clays. The magnitude of the displacements due to soil creep depends on various factors, such as excavation depth, elapsed period, unsupported length, and strut stiffness. Parametric analyses have indicated several effective measures that can be taken in practice to mitigate the detrimental effect of soil creep on the deformation of deep braced excavation. Based on the literature review, potential directions of the related future research work are discussed. This paper should be beneficial for both researchers and engineers focusing on mitigating the adverse effect of soil creep on the stability of deep braced excavations.

Rapidly eroding hilltops are surprisingly smooth: ridgetop curvature varies with the square root of erosion rate

<p>The shape of soil-mantled hillslopes is typically attributed to erosion rate and the transport efficiency of the various processes that contribute to soil creep. While climate is generally hypothesized to have an important influence on soil creep rates, a lack of uniformity in the measurement of transport efficiency has been an obstacle to evaluating the controls on this important landscape parameter. We addressed this problem by compiling a data set in which the transport efficiency has been calculated using a single method, the analysis of hilltop curvatures using 1-m LiDAR data, and the erosion rates have also been determined via a single method, in-situ  <sup>­</sup>cosmogenic <sup>10</sup>Be concentrations. Moreover, to control for lithology, we chose sites that are only underlain by resistant bedrock. The sites span a range of erosion rates (6 – 1373 mm/kyr), annual precipitation (31 – 320 cm/yr), and aridity index (0.08 – 1.38). Surprisingly, we find that hilltop curvature varies with the square root of erosion rate, whereas previous studies predict a linear relationship. In addition, we find that the inferred transport coefficient also varies with the square root of erosion rate but is insensitive to climate. We explore various mechanisms that might link the transport coefficient to the erosion rate and conclude that present theory regarding soil-mantled hillslopes is unable to explain our results and is, therefore, incomplete. Finally, we tentatively suggest that bedrock processes may be responsible for the shape of hillslope profiles at our sites.</p>

Ground Magnetic Surveying and Susceptibility Mapping Across Weathered Basalt Dikes Reveal Soil Creep and Pedoturbation

Ground magnetic survey profiles across a soil-covered and weathered mafic dike in sedimentary host rock not only permit to delineate the strike, width and burial depth of the intrusive basalt sheet, but also reflect the subsurface deformation of its clayey weathering products. We illustrate this finding and its practical geomorphological applicability by an example from the mid-German Heldburg Dike Swarm, where blue- and olive-gray basalt-derived clays inherited not just the dike space previously occupied by the basalt, but also large parts of its magnetic iron minerals and their strong induced and remanent magnetization. Such ductile basaltic “marker soils” deform and move with the surrounding low-magnetic host soils, but remain distinguishable by their contrasting colors and high magnetic susceptibility. Ground magnetic surveys can therefore delineate soil creep distance at meter- and basalt weathering depth at decimeter-precision. Magnetic mapping of a weathered dike’s cross-section from an exploration trench by in-situ susceptometry permits to analyze past soil deformation in great detail. Weathering and solifluction transforms the simple “vertical sheet” anomalies of dikes into complex, but still interpretable composite patterns, providing a new and promising exploratory approach for field studies concerned with soil creep and pedoturbation.

Linking Soil Hydrology and Creep: A Northern Andes Case

Soil creep is common along the hillslopes of the tropical Andes of Colombia, where very heterogeneous soils develop on old debris flow deposits and are subjected to abundant rainfall with a bimodal annual regime. In particular, the western hillside of the city of Medellín, Colombia, is comprised of a series of debris and earth flow deposits in which landslides and soil creep are common. To explore linkages between soil creep and hydrology, we selected an experimental site in the western hillslope of the Medellín valley to assess the behavior of water within the soil mass, its relationship with rainfall, and its connection with soil displacement. In experimental plots, we systematically measured runoff, percolation, water table levels, and volumetric water content, for a period of almost 2 years; we also conducted several alti-planimetric positioning surveys to estimate relative displacements of the soil surface. Moisture content of the soil remained above field capacity for most of the year (~68% of the time) and active and quasi-permanent lateral subsurface flow occurred within the upper 80 cm of the profile. The shallow flow likely facilitates the downslope movement. Additionally, our results suggest that displacement magnitudes are largest during the wet season of September–October–November, when a highly humid soil experiences changes in water content, so it is during this time that the effects of expansion / contraction of the soil particles (associated to wetting / drying cycles) contribute the most to the movement. This observational study represents a contribution to the understanding of soil creep in tropical hillslopes, where it responds to the wetting / drying cycles, with the particularities of a rainy weather (>1500 mm/year), warm temperatures (~22 °C on average), and a bimodal precipitation seasonality.

Supplemental Material: The convexity of carbonate hilltops: 36Cl constraints on denudation and chemical weathering rates and implications for hillslope curvature

Figure S1: 36Cl denudation rates vs. mean annual rainfall of samples categorized by hillslope position and samples types; Figure S2: fd and CDF vs. mean annual rainfall, derived from immobile elements; Figure S3: Values of soil creep efficiency (K) vs. mean annual rainfall, estimated from numerical modelling and steady state, table with coordinates of study sites location; Figure S4: Photographs of hard dolo-limestone and soft chalk rocks; Figure S5: Mean hillslopes slope vs. mean relief.