Unsaturated Soil Layer Research Articles

Research Progress on Key Issues of Shear Strength Theory and New Testing Technology of Roadway Crossing Unsaturated Soil Layer

By studying the relationship between the shear strength and water content of unsaturated soil, summarizing and analyzing the previous methods of preparing soil samples with different water content, it is found that the test phenomenon of "critical water content" exists in the specific gravity method. The research shows that the shear strength of unsaturated soil is the result of the common action of compactness and water content. Nine widely used shear strength measurement methods of unsaturated soil are sorted out, and their common shear test methods are divided into three categories, namely direct shear test method, compression shear test method and rotary shear test method. The advantages and disadvantages of each method are analyzed. Many shear strength formulas are divided into four categories: combined water-soil characteristic curve, piecewise function, mathematical fitting and total stress index. The characteristics and shortcomings of shear strength of different kinds of unsaturated soil are compared, and some suggestions on the theoretical research direction of shear strength in the future are put forward. Finally, a variety of new ideas and methods for studying the shear strength theory of unsaturated soil are introduced. This paper improves the theoretical basis of unsaturated soil, which is of great significance to speed up the engineering practice of shear strength theory of unsaturated soil.

Temperature effects on adsorption and capillarity water retention mechanisms in constrained unsaturated soils

AbstractThis paper focuses on the impact of elevated temperatures on the adsorptive and capillarity water retention mechanisms of unsaturated soils under constrained (constant volume) conditions. This topic is critical for simulating the thermo-hydraulic behavior of soils in hydrogeological or geotechnical applications, including climate change effects on near surface soils, energy piles or soil borehole thermal energy storage systems in unsaturated soil layers, and buffers for geological nuclear waste repositories. A nonisothermal soil water retention curve (SWRC) that separately considers the temperature-dependency of the key parameters governing adsorptive and capillarity water retention mechanisms and soil physical parameters (e.g., surface tension, contact angle, adsorption capacity, cation exchange capacity, mean cavitation suction, air entry value and equilibrium film thickness) was developed to provide insights into the impact of temperature on water retention over the full suction range. The nonisothermal SWRC was validated using experimental data on high plasticity clays, with a good prediction of temperature effects on adsorption and capillarity water retention mechanisms in constrained unsaturated soils.

Dynamic Response Analysis of Non-Uniform Unsaturated Soil Layer Roadbed under Uniform Moving Load

Dynamic Response Analysis of Non-Uniform Unsaturated Soil Layer Roadbed under Uniform Moving Load

Reducing the risk of missing soil contamination by integrating multi-investigated methods in a retired chrome salt factory

An effective contamination screening process is essential for mitigating the risk of overlooking pollution during site investigations. While Electrical resistivity tomography (ERT) and portable X-ray Fluorescence (XRF) offer quick assessments of contaminated areas, there is a notable gap in research regarding ERT’s application in the unsaturated soil layer, and portable XRF’s use for screening polluted samples in chromium-contaminated sites. This study gathered soil pollution and electrical resistivity data from a decommissioned chrome salt factory, employing drilling, sampling, XRF, and ERT methods. Predominantly, sulfate and chromium emerged as the pollutants, with concentrations ranging from 19.24 to 12700.00 mg/kg for sulfate, 0.65 to 3000.00 mg/kg for Cr(VI), and 12.00 to 27800.00 mg/kg for total Cr, respectively. Polluted hotspots were identified by lower apparent soil resistivity compared to the background levels at corresponding depth. Notably, a significant inverse correlation was observed between sulfate and Cr(VI) concentration and soil resistivity, particularly with sulfate (R2 =0.48) demonstrated through a power function fit at depths ranging from −4 to 0 m. The XRF analysis for total Cr concentration exhibited a significant correlation (Pearson’s r = 0.67) with chemical analysis, confirming 1000 mg/kg as an effective screening threshold. In light of these findings, this study proposes a scientific procedure that integrates drilling, sampling, ERT, and XRF for soil contamination investigation. This framework aims to support similar site assessments, reducing the risk of overlooking contamination.

A framework of payments for water-related ecosystem services (PWES) based on green water management

Green water describes precipitation that infiltrates into the unsaturated soil layer and returns to the atmosphere via plant transpiration and soil evaporation, which are two functions of the hydrological cycle. Green water provides the basis for timber growth, crop production, and ecosystem services. However, it has not been adequately accounted for in traditional payments for water-related ecosystem services (PWES), which predominantly consider on blue water and hence underestimate the availability of all water resources. In this study, a new framework of payments for green water-related ecosystem services (PGWES) is proposed based on green water management (GWM) and green water compensation (GWC). This framework is constructed with game theory modelling, hydrological simulation and scenario evaluation. Then, the Xiangjiang River Basin (XRB) is used as a case study to test and evaluate the feasibility of the framework. The results of this study are as follows: (a) the framework of PGWES and its game payoff matrix and optimal payoff function are improved by adding the GWM scenarios. This approach can identify the GWC standard and Pareto optimal solution in different GWM scenarios, thereby optimizing the spatial allocation of blue and green water in the river basin and coordinating conflicts of interest between the upstream and downstream. (b) The average Pareto optimal payoffs of the XRB are increased by 4390 million CNY/year under different GWM and GWC scenarios compared to the baseline scenario, and the average GWC standard is 2.41 CNY/m3 or 1305.98 CNY/hm2. The case study indicates that the socioeconomic and ecological benefits for the whole XRB are expected to significantly improve under this framework. (c) Three aspects of managing water resources and conflict of interest issues noted during the study include “broadening water sources”, “saving water resources” and “coordinating water conflict”. Green water resources are an important component in broadening water sources; green water management provides a basic measurement tool for saving water resources; and green water compensation based on game theory offers a possible way to coordinate water conflicts and should be given more attention. In general, the PGWES framework provides a referenced methodology for the efficient development and equitable allocation of blue and green water resources in the river basin; furthermore, this study provides a scientific basis for the theory and methods of PWES, thus enriching this field of research.

Evaluation of the storage and evapotranspiration terms of the water budget for an agricultural watershed using local and remote-sensing measurements

The understanding of the water budget components, especially evapotranspiration and the amount of water stored in the watershed, is key to water resources management and hydrological modeling in general. In this work, we present the analysis of 13 hydrological years’ measurements of discharge, precipitation, evapotranspiration, and soil moisture over two agricultural watersheds in Eastern Nebraska, United States. These data were used to investigate several facets of the hydrological water budget: water- and energy-balance closures, water budget residues (here, the water budget residue is the remaining imbalance after all water budget terms have been considered; see Eq. (13)), storage in the unsaturated and saturated soil layers, and the evapotranspiration derived from the water budget. Remote-sensing data were also used to assess the spatial variability of net radiation and soil moisture over the basins. Adjusting the eddy-covariance measured evapotranspiration to close the energy budget agreed, in the long term, with the water budget evapotranspiration estimates. The evapotranspiration estimated with storage calculated with soil moisture measurements and recession analysis presented the best agreement with the (adjusted) measured evapotranspiration, but considering only either the vadose zone or the saturated zone for storage estimates still produced reasonable results. We found considerable variability in soil moisture over the basins in the final period of the growing season, coinciding with the time of the year when water budget residues were largest.

Characterization and Analysis of Landslide Evolution in Intramountain Areas in Loja (Ecuador) Using RPAS Photogrammetric Products

This case study focuses on the area of El Plateado near the city of Loja, Ecuador, where landslides with a high impact on infrastructures require monitoring and control. The main objectives of this work are the characterization of the landslide and the monitoring of its kinematics. Four flights were conducted using a remotely piloted aerial vehicle (RPAS) to capture aerial images that were processed with SfM techniques to generate digital elevation models (DEMs) and orthoimages of high resolution (0.05 m) and sufficient accuracy (below 0.05 m) for subsequent analyses. Thus, the DEM of differences (DoD) and profiles are obtained, but a morphometric analysis is conducted to quantitatively characterize the landslide’s elements and study its evolution. Parameters such as slope, aspect, topographic position index (TPI), terrain roughness index (TRI), and topographic wetness index (TWI) are analyzed. The results show a higher slope and roughness for scarps compared to stable areas and other elements. From TPI, slope break lines have been extracted, which allow the identification of landslide features such as scarps and toe tip. The landslide shows important changes in the landslide body surface, the retraction of the main scarp, and advances of the foot. A general decrease in average slope and TRI and an increase in TWI are also observed due to the landslide evolution and stabilization. The presence of fissures and the infiltration of rainfall water in the unsaturated soil layers, which consist of high-plasticity clays and silts, contribute to the instability. Thus, the study provides insights into the measurement accuracy, identification and characterization of landslide elements, morphometric analysis, landslide evolution, and the relationship with geotechnical factors that contribute to a better understanding of landslides. A higher frequency of the RPAS surveys and quality of geotechnical and meteorological data are required to improve the instability analysis together with a major automation of the GIS procedures.

Inverse modeling of nonisothermal multiphase poromechanics using physics-informed neural networks

We propose a solution strategy for parameter identification in multiphase thermo-hydro-mechanical (THM) processes in porous media using physics-informed neural networks (PINNs). We employ a dimensionless form of the THM governing equations that is particularly well suited for the inverse problem, and we leverage the sequential multiphysics PINN solver we developed in previous work. We validate the proposed inverse-modeling approach on multiple benchmark problems, including Terzaghi's isothermal consolidation problem, Barry-Mercer's isothermal injection-production problem, and nonisothermal consolidation of an unsaturated soil layer. We report the excellent performance of the proposed sequential PINN-THM inverse solver, thus paving the way for the application of PINNs to inverse modeling of complex nonlinear multiphysics problems.

Simplified mathematical modeling to plane strain consolidation considering unsaturated and saturated conditions for ground with anisotropic permeability

Road embankment where the unsaturated soil layer lies above the saturated soil layer is often encountered in geotechnical engineering, but the analytical models for evaluating the plane stain consolidation characteristics of an unsaturated–saturated ground are limited. With a combination of the consolidation theories for saturated and unsaturated soils, this paper thus develops a plane strain consolidation model for exploring the transient behavior of unsaturated–saturated soils subjected to instantaneous loading. A semi-analytical solution, which is a simple tool for assessing unsaturated–saturated soil consolidation, is proposed by making use of decoupling strategy, Fourier expansion as well as Laplace transformation. Further, the proposed solution in this paper is validated by the published analytical or semi-analytical solutions in some specific cases. By implementing the proposed solution into a computation, the consolidation behavior of an unsaturated–saturated subgrade is presented, and the effects of important factors, including the anisotropic permeability ratio and interlayer permeability ratio, are studied. The anisotropic permeability ratio and interlayer permeability ratio are significant indexes to change the consolidation rate of lower soils so that the prefabrication of lateral drainage boundary on both sides of the unsaturated–saturated subgrade would accelerate its consolidation.

Semi-analytical solutions to the one-dimensional consolidation for double-layered unsaturated ground with a horizontal drainage layer

In order to accelerate the consolidation rate of double-layered unsaturated ground with poor hydraulic conductivity, the sand blanket is always set between two adjacent unsaturated soil layers to produce drainage paths along the horizontal direction in practical engineering. Based on the one-dimensional consolidation theory of unsaturated soil proposed by Fredlund and Hassan(1979), the semi-analytical solutions of excess pore-air and pore-water pressures, and settlement of the double-layered unsaturated ground with the horizontal drainage blanket are obtained by Laplace transform technique in this paper. Additionally, the current solutions can degenerate into those under the single-layered unsaturated model, which are then utilized to compare with existing solutions in literature, the significant agreement indicates that the present solutions are reliable and general. At last, a case study is adopted to investigate the influence of the vertical position of the drainage blanket, and permeability coefficients ratio of two adjacent soil layers on the consolidation of the double-layered unsaturated ground. Results show that the horizontal drainage layer will effectively accelerate the consolidation process, and the setting depth of 0.7H works best. Moreover, the smaller the ratio of the permeability coefficient of the upper layer to that of the lower layer (with the permeability coefficient of the upper layer remaining constant), the faster consolidation will develop.

Construction of a mathematical model and numerical study of interaction between moisture-, heat-, and mass transfer processes of salt solutions in an unsaturated soil layer

Methods of mathematical modeling and computer simulation were used to investigate the influence of interrelated processes of transfer of saline solutions in the unsaturated soil layer. To this end, a mathematical model has been built for modeling the corresponding processes of the moisture-, heat-, and mass transfer of saline solutions in an unsaturated soil layer. An effective computational algorithm was developed to solve the corresponding nonlinear boundary problem numerically by the method of finite differences; it was implemented in the Asp.net programming environment in the C++ language. Based on the numerical experiments carried out, the distribution of moisture, concentration, and temperature fields in the aeration zone (incomplete saturation) was obtained. To study the influence of mass transfer of salts on moisture transfer, a numerical solution was found to the problem of moisture transfer, the problem of moisture transfer taking into consideration mass transfer and moisture transfer, taking into consideration mass transfer in the presence of osmosis. Analysis of the results showed that the distribution of the concentration of saline solutions over time is slower and more predictable. It was established that the distribution of moisture heads increases with depth and time when saline solutions fall on the surface of the soil massif. With the influence of salt concentration, the distribution of moisture increases with depth and time throughout the entire area of moisture transfer by 1–3 %. The distribution of moisture heads taking into consideration the concentration of salts and osmosis is reduced by 3–5 % compared to the results of the problem without taking into consideration the phenomenon of osmosis. The distribution of the concentration of saline solutions during moisture transfer and osmosis acquires higher values compared to the results without taking osmosis into consideration. The established features can be successfully applied to clean the fertile soil layer and resume agricultural activities

One-dimensional consolidation of unsaturated–saturated soil system considering pervious or impervious drainage condition induced by time-dependent loading

Since the ground with unsaturated soil layer and saturated soil layer is very common in most areas, a more comprehensive analytical solution is essential for the accurate prediction of ground consolidation behavior. In this paper, the one-dimensional consolidation theories for unsaturated and saturated soils are introduced to simulate the consolidation characteristics of the different soil layers, and the hypothetical soil interface of two layers that only allows pore water to flow is supposed in this model. After using the decoupling and Laplace transformation techniques to the coupled governing equations, the general Laplace-domain solutions are proposed under the permeable or impermeable surface for ground consolidation that an unsaturated–saturated ground with finite thickness subject to arbitrary time-dependent loading. The results in the temporal domain for the excess pore pressures and soil settlement are numerically obtained by Crump’s algorithm. The methodology and proposed solution are validated by some published solutions in the literature. Based on the proposed solutions under the three representative boundary conditions, the temporal behavior of ground subjected to ramp loading is explored. The results show that boundary condition would significantly affect the evolutions of excess pore pressures and further change the ground settlement.

Energy pile groups for thermal energy storage in unsaturated soils

• Energy pile groups provide superior thermal energy storage performance over boreholes. • Both energy pile geometry and number of internal heat exchangers are important. • Lower thermal conductivity of unsaturated soils leads to higher heat retention. • Transient decreases in degree of saturation were observed over several years. • Heat transfer in unsaturated soil can be enhanced in some cases via vapor diffusion. A coupled heat transfer and water flow model implemented in COMSOL and validated against measurements from a tank scale test was applied to investigate the application of energy pile groups for thermal energy storage in unsaturated soil layers. The novel focus of the investigation was understanding the long-term thermo-hydraulic response of the unsaturated soil within the energy pile group during heat injection at high temperature up to 90 °C and associated impacts on the heat storage performance. Unsaturated soil layers are advantageous for thermal energy storage due to enhanced convective heat transfer during injection associated with vapor diffusion and favorable insulation properties during storage associated with lower thermal conductivity of soils surrounding a heat storage system. Evolutions in temperature and degree of saturation in soil layers having different hydraulic properties and water table depths were simulated during five years of operating a group of five energy piles with inlet fluid temperatures of 90 °C during heat injection and 30 °C during heat extraction. Transient fluctuations in the degree of saturation were observed in all soil layers simulated, but a permanent decrease was only observed for a soil layer having a greater air entry suction after several cycles of heating and cooling. While the heat storage in energy pile groups in unsaturated soil layers was always between that of dry and saturated soils with no groundwater flow, the soil hydraulic properties and water table depth were found to control both the rate of heat transfer and the total heat stored. When comparing the performance of energy pile groups with a group of borehole heat exchangers commonly used in heat storage applications, the energy piles were approximately 1.2 times more effective in extracting heat with a faster response confirming their suitability for heat storage.

Forward calculation of displacement fields with multilayered unsaturated highway system induced by falling weight deflectometer using dynamic response method

A new dynamic forward model of a multilayered highway structure, which considers the unsaturated characteristics of subgrade soil, is established in this paper to assess the dynamic response induced by the falling weight deflectometer (FWD) load. The Laplace-Hankel transform and dynamic stiffness matrix method are employed to solve the governing equations of the unsaturated soil layer. Combined with the global stiffness matrix and boundary conditions, analytical solutions of the entire system can be derived in the transform domain. A numerical integration method is utilized to perform the Laplace-Hankel inverse transform to obtain time-domain solutions. An analysis of the effects of saturation on the stress, displacement, and pore water pressure is presented. It is revealed that the effect of saturation of unsaturated subgrade soil on vertical stress is negligible, whereas vertical displacement and pore water pressure increase significantly with increasing saturation. Additionally, the vertical displacement response on the pavement surface is sensitive to subgrade deterioration and variations in material properties. Moreover, subgrade deterioration can be reflected in the variation in vertical displacement at approximately 1.2 m from the load center for the studied road structure. The proposed model, which is more realistic and reasonable, can provide a theoretical basis for the back-calculation of the elastic modulus of multilayered highway systems.

Numerical investigation of the performance of engineered barriers in controlling stormwater runoff

In this paper, 2-dimensional, hydro-mechanically coupled finite element analyses are conducted to assess the performance of an engineered barrier, constructed from natural geomaterials, aimed at reducing flood risk in urban environments. The barrier consists of an unsaturated compacted soil layer with water holding properties and a drainage layer of a coarse granular material, that acts as a capillary break, and is constructed on top of the natural soil, in this case London clay. The barrier is vegetated so that its water storage capacity is renewed after each rainfall event. Sophisticated boundary conditions are used to simulate the effect of precipitation and evapotranspiration. The evolution of the rainfall infiltration and runoff rate is simulated both for a treated soil column with an engineered barrier and an untreated one consisting solely of in-situ London Clay. The percolation rate of rainfall water from the bottom of the barrier is also estimated. This comparison highlights the effectiveness of the engineered barrier in reducing the risk of fast flooding, in preventing excessive deformations and in protecting underground infrastructure during wetting and drying cycles. The effect of the hydraulic properties and geometry of the barrier is investigated by means of an extensive parametric analysis. Finally, recommendations for the design of barrier systems are made.

Soil–Foundation–Structure Interaction of Inelastic Structural Systems on Unsaturated Soil Layers

Recently, progress has been made toward understanding the seismic response of structures placed on unsaturated soil layers. A missing link, however, involves the influence and assessment of the underlying soil saturation conditions on the expected superstructure seismic demands. Simplified soil–structure interaction procedures that can be used to predict superstructure seismic demands have not been explicitly extended to incorporate the influence of unsaturated soil on the system response. In this paper, results from a series of six centrifuge tests are compared. In each test, an inelastic single-degree-of-freedom physical model was shallowly embedded in a sandy silt with a distinct water table elevation or a completely dry soil condition. The soil-structure system was subjected to a series of earthquake motions. The response of the system was evaluated to assess the influence of the soil saturation condition on the seismic response. Specifically, a conventional analytical procedure for predicting the influence of inertial interaction on the seismic response of the structure was extended to consider the water table elevation and underlying soil saturation condition and evaluated for its reliability. Analytical flexible-base modal parameters were compared with those determined from experimental results to judge the potential of the analytical procedure to be used in practice. Experimental results suggest that as the water table elevation was lowered from the fully saturated condition, both the flexible-base system period and damping ratio reduced. Therefore, the system behaved stiffer in the unsaturated soil compared with the dry and fully saturated conditions. The stiffer response reduced the seismically induced foundation settlements and rotations but amplified superstructure seismic demands in the form of accelerations, flexural drifts, and bending strains.

Grid-based simulation of soil moisture in the UK: future changes in extremes and wetting and drying dates

Soil moisture, typically defined as the amount of water in the unsaturated soil layer, is a central component of the hydrological cycle. The potential impacts of climate change on soil moisture have been less specifically studied than those on river flows, despite soil moisture deficits/excesses being a factor in a range of natural hazards, as well as having obvious importance for agriculture. Here, 1 km grids of monthly mean soil moisture content are simulated using a national-scale grid-based hydrological model, more typically applied to look at changes in river flows across Britain. A comparison of the soil moisture estimates from an observation-based simulation, with soil moisture deficit data from an operational system developed by the UK Met Office (Meteorological Office Rainfall and Evaporation Calculation System; MORECS), shows relatively good correspondence in soil drying and wetting dates, and in the month when soils are driest. The UK Climate Projections 2018 Regional projections are then used to drive the hydrological model, to investigate changes in occurrence of indicative soil moisture extremes and changes in typical wetting and drying dates of soils across the country. Analyses comparing baseline (December 1981–November 2011) and future (December 2050–November 2080) time-slices suggest large increases in the spatial occurrence of low soil moisture levels, along with later soil wetting dates, although changes to soil drying dates are less clear. Such information on potential future changes in soil moisture is important to enable the development of appropriate adaptation strategies for a range of sectors vulnerable to soil moisture levels.

Moisture influence on structural responses of pavement on expansive soils

Expansive soils are prone to heave and shrinkage due to changes in their natural moisture content. It would be helpful to predict distresses in pavement induced by heave/shrinkage because of long-term moisture variations in the subgrade considering specific environmental conditions. The virtual load method (VLM) has recently been developed to identify virtually distributed load that produces an equivalent deflection of the pavement caused by heave/shrinkage of expansive subgrade soils. In this research, a pavement section of interstate highway 220 (I-220) in Northern Louisiana is considered to implement the VLM and analyze structural behavior due to natural migration of water in unsaturated soil layers. Heave/shrinkage of the soils is computed from the predicted moisture content changes in subgrade using software VADOSE/W. In applying the Winkler’s model in the inverse theory to identify expansive soil-induced stresses in the pavement, a new iteration algorithm is proposed to consider the pavement-soil interaction. Special attention is drawn to the location of absolute maximum shear forces and bending moments in the pavement and their influences on the subgrade deformation. The effects of pavement stiffness and swelling pressure of the subgrade expansive soils are presented to understand the behavior of pavement on expansive soils.

Effects of land use on the hydrologic regime, vegetation, and hydraulic conductivity of peatlands in the central Peruvian Andes

The vegetation, geographic distribution, and pastoralist uses of peatlands in the central Andes have been extensively studied over the past 20 years. However, little information exists to characterize the hydrologic processes that support these groundwater-dependent ecosystems or the hydrologic alterations occurring due to human activities or climate changes. This lack of information significantly limits our understanding of how the central Andean Puna, one of the world's most threatened ecoregions, may be altered by increasing temperature and water extraction for human use. In addition, these peatlands provide critical pasture for native and domestic livestock and carbon storage, but overgrazing is an important modifier of their ecological trajectories.We analyzed three groundwater-fed fen peatlands with differing hydrologic regimes, annual precipitation, land use, and vegetation composition. Water table depth was a key factor significantly contributing to differences in soil hydraulic conductivity (p < 0.05) and vegetation composition (p < 0.05). Saturated hydraulic conductivity (K) varied from 0.57 m day−1 to 0.03 m day−1 for the horizontal component (Kh) and 0.07 to 0.002 m day−1 for the vertical component (Kv). The principal differences in K were in the seasonally unsaturated upper soil layers at 0–75 cm depth. The annual deepening in the water table and Distichia muscoides dominance drive the variability in K for these layers. Those drivers were themselves correlated but can individually modify K, increasing the decomposition rate and porosity (WT deepening) and altering the initial peat structure (D. muscoides dominance). Three vegetation communities were identified, one in sites with the deepest water tables, the lowest hydraulic conductivities, and dominance of Werneria pygmaea and Plantago rigida. The second community, dominated by bunch grasses in the genus Calamagrostis, occurred in areas with the most variable water table and medium hydraulic conductivity. The third community occurred in the most hydrologically stable areas, with the shallowest water table, highest hydraulic conductivities, and was dominated by Distichia muscoides. The study peatlands appear to have originated as the third community – groundwater-supported fens dominated by Distichia muscoides cushion and pool communities. However, modern hydrologic changes caused by human land uses and climate variability have caused a divergence in the vegetation, and in the more disturbed sites created higher decomposition rates in the shallow peat layers, and differences in soil structure.

The other side of tropical forest drought: do shallow water table regions of Amazonia act as large-scale hydrological refugia from drought?

Tropical forest function is of global significance to climate change responses, and critically determined by water availability patterns. Groundwater is tightly related to soil water through the water table depth (WT), but historically neglected in ecological studies. Shallow WT forests (WT < 5 m) are underrepresented in forest research networks and absent in eddy flux measurements, although they represent c. 50% of the Amazon and are expected to respond differently to global-change-related droughts. We review WT patterns and consequences for plants, emerging results, and advance a conceptual model integrating environment and trait distributions to predict climate change effects. Shallow WT forests have a distinct species composition, with more resource-acquisitive and hydrologically vulnerable trees, shorter canopies and lower biomass than deep WT forests. During 'normal' climatic years, shallow WT forests have higher mortality and lower productivity than deep WT forests, but during moderate droughts mortality is buffered and productivity increases. However, during severe drought, shallow WT forests may be more sensitive due to shallow roots and drought-intolerant traits. Our evidence supports the hypothesis of neglected shallow WT forests being resilient to moderate drought, challenging the prevailing view of widespread negative effects of climate change on Amazonian forests that ignores WT gradients, but predicts they could collapse under very strong droughts.