Tropical Soil Profiles Research Articles

Integrating downhole seismic testing into dynamic probing light (DPL): first results

Dynamic probing light (DPL) is available among a wide range of dynamic penetrometers. This equipment can be used in locations where conventional sounding equipment may have limited access. Here, the shear wave velocity (Vs) determination by the downhole seismic technique is incorporated to the DPL to develop a hybrid test, the seismic DPL test, for the preliminary characterization of tropical soils. The corresponding equipment, test procedure and data interpretation of the seismic DPL (S-DPL) test are presented and discussed. S-DPL test campaigns were carried out at two experimental research sites where crosshole seismic tests were conducted to provide reference Vs profiles. The Vs profiles determined with the S-DPL test are in accordance with those determined with the crosshole test for both sites. The proposed hybrid test (S-DPL) has the potential to serve applications in preliminary site characterization, particularly at sites including unsaturated tropical soil profiles.

Geotechnical site characterization by DMT and laboratory tests on an unsaturated tropical soil site for slope stability analysis

The slope stability is an important topic because it presents risks of socio-economic losses caused by eventual ruptures. It is necessary to identify the site profile, as well as obtaining soil strength parameters for the slope stability analysis. This paper presents and discusses the use of the Flat Dilatometer Test (DMT) in the geotechnical site characterization for slope stability analysis in an unsaturated tropical soil site. Six DMTs were performed to define the stratigraphical profile and estimate geotechnical parameters. Shear strength parameters were determined in the laboratory using saturated and unsaturated triaxial compression as well as soil water retention curves (SWRC) to support DMT data interpretation. A commercial software was used to perform the slope stability analysis a cut slope with 6.6 m height and a gradient around 55° to illustrate the application of DMT and triaxial test data. The DMT allowed the representative site profile to be identified, as well as estimating the design parameters that compared well to those interpreted from unsaturated triaxial test data for the in-situ soil suction. The DMT can be used as logging test in the preliminary characterization of studied site specially to define the stratigraphical profile, site variability, select the regions to collect disturbed and undisturbed soil samples and as the first attempt to estimate the geotechnical design parameters via correlations. It is important to emphasize that laboratory tests on undisturbed soil samples are essential in the slope stability analysis of unsaturated tropical soil profiles.

Maximum shear modulus and modulus degradation curves of an unsaturated tropical soil

The maximum shear modulus (G0) and the modulus degradation curve (G/G0 versus γ) are important information in the evaluation of the soil mechanical behavior, both for dynamic and static loads. Dynamic tests (resonant column and cyclic triaxial tests) are not routinely performed in geotechnical practice in Brazil, and the geotechnical literature on the dynamic behavior of unsaturated tropical soils is limited. This paper presents and discusses seismic dilatometer (SDMT), resonant column, and triaxial test with bender elements and internal instrumentation to determine G0 and the modulus degradation curve in an unsaturated tropical sandy soil profile. It was observed that G0 tends to increase non-linearly with soil suction and net stress (σ - ua). It was also observed that the in situ G0 values determined with the SDMT were higher than those from laboratory tests (bender elements and resonant column). The modulus degradation curves determined with resonant column were used to define the reference curve via SDMT for the studied site. Soil suction influence in shear modulus degradation curves determined with unsaturated triaxial compression tests with local instrumentation is also presented and discussed.

Suction and reloading effects on pile behaviour in tropical soil

ABSTRACT The design of foundations for buildings subjected to unload–reload cycles generate the need to understand the reloading effects in piles. Commonly, such foundations are found in unsaturated soils, which also requires considering the effects of matric suction. This paper investigates the effects of soil suction and reloading on the behaviour of piles from an experimental and numerical approach. A bored pile built on a tropical soil profile was subjected to a sequence of load tests, performed at different climate seasons. An average 85% decrease in soil suction was measured from the pile load test in the dry season to the test in the rainy one, which contributed to a reduction in the pile’s ultimate axial load capacity and stiffness of around 50%. The unload–reload cycles caused an increase on the pile’s ultimate load capacity between 7% and 9% and on the pile stiffness between 13% and 24%.

Numerical simulations of displacement piles in a tropical soil

The behavior of pile foundations under axial loading is directly influenced by the effects that its installation process induces in the surrounding soil. Consequently, the consideration of these effects is essential for the correct numerical modeling of these geotechnical structures. In the present study, numerical simulations of driven cast-in-situ piles under axial loading have been carried out using finite element analysis. Three 3.5 m long piles with diameters ranging from 114.3 to 219.1 mm were analyzed. The pile installation effects have been considered indirectly by employing two distinct approaches, both based on the concepts of cylindrical cavity expansion. The behavior of the tropical soil profile is described with the Hardening Soil constitutive model. The load-displacement response and load distribution along the pile obtained with the numerical simulations have been analyzed and compared with in-situ load tests results. In the failure conditions, both approaches accurately predicted the bearing capacity of the piles, with an average error of only 2% compared to the measured values. The results in terms of load distribution over depth were also satisfactory. The difference between measured and numerical ultimate base resistance values ranged from 0% to 30%. The good agreement between the numerical and experimental results indicates that the proposed numerical approaches have been effective in simulating the piles installation process and reinforces the importance of considering the installation effects in the numerical modeling of these geotechnical structures. Both approaches can also be used to predict the bearing capacity of displacement piles.

Experimental and Numerical Behavior of Horizontally Loaded Piled Rafts with a Defective Pile

The present paper aims to investigate the effects of defective piles in horizontally loaded piled raft foundations. Close to real scale foundation models of defective and intact three-piled systems were submitted to horizontal load tests. The models used bored type piles drilled with 5 m in length and 0.25 m in diameter, connected to a concrete raft founded on a tropical soil profile at the University of Campinas research site. Based on the experimental data, a three-dimensional finite element analysis was initially calibrated and consequently used to study the effects of the defective element at the pile load distribution and the horizontal subgrade forces at the pile shaft. The results show that the presence of a defective pile increases the raft tilting, which affects both vertical and horizontal load distributions among the raft and the piles, and among trailing and leading piles.

Case Studies and Challenges of Implementing Geotechnical Building Information Modelling in Malaysia

Unforeseen ground conditions are some of the main contributors to construction cost over-runs and late completion. Recent research trends have witnessed the scopes of building information modeling (BIM) technology being extended to subsoil and substructure constructions, or simply known as geotechnical BIM. This paper aimed to explore the procedures of developing 3D subsoil models through two case studies in Malaysia. The geotechnical BIM processes were performed by commercial software, AutoCAD Civil 3D 2017, with the extension of the Geotechnical Module. The modeling procedures can be divided into three main stages, namely data collection, data interpretation, and data visualization. The subsoil models were successfully developed at different levels of detailing to serve for different applications. The results showed that the 3D subsoil modeling required huge modeling and computational efforts, particularly when dealing with tropical residual soil profiles in Malaysia that are highly intricate. Thus, an adequate soil strata generalization was required to simplify the generated subsoil model. Data collection and management was identified as one of the main challenges of promoting geotechnical BIM in Malaysia at a macro-scale. Despite the challenges, successful implementation of the geotechnical building information in the present case studies were proved to be capable of promoting interoperability of soil data, which is an essential element in sustainable construction.

Tropical soil profiles reveal the fate of plant wax biomarkers during soil storage

The waxy coating that protects the leaves and other soft tissues of plants includes n-alkane and n-alkanoic acid compounds that are commonly used as biomarkers to reconstruct past environment. Plant waxes have geological relevance given their persistence in soils and paleosols, as well as in lake and marine sediments, yet diagenesis may alter their molecular and isotopic signatures from synthesis to deposition. This study seeks to understand the fate of plant wax biomarkers in soils after leaf-fall as characterized by a series of tropical soil profiles. We investigate the changes in abundance, molecular distributions, and hydrogen (δD) and carbon isotopic compositions (δ13C) of plant waxes (n-alkanes and n-alkanoic acids) in six litter-to-soil profiles along a 2740 m elevation transect from the eastern flank of the Andes mountains down to the lowland Amazon floodplain in Peru. From litter to soil, we find acid/alkane ratios increase while absolute abundances decrease. In contrast, within each soil, acid/alkane ratios are roughly constant, and we find an equivalent exponential decline in concentration in both compound classes with depth, with molecular distributions indicating some new production. We observe a 4–6‰ 13C-enrichment from litter to deeper soils for both C29n-alkanes and C30n-alkanoic acids, of which the Suess effect accounts for ≤2‰. We infer that microbial degradation and production (or ‘turnover’) processes influence the δ13C of plant waxes that survive in soils; in contrast, no systematic change in δD values is observed. The plant wax signal in soils includes averaging of inputs and diagenetic effects, so this signature is particularly relevant for the interpretation of plant wax archives in paleosols and the plant waxes eroded from soils and exported to downstream sedimentary archives. We show that soils represent the major stock of plant wax under living ecosystems, suggesting that soils may be a quantitatively-important source of plant waxes available for fluvial erosion, with implications for studies of carbon cycling and paleoenvironmental reconstructions from downstream geological archives.

Site Characterization of a Tropical Soil by In Situ Tests

This paper compiles experimental site characterization of an unsaturated tropical soil profile in an experimental research site in Bauru, Sao Paulo, Brazil. Several in situ campaigns comprised geophysical borehole tests (cross-hole, down-hole and up-hole together with the SPT), as well as mechanical tests (SPT, CPT and SDMT) have been performed. There is an unsaturated sandy soil at the studied site and, according to the MCT Classification System for tropical soils, the first 13 m depth is a colluvium with lateritic soil behavior (LA’) over the residual soil with non-lateritic behavior (NA’). The ratio between seismic ( G o ) and mechanical ( q n , N 60 , E D , K D and M DMT ) parameters was useful to identify the existence of microstructure and to confirm unusual soil behavior for this tropical soil site. CPT and SDMT were efficient for detailed stratigraphic logging, however, the estimative of geotechnical parameters cannot be solely been done base on just conventional correlations defined for sedimentary soil.

Soil organic carbon stocks and their determining factors in the Dano catchment (Southwest Burkina Faso)

Although the evaluation of soil organic carbon (SOC) stocks across different types of land use and major reference soil groups is essential for mitigating climate change, there remains, to date, limited comprehensive understanding of whole tropical soil profiles. Therefore, this study aimed to explain the amount of SOC stocks in different land-use systems and across various soil groups, as well as its spatial pattern in the topsoil (0–30 cm) and subsoil (30–100 cm) within the savannah zone of Burkina Faso. Roughly 70 soil profiles were considered along with additional auger sampling to account for spatial variation in both cropland (CR) and savannah (SA). The machine learning technique random forest regression (RFR) and multiple linear regression (MLR) were used for modeling the surface and subsurface SOC stocks. For model calibration, covariates including land use, topographic, texture, and climatic data were considered as surrogate for soil forming factors. The prediction maps produced by the calibrated models were validated by an independent dataset. The results indicated that about 53% of the SOC stock over 1 m depth was held in the upper 30 cm. Only a marginal difference was recorded between the topsoil SOC stock in SA (41.4 t C ha−1) and CR (39.1 t C ha−1) soils. For the subsoil, a significant difference (p < 0.05) was observed between the SOC stock of CR soils recording about 40.2 t C ha−1 and SA soils with 26.3 t C ha−1. Among the reference soil groups, the Gleysols located at lower elevation positions revealed the highest SOC stocks over 0–30 cm (44 t C ha−1) and 100 cm depth (86.6 t C ha−1). The Stagnosols (45.2 t C ha−1) followed by the Gleysols (42.7 t C ha−1) recorded the highest SOC stocks over 30–100 cm. The variability of SOC stock in the topsoil was primarily related to site-specific elements, such as particle-size fraction and wetness index, while its distribution in the subsoil was mainly associated with the topographical orientation represented by the slope aspect. Compared to the MLR, RFR estimated mean top- and subsoil SOC stocks of the catchment fairly well, along with lower statistical error metrics, though extreme values were not covered. Nevertheless, the findings on SOC stocks reinforce the view that the semi-arid ecosystems of West Africa still offer a significant opportunity for carbon sequestration for both topsoil and subsoil, and these results represent a baseline for future modeling of SOC dynamics in the region.

Drivers and patterns of iron redox cycling from surface to bedrock in a deep tropical forest soil: a new conceptual model

Iron (Fe) reduction and oxidation are important biogeochemical processes coupled to decomposition, nutrient cycling, and mineral weathering, but factors controlling their rates and spatial distribution with depth are poorly understood in terrestrial soils. In aquatic ecosystems, Fe reduction often occurs below a zone of oxic sediments. We tested an alternative conceptual model for Fe redox cycling in terrestrial soils using a deep humid tropical forest soil profile. We hypothesized that Fe reduction in anaerobic microsites scales with depth variation in labile C and Fe availability, as opposed to bulk oxygen (O2). We measured bulk O2 at multiple depths from 0.1 to 5 m quasi-continuously over 18 months and sampled soils from surface to bedrock (~7 m). Median O2 mixing ratios declined from 19.8 ± 1.2 % at 0.25 m to 16.1 ± 1.0 % at 1 m, but did not consistently decrease below 1 m, challenging a recent model of regolith development. Reduced Fe (Fe(II)) extractable in 0.5 M hydrochloric acid was greatest in 0–0.1 m soil and declined precipitously with depth, and did not correspond with visible gleying in B horizons. We observed similar depth trends in potential Fe reduction under anaerobic conditions. Depth trends in Fe(II) also closely mirrored short-term soil respiration and bulk soil C. Labile C stimulated Fe reduction at 0–0.1 m depth, whereas addition of short-range-ordered Fe oxides had no effect. Cultivable Fe-reducing bacterial abundance was four orders of magnitude greater in surface soil (0–0.1 m) than below 1 m. Although cultivable Fe oxidizing bacteria were typically also more abundant in surface soil, addition of labile C and nitrate stimulated Fe oxidizers in deep soil by two orders of magnitude under anaerobic conditions. This implies that infiltration of nitrate (and possibly C) from shallow soil water could potentially promote biotic Fe oxidation, a critical step in bedrock weathering, 7 m below. Together, these data suggest that C, Fe, and nutrient availability increase microbial Fe reduction and oxidation in surface (vs deeper) soil microsites despite high bulk O2, in contrast to the depth segregation of electron accepting processes often observed in aquatic ecosystems. Furthermore, the greatest capacity for Fe redox cycling can occur in A horizons that do not display gleying or mottling.

Influence of biomineralization on a profile of a tropical soil affected by erosive processes

Most of the soils of tropical countries, especially those in South America and Africa, are affected by erosion processes. As a result, researchers in the field of geotechnical engineering, specifically in the context of "biotechnology" or "bioengineering", have been investigating the use of microorganisms to improve the geotechnical properties and stability of soils. Using this approach, this work was developed to analyze the effects of the implementation of a calcium carbonate precipitating nutrient in native microbiota on the mitigation of erosion processes in a tropical soil profile. The methodology used in this research consisted of collecting undisturbed samples in a soil profile located in an area affected by erosion processes. In such samples, the native bacteria were identified, and it was determined that the nutrient B4 induced the precipitation of calcium carbonate. Subsequently, soil samples were characterized physically, chemically, mineralogically and mechanically in their natural state and after the addition of the nutrient. The tests were performed at least fifteen days after treatment with the nutrient. It was concluded that the use of the nutrient B4 enabled the native bacteria present in the soil to precipitate calcium carbonate, resulting in improvements in the physical, chemical, mineralogical and mechanical properties of the soil, which allowed for the mitigation of erosion processes that characterize the soil profile studied. The conclusions derived from the study apply not only to other tropical soil profiles subjected to erosion but also to improvements of the geotechnical behavior of soils in general.

Relating the biological stability of soil organic matter to energy availability in deep tropical soil profiles

Tropical subsoils contain large reservoirs of carbon (C), most of which is stored in soil organic matter (SOM). Subsoil OM is thought to be particularly stable against microbial decomposition due to various mechanisms and its position in the soil profile, potentially representing a long-term C sink. However, few experiments have explicitly investigated SOM stability and microbial activity across several orders of magnitude of soil C concentrations as a function of soil depth. The objective of this study was to evaluate the biological stability of SOM in the upper 1.4 m of tropical forest soil profiles. We did so by measuring CO2 evolution during a 90-day laboratory incubation experiment on a sample set that was previously characterized for C and nutrient concentrations and microbial biomass. We concurrently measured the energy content of SOM using differential scanning calorimetry (DSC) as an index of the energy available for microbial metabolism, with the hypothesis that the biological stability of SOM would be inversely related to the energy contained within it. Cumulative CO2 evolution, mean respiration rates, and the energy density of SOM (energy released during combustion normalized to soil C) all declined with soil depth (P < 0.01). Biological indices of C stability were well correlated with measures of SOM energy. There was no change in the mean respiration rate as a function of depth when normalized to soil C, and a trend toward increased respiration per-unit microbial biomass (P = 0.07). While reduced microbial respiration in subsoils suggests an increase in the biological stability of SOM, we suggest this is driven principally by concurrent declines in energy availability as measured by DSC and the size of the microbial biomass pool. On a per-unit biomass basis, subsoil OM may be as prone to decomposition and destabilization as surface SOM.

Parent material and vegetation influence bacterial community structure and nitrogen functional genes along deep tropical soil profiles at the Luquillo Critical Zone Observatory

Microbial communities mediate every step of the soil nitrogen cycle, yet the structure and associated nitrogen cycle functions of soil microbial communities remain poorly studied in tropical forests. Moreover, tropical forest soils are often many meters deep, but most studies of microbial nitrogen cycling have focused exclusively on surface soils. The objective of our study was to evaluate changes in bacterial community structure and nitrogen functional genes with depth in soils developed on two contrasting geological parent materials and two forest types that occur at different elevations at the Luquillo Critical Zone Observatory in northeast Puerto Rico. We excavated three soil pits to 140 cm at four different sites representing the four soil × forest combinations (n = 12), and collected samples at ten-centimeter increments from the surface to 140 cm. We used bacterial 16S rRNA gene-DGGE (denaturant gradient gel electrophoresis) to fingerprint microbial community structures, and quantitative PCR to measure the abundance of five functional genes involved in various soil nitrogen transformations: nifH (nitrogen fixation), chiA (organic nitrogen decomposition), amoA (ammonia oxidation), nirS (nitrite reduction) and nosZ (nitrous oxide reduction). Multivariate analyses of DGGE fingerprinting patterns revealed differences in bacterial community structure across the four soil × forest types that were strongly correlated with soil pH (r = 0.69, P < 0.01) and nutrient stoichiometry (r2 ≥ 0.36, P < 0.05). Across all soil and forest types, nitrogen functional genes declined significantly with soil depth (P < 0.001). Denitrification genes (nirS and nosZ) accounted for the largest proportion of measured nitrogen functional genes. Measured nitrogen functional genes were positively correlated with soil carbon, nitrogen and phosphorus concentrations (P < 0.001) and all genes except amoA were significantly more abundant in the Inceptisol soil type compared with the Oxisol soil type (P < 0.03). Greater abundances and a stronger vertical zonation of nitrogen functional genes in Inceptisols suggest more dynamic nitrogen transformation processes in this soil type. As the first study to examine bacterial nitrogen functional gene abundances below the surface 20 cm in tropical forest soils, our work provides insight into how pedogenically-driven vertical gradients control the nitrogen-cycling capacity of soil microbial communities. While previous studies have shown evidence for redox-driven hotspots in tropical nitrogen cycling on a watershed scale, our study corroborates this finding on a molecular scale.

Influence of biomineralization on the physico-mechanical profile of a tropical soil affected by erosive processes

Most of the soils of tropical countries are affected by erosion processes. As a result, much attention has been dedicated to the use of microorganisms to improve the geotechnical properties and stability of soils in the context of “bioengineering”. This work was carried out to analyze the effects of the use of a CaCO3 precipitating nutrient on native microbiota with the aim of mitigating the erosion processes in a tropical soil profile. We observed that the use of nutrient B4 enabled native bacteria present in the soil to precipitate calcium carbonate, resulting in improvements in the physical, chemical, mineralogical and mechanical properties of the soil, which allowed the mitigation of the erosion processes that characterize the soil profile studied.

Changes in phosphatase kinetics with soil depth across a variable tropical landscape

Phosphatase enzymes play a key role cycling phosphorus from organic to plant-available pools, particularly in tropical soils where inorganic phosphorus is often limited. However, most studies of phosphatase activity have focused only on surface soils, despite the large quantities of carbon and nutrients stored in tropical subsoils. The goal of this study was to determine how acid phosphatase kinetic parameters change with depth across two parent materials (represented by Oxisols and an Inceptisols) and two distinct forests (lower and upper montane) at the Luquillo Critical Zone Observatory in northeast Puerto Rico. We collected samples from five soil pits at each of four soil × forest types, and measured apparent phosphatase kinetic parameters (AppVmax and AppKm) and soil nutrients at 0, 20, 50, 80, 110 and 140 cm depths. Across all sites, AppVmax declined 97% and AppKm declined 85% from the surface to 140 cm depth. The ratio of AppVmax to AppKm (i.e., Ka) did not change through the first meter of soil profiles but was significantly reduced by 50% 140 cm. Total carbon, nitrogen and extractable phosphorus all declined exponentially with depth. Carbon concentrations and AppVmax were both significantly greater in Oxisols compared with Inceptisols, and in the higher elevation montane forest compared to the lower elevation forest. The scaling relationship we observe between AppVmax and AppKm is common for environmental systems, although the degree of correlation in our study (R2 = 0.48) is unusually high, suggesting these parameters are both driven by changes in energy and nutrient availability along depth profiles. However, the consistency of Ka with depth indicates that overall catalytic capacity of phosphatase is maintained across a range of substrate concentrations. The larger variability in AppVmax compared with AppKm suggests microorganisms exert more control over phosphatase production than substrate availability. Our findings indicate that subsoil microbial communities are not metabolically dormant, but rather contribute to P-cycling at rates comparable to their surface counterparts. Further research on ecology of microorganisms in resource-limited tropical subsoils is warranted to better understand microbial contributions to biogeochemical cycles throughout tropical soil profiles.

Soil–Water Characteristic Curves Obtained for a Colluvial and Lateritic Soil Profile Considering the Macro and Micro Porosity

This study aims to determine and to model the relationship of matric suction versus water content, known as soil–water characteristic curve (SWCC), for a tropical soil profile at the southeastern Brazil. This soil profile consists of a colluvial, lateritic silty clay, with thickness of about 6.5 m. The filter paper and pressure plate techniques were used to determine the SWCC. Specimens were trimmed from the undisturbed soil samples collected along soil profile depth and submitted to drying paths. Impregnated thin-layer plates and a petrographic microscope were used to examine the structure and mineralogical composition of the soil samples. Mercury intrusion porosimetry tests were performed on some soil samples to obtain the frequency histogram of the pores. SWCC with bimodal pore-size distribution were obtained by the filter paper technique and SWCC with unimodal pore-size distribution were obtained by the pressure plate technique. The SWCC showed values of air-entry ranging from 1 to 3 kPa, depending on the technique used, resembling soils with macroporosity, as the lateritic soils. Combining both techniques, the SWCC could be fitted by a model that takes into consideration soils with macro and microporosity.

Mercury distribution in tropical soil profiles related to origin of mercury and soil processes

The aim of the study is to improve our understanding of the vertical and lateral variations in the mercury content [Hg] of tropical soils. In addition to the distance to anthropogenic sources of Hg, the most frequently evoked determining factor is the abundance of Hg-bearing phases. Soil processes (weathering, mass or water transfer) determine the abundance of carrier phases. We assume that soil processes also have a direct impact on the distribution of Hg and that the impact is different according to the lithogenic or atmospheric origin of this element. We compare two types of soil (oxisol and ultisol) in the French Guiana forest, at localities a few tens of metres apart and exhibiting very different Hg contents. We show that vertical profiles of [Hg] are strongly related to the variations of [Hg atmospheric], whereas [Hg lithogenic] varies little. The penetration of Hg atmospheric from the surface deposits is favoured down to a depth of 3 m in the oxisol and limited to the upper horizons of the ultisol because of contrasted hydraulic conductivity between the two soils. Hg is primarily of lithogenic origin in the alteritic horizons of the ultisol. The relative accumulation of Hg lithogenic during the progressive weathering of parental material is limited near the soil surface by the disequilibrium of secondary minerals. Remobilization of Hg atmospheric or Hg lithogenic stored in the soil is a function of the chemical or particulate erosion of Hg-bearing phases, particularly active in the upper horizons of the ultisol, where lateral flow occurs during rain events. The correlations observed between the iron or clay contents and [Hg] can be caused by the affinity of Hg for these carrier phases, but may also reflect the weathering and the transfer processes which affect together the fate of Hg and the mineralogical and chemical composition of the soil.

Multimethod study of the degree of humification of humic substances extracted from different tropical soil profiles in Brazil's Amazonian region

This work consisted of determining the degree of humification of humic substances (HS) extracted from six different Amazonian soils collected from flooded and unflooded regions at different depths (0–10, 10–20, 20–40, and 60 cm). The humic substances were extracted according to procedures recommended by the International Humic Substances Society and characterized using elemental analysis, electron paramagnetic resonance (EPR), and fluorescence spectroscopy. The findings on semiquinone-type free radical concentrations in HS showed variations of 0.10–7.55×10 18 spins g −1 of carbon (g C) −1, indicating considerable differences between the humification levels of HS extracted from Amazonian soils. The results showed an average of 1.71±0.04×10 18 spins (g C) −1, which is congruent with other data reported in the literature on Tropical soils. It was found that, on average, HS extracted from flooded soil contained higher semiquinone-type free radical concentrations than HS extracted from unflooded soils, indicating the influence of humidity in the humification process of organic matter. The humification process varies according to the profile, and the 10–20- and 0–10-cm profiles generally showed more humified HS. The degree of humification of the HS studied here displayed a similar behavior when exposed to fluorescence (excitation at 465 nm) and EPR ( R=0.85). However, the low correlation between the C/H, C/O, and C/N atomic ratios and the semiquinone-type free radical concentration/fluorescence intensities indicated that data obtained by these techniques with regard to the degree of humification of HS may lead to different conclusions.

Yield, Strength, and Critical State Behavior of a Tropical Saturated Soil

The paper reports laboratory investigations carried out on a tropical soil profile to study its compressibility, strength, critical state and limit state conditions, and their variation with depth. The soil profile comprises a reddish lateritic layer (horizon B) underlain by a saprolitic soil (horizon C) from which a number of block samples were taken. A series of isotropic and anisotropic compression tests, and drained and undrained triaxial tests, were conducted on specimens sampled at depths between 1.0 and 7.0 m, and also in the exposed saprolitic soil. Special triaxial tests, with the pore pressure increased to induce failure, were performed to investigate the failure at low stress levels. On this basis a tensile cutoff on the failure envelope was defined. In order to assess the influence of the natural soil structure, drained and undrained triaxial tests were carried out on compacted samples obtained from depths of 1.0 and 5.0 m. Higher strength parameters were measured for the horizon C soil, which...