Soil Strength Values Research Articles

Least limiting water range indicators of soil quality and corn production, eastern Ontario, Canada

The least limiting water range (LLWR) attempts to incorporate crop-limiting values of soil strength, aeration, and water supply to plant roots into one effective parameter (on the basis of soil water content). The LLWR can be a useful indicator of soil quality and soil physical constraints on crop production. This study focused on assessing dynamic cultivation zone LLWR parameters between different cropping/tillage/trafficked clay loam plots at Winchester, Ont., to identify potential management impact on surficial soil physical conditions for contrasting growing seasons. This study also evaluated dynamic cultivation layer LLWR variables as indicators of corn ( Zea mays L.) plant establishment and corn yield. The results suggest that no-till soils had lower average air-filled porosities (AFP) and O 2 concentrations than respectively managed tilled plots for both years of study. Potential trafficking effects on aeration properties were most evident in no-till relative to till; preferentially trafficked no-tilled plots had lower AFP and O 2 concentrations than respective non-preferentially trafficked no-till plots for both years of study. Corn establishment and yield variability were principally explained by cumulative differences between daily AFP and aeration threshold values, and the cumulative number of days daily AFP was below an AFP aeration threshold for specific corn growth stage periods. Lower AFP was linked to lower yields and plant establishments. Soil strength, as measured by cone penetration resistance, was important over certain sites, but not as important globally as AFP in predicting crop properties. Overall, conventional tilled soils that were not preferentially trafficked had most favorable aeration properties, and subsequently, greatest corn populations and yields. No-till soils were at greater risk of aeration limiting conditions, especially those in continuous corn and preferentially trafficked.

Spatial distribution of Eucalyptus roots in a deep sandy soil in the Congo: relationships with the ability of the stand to take up water and nutrients.

Spatial statistical analyses were performed to describe root distribution and changes in soil strength in a mature clonal plantation of Eucalyptus spp. in the Congo. The objective was to analyze spatial variability in root distribution. Relationships between root distribution, soil strength and the water and nutrient uptake by the stand were also investigated. We studied three, 2.35-m-wide, vertical soil profiles perpendicular to the planting row and at various distances from a representative tree. The soil profiles were divided into 25-cm2 grid cells and the number of roots in each of three diameter classes counted in each grid cell. Two profiles were 2-m deep and the third profile was 5-m deep. There was both vertical and horizontal anisotropy in the distribution of fine roots in the three profiles, with root density decreasing sharply with depth and increasing with distance from the stump. Roots were present in areas with high soil strength values (> 6,000 kPa). There was a close relationship between soil water content and soil strength in this sandy soil. Soil strength increased during the dry season mainly because of water uptake by fine roots. There were large areas with low root density, even in the topsoil. Below a depth of 3 m, fine roots were spatially concentrated and most of the soil volume was not explored by roots. This suggests the presence of drainage channels, resulting from the severe hydrophobicity of the upper soil.

QUANTIFYING SOIL TRAFFICABILITY IMPROVEMENTS PROVIDED BY SUBSURFACE DRAINAGE FOR FIELD CROP OPERATIONS IN LOUISIANA

A field experiment was conducted on a Commerce silt loam (alluvial) soil near Baton Rouge, Louisiana, to study the effects of subsurface drainage on soil trafficability for two different water management systems: surface drainage only and subsurface drainage. Following a 30–mm rainfall event there was a significant difference in the decrease of soil moisture at a 10–cm depth directly above the subsurface drains compared with midway between drain pipes spaced at 15 m. Differences in soil moisture content between subsurface drained and surface drained only plots were not statistically significant, however, the plots that were subsurface drained had trafficable conditions one day sooner than the surface drained only plots. Soil strength values above the subsurface drains were consistently higher than at the mid–point between drains and soil strength increased as water table depth increased. A portable capacitance volumetric soil moisture meter was evaluated in this project for the accuracy in obtaining soil moisture content in the field. Results have shown that there was no correlation between the soil moisture obtained in situ by the volumetric moisture meter and by analysis of soil samples.

ANALYSIS OF METHODS FOR DETERMINING SOIL SHEAR STRENGTH DESIGN VALUES/GRUNTO STIPRUMO RODIKLIŲ SKAIČIUOJAMŲJŲ REIKŠMIŲ NUSTATYMO METODŲ ANALIZĖ

Designing foundations by soil safety factors is used for obtaining soil strength design values. The methods [10, 11, 12, 13] used for their calculations and minimal values of investigated coefficients are provided in the standards. The design values of soil characteristics computed by different methods do not match. In reality they should ensure the unique reliability of soil shear strength design values. In the present work, the soil shear strength values are computed by different methods. The experimental data are obtained for sands using direct shear strength apparatus. The comparative analysis of obtained results and methods for predicting the design values of soil strength characteristics is applied. When designing foundations, the relation between soil shear strength and normal stress is treated as linear (1, 2), but the dependency of the lower control limit on normal stress is nonlinear (5). Therefore, methods for getting experimental data of soil shear strength give different results. In corresponding standards, different additional conditions are considered for approximation of the investigated non-linear dependency to the linear one. The soil safety factors used for computing design values of tangent of internal friction angle and cohesion depend on these additional conditions. The approximation of non-linear dependency to the linear one does not allow ensuring the unique probability of soil shear strength design values. It is impossible to ensure the desired unique probability for different values of normal stress using traditional methods and constant soil safety factors. In order to obtain the same probability for soil shear strength values lower than the designed value, the probability-statistical methods should be applied. In this article, the method for determining soil shear strength design values is presented (10, 11). This method ensures the same probability of the soil shear strength design values for any values of normal stress. The probability-statistical methods should be applied for design of foundations when soil strength design values depend on normal stress. In order to apply the investigated methods, the probability of foundation collapse or reliability index β, the mean values of soil strength and their variance should be known. Dimensions of the foundation are obtained from mean value sand variance of soil strength characteristics solving the system of equations (13). The solution of this system is a combination of arguments of foundation design conditions that are investigated as normal random variable. The investigated combination corresponds to the limit state.

Least limiting water range: a potential indicator of physical quality of forest soils

The interactions of the 4 basic soil physical properties—volumetric water content, matric potential, soil strength, and air-filled porosity—were investigated over a range of contrasting textures and for 3 compaction levels of 4 forest soils in New Zealand, using linear and non-linear regression methods. Relationships among these properties depended on texture and bulk density. Soil compaction increased volumetric water contents at field capacity, at wilting point, and at the water contents associated with restraining soil strength values, but decreased the water content when air-filled porosity was limiting. The integrated effect of matric potential, air-filled porosity, and soil strength on plant growth was described by the single parameter, least limiting water range (LLWR). LLWR defines a range in soil water content within which plant growth is least likely to be limited by the availability of water and air in soil and the soil strength. Soil compaction narrowed or decreased LLWR in most cases. In coarse sandy soil, initial compaction increased LLWR, but further compaction decreased LLWR. LLWR is sensitive to variations in forest management practices and is a potential indicator of soil physical condition for sustainable forest management.

Grazing impacts on bulk density and soil strength in the foothills fescue grasslands of Alberta, Canada

Alberta foothills fescue grasslands are very productive ecosystems but there is concern that the traditional season-long (continuous) grazing regimes may be leading to soil deterioration due to compaction and increased soil strength. The objectives of this study were to quantify grazing effects on soil bulk density and soil strength of sloped areas in the Alberta foothills fescue grasslands at the Agriculture Canada Stavely Range Substation. The effects of two grazing intensities (heavy and very heavy) for two treatments (short duration and continuous) on these two parameters were compared to an ungrazed control. Soil bulk density and soil water to a depth of 7.5 cm were measured with a surface water/density gauge. Soil strength was measured with a hand-pushed cone penetrometer to a depth of 45 cm. Cone index, the maximum penetration resistance in a given depth interval, was used as a measurement parameter for soil strength.Grazing affected both soil bulk density and penetration resistance. Even short-duration treatments affected these soil properties, although their effects were similar for both heavy and very heavy grazing intensities. Distinction between heavy and very heavy continuous grazing treatments was clear for both bulk density and penetration resistance, with the very heavy treatment having the greatest detrimental effect on these two soil parameters for all treatments. Bulk density and soil strength values were always lowest in the spring after snowmelt and highest late in the growing season, reflecting the water status of these ecosystems. Identical treatment rankings were obtained using bulk density and penetration resistance, but cone index was a more sensitive indicator of the effects of grazing than bulk density. Key words: Grazing, fescue grasslands, bulk density, soil strength

A Soil Identification Method for Potential Road Construction Problem Sites on the Olympic Peninsula, Washington

Several logging road subgrades and cut slopes failed under wet construction conditions in the 1980's on the Olympic Peninsula in Washington. Although standard laboratory soil test results did not indicate this would occur, field tests showed possible soil failure. A majority of these soils were coarse-grained with significant amounts of fines (silt and clay). Field soil identification methods indicated that these soils had a moisture content above the plastic limit; the laboratory test results were mostly non-plastic. During construction, the soils performed like plastic soils with low strength values, although in the laboratory the soils had suitable strength values for normal road construction methods. An hypothesis that standard laboratory tests for soil strength values do not detect organic and/or clay minerals that lead to failure under wet road construction conditions on the Olympic Peninsula was proven correct. A combined field and laboratory testing procedure was developed for identifying these soils. (A)

Soil Aggregates and Clod Strength Dependence on Size, Cultivation, and Stress Load Rates

AbstractA study was carried out (i) to determine soil clod fragmentation patterns' dependence on specific surface energy, aggregate or clod size, and stress loading rates; and (ii) to examine the hypothesis that the large clods produced during tillage operations performed on air‐dry soils are a result of soil compaction due to overtilling and machinery traffic on a moist soil during the cropping season. It was found that surface energy and specific energy required to reduce aggregate size are dependent on stress loading rates. Under a specific stress loading rate, suspected to be less than or equal to the stress wave velocity in the soil, both common methods—the unconfined compression test and the drop‐shatter test—are assumed to be equally efficient in determining the energy required for soil fragmentation. It was concluded, based on soil strength values obtained, that the fracture strength of the soils tested is attributed to CaCO3 bonding bridges and is practically uniform for all clod and aggregate size ranges tested. The observed huge clods created while tilling the air‐dry soils were size dependent on row spacing and tillage direction. These results are typical of fracturing patterns of material characterized by uniform strength. The fracture patterns for these materials follow the planes of maximal shear stresses unless obstructed by plants of lower strength (e.g., plant rows, major cracks); thus the size of fractured units depend on tillage direction, tillage implement, and row spacing.

Cyclic Triaxial Strength of Standard Test Sand

A cooperative cyclic triaxial strength test program was performed by eight university, government, and consulting laboratories to define the cyclic strength characteristics often called the liquefaction potential or cyclic mobility of a standard test sand. Other laboratories may check the adequacy of their cyclic triaxial test equipment and testing procedures by duplicating the results of these tests. It was found that a good degree of agreement for cyclic soil strength values could only be achieved among different laboratories when proper attention was given to the following testing details: (1) Great care must be taken in measuring specimen dimensions and weight to accurately calculate specimen dry weight; (2) tested specimen dry weights must be within 0.5 pcf (8 kg/m²) of the specified value; (3) specimen preparation techniques must not deviate from the specified method; and (4) a smooth periodic loading wave form, such as a sine wave, should be used for testing.

Stability Problems of Collapsing Soil

Where collapsing soils exist in extensive areas of canal construction, stabilizing the foundation subsoils by ponding method is necessary before embankments and linings are built. As these soils a may still be somewhat critical after collapse, slope stability analyses must include this hazard In interpretation of soil strengths and safety factors. A procedure is presented for delineating collapsible soils and includes settlement records from 16 miles of preconstruction ponding treatment on the San Luis Canal, California, substantiating the criteria. Soil strength values from laboratory tests, which considered the critical nature of such soils, were used in a stability study. Safety factors were adjusted to account for earthquake stresses, and consideration was given to reduced ultimate strength and limits of strain to resist bulging and cracking of the lining. Improved stability was attained by zoning the canal embankments and compacting foundation surfaces with extra heavy pneumatic rollers.

A Study of a Large Compacted Clay Embankment-Fill Failure

The paper describes the failure during construction of a proposed 70 ft. high railway embankment fill. The fill was built of a uniform clay of medium plasticity which was used at an average moisture content of 3 per cent greater than had been provided for in the design. The fill failed under its own weight when it reached a height of 55 feet.In order to assist in the design of stabilizing works three test holes were drilled in the fill and soil samples recovered. Properties of field compacted and laboratory compacted soil samples are compared. The age of both types of samples is shown to have a significant effect on the test results.Both total and effective stress analyses of the embankment at failure have been performed using the laboratory values of soil strength. The total stress analysis gives a safety factor of 1.0 at failure whereas the effective stress analysis gives a safety factor of 1.2. The hazards of choosing the correct value of laboratory shear strength for the total stress analysis are discussed.