Soil Density Changes Research Articles

Influence of sand density and retaining wall stiffness on three-dimensional responses of tunnel to basement excavation

Basement excavation inevitably causes stress changes in the ground, leading to soil movements that may affect the serviceability and safety of adjacent tunnels. Despite paying much attention to the basement–tunnel interaction, previous research has mainly focused on the influence of tunnel location in relation to the basement, tunnel stiffness, and excavation geometry. The effects of sand density and basement wall stiffness on nearby tunnels due to excavation, however, have so far been neglected. A series of three-dimensional centrifuge tests were thus carried out in this study to investigate these effects on the complex basement–tunnel interaction. Moreover, three-dimensional numerical analyses and a parametric study by adopting a hypoplastic sand model were conducted to improve the fundamental understanding of this complex problem, and calculation charts were developed as a design tool. When the basement was constructed directly above the existing tunnel, excavation-induced heave and strain were more sensitive to a change in soil density in the transverse direction than that in the longitudinal direction of the tunnel. Because a looser sand possesses smaller soil stiffness around the tunnel, the maximum tunnel elongation and transverse tensile strain increased by more than 20% as the relative sand density decreased by 25%. Moreover, the tensile strain induced along the longitudinal direction was insensitive to the stiffness of the retaining wall, but the tensile strain induced along the transverse direction was significantly reduced by a stiff wall. When the basement was constructed at the side of the existing tunnel, the use of a diaphragm wall reduced the maximum settlements and tensile strains induced in the tunnel by up to 22% and 58%, respectively, compared with the use of a sheet pile wall. Under the same soil density and wall stiffness, excavation-induced maximum movement and tensile strains in the tunnel located at a side of the basement were about 30% of the measured values in the tunnel located directly beneath the basement centre.

Effects of soil bulk density on sessile oak Quercus petraea Liebl. seedlings

This article presents the results of laboratory examinations concerning the effects of soil bulk density on the growth of sessile oak Quercus petraea Liebl. seedlings. The soil for the study was collected from a nursery plot and then compacted into PCV pots in eight different bulk density levels ranging from 0.81 to 1.32 g cm−3. Oak seedlings were cultivated in controlled conditions for 76 days after sowing. The growth and mass parameters of particular seedlings together with the parameters of the assimilation apparatus and roots were measured, taking into account the share of various diameter roots in the root system. For the purpose of the experiment, 120 acorns were sowed, of which 73 seedlings were cultured. An analysis of the growth of the sessile oak seedlings revealed that a change in soil density significantly affected root system development, total height of seedlings and dry mass. It was observed that the dry mass of the root system as well as the area and length of the roots decreased with an increase in soil density. Root system reduction was noted, the size of which determines the proper development of the tree and ensures its stability. Besides a decrease in the root system, an increase in soil compaction also resulted in a reduction in the number of roots of a specified diameter. The first to be affected were the smallest roots, i.e., those with diameters up to 0.2 mm, which are responsible for the uptake of mineral components. It was confirmed that even a slight increase in soil compaction might negatively result in the growth of young seedlings, impeding root system development.

Investigation of soil density effect on suction induced due to root water uptake by Schefflera heptaphylla

AbstractAn increase in soil water suction due to root water uptake is significant for stability of slopes and landfill covers as it increases shear strength of soil by 50%. Coupled effects of soil density changes in soil hydraulic properties [i.e., soil water characteristic curve (SWCC) and hydraulic conductivity] and root characteristics (root depth, vertical root distribution) found in past experimental studies may affect soil water suction, which are not fully understood. The objective of this study is to numerically investigate these coupled effects of soil density by systematically comparing suction profiles under three different scenarios: (1) changes in only soil hydraulic properties, (2) changes in only root characteristics, and (3) changes in both soil properties and root characteristics. Silty sand with two different degrees of compaction (DoC) (i.e., 82% and 89%), which are typically found on man‐made slopes in subtropical regions, is considered. Schefflera heptaphylla is selected as vegetation species, which is commonly used for ecological restoration for man‐made slopes. A laboratory experiment was conducted to quantify suction distribution in silty sand vegetated with Schefflera heptaphylla at DoC of 89%. Vertical root distribution was measured using image analysis, while an axi‐symmetric finite element analysis was conducted to calibrate transpiration rate from measured suction. Root characteristics corresponding to DoC of 82% were then deduced using observations from literature. Richards equation coupled with sink term was numerically solved using Vadose/W to simulate water flow and root uptake on a flat ground (5 m depth, 10 m wide). Soil hydraulic parameters from a completely decomposed granite soil for two different DoCs (82% and 89%) were considered. Simulations assume constant transpiration rate for a period of 10 d. With increase in soil density from DoC of 82% to 89%, computed suction increases by 110% for scenario 1 (only effect on soil hydraulic properties), whereas it decreases by at least 21% for scenarios 2 (only root characteristics) and 3 (coupled effect). Here, a higher unsaturated hydraulic conductivity and a lower actual transpiration rate balance effects of a lower desorption rate of SWCC.

Drained cyclic capacity of plate anchors in dense sand: Experimental and theoretical observations

This paper provides experimental evidence that shows that the drained cyclic capacity of a plate anchor in dry dense sand may be higher than the equivalent monotonic capacity. The experimental data show that when cyclic loading is low relative to the monotonic capacity, increases in the eventual capacity are observed; when the magnitudes of the cyclic loads are closer to the monotonic capacity, no increases in capacity are observed. These responses are explained in the paper using an elasto-plastic macro-element model extended with expandable bounding and memory surfaces that address the increase in strength or stiffness caused by changes in soil density and fabric when the anchor is subject to cyclic loading in dense sand.

Comparison of Soil Sampling Devices for Soil Bulk Density Determination for Carbon Sequestration Monitoring

Soil bulk density is often used to adjust soil carbon (C) values for changes in soil density because of management differences in C-sequestration studies. Often hand-sampling devices used for soil fertility monitoring may be used for sampling soil to a depth of up to 30 cm. Four sampling devices of differing characteristics that can be used with hand-sampling equipment across full tillage, reduced tillage, and no-tillage to determine soil bulk density were evaluated. There were no significant differences in soil bulk density values between three cylindrical devices, which measured from 19 to 31 mm in diameter. However, a 21 mm × 76 mm rectangular device gave significantly lower bulk density values than the cylindrical devices. Calculation of soil organic carbon (SOC) mass per unit area showed similar results for the cylindrical devices with the rectangular device giving a significantly lower value than the cylindrical devices. Using a cylindrical device on a hand probe in a consistent manner will give satisfactory results when sampling surface soils.

WATER ABSORPTION BY SWELLING LEGUMINOUS SEEDS AS AFFECTED BY WATER POTENTIAL AND EXTERNAL MECHANICAL CONSTRAINTS

The Collis-George/Melville model of water uptake by swelling seeds was used to determine the diffusivity to water of pea, chickpea and vetch seeds as affected by external solution and water potential. These values were compared with the Hadas/Russo results for the same seeds. Soil density changes at the seed-soil contact interface were calculated and the approximate pressures exerted on the soil by the swelling seeds were evaluated from consolidation test data for the sand and silty soils used in the experiments. Seed diffusivity to water according to the Collis-George/Melville model was in close agreement with the Hadas/Russo data only for the later part of the imbibition process when the seeds are fully imbibed or nearly so. The non-swelling Hadas/Russo model cannot be expected to yield accurate values at low seed moisture contents. The Collis-George/Melville model failed to yield consistent values for vetch seeds due to their partially impermeable seed coat. As expected, the calculated diffusivities to water of the seeds decrease as the external potential becomes more negative. Due to soil compaction during the imbibition process, the volume of the germinating seeds was smaller than that of free-swelling seeds under the same external water potential conditions. Soil mechanical constraints and water stress caused delayed germination but did not affect final germination. Their specific effects could not be differentiated under the present experimental conditions.

Soil Compaction Caused by Cut-to-Length Forest Operations and Possible Short-Term Natural Rehabilitation of Soil Density

Our research explored the impact of forest machinery on soil when trafficking off-road through forest stands. In particular, we assessed soil compaction caused by harvesting operations. This study had two objectives: (i) Quantify the increase of soil bulk density (absolute and relative density) by forest machinery; and (ii) Analyze the persistence of soil compaction over a 5-yr period. Our research was innovative in three respects; 1. We assessed in-place soil density at exactly the same locations pre- and posttreatment with a nuclear moisture and density gauge. In this context, we consider treatment as forest machinery (harvester and forwarder) trafficking on forest soil. 2. After the treatment, we monitored soil density at identical locations through yearly assessments for 5 yr to identify possible natural rehabilitation patterns. 3. We related the measured field bulk densities to site specific maximum bulk densities derived by standard Proctor tests (concept of relative bulk density) to get a better understanding of the severity of off-road traffic impact on soil density changes. Our key findings on two research sites were: 1. On average, dry soil bulk density increased by 19% in machine tracks. 2. Machine impact was not just limited to vehicle tracks; we noticed an increase of soil bulk density >10% in 14 of 65 (21.5%) locations extending up to 1 m away from tracks. 3. Due to machine impact, field bulk density increases exceeded the 80% maximum bulk density threshold at 32% of all track locations, mostly in soil depths of 20 to 30 cm. 4. Monitoring soil density for 5 yr after the treatment indicated no natural rehabilitation (decrease) of soil density down to pretreatment levels.

Numerical simulation of pile installation

During installation of a displacement pile, the soil around the pile is heavily distorted. The resulting changes in soil density and stress state around the pile determine the ultimate pile capacity. In most finite element models, the installation phase is not explicitly modelled. In this paper the full installation phase is modelled in two different ways, in a numerical code capable of large deformations. In the first approach the pile is fixed and the soil flows around the pile. This modelling approach requires somewhat unrealistic boundary conditions and requires that the results for the pile installation are for a non-stationary phase of the calculation, whereas formally only the values at the stationary full penetration phase are reliable. A second modelling approach has been introduced to overcome these limitations. In the new approach the initial conditions are set as a stress level, increasing linearly with depth, resulting from a gravity loading stage, and a stepwise penetration of the pile into the soil is prescribed. This approach updates the geometry of the problem domain and keeps updating the convective terms in a fixed mesh. Results are compared with experimental results from centrifuge tests. Although both models show the development of large effective vertical stress below the pile base and porosity change near the pile shaft, also differences with the experimental results are found. Especially the stiffness response during pile installation is difficult to model correctly.

Filtration behaviour of soil-nonwoven geotextile combinations subjected to various loads

Geotextiles are often subject to different load types in their filtration applications. The load action can cause changes in soil density, geotextile stretching and flow interaction at the soil-geotextile interface. All of these load-induced changes to a geotextile may affect the filtration behaviour of the soil-geotextile system. The impact of load type on the filtration behaviour of soil-nonwoven geotextile combinations has been studied through a series of tests using an experimental apparatus designed specially for the laboratory tests. In these tests, the soil-geotextile combination was fabricated by inserting a piece of nonwoven geotextile between a 50 mm thick soil layer and a layer of steel beads. Two chemical-bonded nonwoven geotextiles were employed in this study. One of the three load types, namely sustained, pulsatory and a combination of both was applied to the combination prior to each filtration test. The frequency of the pulsatory load was 0.1 Hz and a total of 5000 cycles of repeated load applied to the combination for each load type test. After applying this specific type of load on a soil-geotextile combination, water was allowed to flow down through the combination from the soil into a drainage layer set at various hydraulic gradients. The flow rates corresponding to elapsed times were measured and the average hydraulic conductivity value was extracted by using Darcy’s law to characterize the filtration performance of the entire soil-geotextile combination. Variations in the average hydraulic conductivity value with respect to the soil void ratio, magnitude and type of normal load were examined. The experimental results revealed that the void ratio of soil decreased with the increase of total load. Although two parent geotextiles under study, namely GT1 and GT2, have similar filtration characteristics, soil-geotextile combinations composed of these two geotextiles exhibited different filtration responses to the normal load. Soil-GT1 combinations exhibited a normal relationship between the average hydraulic conductivity and the normal load applied; the average hydraulic conductivity increased with an increase in the total load. Soil-GT2 combinations exhibited different load-dependent responses to a normal load with the average hydraulic conductivity depending on the magnitude and type of load. Such load-dependent hydraulic conductivity changes are attributed mainly to the geotextile in-plane strain and the pumping action in the combination.

Localized application of soil organic matter shifts distribution of cluster roots of white lupin in the soil profile due to localized release of phosphorus.

Phosphorus (P) is a major factor controlling cluster-root formation. Cluster-root proliferation tends to concentrate in organic matter (OM)-rich surface-soil layers, but the nature of this response of cluster-root formation to OM is not clear. Cluster-root proliferation in response to localized application of OM was characterized in Lupinus albus (white lupin) grown in stratified soil columns to test if the stimulating effect of OM on cluster-root formation was due to (a) P release from breakdown of OM; (b) a decrease in soil density; or (c) effects of micro-organisms other than releasing P from OM. Lupin plants were grown in three-layer stratified soil columns where P was applied at 0 or 330 mg P kg(-1) to create a P-deficient or P-sufficient background, and OM, phytate mixed with OM, or perlite was applied to the top or middle layers with or without sterilization. Non-sterile OM stimulated cluster-root proliferation and root length, and this effect became greater when phytate was supplied in the presence of OM. Both sterile OM and perlite significantly decreased cluster-root formation in the localized layers. The OM position did not change the proportion of total cluster roots to total roots in dry biomass among no-P treatments, but more cluster roots were concentrated in the OM layers with a decreased proportion in other places. Localized application of non-sterile OM or phytate plus OM stimulated cluster-root proliferation of L. albus in the localized layers. This effect is predominantly accounted for by P release from breakdown of OM or phytate, but not due to a change in soil density associated with OM. No evidence was found for effects of micro-organisms in OM other than those responsible for P release.

Lime and gypsum to improve root depth of orange crop in an Ultisol of the Coastal Tablelands

Coastal Tableland is a landscape unit in the North East of Brazil in which the main soils are Ultisols. In these soils, a compacted layer denominated "cohesive horizon" occurs and root growth is limited by it. An experiment with five treatments and six replications was set up in order to study how liming and gypsum could improve root depth of orange (Citrus sinensis L. Osbeck) crop in an Ultisol in which a compacted layer was found at 0.3 m. Treatments were: A - No liming and no gypsum; B - Liming to achieve 60% base saturation; C - B + 1 t of gypsum ha-1 ; D - B + 2 t of gypsum ha-1 and E - B + 3 t of gypsum ha-1. Gypsum increased calcium and sulfate in the cohesive horizon. Surface application of lime and gypsum did not cause changes in soil density and total porosity in the cohesive horizon. An improvement of root length was observed at the cohesive horizon.

The effect of tyres and a rubber track at high axle loads on soil compaction—Part 2: Multi-axle machine studies

This paper reports on a study of the effect of the passage of multi-axle harvesting machines on the soil physical properties. In particular, it investigates the effect of the rear tyre of a combine harvester on the amount of soil compaction subsequent to the passage of the front tyre/track. The work was conducted in controlled laboratory conditions to determine the effect of a simulated self-propelled combine harvester with a total machine weight of 30–33 t. This was assessed by embedding talcum powder tracer lines in the soil to measure soil displacement and soil density changes. Dry bulk density and penetrometer resistance were also measured. The results showed that the benefit of the rubber track found by Ansorge and Godwin [2007a. The effect of tyres and a rubber track at high axle loads on soil compaction: Part 1: Single Axle Studies. Biosystems Engineering 98 (1), 115–126] was maintained after the additional passage of the rear tyre. After the passage of a track the effect of rear tyre size was insignificant, but the rear tyre size had a significant influence on soil density when following a leading tyre. This was due to a higher strength layer at the soil surface created by the track which was able to withstand the load of the subsequent passes and protect the soil below from further compaction. Results similar to those found for a tracked machine were also achieved by three passes of a 900 mm section width tyre at 5 t load and 0.5 bar inflation pressure. The track results for the 33 t machine were very similar to those of a smaller combine harvester with a total load of 11 t and similar rut width. The study confirmed the benefit of tracks with regard to soil compaction and emphasised the fact that total axle loads and machine weights are less important than how the loads are distributed to the soil.

The effect of tyres and a rubber track at high axle loads on soil compaction, Part 1: Single axle-studies

One option for ground drive systems with large harvesting equipment is to use rubber-belted tracks; however, little is known about the performance of these systems relative to appropriately sized pneumatic tyres. The effect of self-propelled wheels and a track with high axle loads (9–24 t) on soil compaction were studied. Soil displacement and soil density changes were assessed by embedding talcum powder lines as tracer into the soil during preparation. In addition, soil dry bulk density and penetrometer resistance were measured. The track with loads of both 10.5 and 12 t compacts the soil less than wheels at a 10.5 t load in both weak uniform and stratified soil. Towed implement wheels with a 4.5 t load caused similar soil displacement to the track with a load of 12 t. Tyre inflation pressure had a significant influence on soil parameters. To reduce compaction a larger overall diameter was found to be more beneficial than a wider tyre. This emphasised the importance of contact pressure and its distribution with respect to soil density changes. Total axle loads are less important than how these loads are distributed on the ground.

THE HYDRAULIC PROPERTIES OF SOIL AT ROOT-SOIL INTERFACE

Root penetration modifies the soil in the rhizosphere, which affects the transport of water. To study this phenomenon, we developed a soil microcosm for growing roots that allowed our infiltration measurements easy access to the soil. The infiltration of water through small circular areas into the rhizosphere, where roots had penetrated, and into bulk soil, was measured at negative pressures of −1, −2, and −3 hPa. The infiltration data were used to obtain sorptivity values and an estimate of the hydraulic conductivity of saturated soil. At a given negative pressure, the infiltration of water was less through soil that had been deformed by root growth than through bulk soil. To explain these differences we considered how changes in soil density and the effect of root mucilage might influence infiltration into the soil. Our data suggested that it is the increase in density of the rhizosphere soil that is responsible for the change in its hydraulic properties. In other works, root mucilage has been shown to make the soil water-repellent, but we were unable to confirm that finding in this work. The effect of mucilage on soil may depend on the organic matter content of the soil. Further research in this area is needed.

Stress-strain condition around a cylinder expanding in the soil : J. Mecsi, Periodica Polytechnica: Civil Engineering, 38(1), 1994, pp 67–84

The problem of expansion of an underground cylindrical cavity emerges in a number of geotechnical problems for example when the bearing capacity and load-settlement diagram of deep foundations determined, pressuremeter tests are interpreted, or the tension-strain diagram of grouted soil anchors is caiculated and so on. This paper presents a new method to calculate stress/displacement conditions around an expanding cylinder, and the pulling resistance of an axiaily loaded cylinder after its expansion. The cylindrical expansion in the soil examined as a process, to respect the interaction of the surrounding soil mass, determinated the change in soil density. The paper shows example of calculation and simple diagrams to determine the radius of the compaction zone and the distribution of stresses, and volume changes in the soil.

Natural Alleviation of High Axle Load Induced Compaction in a Loam Soil

STUDIES were carried out to determine the natural alleviation of compaction induced by heavy axle loads in a loam soil. 10 and 20 t/axle were applied at two soil moisture contents in May, 1982, following which soil density changes and crop growth and yields were monitored for three growing seasons. Initial soil density changes were minimal due to an existing coarse, naturally dense subsoil and third year investigations showed that soil density and crop yields in compacted plots were not significantly different from those in control plots.

Closed-system freezing of soil in earth dams and canals

Field and associated laboratory experiences with closed-system freezing in earth embankment dams and canals are presented. This type of freezing can cause a redistribution of soil density and moisture, which needs to be considered in the design of soil structures used as water barriers in cold regions. Frost-depth measurements made during the winter of 1978–1979 on two earth dams during interruption of their construction and on the Teton Dam remnant in the State of Idaho, U.S.A., are shown, along with associated soil conditions, air-freezing indexes, and insulating effects of snow and, for one dam, a loose soil cover. Some trends of density and moisture changes near the embankment surfaces were established. Keeping in mind known effects of freezing on soils, the paper raises questions about possible changes in soil density and shrinkage of embankments in contact with bedrock abutments that might adversely affect the safety of earth dams.Key words: dams (earth), canals, frost action, frost protection, soil density, soil moisture, soil shrinkage, soil tests, Teton Dam.

Impact of Tracked and Rubber-Tired Tractors on a Forest Soil

ABSTRACT DESPITE a three-fold difference in mean ground pressure, changes in soil density and porosity caused by an unloaded, rubber-tired log skidder did not exceed those caused by a crawler. The changes in soil density and porosity caused by these machines increased with soil moisture content. Soil density and porosity change proportionally with the square root of the number of passes over the same area.

Experimental studies of coupled heat and moisture transfer in soils during freezing

Moisture and heat flow were measured during the freezing of a colum of Tomakomai silt soil. A dual-beam gamma apparatus was used to measure changes in soil density and moisture content. The experimental data showed movement of water through the frozen layer.

DEVELOPMENT OF COMPACTION PATTERNS DUE TO MACHINERY OPERATION IN AN ORCHARD SOIL

A series of tests was performed in a field, freshly prepared and ready for the planting of new trees, to study the pattern of soil density changes under different loads, soil conditions, tire sizes and numbers of passes using a gamma-ray density meter. The change in soil bulk density varied from 0.08 g/cc to 0.48 g/cc for increasing numbers of traverses of tractor and sprayer. The soil bulk density achieved after 15 passes with a tractor and sprayer approximated both the maximum bulk density obtained with a standard Proctor compaction test and the maximum bulk density that has been observed in adjacent orchards that are 30–40 yr old.