Sandy Silty Soils Research Articles

Analysis of the problems connected to the sinking of micro-TBMs in difficult grounds

Abstract In spite of the many advantages offered by micro-tunnelling, significant problems of a technical nature can be encountered when the excavation machine tends to sink under its weight in poor sandy–silty soils and there is deviation from the theoretical advancement direction. This work presents a new calculation procedure that is able to evaluate the settlements and rotations of a Microtunnel Boring Machine (MTBM) and, therefore, to signal the danger of sinking of the excavation machine. The procedure involves the combined use of the hyperstatic reaction method (with a Winkler approach) and of a finite difference numerical method. A parametric study developed with the proposed procedure for poor grounds, from the geotechnical point of view, has led to useful indications on the courses of action to take (integrative structural works or ground reinforcement) when it is possible that the phenomenon of machine sinking could occur.

Stone movements and permeability changes in till caused by freezing and thawing

Vertical uplifting of boulders and stones is well known to take place in cold regions. Movements of stones in roads might lead to traffic danger, vehicle failures, and cause breakdown of the road surface with the need of expensive repair as a consequence. In addition, freeze/thaw and associated stone movements may cause an increase in permeability, which can lead to contamination of soils and ground water if used as soil liners in landfill areas or even dam failures if used as hydraulic barriers in earth dams. Freeze/thaw tests were carried out in the laboratory on a silty sandy soil in order to study movements of embedded stones and to measure how the overall permeability was influenced by freeze/thaw cycles. The soil samples were compacted at three different water contents, i.e. 11.5% (optimum), 14.5%, and 17.5%. Each sample contained one stone, placed at a predetermined depth. The soil samples were subjected to one-dimensional open system freeze/thaw. Soil temperatures, volume changes, and stone movements were measured. The results showed that upward stone movements took place due to freeze/thaw in the frost susceptible soil compacted at and 3% above the optimum water content. In addition, the permeability increased in samples with initial water contents of 11.5% and 14.5%. This permeability increase was as much as 81 times after six freeze/thaw cycles. For the samples with initial water contents of 17.5%, very small changes in permeability were measured.

Liquefaction Testing of Stratified Silty Sands

The cyclic behavior of stratified silty sandy soils is at present poorly understood, yet these materials are commonly found in alluvial deposits and hydraulic fill, which have a history of liquefaction during earthquakes. The main objective of this research project was to compare the behavior of stratified and homogeneous silty sands during seismic liquefaction conditions for various silt contents and confining pressures. A comprehensive experimental program was undertaken in which a total of 150 stress-controlled undrained cyclic triaxial tests were performed. Two methods of sample preparation were used for each soil type. These methods included moist tamping (representing uniform soil conditions) and sedimentation (representing layered soil conditions). The silt contents ranged from 10 to 50%, and confining pressures in the range of 50 to 250 KPa were considered. The results indicated that the liquefaction resistances of layered and uniform soils are not significantly different, despite the fact that th...

Technical Note Hydraulic conductivity of a tropical residual soil prone to landslides

Abstract Saturated hydraulic conductivity or the coefficient of permeability (κ) of a residual soil prone to landslides was determined for a study area in Fiji. The techniques used were not new but the approach and the resulting κ-values may be useful for other studies (e.g. landslides). Inverse auger-hole tests are used to estimate κ-values. Tests conducted in sandy silty soils indicate values in the order of 10 -6 ms -1 . No significant difference was found between calculated values at the surface and at 3 m depth. Infiltration tests showed saturated infiltration rates of 10 -5 ms -1 .

Bioaccumulation of manganese by plants: influence of MMT as a gasoline additive

This study on the bioaccumulation of manganese (Mn) by plants was motivated by the increased use of Methylcyclopentadienyl manganese tricarbonyl (MMT) as a replacement for lead in gasoline. Oats and beans were grown in sandy and organic soils at a control site (E o) and at two other sites weakly (E +) and highly exposed (E ++) to Mn contamination, potentially from a MMT source. Total Mn, Ca, Mg, Fe, Zn and Al were measured in the soils and in the plants (roots, stems/leaves and fruits). Exchangeable Mn was measured in the soils at the beginning and at the end of the exposure period. The pH of the organic and sandy silty soils were found to be lower outdoors (E + and E ++) than in the greenhouse (E o) and exchangeable Mn was found to be significantly higher in the organic soil at E ++ (1.03–1.36 ppm). Higher Mn accumulation was also found in the fruits and stems of oats grown in the organic and sandy soils at E ++. This Mn accumulation is often associated with increased Fe and Al in the plant. These results suggest that the addition of MMT to gasoline may result in an increase in exchangeable Mn in organic soils. However, it has not been proven that the source of the increased Mn accumulation is indeed MMT in gasoline.

Soil Characteristics Associated with a Primary Plant Successon on a Mojave Desert Dry Lake

The playa of Rabbit Dry Lake has a fine—textured soil high in Na, Cl, and NO3 but low in K and Ca + Mg. Kochia or sometimes Suaeda plants establish in mud cracks on the playa and accumulates small mounds of silty—sandy soil. High levels of NO3, Na, Cl, K, and Ca + Mg accumulate in soil of small mounds but decrease in playa—level soil under small mounds. Atriplex torreyi invades small mounds and accumulates large mounds of wind—deposited sandy soil. Continued increase of NO3 and large increase of Na, Cl, K, and Ca + Mg occur in soil of large mounds as a result of Atriplex leaf deposition. Large mounds rapidly increase in size and coalesce to form giant mounds of sandy soil with levels of NO3, Na, Cl, K, and Ca + Mg sharply lower than those in large mounds but still higher than those in playa—level soil under the giant mounds. Soil of giant mounds is suitable for colonization by Atriplex confertifolia and Haplopappus acradenius. Following death of shrubs like Atriplex torreyi, giant mounds erode, and soil is deposited between eroding giant mounds at a level above the original playa. Such deposited soil is chemically similar to soil in giant mounds but Na and Cl are leached down to the playa level. The accumulation of soil and the redistribution of minerals by plants permit a primary plant succession to occur.