Frozen Soil Samples Research Articles

Content of soil mineral nitrogen as influenced by sample extraction and preservation

为探明影响土壤矿质氮测定的因素, 从棕壤、潮土和黄棕壤3种类型土壤中各采集10个经不同施肥处理的土样, 用连续流动注射分析仪测定经不同浸提剂以及不同保存方法处理后土样的NO₃-N和NH₄-N含量。结果表明: 不论是棕壤、潮土还是黄棕壤, 2 mol·L^-1 KCl提取硝态氮的数量与0.01 mol·L^-1 CaCl2提取的数量相关性均达到<i>P</i><0.01水平; 3种土壤各个土样硝态氮含量的测定值多表现为新鲜土<冷冻土<风干土; 将鲜样浸提后作短时间的冷冻处理, 其效果与鲜样24 h内的测定结果较接近; 土样不同保存方式以及浸提液的保存时间对3种土壤NH₄-N测定结果的影响规律不及NO₃-N明显。

Stress relaxation characteristics of warm frozen clay under triaxial conditions

Stress relaxation is one of the main concerns in geotechnical engineering, especially in permafrost regions with warm frozen soil. This paper investigated the mechanical behaviors of a warm frozen soil during relaxation process. A series of triaxial relaxation tests were carried out on the warm frozen soil samples under the temperature from − 1.0 °C to − 0.2 °C and confining pressure of 0.5 MPa to 3.0 MPa. Three stages were observed during the whole stress relaxation process, i.e. instantaneous relaxation, intense relaxation and slow relaxation. Relationship between deviatoric stress and time was described by a hyperbolic relationship. With relatively small pre-strain (3% and 4%), volumetric changes during stress relaxation contracted initially followed by expansion, but contraction dominates the relaxation process with the magnitude of pre-strain over 6%. Characteristic parameters such as the instantaneous stress relaxation, the critical relaxation duration and relaxation rate are defined and analyzed. It is found that the parameters are all dependent on soil temperature, confining pressure and magnitude of pre-strain. ► The relaxation process of warm frozen clay consists of three typical stages. ► The magnitude of pre-strain influences the volumetric changes during relaxation. ► Deviatoric stress–time relationship can be well described by a hyperbolic relation. ► Characteristic parameters during relaxation depend on the initial test condition.

Dynamic elastic modulus for frozen soil from the embankment on Beiluhe Basin along the Qinghai–Tibet Railway

Abstract Dynamic tri-axial tests were conducted to study the dynamic properties of the frozen soil from the embankment on Beiluhe Basin along Qinghai–Tibet Railway. The dynamic tri-axial tests included frozen soil samples subjected to fifteen test events with five different temperatures, five different water contents and five different confining pressures. Experimental expressions of the frozen soil parameters are obtained, including the maximum dynamic elastic modulus, the dynamic elastic modulus ratio, and the reference linear strain amplitude, based on the test data and an equivalent linearization model. The effects of temperature, water content and confining pressure on the dynamic elastic modulus ratio and the backbone curves of the frozen soil are discussed. The results provide basic experimental data for establishing dynamic constitutive law of the frozen soil and meaningful information for further studies on dynamic response of railway embankment in the permafrost regions caused by passing train.

Triaxial constant stress and constant strain rate tests on ice-rich permafrost samples

A range of mechanical compression tests were conducted on frozen soil samples at temperatures close to the melting point of ice to determine both the creep and the strength properties under triaxial stress conditions. The samples were obtained from two rock glaciers in the Swiss Alps. For comparison, additional samples were prepared artificially for testing. The results showed that the minimum creep strain rate increases exponentially with increasing temperature and applied deviatoric stress. The applied strain rate is the main parameter influencing resistance. However, test results show that the micromechanical behaviour is similar for all tests and independent of the applied loading condition. Measurements of the volumetric strain showed that the sample composition has a major influence, so samples with high air void ratios resulted in continuous contraction (i.e., reduction of the sample volume), whereas samples with low volumetric air and ice contents demonstrated dilatant behaviour.Key words: permafrost, frozen soil, creep, strength, triaxial testing, self-healing.

Effects of volumetric ice content and strain rate on shear strength under triaxial conditions for frozen soil samples

AbstractA set of triaxial constant strain rate and constant stress tests was performed on artificially frozen soil samples to study the effect of the volumetric ice fraction, strain rate and also confining stress on the mobilized shear strength. In general, the peak shear strength increased with decreasing volumetric ice content and increasing strain rate. Pure ice samples, however, showed a peak shear strength that was higher than that for those containing about 80% ice by volume for a similar strain rate, although the loss in strength after this maximum was usually much more pronounced, reaching a lower large strain strength than that for frozen sands. The influence of the ice in samples with low ice contents was primarily noticeable at low strains, whereas the large strain behaviour was very similar to that of the unfrozen material. The tests showed the dependency of the mechanical failure and deformation mode of frozen soils on the loading conditions. It could be demonstrated further that large strains have a significant influence on the strength of frozen geo‐materials and therefore efforts should be made to establish in situ strain states when analysing the stability of frozen slopes. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Nitrous oxide production in a forest soil at low temperatures – processes and environmental controls

Recent investigations have highlighted the relative importance of the winter season for emissions of N 2O from boreal soils. However, our understanding of the processes and environmental controls regulating these emissions is fragmentary. Therefore, we investigated the potential for, and relative importance of, N 2O formation at temperatures below 0 °C in laboratory experiments involving incubations of a Swedish boreal forest soil. Our results show that frozen soils have a high potential for N 2O formation and subsequent emission. Net N 2O production rates at −4 °C equaled those observed at +10 to +15 °C at moisture contents >60% of the soil's water-holding capacity. The source of this N 2O was found to be denitrification occurring in anoxic microsites in the frozen soil and temperature per se did not control the denitrification rates at temperatures around 0 °C. Furthermore, both net nitrogenmineralisation and nitrification were observed in the frozen soil samples. Based on these findings we propose a conceptual model for the temperature response of N 2O formation in soils at low temperatures.

A comprehensive method of determining the soil unfrozen water curves: 1. Application of the term of convolution

Because none of the existing methods of measurements of the unfrozen water content are able to indicate the real phase changes in individual soil sample in detail, a method based on the differential scanning calorimetry (DSC) is presented. The method consists in determining the real heat flux function q( T) absorbed by the frozen soil sample during the warming DSC run. It is based on searching for a distribution of “heat impulses” in relation to temperature, which convoluted with the apparatus function a( T) that gives a minimal deviation from the observed heat flux function h( T). The function q( T) can be easily related to the function of unfrozen water content u( T). The latter determines such important parameters of the soil freezing process as the melting point T 0 and the content of “nonfreezable” water u n.

A comparison of hydraulic conductivities, permeabilities and infiltration rates in frozen and unfrozen soils

In order to protect water resources, fuel-storage facilities in Alaska are required to provide secondary containment for fuel spills. Secondary containment generally consists of a lined catchment basin. The lining must have a hydraulic conductivity of less than 1e−6 cm/s [ADEC, 1996. Alaska Department of Environmental Conservation Division of Spill Prevention and Response Industry Preparedness and Pipeline Program. “Sufficiently Impermeable Secondary Containment Systems.” March 11: 19 pages.]. A series of studies were conducted to evaluate the potential for frozen soil to serve as a secondary containment lining for fuel storage facilities in Alaska. Laboratory tests quantified hydraulic conductivities, permeabilities and fuel infiltration rates for three soil types collected at a fuel storage facility in Bethel, Alaska. Two of the soils, organic-rich silty sand and sandy silt, represented indigenous soil at the site. The third type, silty sand, was fill material imported to the facility from a nearby materials source. Unfrozen and frozen soil samples were placed in cast acrylic permeameter columns. Tests performed on unfrozen samples were conducted at room temperature (19.8 to 24.0 °C). In unfrozen soils, fuel-infiltration rates were similar at the three volumetric moisture contents tested, 15%, 30% and saturation. Tests for frozen samples were conducted in a cold room at −4 °C. Fuel-infiltration rates in the frozen soils decreased as ice-saturation increased. Hydraulic conductivity, permeability and infiltration rates for ice-rich soils were approximately 1e−9 cm/s, 1e−13 cm 2 and 1e−8 cm/s, respectively.

Spatial and Temporal Variability of Water-Soluble Organic Carbon in a Cropped Field

Little is known about the spatial and temporal variability of water-soluble organic carbon (WSC) within agricultural fields. The purpose of this research was to characterize the distribution, the mean, and the variance of WSC that occurs within a cropped field as a function of space and time. Freezing of soil samples showed no significant change in water-soluble organic carbon compared with that in fresh samples taken from a depth of 0.2 m. Thus, water-soluble organic carbon was extracted from frozen soil samples taken from a 200-point grid established on a 1.2-ha field, a 60-point grid within a 0.4-ha field, and a 55-point transect of a field amended with 45 Mg/ha of manure. The initial sampling was in the fall after the harvest of a sorghum crop. The concentrations ranged from 23 to 274 mg/kg within the 1.2 ha field. Over 90% of the concentrations were grouped around the mean of 39.8 mg/kg. The higher values caused the distribution to be greatly skewed, such that a ln-normal distribution characterized the data better than a normal distribution. To be within 10% of the population mean at the 95% confidence level, 26 samples would be required if ln-normal distribution was assumed compared with 88 samples under normal distribution. Soil-water content was found to be well described by a normal distribution for the 200-point grid. The means, variances, and frequency distributions of WSC changed dynamically throughout a 1-year period during which wheat and sorghum were grown. Mean WSC concentrations were highest in the spring and fall and lowest in the summer. The fall samplings had large variances and were best described by ln-normal frequency distributions, whereas other samplings during the year could be adequately described by normal distributions. The WSC concentrations showed only slight spatial dependency for samples taken 1.37 m apart and thus can be assumed to be randomly distributed. The addition of manure to the field resulted in higher WSC concentrations than in the unamended field, but concentrations were nearly equal after a year. Changes in soluble organic carbon in the field are closely related to the degree and duration of drying of the soil before extraction. These changes occur primarily in the upper 0.05 m of soil. Irrigation causes a drastic decrease in the soluble organic carbon in the surface soil, most likely because of leaching of soluble carbon.

Unfrozen water content in saline soils: results using time-domain reflectometry

The use of time-domain reflectometry (TDR) for determining the phase composition of saline permafrost from measurement of the apparent dielectric constant, Ka, is examined.Combined TDR–dilatometry experiments were performed to assess whether the TDR method could be used on frozen soil samples with high pore water salinity. In general, unfrozen water content determinations by TDR were within ±0.025 cm3∙cm−3 of those obtained by dilatometry, with no marked influence due to salinity. A novel probe design for use on saline core samples shows promise as a means for determining unfrozen water contents in the field.

Thermally induced water migration in frozen soils

An apparatus has been developed for the direct measurement of thermally induced water migration in saturated frozen soils. The frozen soil sample and reservoirs are sandwiched between two end plates containing Peltier modules. These cooling devices are controlled by a thermoelectric cooling control system, which can maintain temperatures constant to within ± 0.02° C under steady state conditions. The water in the reservoirs remains unfrozen because it contains dissolved lactose. When a linear temperature gradient is established across the system, water passes from the ‘warm’ reservoir into the prefrozen sample and out into the second, colder, reservoir. Observed rates of flow are compatible with permeability values for frozen soils measured earlier. The results are discussed with regard to current theoretical and applied studies of frozen ground. Unfrozen water in frozen soils has a potential which is lower than pure bulk water by an amount increasing as the temperature decreases. Thus, a temperature gradient in frozen soil implies a flux of water in the direction of decreasing temperature, at a rate dependent on the permeability in the frozen state.

Transportation, preparation and storage of frozen soil samples for laboratory testing : ASTM STP 599, June 1976, P88–112

Transportation, preparation and storage of frozen soil samples for laboratory testing : ASTM STP 599, June 1976, P88–112

Ultrasonic wave measurements on frozen soils at permafrost temperatures

An ultrasonic pulse technique was used in the laboratory to measure compressional and shear wave velocities on frozen soil samples with various ice contents. Measurements were made on a variety of samples, which differed in type of material, geographical location, and ice content, at temperatures ranging from −7 to +1 °C. Three series of tests were run on specimens from the Norman Wells, Fort Good Hope, and Eureka areas, Northwest Territories. The first series of specimens was tested under uniaxial stress conditions, whereas the measurements for the second and third series were done under triaxial stress conditions. The results measured on samples from the Norman Wells and Eureka areas were compared with the results obtained from shallow seismic profiles shot in the field at the sampled drill sites at the time of drilling. The values of field velocities lie within the range of laboratory measurements, and therefore prove that the laboratory tests provide a measure of control in the interpretation of seismic records.

The preparation of microtome sections of unaltered soil for the study of soil organisms in situ

A new technique for study of small soil organisms in situ in unaltered soil is described. The soil samples are cooled in a refrigerator at — 10°C to kill the animals. A small portion taken from a frozen soil sample, is slowly immersed in a solution of gelatin. When the specimen is infiltrated with gelatin and the whole cooled it is fixed in formalin to enable it to withstand treatment with hydro-fluoric acid for removal of sand grains. Subsequently the specimens are immersed in gelatin solution for a second time after which the specimens are affixed to wooden blocks which can be clamped in the microtome. Before sectioning, the embedded specimen affixed to the wooden block is hardened in methylalcohol after which it is possible to cut sections 7,5–10µ thick. The most satisfactory staining procedure proved to be the quadruple staining method of Johansen. By this method nematodes, fungi, bacteria and amoebae are easily distinguishable from the soil particles.