Maximum Frost Depth Research Articles

Permafrost thermal dynamics at a local scale in northern Da Xing’anling Mountains

Permafrost in Northeastern China is not only controlled by latitude and elevation, but also locally environmental factors, such as vegetation cover and human activities. During 2009–2022, thinning active layer, increasing annual maximum frost depth in talik zones and lowering ground temperature above the depth of dividing point (DDP) between permafrost cooling and warming have been observed in many places, possibly due to the global warming hiatus (GWH). However, the responses of permafrost below DDP did not show a clear trend to the GWH, despite an evident ground warming. The warming and degradation of permafrost below DDP in the Da Xing’anling Mountains are more strongly influenced by the overall climate warming than by regional GWH. This study improves our understanding of changing permafrost temperature and its drivers. It also helps to provide data support and references for the management of the ecological and hydrological environment of the northern Da Xing’anling Mountains and the Heilongjiang-Amur River Basin.

Current state and past changes in frozen ground at the Third Pole: A research synthesis

The thermal state of frozen ground and its changes are important for understanding environmental change and supporting related applications to the Earth’s Third Pole, which is a hotspot area for science research. However, challenges remain in data and modelling, meaning that much information is unavailable, especially for the entire Third Pole region. Here, we provided basic statistical data regarding the current state of frozen ground and its changes over the 1960s–2010s across the entire Third Pole by integrating nearly all currently available ground observation data and high-quality spatial data using machine learning models and existing high-quality frozen ground data products. The results show that the current (2000–2018) areal extents of permafrost and seasonally frozen ground in the Third Pole are approximately 1.27 × 106 km2 (1.15 × 106 to 1.39 × 106 km2) and 2.59 × 106 km2, accounting for 28% and 58%, respectively. The areal extent of permafrost region is approximately 50% (23%–93%) larger than that of permafrost area (land underlain by permafrost), especially in some early maps. The corresponding regional average of the mean annual ground temperature is approximately −1.51 °C (−1.75 to −1.27 °C) in the permafrost area. The regional average of active layer thickness overlying the permafrost and the maximum frost depth for regions of seasonally frozen ground are 235 cm (233–237 cm) and 92 cm, respectively. From the 1960s to the 2010s, on average, permafrost in the Third Pole warmed at a rate of 0.17 °C per decade, which was associated with increases in the maximum thaw depth at a rate of 4.42 cm per decade. The regional average of the maximum frost depth declined at a rate of 2.34 cm per decade over the same period. This synthesis highlights the differences between the two terms (permafrost region and permafrost area) and provides crucial information for frozen ground in the Third Pole with higher accuracy for the scientific community and the public.

Assessment and projection of ground freezing–thawing responses to climate change in the Upper Heihe River Basin, Northwest China

Study regionUpper Heihe River Basin, Northwest China. Study focusWe investigated potential climate change under three Representative Concentration Pathways (RCP 2.6, 4.5, and 8.5) and their impacts on frozen ground in the upper Heihe River Basin using the ensemble climate data from eight general circulation models and the Soil and Water Assessment Tool (SWAT). New hydrological insights for the regionAir and ground freezing indices declined significantly during the baseline period (1976–2015), whereas the thawing indices increased, indicating the heat accumulation in study area. The frost depth, which refers to the potential frost depth of active layer in permafrost areas and the maximum frost depth in seasonally frozen areas, decreased significantly at the rate of 3 cm/10 yr. The SWAT-simulation and gray relational analysis revealed that soil water was controlled by precipitation and frost depth in spring and autumn. Compared to that of the baseline, the projected frost depth is projected to decline by 0.07–0.1 m during the near future (2020–2059) and 0.08–0.36 m for the far future (2060–2099). In addition, we developed a long-term warning system, which indicates that the degree of frozen ground degradation would be mild during the near future and would be severe for the far future under RCP 8.5. This study provides valuable insights into the protection of frozen-ground in the Upper Heihe River Basin.

Optimal Design of Central Drainage Ditch Buried Depth for Highway Tunnel in Seasonally Frozen Region

In seasonally frozen regions, the drainage ditch of highway tunnels is usually set below the maximum frost depth. Hence, the deeply buried drainage ditch is often used, which increases the cost, difficulty, and time of tunnel construction. And it affects the stability of the surrounding rock and the tunnel structure safety. To solve these difficulties, this study investigated the optimal design of buried depth of central drainage ditch by numerical simulation. Based on a tunnel in Northwest China, a transient heat transfer model, which involves heat conduction and phase transition, was established. Then, the influences of different water amounts and thermal insulation technologies on the minimum buried depth of central drainage ditch were investigated. The results indicate that the thermal insulation layer, regardless of whether it is an internal thermal insulation layer, external thermal insulation layer, or foam concrete backfilled in invert arch, can prevent the drainage ditch from freezing and increase its elevation. The findings of this study can be helpful in optimizing the buried depth, selecting reasonable thermal insulation technology for central drainage ditch in seasonally frozen regions.

Land Surface Model CAS‐LSM: Model Description and Evaluation

AbstractComprehensive land surface models are very important for climate and weather forecasting and for improving our understanding of the relationships between humans and the Earth system. This work presents a land surface model of the Chinese Academy of Sciences (CAS‐LSM) that considers groundwater lateral flow, human water regulation, soil freeze‐thaw front dynamics, riverine dissolved inorganic nitrogen transport, anthropologic heat release, urban water usage, and urban planning based on the Community Land Model. Global simulations with the updated CAS‐LSM were conducted to evaluate its performance, and the results show its advantages in reproducing ecohydrological processes. Groundwater lateral flow can replenish the groundwater depletion cone caused by overexploitation, where the offset rate increases as the aquifer thickness increases. The maximum thaw depth increases by 3.62 mm yr−1 in permafrost areas, and the maximum frost depth decreases by 1.89 mm yr−1 in seasonally frozen areas. Nitrogen fertilizer use and point‐source pollution increase the riverine dissolved inorganic nitrogen flow rate in western Europe and eastern China. To further verify the model's ability to simulate the effects of urban planning implementation, simulations over Beijing were conducted using the Weather Research and Forecasting (WRF) model coupled with CAS‐LSM. The results show that anthropogenic heat release enhances the sensible heat flux and temperature in the city center. In terms of urban water planning, most of the total water supply in Beijing should be used for urban irrigation, and a small part should be used for road sprinkling to obtain a greater cooling effect.

Temperature field characteristics and influencing factors on frost depth of a highway tunnel in a cold region

Frost damage frequently occurs in cold-region tunnels due to temperature changes, posing risks to the structural stability and traffic safety. Therefore, temperature variations and its distribution in tunnels are crucial factors that have to be considered. Temperature measurements were conducted in the Xing'anling tunnel in northeast China from 2016 to 2019 to investigate the spatiotemporal characteristics of the temperature field. An equation to determine frost depth based on the Stephen's method was established to evaluate the influence of different factors on frost depth. The sensitivity of frost depth to different influencing factors was investigated using system stability analysis. The results showed that mean annual temperature inside the Xing'anling tunnel has gradually increased from 2016 to 2019 due to global warming. Freezing front was characterized by three stages based on the freezing rate, i.e., the fast freezing stage, transitional freezing stage, and stable freezing stages. The maximum frost depth ranged from 1.5 to 5.1 m without an insulating layer covering the lining, and a lag of about 2 months was observed with air temperature. Along the length of the entire tunnel, four stages of frost depth were observed, namely, fluctuations and a decrease, a steady decrease, fluctuations and an increase, and a steady increase. Frost depth exhibited an asymmetrical distribution to the length of the entire tunnel. Temperature parameters had the largest influence on frost depth, followed by water parameters and thermal parameter. Density parameters had relatively little influence on frost depth.

On the Eccentrically Loaded Socket Footings With Cut - Off Pyramid Shaped Socket

Abstract In this work considerations concerning eccentrically loaded socket footing with cut-off pyramid shaped socket were presented. As an object of study sloped footing with 1.40 m height, corresponding to the maximum frost depth has been adopted. Knowing that in practice there are no perfect pure axial loads, load applied on the eccentricity has been taken into considerations. Eccentric loads result in footing rotation in the direction of eccentricity and acting load, hence one footing end is imbedding into the ground, whereas second end tries to rise up. To observe that phenomenon, elastic type of support under the foundation was introduced corresponding to the naturally humid sand with medium compaction. Presented in this paper considerations of innovative connection technology between footing and column were based on performed numerical studies. Advantages and disadvantages of presented footing in comparison to normal socket footings solutions were widely discussed. Numerical analyses were performed with the utilization Finite Element Method based SolidWorks software.

Simulation of Soil Freezing and Thawing for Different Groundwater Table Depths

Core Ideas The SHAW model was used to simulate the freeze–thaw process during freeze–thaw periods. It revealed the effects of soil texture and groundwater table depth on soil freezing and thawing. The frost depth and accumulated negative soil surface temperature relationship was determined. During freeze–thaw periods, the transformation between phreatic water and soil water will change the soil hydrothermal properties and affect the soil freezing and thawing in shallow groundwater areas. The purpose of this study was to determine the effect of four different groundwater table depths (GTDs) and two soil textures on the process of soil freezing and thawing during two successive freeze–thaw periods using the Simultaneous Heat and Water (SHAW) model. The results show that the frost depth was the maximum when the GTD was 1.0 m, and the maximum frost depths of sandy loam and fine sand were 97.6 and 98.9 cm, respectively. When the GTD was larger than 1.5 m, the maximum frost depth decreased with an increase in GTD, and the maximum frost depth of the soil profile was more sensitive to changes in the air temperature. The frost depth of the soil profile was linear with the square root of the accumulated negative soil surface temperature (ANST) under different GTDs. The ANST was influenced by the phreatic evaporation, and the soil freezing rate increased with an increase in GTD under the same ANST. This research is significant for the rational development of soil water and heat resources and the study of soil water–heat transfer in shallow groundwater areas.

Artificial warming-mediated soil freezing and thawing processes can regulate soybean production in Northeast China

Northeast China with seasonally frozen soil is quite sensitive to global warming. The changes in soil freezing and thawing processes initiated by global warming could alter the hydrological cycle of agricultural fields. A paired-plot experiment was conducted in frozen agricultural soils in Northeast China to examine the impacts of simulated warming on soil freezing and thawing processes and on soybean production. Infrared radiators were used to simulate global warming, rising surface soil temperature (5 cm depth) by 2.86 °C. We showed that, artificial warming caused the freeze duration shortened by 22 days, and the thaw duration shortened by 17 days resulting in the mean duration of soil freezing-thawing significantly shortened by 39 days and the maximum frost depth reduced by about 40 cm. Simulated warming had no significant effect on the average annual freeze-thaw cycle frequency. Warming induced a larger water accumulation in the 0–100 cm soil layer during 2014–2015 soil freezing period. In the dry year of 2015, warming did not significantly affect surface soil moisture during period from sowing date to VC (soybean cotyledon) date. Thus, warming-induced an increase in soybean yield in the dry year may be attributable to the positive effect of enhanced soil temperature on soybean growth (aboveground dry matter accumulation) and consequent on soybean production. In the wet year of 2014, warming decreased surface soil moisture from sowing date to the date of VC stage because warming advanced the soil thaw-end date in 20–60 cm layer by 15 days. This decline in surface soil water availability may potentially offset the positive effects of increased soil temperature on soybean yield, thus warming effects on soybean production was neutral in the wet year. Our findings highlight the potential role of seasonally soil freezing and thawing dynamics in regulating soybean to global warming and suggested that warming effects on soil water dynamics during soil freezing and thawing periods, and subsequent on the surface soil water availability at the early vegetative stage and soybean production were associated with the hydrological year. We conclude that under current precipitation patterns, the no response of soil surface water availability to warming during early vegetative growth, coupled with warming-mediated increases in soil temperature, might improve soybean production during dry years in Northeast China.

Mechanism of slope failure in loess terrains during spring thawing

Slope failure in loess terrains of Northern China during spring thawing period is closely related to the freeze-thaw cycling that surface soils inevitably experienced. Field surveys were carried out on natural and artificial slopes in thirteen surveying sites located in the Northern Shaanxi, the center of Loess Plateau, covering five characteristic topographic features including tablelands, ridges, hills, gullies and valleys. Based on the scale that is involved in freeze-thaw cycling, the induced failures can be classified into three main modes, i.e., erosion, peeling and thaw collapse, depending on both high porosity and loose cementation of loess that is easily affected. Model tests on loess slopes with gradients of 53.1°, 45.0° and 33.7° were carried out to reveal the heat transfer, water migration and deformation during slope failure. The surface morphology of slopes was photographed, with flake shaped erosion and cracks noted. For three slope models, time histories for the thermal regime exhibit three obvious cycles of freeze and thaw and the maximum frost depth develops downwards as freeze-thaw cycling proceeds. Soil water in the unfrozen domain beneath was migrated towards the slope surface, as can be noticed from the considerable change in the unfrozen water content, almost synchronous with the variation of temperature. The displacement in both vertical and horizontal directions varies over time and three obvious cycles can be traced. The residual displacement for each cycle tends to grow and the slopes with higher gradients are more sensitive to potentially sliding during freeze-thaw cycling.

Modification of the U.S. Army Corps of Engineers Model 158 for Prediction of Frost–Thaw Profiles in Northern New England

Knowledge concerning frost–thaw profiles is important in the design of pavements for low-volume roads located in seasonal frost areas. In addition, because these roads are highly susceptible to damage during the spring thaw–weakened period, this knowledge is helpful in making decisions about seasonal load restriction (SLR) policies. Direct measurement of frost depth is expensive and usually limited in scope. Therefore, several highway agencies have expressed the need for a prediction model to help them in estimating frost–thaw profiles beneath roadways. A report from the U.S. Army Corps of Engineers New England Division examined frost prediction models. One of the equations in that report was Model 158, which was similar to the modified Berggren equation. Model 158 uses air temperature indexes as well as pavement material properties to integrate heat flow into the calculation of maximum seasonal frost depth. That model has been modified slightly and programmed in an Excel spreadsheet to predict frost and thaw depths on a daily basis. This paper presents an overview of the modified Model 158 and compares predictions obtained from that model with measured frost–thaw profiles obtained at several test sites located in northern New England during a period of five freeze–thaw seasons. Results suggest that the model shows much promise although it generally tended to slightly underpredict maximum frost depths. In tracking the thaw process for SLR posting, the model tended to be conservative in estimating end-of-thaw dates (especially during rapid thawing events); in many instances, estimated end-of-thaw dates fell after measured dates.

실내실험과 현장실험을 통한 Methylene Blue 동결깊이 측정장치 신뢰성 검토에 관한 연구

동결깊이는 영구 동토지역 및 계절성 동토지역에서 도로의 동상방지층, 매설관의 깊이 및 건물의 기초를 설계하는데 중요한 고려사항이다. methylene blue 동결깊이 측정장치는 지중 온도센서를 이용한 계측시스템과 비교하여 저렴하고 유지관리와 측정이 용이한 장점을 지니고 있다. 그러나 지반공학적 관점에서 지중에 매설된 methylene blue 동결깊이 측정장치의 신뢰성을 검토한 연구는 미미하다. 본 논문에서는 실내실험과 현장실험을 통해 methylene blue 동결깊이 측정장치의 신뢰성을 검토하고, 나아가 국내 현장에서 검증된 측정장치로 장기간 측정된 동결깊이 실측 데이터를 이용하여 최대 동결깊이 분포도를 작성하였다. 본 연구를 바탕으로 실측된 동결깊이는 향후 국내 지반구조물 설계에 중요한 자료로 활용될 수 있을 것으로 판단된다. The frost depth is one of important factors in the design of structures such as roadways, buried pipeline, and foundations. A frost indicator with methylene blue solution has several advantages with respect to installation cost, maintenance, and simple measurement. However, as a geotechnical engineering aspect, the accuracy of the frost indicator has not been proved yet. This paper presents experimental and field investigations of the accuracy of the frost indicator and contour maps of maximum frost depth. The contour maps of maximum frost depth can be applied to design geo-infrastructure in South Korea.

Field Test Study on Frost-Heave of Subgrade in Seasonal Frozen Region along Haerbin-Dalian Passenger Dedicate Line

The new constructing railway Haerbin-Dalian Dedicate Line runs through the typical seasonal frozen region. For the purpose of estimating the quality of subgrade construction, a field test was played. Placed the monitoring instrument and measure the soil temperature, settlement and soil pressure etc.. The monitoring data shows that the change of soil temperature is less than the environment temperature. The maximum frost depth is 1.66m. In this depth, the fine-grained soil with high frost-heave characteristic should not be as the subgrade. The effective measure to prevent the rain percolated through the subgrade is important. And the filling of subgrade should be constructed strictly according to the maximal degree of compaction and keep the best water content. The settlement of subgrade is less than 15mm, the water content is also less than the prime frost-heave water content and the frost-heave force is very small. Thus, the construction technique subgrade is feasible.

Water and nitrate movements in an agricultural field with different soil frost depths: field experiments and numerical simulation

AbstractTo evaluate water and anion movement in an agricultural field in different frost conditions, a paired-plot field experiment was conducted at an agricultural site in northern Japan where a significant decrease in the frost depth has occurred during the past 20 years. Snow cover was removed to enhance soil freezing in one plot (treated plot), whereas natural conditions were maintained in a control plot. The maximum frost depth was 0.43 m in the treated plot and 0.11 m in the control plot, which induced substantial differences in water movement throughout the winter. A substantial amount of water moved upward before the onset of snowmelt. However, nitrate did not move markedly before the snowmelt period in either plot. The amount of snowmelt infiltration in the control plot was larger than in the treated plot. Correspondingly, the peak of nitrate content in the control plot was deeper than that in the treated plot after the snowmelt period. Soil freezing, snow accumulation and snowmelt processes were simulated reasonably well using a one-dimensional numerical model: Simultaneous Heat and Water (SHAW). Nevertheless, the model performed poorly for simulating soil thawing and soil water movement, suggesting a need for improvement.

Climate and geomorphology in the uppermost geomorphic belts of the Central Mountain Range, Taiwan

The Central Mountain Range of Taiwan is in its lower parts exposed to strong denudation by mass movements caused by extreme rainfalls and active tectonics. Even though the uppermost regions of the mountains are also influenced by earthquakes and typhoon rains, present geomorphic activity is low. Investigations based on field work and air photo study in Nanhuta Shan (3742 m) and Yushan (3952 m) and the analysis of climate data indicate the presence of a periglacial belt with a lower limit at 3600–3700 m. The upper limit of the periglacial belt in Taiwan is not met. Ground temperature measurements in Nanhuta Shan show that freeze-thaw activity at 3560 m during the winter is restricted to the top few centimetres of the ground and the maximum frost depth is ∼25 cm.The only indicator landforms within the periglacial belt are smooth slopes. The high relief energy and a lack of fine material account for the absence of other periglacial landforms. Relict solifluction terraces, debris cones and slope failures found between 3400 and 3600 m in Nanhuta Shan are strong evidence for a period of increased slope activity during the Holocene, which in two cases were dated by OSL to approx. 3 ka. At least one solifluction unit developed during a cooler phase between 3 ka and today.

Microwave L-band emission of freezing soil

We report on field-measured microwave emission in a period of frost penetration into a grassland soil. The measurements were recorded with a high temporal resolution using an L-band radiometer mounted on a 7-m high tower. The observation period (December 2002 to March 2003) included two cycles of soil freezing and thawing with maximum frost depth of 25 cm. In situ soil temperature and liquid water content were measured at five depths down to 45 cm. Soil moisture profiles were calculated using the COUP numerical soil water and heat model in combination with measured soil properties and meteorological data monitored at the site. The L-band radiation data clearly showed the penetration and thawing of seasonal soil frost. We calculated soil reflectivities based on in situ measured and modeled soil moisture profiles by applying a coherent radiative transfer model. The calculated reflectivities were compared with the radiometrically determined soil reflectivities. It was demonstrated that the quantitative consistency between these reflectivities was significantly improved by applying an impedance matching approach accounting for surface effects. In this particular case, the dielectric structure of the uppermost soil horizon was largely influenced by soil roughness, vegetation, and snow cover. The radiometrically measured soil reflectivities were fitted using a radiative transfer model in combination with a roughness model assuming a soil surface roughness of 25 mm. The analysis during a period of frost penetration shows coherent behavior of the soil reflectivity. Temporal oscillation of the measured L-band radiation appears to be a coherent effect. This effect has the potential to be used for estimating the frost penetration velocity.

Spatial and Temporal Variations of Frozen Ground and Snow Cover in the Eastern Part of the Tibetan Plateau.

The observations of frost depth and snow water equivalent were carried out at Lhasa, Rikeze and Nagqu in the eastern part of the Tibetan Plateau from July 1993 to March 1999. Also, the routine data at the ten meteorological stations in the same part was analyzed, which covers the observation period. All the stations are located in the seasonally frozen ground region. The main results are as follows: The development of seasonally frozen ground was most remarkable at Nagqu, where the maximum frost depth was 160-200 cm. At Nagqu, the frost penetration began in October and continued until the middle of March. During the frost penetration, except in the midwinter, the soil layer near the surface froze in the night and melted in the daytime. At the thawing stage from April to May, the frozen ground melted from the surface and the frozen soil layer left underground became thin. The efficiency of frost penetration, α, under certain temperature conditions tends to increase with the altitude and the precipitation in the preceding autumn, but α is not influenced by the snow cover. Exceptionally, α is large at Lingzi and Qamdo in spite of their low altitudes. This may be ascribed to the less solar radiation at these stations than other stations, which enhances the frost penetration. The maximum frost depth increases with the altitude except for Lingzi and Qamdo.The days of snow cover increases with the latitude, but long lasting snow cover is not usual. At Nagqu, where the days of snow cover is the second largest among the ten stations, the maximum snow water equivalent was 20 mm, the maximum snow depth was 9 cm and the mean snow density was about 0.2 g cm-3. inter-annual variation of the days of snow cover at Nagqu is almost similar to that at Sogxian. The localized negative correlation between the winter air temperature anomaly and the days of snow cover was found at Nagqu and Sogxian in the winter of 97/98, suggesting the albedo effect of snow cover. The correlation is positive between the precipitation is summer and the days of snow cover at Sogxian in the preceding winter, which apparently corresponds to the snow-hydrological effect. The warm (cold) winter tends to follow the wet (dry) summer with much (little) precipitation respectively. But the relationship between air temperature and precipitation in the summer is not clear.

The effect of frozen soil on snowmelt runoff at Sleepers River, Vermont

Soil frost depth has been monitored at the Sleepers River Research Watershed in northeastern Vermont since 1984. Soil frost develops every winter, particularly in open fields, but its depth varies greatly from year to year in inverse relation to snow depth. During the 15 years of record at a benchmark mid-elevation open site, the annual maximum frost depth varied from 70 to 390 mm. We empirically tested the hypothesis that frozen soil prevents infiltration and recharge, thereby causing an increased runoff ratio (streamflow/(rain + snowmelt)) during the snowmelt hydrograph rise and a decreased runoff ratio during snowmelt recession. The hypothesis was not supported at the 111 km 2 W-5 catchment; there was no significant correlation of the runoff ratio with the seasonal maximum frost depth for either the pre-peak or post-peak period. In an analysis of four events, however, the presence of frost promoted a large and somewhat quicker response to rainfall relative to the no-frost condition, although snow cover caused a much greater time-to-peak regardless of frost status. For six years of flow and frost depth measured at the 59 ha agricultural basin W-2, the hypothesis appeared to be supported. The enhancement of runoff due to soil frost is evident on small plots and in extreme events, such as rain on frozen snow-free soil. In the northeastern USA rind eastern Canada, the effect is often masked in larger catchments by several confounding factors, including storage of meltwater in the snowpack, variability in snowmelt timing due to elevational and aspect differences, interspersed forested land where frost may be absent, and the timing of soil thawing relative to the runoff peak.

The effect of Initial Water Content on Maximum Frost Depth and Heat Balance —Studies on the analysis of freezing and thawing processes of soils (II)—

The effect of Initial Water Content on Maximum Frost Depth and Heat Balance —Studies on the analysis of freezing and thawing processes of soils (II)—

Stresses in buried conduits caused by freezing front

A laboratory experiment modeling the buried conduit problem was performed to study the stress changes in the conduit as a result of frost penetration in the soil. The model consisted of a large-scale soil container insulated on all sides except the top. The level and temperature of the ground water in the container, and the end restraints of the buried conduit, could be controlled. The model was placed in a cold room where the air temperature could be reduced to −25°C. The experimental results, supplemented by the available field data, demonstrate the influence of soil freezing on pipe bending stresses. Maximum frost depth, which can vary with the rate of freezing even for comparable air freezing indexes, is a significant soil thermal parameter that correlates with pipe stresses. Availability of ground water significantly increases pipe stresses while pipe-end restraints tend to decrease them slightly. The results suggest that the water expulsion by the advancing freezing front causes loss of soil stiffness due to the increased pore water pressures and decreased effective stresses in the soil.