Study on the Changing Rules of Silty Clay’s Pore Structure under Freeze-Thaw Cycles

For engineering construction in seasonally frozen regions, when the soil below the frost depth without freeze-thaw effect was exposed or reclaimed after excavation, long-term freeze-thaw will change soil skeleton and porous characteristics, thereby leading to the deterioration of soil engineering properties. This study focused on seasonally frozen silty clay from Changchun, China, and conducted different freeze-thaw cyclic tests on remoulded soil samples, during which both freezing temperature and the number of freeze-thaw cycles were varied. The related data of pore structures under different test conditions were acquired through mercury injection porosimetry (MIP) tests, and the effects of the number of freeze-thaw cycles and freezing temperature on the change of the soil’s pore structure were investigated in detail in combination with fractal theory. The variation rules of pores in the soil after freeze-thaw cycles were investigated from a microperspective so as to essentially analyze the mechanism of the deteriorating effect of freeze-thaw on a soil’s engineering properties.

Compression behavior of reconstituted clay: A study on black clay

The soft and black clay found in the Kathmandu Valley is locally known as Kalomato which is highly compressible and weak. The Kalomato from the Khasibazaar is taken as study material. Experimental study is carried out on undisturbed, remoulded and reconstituted soil samples to determine the compression behavior. The study revealed that the undisturbed soil sample has the highest compression index among all samples while the remolded sample has the lowest compression index. The reconstituted soil sample using cement shows that the compression index increases with the increase in cement content while decrease with the increase in curing time. When cement content increases to 15%, the load required to compress the soil to the equal void ratio also increases to almost three times of the undisturbed soil and seven times of the remoulded soil. Therefore, reconstitution of soil using cement is found as effective method for improvement of compression behaviour. This study also has established graphical interrelations between the compression index, swelling index and the cement content which can be used in the study of Kalomato.

Experimental Study of Remoulded Soil Properties Based on Freezing and Thawing Cycle

The experiments were carried out on the remolded soil to study the change law of the soil properties with the numbers of freezing and thawing cycles. It aims at simulating the effect of natural environmental factors on the mechanical performances artificial filling roadbed in Songbei region. After the soil sample was compacted in the optimum moisture content, and undergone limited numbers of freezing and thawing cycles, physical and mechanical properties were measured respectively. The experimental results indicated distinct change trend on physical and mechanical properties of soil after limited numbers of freeze-thaw cycle to confirm that the fillings was susceptible of temperature and moisture in this region.

An analysis of the fragmentation of remolded soils, with regard to self-mulching behavior

A power-law relation was used to analyse the (mass-derived) number-size distributions of fragments generated by wetting and drying remoulded soils. Various soils from Europe and Australia produced a range of values for the two fragmentation coefficients, d and k, generated by the power-law function. Both coefficients had physical significance with respect to self-mulching behaviour. Likened to a fractal dimension, the d coefficient varied directly with the tendency of the remoulded soil to fragment during wetting and drying. Assessment of the number of generated fragments >1 cm was made with the k coefficient. Consideration of both coefficients together in a plot of k v. d enabled similar soils to be grouped and falsely large values of d to be identified; k values were small for limited fragmentations even if the size distribution of the fragments that were produced gave large values of d. Most strongly self-mulching soils produced d values >1 . 5 after three wet/dry cycles, and k values that increased sharply after one cycle and declined with subsequent wetting and drying. Other soils with lesser abilities to self-mulch generally produced smaller d values and more variable k values. Reasonable correlations were found between these two coefficients and other measurements of self-mulching behaviour, particularly after three cycles of wetting/drying. Examination of the aggregate size distributions produced from remoulded soils in this way offers the potential to understand more clearly the dynamics of structure regeneration in soils exhibiting various degrees of self-mulching behaviour.

Structural decline of soils, assessment and prevention

Two extreme textural types of cultivated surface soils are mainly considered here, non-shrinking red-brown earths and highly shrinking cracking clays. Total porosity is used to assess the structural status of the former. Values are compared with the highest and lowest values found in the field. For the latter, the criterion used is the porosity of dry aggregates or clods. Values here are taken from the literature. To find out why inter-particle bonding in soil aggregates is insufficient to stop structural decline, a scheme has been developed which includes a modified version of Emerson's (1967) classification of soil aggregates. Slaking is carefully assessed. The bulk density of a cube made from soil at 'field capacity' is measured as well as testing another for dispersion. Class 3 is now divided into 3a and 3b, according to the degree of dispersion of remoulded soil in water. Also apart from soils which disperse spontaneously from dry, classes 1 and 2, the dispersion of all soils is assessed after remoulding at 'field capacity'. It has been found that the red-brown earth site which had the best visual structure also had the largest total porosity and aggregates in class 4. At the worst site, aggregates were in class 3a and the porosity had been reduced to that of the soil cube. For cracking clays, porosity is appreciably higher where the aggregates are in class 4 rather than class 3a. Water content/dispersion curves are presented for the clays showing the extent of the increase in OD apparently associated with the presence of carbonate. Dispersion of sheared, class 3a soil immersed in water is only an outward sign of the structural damage caused when the soil is sheared too wet. If the soil is dried instead, porosity is still lost. Mechanisms are suggested by which the structure of class 3a clay soils are improved by adding carbonate. The slumping of red-brown earths and the use of surface dressings of gypsum to prevent severe dispersion after cultivation wet are discussed. The structural stability of aggregates in the other five classes is briefly considered. Classes 1 and 2 require an ameliorant to be added, the rest pose few problems.

Mercury intrusion and permeability of Louiseville clay

Several theories have been proposed to correlate the permeability (hydraulic conductivity) with the pore-size distribution of soils, and it seemed interesting to determine if these theories could be applied to natural clays. For this purpose permeability and mercury-intrusion porosimetry tests were performed on a Champlain Sea clay for both intact and remoulded conditions after compression of the specimens to strains varying from 0 to 42%. The results show that for either intact or remoulded soil conditions, there is a relationship between the pore-size parameters and the permeability of the clay. However, there is no unique relationship for both intact and remoulded clays and for the silt and clay considered in previous studies. Consequently, mercury-intrusion porosimetry alone cannot be used to evaluate the permeability of soils in general. Key words: permeability, clays, mercury-intrusion porosimetry, fabric.

Degradation and restoration of soil structure in a cracking grey clay used for cotton production

A field experiment involving irrigated cotton investigated the effect of tillage on a self-mulching cracking grey clay at three different soil water contents: dry (close to permanent wilting point to over 1 m depth), moist (subsoil close to the lower plastic limit) and wet (just trafficable). These treatments had been repeated over three years prior to the plant and soil measurements reported in this paper. Compared with tillage of dry soil, tillage of moist or wet soil depressed lint yield by 35% (P<0.001). Shrinkage curves of resin-coated, intact soil clods showed lower clod bulk density at a standard water content in the dry treatment than in the wet treatment (P<0.001). This difference was most marked in the 0.2-0.3 m depth, where clods from the wet treatment had a bulk density approaching that of clods made from hand-remoulded soil. Subsequent restoration treatments showed that, although wheat improved soil physical condition, the yield of a following cotton crop was reduced due to lower nitrogen uptake (P<0.001). Deep tillage alone increased clod bulk density but deep tillage after wheat decreased density (interaction P<0.001 for the 0.2-0.3 m depth, P<0.05 for the 0.3-0.4 m depth). These effects of deep tillage were not reflected in yield of cotton. Effects of the previous dry, moist and wet tillage treatments persisted but there was some improvement, even in the absence of restoration treatments. To preserve the structure and productivity of cracking clay soils, they should be tilled only when dry to permanent wilting point through the full depth of tillage.

Some mechanical properties of aggregates of top soils from the IJsselmeer polders. 2. Remoulded soil aggregates and the effects of wetting and drying cycles.

The mechanical properties of remoulded samples of eight soils from the IJsselmeer polders were investigated. The Atterberg (Plastic and Liquid) Limits were positively correlated with the soil clay content, but were not significantly correlated with the organic matter content. Artificial aggregates of the remoulded soil age-hardened when moist. The tensile strength of artificial aggregates was positively correlated with the soil clay content and negatively correlated with the porosity. Friability of the dried, remoulded soil was much lower than that of natural, undisturbed soil. Factors affecting the rates of wetting and drying of aggregates in the laboratory were examined in detail. Wetting resulted in mellowing (or softening) of artificial aggregates. Mellowing was investigated as a function of the potential of the source of water causing the wetting. Multiple wetting and drying cycles may result in increased mellowing or in no mellowing depending on the water potential. New terminology is introduced to facilitate discussion of these phenomena. (Abstract retrieved from CAB Abstracts by CABI’s permission)