Cyclic secant shear modulus versus pore water pressure in sands at small cyclic strains

Cyclic strain-controlled behavior of fully saturated sands in undrained condition is analyzed at small cyclic shear strain amplitudes, γc, around the threshold shear strain for cyclic pore water pressure buildup, γtp≈0.01%. The cyclic triaxial and simple shear test results obtained in the past by different researchers and the results of new cyclic simple shear tests reveal that: (i) at very small γc below γtp where there is no buildup of cyclic pore water pressure, ΔuN, with the number of cycles, N, the cyclic secant shear modulus, GSN, initially increases with N for 10-20% of its initial value GS1 and then levels offor just slightly decreases, (ii) at small γc between γtp ≈0.01% and 0.10-0.15%, ΔuN continuously increases with N while the modulus GSN first increases for up to 10% of GS1 and then gradually decreases, and (iii) at γc larger than approximately 0.15%, relatively large ΔuN develops with N while the modulus GSN constantly and significantly decreases. This means that at γc between γtp and 0.10-0.15% the sand stiffness initially increases with N in spite of the reduction of effective stresses caused by the cyclic pore water pressures buildup. In this range of γc, the pore water pressure ΔuN can reach up to 40% of the initial effective confining stress before GSN drops below GS1. The microstructural mechanisms believed to be responsible for such a complex behavior are discussed. It is suggested that during cyclic loading the changes at mineral-to-mineral junctions of grain contacts can cause soil stiffening while, at the same time, the buildup of cyclic pore water pressure causes the softening

Cyclic secant shear modulus versus pore water pressure in sands at small cyclic strains

Cyclic strain-controlled behavior of fully saturated sands in undrained condition is analyzed at small cyclic shear strain amplitudes, γc, around the threshold shear strain for cyclic pore water pressure buildup, γtp≈0.01%. The cyclic triaxial and simple shear test results obtained in the past by different researchers and the results of new cyclic simple shear tests reveal that: (i) at very small γc below γtp where there is no buildup of cyclic pore water pressure, ΔuN, with the number of cycles, N, the cyclic secant shear modulus, GSN, initially increases with N for 10-20% of its initial value GS1 and then levels offor just slightly decreases, (ii) at small γc between γtp ≈0.01% and 0.10-0.15%, ΔuN continuously increases with N while the modulus GSN first increases for up to 10% of GS1 and then gradually decreases, and (iii) at γc larger than approximately 0.15%, relatively large ΔuN develops with N while the modulus GSN constantly and significantly decreases. This means that at γc between γtp and 0.10-0.15% the sand stiffness initially increases with N in spite of the reduction of effective stresses caused by the cyclic pore water pressures buildup. In this range of γc, the pore water pressure ΔuN can reach up to 40% of the initial effective confining stress before GSN drops below GS1. The microstructural mechanisms believed to be responsible for such a complex behavior are discussed. It is suggested that during cyclic loading the changes at mineral-to-mineral junctions of grain contacts can cause soil stiffening while, at the same time, the buildup of cyclic pore water pressure causes the softening

Continuous Measurement of Matrix Suction and Degree of Saturation of Unsaturated Soils with a New Soil-Water Retention Curve Device

The relationship between matrix suction and degree of saturation within the soil is a fundamental parameter in studying many behavioral aspects of unsaturated soils and is referred to as the soil-water retention curve (SWRC). Due to difficulties associated with negative pore water pressure measurements in unsaturated soils, most SWRC measuring techniques benefit from suction controlling methods such as axis translation or osmosis techniques. These methods provide only a few data points on the matrix suction-degree of saturation relationship and cause a discrete measured soil-water characteristic curve. However, many SWRC elements like drying and wetting curve slopes, air entry, and air expulsion values are fundamental parameters in describing the hydro-mechanical behavior of unsaturated soils. Therefore, a realistic understanding of these parameters requires continuous measurement of the degree of saturation-matrix suction relationship at more points. To this end, this paper examines the performance of a new SWRC device developed for the continuous measurement of the soil-water retention curve of unsaturated deformable soils along drying paths. The new apparatus is equipped with new miniature tensiometers enabling direct measurement of soil suction without the need for an artificial increase in pore air pressure. The variation of the degree of saturation is calculated by contiguous weighing of soil samples along drying paths. The credibility of the new SWRC apparatus is examined to investigate the influence of initial compaction on the soil-water retention response of sandy soil along with drying. This was experimentally achieved by SWRC tests on compacted soil samples with a range of void ratios between maximum and minimum void ratios to examine the influence of compaction on the slope of SWRC along with drying and variation of air entry value. The results are thoroughly discussed and compared against other available data in the literature. Also, the results suggest the fast performance of newly developed tensiometers for direct measurement of soil suction with a minute without the need for the application of elevated pore air pressure, which leads to the continuous SWRC measurement of the soil samples within 3 to 5 days along drying paths. The credibility of the new SWRC device is also examined with additional suction measurement tests using conventional jet-fill tensiometers, showing consistent results

Threshold shear strains for cyclic degradation and cyclic pore water pressure generation in two clays

Cyclic threshold shear strains are fundamental cyclic soil properties that have not been fully investigated. To learn more about the threshold shear strains for cyclic degradation, γtd, and cyclic pore water pressure generation, γtp, in fully saturated clays, nine multistage cyclic strain-controlled NGI direct simple shear tests are conducted on laboratory-made kaolinite clay (PI=28) and kaolinite-bentonite clay (PI=55). Three levels of vertical effective consolidation stress, σ'vc (113 kPa, approximately 216 kPa, and approximately 674 kPa); three OCRs (1, 4 and 7.8); and two cyclic loading frequencies, f (0.01 and 0.1 Hz), were applied. In three tests on the normally consolidated (NC) kaolinite clay, γtd varied between 0.012 and 0.014% and γtp between 0.014 and 0.034%. In two tests on the overconsolidated (OC) kaolinite clay with OCR=4, γtd was 0.013% and γtp 0.016 and 0.017%. In two tests on the NC kaolinite-bentonite clay, γtd was 0.013 and 0.016% and γtp 0.052 and 0.078%. In the test on OC kaolinite-bentonite clay with OCR=4, γtd was 0.014% and with OCR=7.8 it was 0.012%. For the same soil γtp is typically slightly greater than γtd. Clear trends of γtd and γtp with σ'vc, OCR, and f could not be identified given the relatively small number of tests. The results indicate that if these trends exist they are small. The comparison of the above results with those from the literature shows that γtd for six different soils ranges between 0.006 and 0.05% and γtp for eight soils between 0.014 and 0.1%, and that there is a modest trend of γtd and moderate trend of γtp increasing with PI

Excess Pore-Water Pressure Generation and Mud Pumping in Railways Under Cyclic Loading

2019, Springer Nature Singapore Pte Ltd. High-speed heavy haul trains have become one of the most popular and economical modes of transportation in the modern world to cater for increased demand in freight for agricultural and mining activities. However, when these trains travel through vulnerable areas occupying soft subgrade formations, frequent maintenance is required to prevent differential settlement and localized failures of track. The poor performance of track caused by ballast fouling is also often observed where fines are fluidized and pumped into the ballast voids (mud pumping), which in turn create ballast pockets, mud holes and track instability. When saturated subgrade is subjected to short-term undrained cyclic loading, the pore-water pressure can accumulate inducing fine particles to migrate upwards into the ballast layer. Mud pumping causes millions of dollars of damage to heavy haul rail networks every year in Australia. This paper presents a critical review primarily focused on the role of excess pore-water pressure generation on mud pumping under cyclic loading. Mitigation of these issues can result in considerable savings to rail authorities on recurrent track maintenance activities