Long-term decay of the water pressure in the osmotic tensiometer

Matric suction is an important state variable required for the assessment of unsaturated soil properties.Tensiometers are commonly used for direct matric suction measurement but have a limited measuring range up to 90 kPa due to the cavitation problem.Osmotic tensiometer(OT)can improve the measuring range of tensiometers by increasing the osmotic pressure of water to avoid the cavitation.However,the long-term water pressure decay that appeared in OTs caused a gradual decrease in their measuring range.In this study,crosslinked poly(acrylamide-co-acrylic acid)potassium salt(PAM-co-PAAK)was used for the preparation of OTs(five in total)to explore the mechanism of water pressure decay of OTs.The maximum water pressure in the OT versus the volume fraction of polymer filled in the OT was described based on the Flory-Huggins polymer theories and validated using WP4C dewpoint hygrometer.The long-term pressure decay of OT-1,OT-2,and OT-3 was observed for 130 d and constant pressures were found for OT-1 and OT-2,indicating that the pressure decay of OT was mainly caused by the stress relaxation of the polymer hydrogels,and standard linear solid(SLS)rheological model was appropriate to fit the decay data.For OT-1,OT-2 and OT-3,the theoretical osmotic pressure that was calculated based on the mass of retrieved polymer from OTs after 130-d pressure observation was higher than the actual osmotic pressure as observed,indicating that polymer leakage cannot explain the pressure decay of the OT.The ultravioletevisible(UVevisible)spectrophotometry examined the change in polymer concentrations in the water containers of OT-4 and OT-5 and demonstrated that there was no increase in polymer leakage during the period of pressure decay of OT-4 and OT-5.As a result,the pressure decay of OT was not caused by polymer leakage.The results of this research suggested that the viscoelastic properties of polymers should be taken into consideration in further OT development.

综合蒸发法和冷镜露点法测量水-土特征曲线和非饱和渗透系数（英文）

目的:综合使用HYPROP和WP4C两种仪器,探索采用蒸发法和冷镜露点法测量大吸力范围的水-土特征曲线,并采用收缩曲线对土体变形进行修正。方法:1.采用HYPROP仪器,在0～100 kPa吸力范围内测量土体的水-土特征曲线。2.当吸力大于600 kPa时,采用WP4C仪器,测量土体的水-土特征曲线。3.采用传统张力仪吸力板仪器,在0～500k Pa吸力范围内测量土体的水-土特征曲线。4.采用Satyanaga et al.(2017)公式,对实验数据进行拟合。5.采用HYPROP仪器直接测量土体的非饱和渗透系数。6.采用蒸发法,获取干缩曲线。结论:1.采用Satyanaga et al.(2017)公式,对实验数据进行拟合,拟合参数直接关联到进气值、拐点及方差。拟合结果显示,不同仪器所收集的水-土特征曲线的数据相互吻合,为水-土特征曲线的实验测量提供了更大的吸力范围。2.采用HYPROP仪器测量的非饱和渗透系数和采用传统改装的非饱和三轴实验数据基本吻合,因此,HYPROP可以作为在低吸力范围内(<100 kPa)测量非饱和渗透系数的有效仪器。3.当进气值小于100 kPa或者大于500 kPa时,综合采用HYPROP和WP4C可以有效缩短实验周期。

Effect of the uncertainty in soil-water characteristic curve on the estimated shear strength of unsaturated soil

Most failures or instabilities of geotechnical structures commonly result from shear failure in soil. In addition, many infrastructures are constructed within the unsaturated zone. Therefore, the determination of shear strength of unsaturated soil is crucial in geotechnical design. The soil-water characteristic curve(SWCC) is commonly used to estimate the shear strength of unsaturated soil because the direct measurement is time-consuming and costly. However, the uncertainty associated with the determined SWCC is rarely considered in the estimation of the shear strength. In this paper, the uncertainties of SWCC resulted from different factors are reviewed and discussed. The variability of the estimated shear strength for the unsaturated soil due to the uncertainty of SWCC associated with the best fit process is quantified by using the upper and lower bounds of the determined SWCC. On the other hand, the uncertainties of the estimated shear strength due to different initial void ratios or different confining pressures are quantified by adopting different SWCCs. As a result, it is recommended that the measured SWCC from the conventional Tempe cell or pressure plate needs to be corrected by considering different stress levels in the estimation of the shear strength of unsaturated soil.

Effect of hysteresis on the stability of residual soil slope

Soil-water characteristic curve (SWCC) is an essential parameter in unsaturated soil mechanics.Matric suction in unsaturated residual soils changes with varying climatic conditions associated with cyclic drying-wetting conditions that result in hysteresis in the SWCC.The soil mechanical behaviour under wetting is crucial since numerous rainfall-induced slope failures occur during the wetting process.However,many slope stability analyses were carried out using drying SWCC.Consequently,the factor of safety (FoS) calculation may not represent the actual field condition.This paper presents the effect of hysteresis in SWCC on the stability of unsaturated residual soil slopes from Bukit Timah Granite in Singapore.The study focused on the analyses considering the differences in pore-water pressure and water content variations under drying and wetting conditions by performing numerical seepage and stability analyses.Each analysis was carried out on a slope subjected to dry and rainy periods under three different conditions:i) using only drying SWCC;ii) using only wetting SWCC;iii) using combined drying and wetting SWCCs.The results indicated that the FoS variations obtained from the numerical analyses incorporating combined SWCC matched more closely those obtained by incorporating only wetting SWCC than those obtained by incorporating only drying SWCC,regardless of the wetting or drying processes that the soil experienced.Moreover,the numerical analyses under high rainfall intensity by incorporating only wetting SWCC gave a more conservative FoS as compared to those obtained by incorporating combined SWCC.Numerical analyses incorporating only drying SWCC gave the most conservative FoS regardless of rainfall intensity.

Effect of variations in rainfall intensity on slope stability in Singapore

Numerous scientific evidence has given credence to the true existence and deleterious impacts of climate change.One aspect of climate change is the variations in rainfall patterns,which affect the flux boundary condition across ground surface.A possible disastrous consequence of this change is the occurrence of rainfall-induced slope failures.This paper aims to investigate the variations in rainfall patterns in Singapore and its effect on slope stability.Singapore's historical rainfall data from Seletar and Paya Lebar weather stations for the period of 1985–2009 were obtained and analysed by duration using linear regression.A general increasing trend was observed in both weather stations,with a possible shift to longer duration rainfall events,despite being statistically insignificant according to the Mann-Kendall test.Using the derived trends,projected rainfall intensities in 2050 and 2100 were used in the seepage and slope stability analyses performed on a typical residual soil slope in Singapore.A significant reduction in factor of safety was observed in the next 50 years,with only a marginal decrease in factor of safety in the subsequent 50 years.This indicates a possible detrimental effect of variations in rainfall patterns on slope stability in Singapore,especially in the next 50 years.The statistical analyses on rainfall data from Seletar and Paya Lebar weather stations for the period of 1985–2009 indicated that rainfall intensity tend to increase over the years,with a possible shift to longer duration rainfall events in the future.The stability analyses showed a significant decrease in factor of safety from 2003 to 2050 due to increase in rainfall intensity,suggesting that a climate change might have existed beyond 2009 with possibly detrimental effects to slope stability.

孔径分布函数对水分在非饱和土体中迁移的作用（英文）

目的:了解孔径分布函数对水分在非饱和土体中迁移的影响,并提出相关数学模型,量化非饱和土体在不同吸力作用下的渗透系数。创新点:从孔径分布函数出发探讨土体的工程性质,以日常生活中所用的阀门模型解释非饱和土体在不同吸力作用下的渗透系数。提出采用非饱和土渗透系数的实验数据,间接估算土体的孔径分布函数。方法:从物理模型推导相关数学公式,并用实验结果对数学公式进行验证。结论:孔径分布函数主导非饱和土的渗透系数;孔径分布函数是连接土体两大水力特性(包括水土特征曲线和渗流方程)的桥梁。孔径分布函数对水分在土体中迁移的作用可以用日常生活中使用的阀门模型简单描述。基于阀门模型,土体的孔径分布函数也可以由土体的非饱和渗透系数的相关实验数据间接估算得到。

Field instrumentation for real-time measurement of soil-water characteristic curve

Suction measurement has a vital role in unsaturated soil analysis.However,measuring soil suction remains a challenging task due to a number of issues such as the limited range of suction measurement,cavitation,or long equilibrium time.It is even more challenging when the suction measurement is to be carried on the field.Hence,the development of a new suction measurement device which is able to measure high suction range for a long duration without tedious maintenance and yet portable enough for site measurement is required.In this study,a new sensor which is referred to as NTU Osmotic Tensiometer was developed along with the method to correct for decay and temperature.The NTU osmotic tensiometer is based on polymer swelling capacity in order to measure in-situ soil suction in real time for a very long duration.It requires minimum maintenance as the polymer-based sensor is not affected by the cavitation phenomenon.However,correction for decay and temperature is of paramount importance and therefore explained in this paper.Verification of the NTU osmotic tensiometer was carried out by comparing the field measurement results from the NTU osmotic tensiometer and the small tip tensiometer.The performance of the NTU osmotic tensiometer was found to be comparable with that of the small tip tensiometer,but the NTU osmotic tensiometer is able to measure more than 100 kPa soil suction.Therefore,it is possible to obtain the field soil-water characteristic curve by combining the measured in-situ soil suction from the NTU osmotic tensiometer with the measured in-situ water content from the moisture sensor as illustrated in the paper.

Characteristics of unsaturated soil slope covered with capillary barrier system and deep-rooted grass under different rainfall patterns Check for updates

Rainfall-induced slope failures commonly occur in residual soil slopes.Slope failures are triggered by the reduction in soil strength.This is attributed to the decrease in soil suction due to rainwater infiltration.Slope covers like capillary barrier system and vegetative cover are effective methods that can be used to prevent rainfall-induced slope failures.The capillary barrier system is able to limit the rainwater infiltration,and vegetation can contribute to the increase in soil strength.Vetiver grass is widely planted in tropical and subtropical areas of the world for soil and water conservation.This study investigates the characteristics of unsaturated soil slope covered with capillary barrier system and Vetiver grass in comparison with the original slope through numerical analyses and field measurements.The analyses were carried out under the advanced,normal,and delayed rainfall patterns.The results of the analyses indicated that the capillary barrier system played a more significant role than the Vetiver grass in maintaining slope stability,although both the capillary barrier system and Vetiver grass contributed to the slope stability.In addition,both numerical analyses and field measurements showed that under the delayed and normal rainfall patterns,when antecedent rainwater infiltration could increase the soil moisture,the capillary barrier system performed much better compared to Vetiver grass in maintaining soil matric suctions and slope stability.