Soil Investigation and Testing Research Papers (original) (raw)

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- Numerical Modeling, Unsaturated soil, Geotechnical Engineering, Genetic Algorithms

Presentation paper in Material testing of low volume roads in Tanzania using Low Volume Roads Manual 2016. Material Testing based on DCP DN Methods on site where the quality of materials is determined and to enable one understand the... more

UNIVERSITY OF ENGINEERING AND TECHNOLOGY PESHAWAR

... Lourenco, SDN, Gallipoli, D. , Toll, DG, and Evans, F. (2006) Use of Tensiometers for the Measurement of Soil Suction. In: 10th Confresso Nacional de Geotecnia, 22 - 26 May 2006, Lisboa, Portugal. ... Authors: Lourenco, SDN,... more

UNIVERSITY OF ENGINEERING AND TECHNOLOGY PESHAWAR

Buku ini berisi kumpulan korelasi untuk menentukan berbagai parameter geoteknik dari hasil pengujian laboratorium dan pengujian lapangan (Uji SPT, Uji CPT, Uji Dilatometer, Uji Pressuremeter, Uji Vane Shear). Buku ini juga memiliki bab... more

- by Irene Lumban Raja
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- Soil Investigation and Testing

“Raw earth” (“terre crue” in French) is an ancient building material consisting of a mixture of moist clay and sand which is compacted to a more or less high density depending on the chosen building technique. A raw earth structure could in fact be described as a “soil fill in the shape of a building”. Despite the very nature of this material, which makes it particularly suitable to a geotechnical analysis, raw earth construction has so far been the almost exclusive domain of structural engineers and still remains a niche market in current building practice. A multitude of manufacturing techniques have already been developed over the centuries but, recently, this construction method has attracted fresh interest due to its eco-friendly characteristics and the potential savings of embodied, operational and end-of-life energy that it can offer during the life cycle of a structure. This paper starts by introducing the advantages of raw earth over other conventional building materials followed by a description of modern earthen construction techniques. The largest part of the manuscript is devoted to the presentation of recent studies about the hydro-mechanical properties of earthen materials and their dependency on suction, water content, particle size distribution and relative humidity.

The paper gives a brief description of double wall cell triaxial equipment for testing soil samples under unsaturated conditions and presents results of some of the calibrations necessary to achieve accurate measurements of inner cell water volume, pore water volume, pressure, load and displacement. The calibration of the measurement devices showed linear relationships between the raw readings and applied values with the regression constants differ from 1 and 0. The result of apparent volume change calibrations showed significant correlation between cell water volume fluctuation and temperature fluctuation inside testing room. A correction to the measured volume was therefore required to reduce the effects of long-term temperature fluctuation on the measured volume. Investigation of the accuracy of volume change measurement with the double wall cell showed excellent matching between the inner cell volume change and pore water volume change under saturated conditions, indicating the high reliability of the double wall system for apparent volume change measurements.

Au cours des dernières décennies, un intérêt croissant est porté à la terre crue en tant qu'éco-matériau de construction pour son caractère local et la faible consommation d’énergie qui caractérise son cycle de vie global. Toutefois, sa résistance mécanique relativement faible et sa sensibilité à l'eau freinent le développement de ce matériau. Le principal but de cette étude est d’améliorer les propriétés mécaniques de briques de terre crue par l’emploi d’une nouvelle procédure de compactage à pression élevée maintenue pendant une durée correspondante au temps de consolidation. Un système de compactage a été élaboré afin de compacter le sol à haute pression en permettant le drainage de l’eau interstitielle. Dans un premier temps, les caractéristiques géotechniques du matériau d'étude ont été déterminées. Après compactage à 25, 50 et 100 MPa, les échantillons ont été placés dans une enceinte climatique (25°C, humidité relative de 62 %) jusqu’à équilibre hydrique, puis testés afin d’évaluer leur module d’Young et leur résistance mécanique en compression. Cette nouvelle méthode de compactage à haute pression a permis d'atteindre des performances mécaniques élevées. Ainsi, un des principaux freins à l'utilisation de la terre crue comme matériau de construction serait levé.

The paper reveals the results of the study on the validation of the DCP (Dynamic Cone Penetrometer) method to find a CBR (California Bearing Ratio) in situ that can be compared with a CBR obtained in laboratory. Samples were taken from four construction material quarries in the area of Tegucigalpa, Honduras; soil types were mainly sandy silt and gravel with presence of sand. To perform such investigation, it was necessary to determine in situ conditions of density and moisture content using a nuclear densimeter. Subsequently, these conditions were reproduced in laboratory with the compaction method of Proctor Modified, to find a laboratory CBR. This was done in order to have the same conditions that allow a proper verification of the correlation between the CBR found using the DCP with the CBR measured in laboratory. Below, it is explained in detail the analysis process where it were verified that those study soils selected met the correlation of the DCP method with the CBR according to the equation proposed by ASTM D 6951 standard for the soils analyzed. Samples are highly representative, therefore it's validated that the ASTM D 6951 standard can be applied.

- by Irene Lumban Raja
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- Soil Investigation and Testing

The Spectral Analysis of Surface Wave (SASW) method is a non-destructive in situ seismic technique used to assess and evaluate the material stiffness (dynamic elastic modulus) and thickness of pavement layers at low strains. These values can be used analytically to calculate load capacities in order to predict the performance of pavement system. The SASW method is based on the dispersion phenomena of Rayleigh waves in layered media. In order to get the actual shear wave velocities, 2-D and 3-D models are used in the simulation of the inversion process for best fitting between theoretical and empirical dispersion curves. The objective of this study is to simulate and compare the 2-D and 3-D model of SASW analysis in the construction of the theoretical dispersion curve for pavement structure evaluation. The result showed that the dispersion curve from the 3-D model was similar with the dispersion curve of the actual pavement profile compared to the 2-D model. The wave velocity profiles also showed that the 3-D model used in the SASW analysis is able to detect all the distinct layers of flexible pavement units. Keywords: 2-D and 3-D stiffness matrix model, SASW, pavement structure.

The paper presents a bounding surface model that describes the gradual yielding of unsaturated soils subjected to isotropic loads. The model originates from consideration of the capillary bonding between soil grains, which leads to the definition of a “unified normal compression line” that is valid in both saturated and unsaturated conditions. This line has the same slope and intercept of the saturated normal compression line but is formulated in terms of a “scaled stress” variable, which takes into account the mechanical effect of capillarity by factoring the average skeleton stress (also known as Bishop’s stress) with a power function of degree of saturation. The normal compression behaviour of unsaturated soils is therefore described by only one additional parameter, which is the exponent of the degree of saturation in the scaled stress expression. For over-consolidated soils, the occurrence of gradual yielding is introduced by assuming that, as the soil state moves towards the unified normal compression line, the slope of the loading curve tends towards the slope of the unified normal compression line according to an expression requiring only one extra parameter. Interestingly, this expression can be integrated in a closed form to provide a general equation for all loading paths in saturated and unsaturated conditions. Different loading curves are simply distinguished by the different values of the integration constants. Unloading paths are also simulated in a similar way. The proposed model requires a total of five parameters, which include the three standard parameters for saturated soils (i.e. the slope and intercept of the saturated normal compression line and the slope of the swelling line) plus one parameter to describe unsaturated behaviour and one parameter to describe the gradual yielding of over-consolidated soils subjected to loading.

- by Domenico Gallipoli
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- Civil Engineering, Soil Science, Unsaturated soil, Geotechnical Engineering

Rainfall is the most frequent triggering factor for landslides and the development of early warning systems has to take account of this. It is suggested that direct measurement of pore pressure gives the most reliable prediction of failure of a slope. The amount of rainfall can be only a crude indicator of failure as the processes that occur between rain falling on a slope and the resulting pore water pressure change are complex, highly non-linear and hysteretic. The paper describes high capacity tensiometers developed within the EU-funded MUSE Research Training Network that have been used for measuring suctions in slopes. High-capacity tensiometers are capable of direct measurement of pore water pressure down to −2 MPa and are also able to record positive pore water pressures. Two methods of field installation are discussed; one developed by ENPC in France uses a single tensiometer per hole, and the second technique, developed by Durham University in the UK, allows multiple tensiometers to be used at different depths within a single borehole. Continuous monitoring of pore water pressure has been carried out over several months and shows the responses to climatic events.

- by Domenico Gallipoli and +3
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- Numerical Modeling, Unsaturated soil, Geotechnical Engineering, Genetic Algorithms

The use of raw earth as construction material can save embodied and operational energy because of low processing costs and passive regulation of indoor ambient conditions. Raw earth must however be mechanically and/or chemically stabilised to enhance stiffness, strength and water durability. In this work, stiffness and strength are enhanced by compacting raw earth to very high pressures up to 100 MPa while water durability is improved by using alkaline solutions and silicon based admixtures. The effect of these stabilisation methods on hygro-mechanical behaviour is explored and interpreted in terms of the microstructural features of the material. Stiffness and strength are defined at different humidity levels by unconfined compression tests while the moisture buffering capacity is measured by humidification/desiccation cycles as prescribed by the norm ISO 24353 (2008). As for the microstructural characterisation, different tests (i.e. X-Ray diffractometry, Infrared Spectroscopy, Mercury Intrusion Porosimetry, Nitrogen Adsorption) are performed to analyse the effect of stabilisation on material fabric and mineralogy. Results indicate that the use of alkaline activators and silicon based admixtures significantly improves water durability while preserving good mechanical and moisture buffering properties. Similarly, the compaction to very high pressures results in high levels of stiffness and strength, which are comparable to those of standard masonry bricks. This macroscopic behaviour is then linked to the microscopic observations to clarify the mechanisms through which stabilisation affects the properties of raw earth at different scales.

- by Domenico Gallipoli
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- Civil Engineering, Soil Science, Microstructure, Geotechnical Engineering

This paper presents a series of consolidated drained triaxial tests performed on weakly bonded soil. Samples were artificially prepared by mixing sand and kaolin (87% sand: 13% kaolin) and firing at 500ºC in a furnace. The weak bond strength is provided by the fired kaolin. The results are explained according to the critical state concept in order to interpret the behaviour of the studied soil. Destructured soil was also prepared and tested in the same manner as the bonded soil but without firing. The bonded samples showed more pronounced peaks in stress-strain behaviour compared to the destructured samples. The critical state line (CSL) could not be easily identified as some of the drained tests show changes in deviator stress and volumetric strain up to the end of tests. Therefore, careful assessment of stress-strain behaviour, pore-water pressure responses, stress paths and volumetric changes was made to identify the critical state line. Samples sheared at low and medium stress levels displayed contraction prior to dilatant behaviour whilst those samples sheared at higher stresses showed contraction up to the end of shearing. A " discontinuity " approach was applied to position the critical state on dilatant 2 paths in v-ln p space after considering other stress-strain and volumetric strain curves. The critical state line (CSL) in v-ln p space for bonded samples plotted above the CSL for destructured samples but bent down toward the destructured line at higher stress levels. This clearly indicates the effect of cementation bonds on the CSL for the bonded samples, as the influence of cementation remains until the stress levels increase beyond the yield stress of the bonded material. In addition, the effect of bonding can be visualized from the normalised stress paths of destructured and bonded samples.

- by Domenico Gallipoli and +2
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- Civil Engineering, Soil Science, Unsaturated soil, Geotechnical Engineering
- by shahidul islam and +1
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- Microstructure, Modeling, Peatland Ecology, Removal Of Heavy Metals In Contaminated Soils

The paper presents a benchmarking study carried out within the ‘Mechanics of Unsaturated Soils for Engineering’ (MUSE) network aimed at comparing different techniques for measurement and control of suction. Techniques tested by the eight ‘Mechanics of Unsaturated Soils for Engineering’ research teams include axis-translation (pressure plate and suction-controlled oedometer), high capacity tensiometer and osmotic technique. The soil used in the exercise was a mixture of uniform sand, sodium bentonite (active clay) and kaolinite (non-active clay), which were all commercially available. Samples were prepared by one team and distributed to all other teams. They were normally consolidated from slurry under one-dimensional conditions (consolidometer) to a given vertical stress. The water retention characteristics of the initially saturated specimens were investigated along the main drying path. Specimens were de-saturated by applying suction through the liquid phase when using an axis-translation technique or osmotic method and de-saturated by air-drying, when suction was measured using high-capacity tensiometers. In general, the same technique was tested by at least two teams. The water retention curves obtained using the different techniques are compared and discrepancies are discussed in the paper.

- by Domenico Gallipoli and +2
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- Numerical Modeling, Unsaturated soil, Geotechnical Engineering, Genetic Algorithms

Past studies on the use of high suction tensiometers for measuring negative pore water pressure have focused on three different aspects, namely initial saturation of the probe, calibration over both positive and negative pressure ranges and measurement procedures. Among these three areas, the one focusing on measurement procedures has undeservedly received less attention. Aspects related to measurement conditions during laboratory or field testing are as important as the initial pre-conditioning or calibration of the probe. According to the particular type of measurement, different aspects of the testing procedure have to be considered in order to obtain accurate readings of pore water pressure. This note presents preliminary data to highlight the importance of factors such as measurement time, soil-probe contact and material type when measuring suction by means of high suction tensiometer.

- by Domenico Gallipoli and +1
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- Numerical Modeling, Unsaturated soil, Geotechnical Engineering, Genetic Algorithms

ini adalah contoh laporan mingguan pekerjaan

Compacted clay fills are generally placed at the optimum value of water content and, immediately after placement, they are unsaturated. Wetting might subsequently occur due, for example, to rainfall infiltration, which can cause volumetric deformation of the fill (either swell or collapse) with associated loss of shear strength and structural integrity. If swelling takes place under partially restrained deformation, due for example to the presence of a buried rigid structure or a retaining wall, additional stresses will develop in the soil and these can be detrimental to the stability walling elements and other building assets. Factors such as dry density, overburden pressure, compaction water content and type of clay are known to influence the development of stresses. This article investigates these factors by means of an advanced stress path testing program performed on four different clays with different mineralogy, index properties and geological histories.
Specimens of kaolin clay, London clay, Belfast clay and Ampthill clay were prepared at different initial states and subjected to “controlled” wetting whereby the suction was reduced gradually to zero under laterally restrained conditions (i.e. Ko conditions). The results have shown that the magnitude of the increase in horizontal stresses (and therefore the increase of Ko) is influenced by the overburden pressure, compaction water content, dry density at the time of compaction and mineralogy.

- by Gabriela Medero and +2
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- Unsaturated soil, Geotechnical Engineering, Unsaturated Soil (Engineering), Soil Investigation and Testing

The primary superficial deposit within the greater Dublin region (Ireland) is a stiff lodgement till, known colloquially as Dublin Bolder Clay (DBC). This material’s high stone content and strength often make good-quality undisturbed sampling extremely difficult. CPT testing is still rare, with present site investigation practice in DBC for routine design generally involving standard penetration testing. This paper brings together the body of knowledge and experience gained in the use of piezocone (CPTu) technology in characterising DBC deposits for 3 different sites, including the relationship be-tween CPT cone tip resistance, qc, and peak undisturbed undrained strength. Although the CPT data were ‘spiky’ on account of the high stone content and possible frequent dense sand/gravel lenses present, significant features were identifiable including coarse seams present and the transition boundary between the brown DBC and underlying stiffer black DBC.

- by Domenico Gallipoli and +1
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- Numerical Modeling, Unsaturated soil, Geotechnical Engineering, Genetic Algorithms

Rammed earth is a manufactured material comprising sand, gravel and clay, which is compacted between forms to build walls. Primarily a historic method of construction, it is now receiving considerable interest worldwide owing to its zero reliance on materials such as cement, and its potential for recycling. Despite its longevity, the source of its shear strength is poorly understood. This paper presents initial laboratory test results that point to the main source of strength in rammed earth being suction, and indicating that recent advances in unsaturated soil mechanics may also be applied to this material.

- by Paul Jaquin and +2
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- Unsaturated soil, Geotechnical Engineering, Sustainable Building Design, Unsaturated Soil (Engineering)

Analyzing soil samples for prescribing soil test based fertilizer doses after each crop in a sequence is time, labour and energy consuming and involves additional cost. With a view to develop post-harvest soil-test values (PHSTVs) prediction equations with multiple linear regression, field experiments were conducted during 2015-16 by adopting Inductive cum Targeted yield model, on an Inceptisol (Vertic Ustropept) with pearl millet under integrated plant nutrition system (IPNS). The experiment was laid out in a fractional factorial design comprising twenty-four treatments. When grain yield was considered, the predictability values of KMnO4-N, Olsen-P, and NH4OAc-K under NPK plus FYM @ 12.5 t ha-1 treatment were 98.4, 97.5, and 98.1%, respectively. When uptake was considered, the predictability values of KMnO4-N, Olsen-P, and NH4OAc-K under NPK plus FYM @ 12.5 t ha-1 treatment were 98.4, 97.6, and 98.1%, respectively. Significant R2-values (>0.65) were recorded for these regression equations which could be used with confidence for the prediction of post-harvest KMnO4-N, Olsen-P and NH4OAc-K. The data on observed and predicted soil test values of available N, P and K were in good agreement with each other, proving the validity of the post-harvest soil test values prediction equations as evidenced by highly significant correlation.

- by Udayakumar Sekaran and +2
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- Soil Science, Soil Analysis, Soil Investigation and Testing

The influence of cropping systems on the characteristics and fertility of soils derived from beach sands was investigated. The study site was the Cross-River Basin farm, Onna Local Government, in the Niger Delta area of Akwa Ibom State, Nigeria. Two cropping systems-sole cropping (SC) and mixed cropping (MC) and fallow plot [FP] as control, were examined. Pineapple (Ananas commosus) plot and water melon (Citrullus lanatus) plot, represented SC. Plantain/cassava (Musa spp/Manihot esculenta) plot and water leaf/scent leaf (Talinum triangulare L./Ocimum grattissimum) plot represented MC. Representative soil samples were collected at 0-15 and 15-30 cm depths from each plot for laboratory analysis. The result showed that irrespective of cropping system or soil depth, total nitrogen [N] and exchangeable potassium (K) were low – (values below critical levels: < 0.15% and < 0.20 cmol/kg, respectively). Available phosphorus (P) was medium 8 – 20 mg/kg) at both depths in the pineapple plot, and high (> 20mg/kg) at both depths in all others cropping systems. Organic matter (OM) was low (< 2%) in the plantain/cassava MC, and medium (2 – 3%) in other cropping systems, at 0 – 15 cm depth, but at 15 – 30 cm, OM was medium in water melon SC and plantain/cassava MC and low in other cropping systems. Exchangeable bases (Ca, Mg, K) and P were highly variable (coefficient of variation (CV), > 35%) within each cropping system. Analysis of variance (ANOVA) test showed that, irrespective of soil depth there were significant differences (P ≤ 0.05) among the cropping systems in 11 (79%) of the 14 soil characteristics tested. In view of the deficiencies and variations in soil fertility, regular soil testing for proper fertilizer recommendations is essential to ensure balanced soil nutrient application. Also, well planned crop combinations and rotation programmes will ensure optimum and sustainable land productivity, improved ecological balance and environmental quality.

- by Gabriela Medero and +1
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- Numerical Modeling, Unsaturated soil, Geotechnical Engineering, Genetic Algorithms

An-Najaf province considered one of the most important cities in Iraq, which is facing a rapid population growth and continuous development in constructions such as housing, hotels, bridges and shopping malls. Therefore, this study aims to create database for the gypsum content in An-Najaf province (Najaf center and Al-Kufa city). To evaluate the spatial variability of gypsum content, 464 boreholes and in situ tests were selected and analyzed using Geographic Information System (GIS). To achieve that, the adopted method consisted of four steps: (1) data collection; (2) geotechnical data georeferencing; (3) interpolation methods and (4) establishment of maps. Nine geotechnical maps were produced for depths (0, 2, 4, 6, 8, 10, 12, 14, 16, and 35) m. Results showed that the largest part of the study area for the depths from 0-4 m had a gypsum content ranged between 10-25% (moderately gypsiferous), whereas the depths from 4-8 m had a gypsum content ranged between 3-10% (slightly gypsiferous), and the depths from 8-35 m had a gypsum content ranged between 0.3-3% (very slightly gypsiferous). This conclude that the layers from 0-4 m are exceeding the allowed limits (10% gypsum content), which may affect the stability of constructions due to the high levels of groundwater in Najaf province.

High-suction tensiometers are able to measure suctions up to 2 MPa. Direct calibration at such high suctions requires the imposition of negative water pressures, which are difficult to achieve using facilities commonly available in soil mechanics laboratories. For this reason, tensiometers are usually calibrated in the positive pressure range, and such calibration is subsequently extrapolated to negative pressures. This paper examines different experimental techniques to assess the accuracy of such extrapolation. Any error in the calibration process would be directly reflected in the measured values of suction, and might be particularly significant (in relative terms) for the measurement of low suctions. In addition, the results of this study show that calibration in the positive range is affected both by the physical configuration of the tensiometer during calibration and by aspects of its design. The paper concludes that linear extrapolation of the calibration from the positive to the negative range is sufficiently accurate provided that calibration is done under conditions that closely match the conditions in which the tensiometer will be used. Owing to structural differences between tensiometers, and also to suction induced ‘calibration hysteresis’, at least one check on the accuracy of the extrapolated calibration equation over a range of negative pressure should be performed, even if at low values of suction.

- by Domenico Gallipoli and +1
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- Numerical Modeling, Unsaturated soil, Geotechnical Engineering, Genetic Algorithms

In their paper (Llano-Serna & Contreras, 2020), the authors provide valuable insights into the effects of surface roughness and shear rate during 30°, 80 g fall-cone calibration, investigating a standardised, smooth, polished cone (arithmetical mean roughness (Ra) value of 0·8 μm, i.e. compliant with ISO 17892-6 (CEN, 2017)) and two rougher cones with Ra values of 1·5 and 4·7 μm. The authors report that, for the investigated tailing, kaolin and sediment soils, the experimental result presented in Fig. 6 ‘seems counterintuitive and does not support a generally accepted concept … the idea of the roughness as an important influencing factor on the cone mechanics’ (Llano-Serna & Contreras, 2020; p. 338) from which they conclude that ‘for the scale of the fall-cone test, roughness changes within the scale of variation analysed here (Ra = 0·8–4·7 μm) do not affect the fall-cone penetration considerably’ (Llano-Serna & Contreras, 2020; p. 338). In this submission, the discusser makes two main contributions: (a) presentation of some experimental data in support of the above finding; (b) the significance of natural variation in the rate dependence parameter (μ) between different fine-grained soils on the determination of the cone factor, K.

UNIVERSITY OF ENGINEERING AND TECHNOLOGY PESHAWAR

The effectiveness of colloidal silica (CS) treatment in increasing the liquefaction resistance of sandy soils is by now amply demonstrated. However, the best value of the CS content to achieve high performance, minimising economic cost and impact on buildings and the environment, has not yet been quantified. This paper presents the results of a laboratory study aimed to evaluate the influence of different CS contents on the behaviour of a liquefiable sand. The investigation included direct shear, cyclic triaxial, hydraulic conductivity and oedometer tests. CS contents 0, 2 and 5% (by weight) were used for all tests, except for direct shear tests (CS contents 0 and 2%) and oedometer tests (CS contents 0, 2, 3, 5, 10 and 13%). The test results showed that 2% CS content was enough to increase the soil strength under cyclic and monotonic loading conditions. The hydraulic conductivity of treated soil decreased significantly as CS content increased. Oedometer tests pointed out that the main disadvantage of CS treatment is the soil compressibility increase. On the basis of the obtained results, 2% CS content seems to be recommended because it shows effectiveness and capability to improve the liquefaction resistance of sand while minimising the soil compressibility increase.

- by Giovanni Ciardi and +1
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- Geotechnical Engineering, Liquefaction, Soil Dynamics, Earthquake Geotechnical Engineering

ABSTRACT The results of a series of large-scale triaxial tests performed on dense, prismatic gravel specimens, with a height of 50 cm and a cross-section of 23 cm×23 cm, are described. The specimens were prepared at a density equal to approximately 95% of the maximum density at the optimum moisture content. Deformations were measured locally using vertical and horizontal local deformation transducers. Stress conditions with selected levels of very low confining pressure were used to simulate specific conditions in the case of road and railway embankments. Particular attention was paid to the bedding error at the top and the bottom ends of the specimens, and to fixing transducers onto the membrane to be used under low confining pressure. The confining pressure was applied by vacuum and varied from 10 kPa to 75 kPa. Unsaturated specimens were tested under drained triaxial compression using monotonic and cyclic loading with frequencies in the range of 0.5–5 Hz. The effects of a large number of load cycles and of specimen preloading were investigated.

- by Stanislav Lenart
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- Civil Engineering, Geology, Materials Science, Geotechnical Engineering

In high suction tensiometers, cavitation occurs when the tension in the water inside the device exceeds a certain threshold. The threshold corresponds to the formation of air bubbles either by air entry into or formation within the porous stone or reservoir. The standard procedure to inhibit air bubble formation in
the device is to flood under vacuum and apply water at high pressure to force any remaining air bubbles to dissolve. A series of factors are, however, likely to influence the cavitation behaviour of high suction tensiometers. This paper demonstrates that: (1) there are no precursory phenomena leading to cavitation, (2) higher pressures are not essential for saturation, (3) cooler temperatures and tensiometer usage decrease the pore water pressure at cavitation, and (4) high suction tensiometers record increasing pressures when plunged dry in free water. The findings provide useful guidance to researchers using these devices for suction measurement.

- by Domenico Gallipoli
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- Civil Engineering, Unsaturated soil, Geotechnical Engineering, Soil Mechanics

This study was carried out to investigate the undrained behaviour of weakly bonded soil. Artificial specimens were used and were prepared from a mixture of sand and kaolin. The mixture was fired at 500°C for 5 h to create uniform strength of artificial weakly bonding between sand particles. Based on microscopic observation of the specimens revealed that the fired kaolin formed bridges between the sand particles and most of the sand particles were entirely coated by kaolin. It was observed that very few sand grains made a direct grain to grain contact. The occurrences of inter-particle voids either isolated or connected to form larger voids. Fissures found in between the bonding and the grains were due to high temperature firing. A series of consolidated undrained tests (CU) were performed using conventional triaxial compression test. The stress ratio curves for the bonded specimens showed a significant peak at early strain and the values of maximum stress ratios dropped with increasing applied confining stresses. For the destructured (or unbounded) specimens, no apparent peaks were observed and the stress values were slightly lower compared to that of bonded specimens. The bounding surface for the destructured specimens is represented by a straight line equivalent to stress ratio of 1.24. This line is equivalent to strength parameters of internal friction angle, φ’ and cohesion intercept, c’ of 31° and 21 kPa, respectively. Meanwhile, the bounding surface for the bonded specimens is indicated by some curvature at low stress (p′<150 kPa) and high stresses (p′>1100 kPa). The study clearly suggested the role of bonding on the engineering behaviour of residual soils in undrained conditions.

This paper presents the development of an automated tensiometer based suction control system for testing unsaturated soil samples. The system is able to dry and wet soil, while measuring suction (pore water pressure – air pressure) and water content. The system uses air circulation within a closed loop to dry the soil, or water injection to wet the soil, to achieve the required pore water pressure while the air pressure is kept atmospheric. Pore water pressure is controlled by using a feedback computer system that dries or wets soil samples according to measurements obtained from sample-mounted high suction tensiometers. Excess moisture in the air circulation loop is captured by an in-line moisture trap consisting of a sealed cell containing a desiccant (silica gel), which is placed on an electronic balance to give continuous measurements of retained water. Changes of the sample water content are measured as the difference between the amount of water injected and that retained by the moisture trap. The system is fully automated and runs controlled by software with minimum assistance. The proposed suction control system presents advantages over the conventional axis translation technique as it avoids the need for elevated pore air pressures and hence better replicates the natural processes of wetting and drying of soils. The system was developed for use in a triaxial cell, but could also be used with other instruments and for the determination of water retention curves.