Evaluation of existing CPT correlations in silt
A. S. Bradshaw, A. C. Morales-Velez and C.D.P. Baxter
ABSTRACT: This paper evaluates the applicability of existing Cone Penetration Test (CPT) soil type and soil properties correlations in two different deposits of uniformly graded non-plastic silt. Current CPT correlations for soil engineering properties are based largely on experience in either sands that are typically drained during penetration, or clays that are typically undrained. Silts may exhibit partially drained conditions during penetration that introduces uncertainty when applying correlations from the literature. The assessment is based on an analysis of existing CPT data collected at two study sites in Rhode Island, U.S.A. that are underlain by thick deposits of non-plastic silt. Existing CPT correlations were used to predict the soil type and selected geotechnical properties of the silt, which were compared to laboratory test results to evaluate the quality of the predictions. The silts in this study exhibited partially drained to drained behaviour during cone penetration. Existing CPT soil classification charts were ineffective in identifying the silt but correctly characterized its engineering behaviours. Existing CPT correlations accurately predicted the friction angle, shear wave velocity, and cyclic resistance of the silts investigated in this study.
Characterisation of quick clay at Dragvoll, Trondheim, Norway
A. Emdal, M. Long, A. Bihs, A. Gylland and N. Boylan
ABSTRACT: A detailed characterisation of the quick clay underlying the NTNU research site at Dragvoll, Trondheim is presented. The objective of the work is to provide guidance on quick clay parameters to engineers and researchers working with similar clays in Scandinavia and North America. Dragvoll clay is characterised by its high sensitivity and is quick at relatively shallow depth. The material exhibits low undrained shear strength and high compressibility except over a shallow overconsolidated zone. Its properties are similar to other quick clays in the area and are consistent with well know correlations for Norwegian clays. A combination of simple index tests such as water content and Atterberg limits and CPTU testing proved very useful in characterising the material. The material is very sensitive to sampling and laboratory handling. Results of standard and non standard tests, such as piezoball testing, are presented.
Field response of push-in earth pressure cells for instrumentation and site characterization of soils
Alan J. Lutenegger
ABSTRACT: The use of Push-In Earth Pressure Cells in Geotechnical Engineering is described. Applications for both instrumentation and site characterization are discussed. Several examples of the field response of Push-In Earth Pressure Cells in soils are shown to illustrate their behaviour. These instruments can provide a reasonably simple, economical and reliable tool for a wide range of applications in geoconstruction and site characterization and should be considered by engineers more frequently. A discussion of the interrelationships between the Initial Lateral Stress Ratio and the Reconsolidation Lateral Stress Ratio suggests that in fine-grained soils these values should be related to the stress history of the soil through the overconsolidation ratio.
Frequent-interval SDMT and continuous SCPTu for detailed shear wave velocity profiling in soils
T. Ku and P.W. Mayne
ABSTRACT: Two new approaches to downhole shear wave velocity (Vs) measurements are presented, including frequent-interval method by seismic flat dilatometer (FiSDMT) and continuous-interval method by seismic piezocone testing (CiSCPTu). A recently-awarded patent for a roto-autoseis source assists in both methods by generation of fast and repeatable wavelets that are recorded by the probes during advancement. In the case of frequent-interval SDMT, either pseudo-interval or true-interval Vs data are procured at the same depth intervals of 0.2-m as the normal lift-off pressure (p0) and expansion pressure (p1) and therefore is a slowest version of downhole testing. This offers the advantage of accurate and detailed small-strain stiffness measurements (i.e., Gmax) that can be useful in careful settlement calculations, pavement subgrade designs, and paleoliquefaction studies with shallow fine resolution requirements. In the continuous SCPTu, the autoseis generates wavelets as frequently as every 1 or 2 seconds, thus a fastest type of downhole testing. As there are considerable issues with signals that are complex because of refraction effects, variable penetration rates, noise, and vibration, special measures in processing are required in order to extract the Vs profile. The result offers continuous profiles of qt, fs, u2, and Vs with depth from a single sounding.
KEYWORDS: cone penetration, dilatometer, geophysics, in-situ testing, shear wave velocity, seismic flat dilatometer, seismic piezocone
In situ testing of peat – a review and update on recent developments
M. Long and N. Boylan
ABSTRACT: This paper reviews the techniques used and some recent developments on in situ testing of peat for the purposes of the design and surveillance of engineering structures. Geophysical techniques, especially ground penetrating radar, are now being used extensively in peatlands. All geotechnical in-situ tests in peat can be influenced by partial drainage and therefore can give misleading results if not used carefully to well established guidelines and if not interpreted correctly. There is therefore a benefit in multi-measurement tests (e.g. CPTU and piezoball) which give additional information to help assess the drainage condition. There seems promise in the use of the pore pressure measurements for both CPTU and piezoball for the purposes of profiling peat decomposition and possibly shear strength assessment. Field vane testing will frequently give misleading results. Other standard geotechnical techniques may only be useful when used in conjunction with locally derived empirical correlations. Pore pressure measurements in peat may be influenced by the presence of gas in the deposits.
Understanding the stiffness of soils in Singapore from pressuremeter testing
K.H. Goh, K. Jeyatharan and D. Wen
ABSTRACT: The stiffness of soil is an important parameter that affects the prediction of ground deformation and impact on adjacent structures due to construction activities such as deep excavations and tunnelling. Whilst constitutive models and analytical methods have been derived to predict soil deformation from its stiffness, engineers face a difficult task of identifying soil stiffness from routine site investigations. This paper discusses the use of pressuremeter testing in site investigations to estimate the soil modulus for design.
In recent years, an intensive regime of pressuremeter testing was implemented along with conventional soil investigation works for the land transportation infrastructure construction in Singapore. These include the investigations in the Downtown Line project on Kallang Formation and the Old Alluvium soils, investigations in the North-South Expressway and Thomson Line projects on the Bukit Timah Granite Formation, and investigations in the Tuas West Extension project on the Jurong Formation. This paper reports on the use of various types of pressuremeter testing in Singapore – namely the Menard pressuremeter, the OYO pressuremeter and the self-boring pressuremeter – in terms of the practical experience and the interpretation of pressuremeter test results to understand the stiffness of local soils.
The paper begins by summarising the applications and limitations for various types of pressuremeters in in-situ testing, before discussing the lessons learned from using these pressuremeters in Singapore. Some of the improvements include relating the pressuremeter modulus to the corresponding strains from which they are derived, as well as developing guidance for operators on when to start the unloading cycle. Empirical relationships using SPT-N correlations would also be recommended based on the pressuremeter tests for the various local soils. Other than its elastic modulus, the small strain stiffness of soil has also been investigated to various extents depending on the type of pressuremeter test. These in-situ investigations will improve on the understanding of soil stiffness in Singapore.
Rate effect on cone penetration test in sand
F. A. B. Danziger and T. Lunne
ABSTRACT: A literature survey has provided quite variable results with respect to rate effect on cone penetration test(CPT) in sand. Most of the available data refer to the penetration rate in the range 2 mm/s – 20 mm/s, and show some rate effect. The analysis of the data shows that the factors controlling rate effect on CPT in sand are excess pore pressure generation (in the case of loose silty sands and loose fine sands) and grain crushing (especially in the case of dense sands). Excess pore pressure generation produces a reduction in cone resistance when the rate is increased from 2 mm/s to 20 mm/s, while the opposite occurs in the case of grain crushing. Since the stress level influences grain crushing, the higher the stress level the higher the rate effect. Moreover, the higher the crushability of the sand grains, the higher the rate effect. There is scarcity of tests at high rates. Few data available from tests with variable high rates indicate a significant rate effect.
In situ measurement of hydraulic conductivity of saturated soils
D.J. DeGroot, D.W. Ostendorf and A.I. Judge
ABSTRACT: The hydraulic conductivity of saturated soilsvaries significantly from approximately 10-13 m/s for high plasticity clays to 1 m/s for clean, uniformly graded, coarse gravels. This very large range in possible values has resulted in the developed of numerous fieldmethods that cater to the soil type being tested and the anticipated hydraulic conductivity. The most common in situ method used in practice is the slug test as performed in open standpipe piezometers. Other methods include in situ dissipation tests and large scale pumping tests. This paper describes these various in situ measurement and analysis options and presents results obtained for a variety of soils including clays, silts, sands and gravels. The examples highlight the major influence of soil type on measured hydraulic conductivity values and also show the secondary influence of soil fabric and scale effects.
KEYWORDS: hydraulic conductivity, in situ testing, slug tests, dissipation testing, pump tests
