CONSOLIDATION CHARACTERISTICS OF A SILTY CLAY: CONCERNING THE EFFECT OF SOIL DISTURBANCE

In this paper, undisturbed specimen of a silty clay constituting of Phu Bai formation (ambQ21-2 pb) was collected from boreholes in Hue city and surrounding areas. The soil, under both undisturbed and disturbed conditions, was then subjected to standard one-dimensional consolidation tests with 7 loading increments. It is shown from the experimental results that the time to the end of primary consolidation (EOP), determined by Log Time method (tLT) and 3-t method (t3T), decreases with the load increment and under the same vertical stress, the primary consolidation of disturbed silty clay finish at a shorter time than those of the undisturbed one. The coefficient of secondary consolidation (Cα) increases with the vertical stress and reaches the maximum values before decreasing. The obtained values of Cα = 0.005 - 0.020 suggest a relatively low secondary compressibility of the silty clay constituting of Phu Bai formation.

Influence of Structure and Liquid Limit on the Secondary Compressibility of Soft Soils

The macroscopic mechanical properties of natural sedimentary soft soils, which are usually linked to their microstructure, are different from those of remolded soils. The interaction between soil structure and mechanical behavior is a manifestation of structural mechanics effects. It is essential to understand the effects of secondary compressibility to predict long-term foundation deformations. The effects of soil composition on secondary compression deformation are little studied, and the soil structure is rarely involved in the compression process. The sedimentary environment creates the initial composition and structure of soft soil, and it also basically determines its grain size and mineral composition, while different depths give soft soil different overburden pressures, and the soil composition and depth directly affect its yield stress during compression. So, natural sedimentary soft soils sampled at different depths and from different sedimentary environments (such as marine-neritic facies, sea shore facies and limnetic facies) were selected to study the influence of structure on the secondary compression coefficient Cα during pressure change and the relationship between soil composition and Cα. One-dimensional compression and consolidation creep tests were carried out on undisturbed and remolded samples. The undisturbed samples were obtained by the thin-wall samplers in rotary wash borings, and the quality of the samples met the test standard. Based on the concept of the void index Iv and the intrinsic compression line (ICL) proposed by Burland, the role of structure in the compression process was studied, and the influence of soil composition and structure on secondary compression characteristics was summarized. The Cα/Cc values are 0.031, 0.034, 0.030, and 0.036 for Shanghai, Tianjin, Suzhou, and Ningbo soft soils, respectively, within the range of inorganic clays and silts (0.04 ± 0.01) given by Mesri. According to the compression index Cc obtained by compression test, Cα/Cc can be used to estimate Cα. The yield stress of normal consolidated soil is near pre-consolidation pressure, while that of structural soft soil is greater than its pre-consolidation pressure. Natural sedimentary soft soils show over-consolidation characteristics due to the action of the structure; the soil structure resists the external load and hinders secondary compression. When the soil structure is almost destroyed, the pressure reaches the structure full yield stress P′. The tests of structural soft soils show that Cα changes with pressure before the structure completely yields, first increasing and reaching peak Cαmax near P′; the value of P′ is approximately 1.6–3.0 σ’k, where σ’k refers to the structure yield stress of soil obtained by the Casagrande method. After the structure disappeared, Cα gradually decreased and then stabilized, which is considered to be independent of the load. The Cαmax is positively correlated with the liquid limit, indicating that the peak value that can be reached by the Cα is related to the maximum content of bound water in soft soil, thus the soil composition has a significant influence on secondary compressibility, which contributes to the prediction of long-term foundation deformation.

Moisture Content Impact on Mechanical Properties of Selected Cohesive Soils from the Wielkopolskie Voivodeship Southern Part

Results of investigations of shearing resistance and compressibility of fine-grained cohesive soil from the southern part of the wielkopolskie voivodeship in relation to the increasing moisture content are presented. The analysis of two series of samples, using soil paste for the consistency index of 0.9 and 0.4–0.3 was carried out. The results imply that the increasing moisture content causes a decrease in the angle of shearing resistance and cohesion and is also reflected in the higher compressibility of the soil. It was observed that regardless of the soil consistency, the angle of shearing resistance decreases and the cohesion value and the oedometric modulus of primary (consolidation) and secondary compressibility grows with the increase in the clay fraction.

Estimation of total settlement of embankments by field measurements

In the absence of compressibility data from laboratory tests, the total primary consolidation settlement of a structure founded on clay can be estimated from settlement measurements taken over a period of time. In this note, the hyperbolic and Asaoka methods are applied to a case history where an embankment fill was placed on a clay exhibiting secondary compression. It is shown that significant inaccuracies in time–settlement predictions can result if an appropriate time factor versus degree of consolidation relationship is not taken into account. Key words: primary and secondary compressibility, coefficient of consolidation, field measurements, hyperbolic method, Asaoka method.

Settlement analysis of the Gloucester test fill

This paper reports the observation and analysis of the rate and magnitude of settlement under the test embankment at Canadian Forces Station Gloucester. The embankment has been in existence for 7 years and, because of extensive instrumentation, a sufficiently complete record has emerged. An advanced finite element method has been used for the computation of the induced stresses in the foundation soil.Both block samples and 5 in. (12.7 cm) diameter Osterberg samples were recovered at various depths from the site. An experimental program, including the use of the 6 in. (15.2 cm) Rowe cell, has been carried out and an analysis of the test results based on the Gibson and Lo theory. The test results have also been used in the estimation of the field performance.From the present study it is found that: the coefficient of consolidation and the primary and secondary compressibility can be adequately determined from samples of size 4.5 in. (11.3 cm) diameter by 2 in. (5.1 cm) high or larger; the secondary compression contributes significantly to the total settlement for the soil considered; and the Gibson and Lo theory predicts fairly accurately both the time rate and magnitude of settlement in the field.