Long-term settlement of reclamations constructed over soft soils may be reduced by the use of temporary pre-loading, although there is often uncertainty over how much surcharge to use, and how long it should be maintained. An elastic visco-plastic constitutive model is described and its application in assessing surcharging strategy for reclamation schemes is discussed. The model is incorporated in a one-dimensional finite difference consolidation analysis with vertical and radial drainage of a multi-layer soil profile. The analysis allows the inclusion of vertical drain resistance, a zone of peripheral smear around the drain, permeabilities that vary with void ratio, and creep both during and after primary consolidation. Drawing on data from the Bothkennar soft clay research site, the model is used to predict settlements beneath a hypothetical test fill, including the creep settlement behaviour when a surcharge is removed after different time periods. The behaviour during primary consolidation is back-analysed using Asaoka's and the hyperbolic observational methods, and both are found to under-predict the magnitude of the settlement.

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In order to be able to design the electro-osmosis scheme, two major properties of clay, electro-osmosis permeability and coefficient of consolidation, need to be known. These properties can be measured in the laboratory by running the electro-osmosis consolidation test. This type of test is carried out with the help of modified triaxial apparatus in which the soil sample has an electric current applied, and its volumetric change accurately measured. It was found that the electro-osmosis permeability (k_e) of Singapore marine clay ranges between 10^-8 and 10^-9m/v-s, whereas the electroosmosis coefficient of consolidation (C_ve) varied between 0.17 and 2.81m²/yr for tests under an electric potential gradient between 2 to 12 volts ; c_ve increased with voltage applied. Physical and compressibility properties changes were noted after the electro-osmosis process. Gain in effective stress due to electro-osmosis was significant and magnitude of effective stress gain was directly proportional to the voltage applied. However vertical displacement was found to be smaller and significant magnitude of volume change was contributed from lateral deformation. This paper describes the test apparatus, and procedures and discusses the results from electro-osmosis tests, carried out on Singapore Marine Clay.

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Kinking deformation by buckling of a PVD (Prefabricated Vertical Drain) due to consolidation settlement of the clay layer around it is discussed in this paper. A new testing device is manufactured for the purpose, and the deformation properties of two-kinds of PVD during consolidation of the surrounding clay are compared. It is concluded that a fibredrain made only of natural fibres can sustain the vertical permeability during a settlement strain up to 24%, while a PVD made of plastic loses its permeability before 19% strain. The field performance of both materials is also referred to.

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The in-situ deep mixing technique has been established as a means to effect columnar inclusions into soft ground to enhance the bearing capacity and reduce settlement. Since the inception of this method, developments in the plant and machinery, as well as associated field techniques, have surpassed the basic understanding of strength developments in high water-content clays admixed with cementing agents. In this paper an attempt is made to identify the critical factors governing the engineering behavior of cement-stabilized clay, which helps not only to control the input of cementing agent to attain strength development with curing time and clay water content, but also to understand the subsequent engineering behavior. It is revealed that the clay-water / cement ratio, wc / c is the prime parameter for the above purposes.

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This paper describes a geotechnical survey evaluating the shear strength of Pleistocene clay in Osaka Bay. In the stability evaluation for a seawall structure of Kansai International Airport, as the slip circle giving the minimum safety factor possibly passes through the Pleistocene clay layer, the stability could be greatly affected by the shear strength profile of the Pleistocene clay. The result of the stability analysis is very sensitively affected by the undrained shear strength profile, which is roughly estimated based on a ratio of undrained shear strength to consolidation yield stress (s_u / p_y) of natural clay deposit (Mesri, 1975 ; Tanaka and Tanaka, 1994) and the average apparent overconsolidation ratio (OCR=p_y / σ'_v0) at this site (Horie et al., 1984). In this study, in order to evaluate the undrained shear strength obtained by typical testing methods, UC (unconfined compression) test, UU, CU, K₀CU compression and extension triaxial tests and DS (direct shear) test were carried out for undisturbed samples of the Pleistocene clays collected for the second phase construction of Kansai International Airport. Mechanical behaviors and shear strength profiles obtained by each test were compared and discussed. The undrained shear strength and its scattering depend on the testing methods. The recompression method, in which a specimen is consolidated under the in-situ stress, gives many reliable test results. The UC and UU tests, in which a specimen is not consolidated, give very scattered and unreliable test results. From the comparison between undrained shear strengths for design determined by each test with correction factors, it is found that the strength of the DS test is smaller than those of K₀CU and CU tests. This fact is derived from the relatively larger strength anisotropy of the Pleistocene clay. The undrained shear strength increase ratio with depth Δs_u / Δz for design is obtained as 2.09kPa / m, considering strength anisotropy.

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Unconfined compression tests have been widely used in Japan for the purpose of determining undrained strengths of clay samples, but the unconfined compressive strengths are usually scattered even if specimens tested seem to have been subjected to the same stress history. This is due to the characteristic of the test itself, which is performed under unstable confining stress given by capillary pressure of the sample itself. In order to make clear the cause of scattering of measured strengths, three series of tests simulating the process from sampling to unconfined and triaxial compression tests on saturated soil samples are performed with three remolded and three undisturbed clays together with an undisturbed peat, and the influence of stress release and mechanical disturbance of the test specimen on the undrained strength is examined. Based on the test results, a practical method for estimating the in-situ undrained strength of soft soils from the results of a routine unconfined compression (UC) test together with suction measurement is proposed. From comparisons of the undrained strength profiles estimated by the proposed method with the strength profiles obtained by in-situ sounding test, it is found that the strength obtained by UC test can be reasonably corrected to estimate the in-situ undrained strength of soft soil deposits.

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When pump-dredged clays are discharged into a pond, suspended soil particles settle loosely to create sediments, which subsequently consolidate due to their own weight over a long period of time. In planning the capacity of such a disposal pond, it is important to estimate the time dependent volume change of dredged clays during the reclamation process. As the land thus created is extremely soft, ground improvement work is necessary if there exists a further plan of utilising the land. Predicting the soil profile of reclaimed land is absolutely necessary for designing the ground improvement. In this paper the authors i) outline the 18-year reclamation history of the Kanda Disposal Pond, ii) introduce a program to analyse self-weight consolidation under fully submerged condition, iii) modify the program to represent the suction effect due to ground water lowering, and iv) verify the applicability of the modified program by comparing the calculated results with the measured data. A simple analysis under fully submerged condition cannot explain the record of an 18-year reclamation, even if various combinations of consolidation parameters are used. The code modified to include the suction effect successfully simulates the actual behaviour when the appropriate parameters concerning the suction are chosen.

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Effects of creep as well as initial water content at deposition on the pseudo-elastic shear modulus, G_max, of finegrained soil are studied in a consolidometer equipped with bender elements. The state boundary (SB) concept is discussed in void ratio (e)-G_max-Effective Vertical Stress (σ'_v) space and the condition of metastability associated with short-term creep and long-term natural creep are exemplified from laboratory creep tests and in-situ soil test results, respectively. In addition to the conventional laboratory reconstitution method adopted for almost all clay samples that use an initial water content of twice the liquid limit, the soil samples from Minato-Mirai (MM) site were also prepared by directly consolidating the slurry with a water content ranging from 1.5 to 5.0 times the liquid limit. The larger primary metastable region or higher degree of on-depositional structuration associated with the samples at higher initial void ratio is confirmed. However, no appreciable difference was found in the increase in G_max with time, represented by N_G, between the samples prepared by the two different methods. In addition to the highly plastic MM clay, the study of N_G also covered other clayey soils of different origin and plasticity index range from 29 to 78. Metastability index, MI (G_max)^e that manifested the aspect of structuration and destructuration reaches a maximum value at the end of sustained loading and vanishes slowly with any increase in stress level. By using liquidity index instead of void ratio in the e-log G_max plot, the metastability index, MI (G_max)^LI, is found to represent a wider variety of soils with minimum scatter. A slight stress level dependency of metastability increase was observed yielding smaller values at higher stresses. For the present test conditions and duration, subsequent stressing of 1.5-2.0 times the creep stress brought about the complete destructuration of the creep-added soil-structure formed in the previous creep step.

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Reclamation is a major construction activity in Singapore, and this often entails the construction of a vertical seawall to contain the reclamation. In Singapore a frequently used technique is the use of L-Blocks. However, there are concerns about the movement and tilt of such relatively light blocks. An ongoing reclamation in the northeast of Singapore makes use of nearly 5.6km of such wall. As a requirement, prior to commencement of construction, a finite element study was carried out to analyze the possible movement and tilting of the blocks as well as the long-term settlement of the seabed. Instrumentation clusters were then installed to provide valuable data to validate the accuracy of the design and also allow lessons to be learned from the difference between prediction and measurement. This paper is a report of the comparison between prediction and performance that was carried out at this site. As is the case in many actual constructions projects, the effect of certain activities unanticipated in the design caused the measured value to differ significantly from prediction. When these are factored out, it is evident that the finite element analysis can produce reasonable trend, though not the magnitude. However, the results thus far suggest that two simple steps can greatly improve performance, namely a more accurate soil profiling and accounting for the higher stiffness of stiffer strata due to non-linearity at small strain.

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This paper is part os a Special Lecture presented at the International Symposium on Coastal Geotechnical Engineering, IS Yokohama 2000, held in Yokohama in September 2000. It examines the different approaches that have been proposed for the evaluation of stability of embankments on soft clays. Stability of embankments constructed in one stage is well controlled. However, experience shows that engineers have to be careful when they want to apply in a given geological context an empirical approach that has been developed in a different one. For stage-constructed embankments, there are indications that the methods proposed up to now have a tendency to underestimate stability.

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Dredged soils treated with cement are used popularly as fillings in many reclamation projects. But the basic property of the strength uniformity has not been clarified for this artificial geomaterial, due to the fact that the strength is easy to obtain by increasing cement content. It is considered important to assess the strength variance properly when we intend to provide a recycled geomaterial with high quality at low cost. An empirical and simple correlation is proposed concerning compressive strength, cement content and water content. Based on the empirical relation, the variance property of compressive strength is interpreted, and some data actually obtained during construction are presented. Aiming to overcome the deficiencies of the conventional method by means of unconfined compression tests, a quality assessment is experimentally performed using the cone penetration test. In order to reduce the construction cost, special working ships have been introduced in practical engineering. The working ships are equipped with a cement treatment system, handling the soft dredging in a large quantity. Improved dredgings have already found applications in various marine construction projects. In particular, clayey dredgings usually show very small permeability, offering the possibility of reusing the dredgings in different seep-proof structures. Finally, difficulties are mentioned concerning the reuse of dredging from a practical viewpoint.

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A lightweight treated soil method has recently been developed to reuse dredged soils as artificial lightweight geomaterials, the density of which ranges from 1.0∼1.2g/cm³, for coastal construction projects. In the two types of lightweight soils, foam-treated soil (FTS) and bead-treated soil (BTS), the slurry of dredged soil is mixed with cement and air foam, or cement and EPS beads (the diameters are 1∼3mm), respectively. The lightweight treated soil method have been applied in several seaports and coastal airport projects, but only in the sites at a level shallower than -3m to avoid density change due to large water pressure and the possibility of washout during underwater placing. In this study, full scale field placing tests of lightweight soils were carried out at sites 10m below the sea level in Kumamoto Port with the purpose of investigating the material properties of lightweight soils placed under deep water. Especially the change of density through the process of mixing, transportation, placing and hardening was examined in detail. It was found that the lightweight soil method can be applied under sea water of -10m, although a part of the total volume of mixed foam of mixed EPS beads is swept away during the construction process. However the wet-density of lightweight soil measured 1 year after the construction was almost the same as that of a 28-day specimen, while the 1 year strength was 40% larger than the 28-day strength.

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