Geotechnical aspects of earthquake‐induced settlement of clay layer

Abstract

It has been observed that earthquake‐induced settlement depends on the excess pore water pressure accumulated during an earthquake. In particular, in the case where a clay layer is overconsolidated, excess pore water pressure is produced and settlement occurs by dissipation of the excess pore water pressure, which is very large in comparison with the coefficient of secondary compression. Therefore, if the settlement of clay ground induced by secondary compression becomes a serious problem, careful consideration of the earthquake‐induced settlement is needed. In this article, the settlement characteristics of a clay layer induced by cyclic shear are discussed, including the effects of loading period, the threshold shear strain below which no excess pore pressure or no settlement takes place, and the relationships between uniform shear strain cycles and irregular strain‐time histories. Then a calculation procedure for estimating the earthquake‐induced settlement is developed and applied to three soil profile cases, including the clay layers in Mexico City and Osaka Bay in Japan. In the case of a soil profile in Mexico City, settlements of about 0–3 cm are estimated and these values agree reasonably with the leveling results for the Mexico City earthquake of 1985. Furthermore, it is pointed out that the settlement induced by earthquakes is considerably affected by differences in the accelerograms.     

Soil exploration in a 500 m deep fjord,Western Norway

Abstract

A number of engineering organizations and individuals have contributed toward a comprehensive feasibility study made in 1973–1974 in connection with a submerged floating tunnel project in Norway. The tunnel is planned to cross the 500 m deep Eidfjord in Hardanger over a length of about 1.3 km. The main findings from geophysical explorations, subsoil sampling operations, and an extensive laboratory testing program on extracted soil samples are described here.

A main part of this paper is devoted to the study of a full‐scale field test with a gravity anchor block weighing 180 tonne. This study reports on the behavior of the block during launching from its sloping construction ground, the sinking operation, and the behavior of the block after it reached the bottom at 450 m depth. Observations of settlement and tilt are available, and a comparison is made between the observed and computed behavior. Broadly speaking, a fairly good agreement was found.     

Sedimentation properties and modeling of dredged soil

ABSTRACT

When dredged soil containing coarse soil is used for the construction of reclaimed ground that is in contact with the surface of seawater, there is a high possibility of the generation of nonuniformly reclaimed ground due to the segregation of fine-grained soil from coarse-grained soil. It is difficult to assume uniform properties of reclaimed ground because the properties are defined and formed by the spontaneously segregating sedimentation. Estimation of the soil’s volume change lacks accuracy if the properties of the reclaimed ground are assumed to be always uniform. Therefore, for pump-dredged reclamation, a predictive study and various experiments are required to estimate the physics and properties of the dredged soil sedimentation. Accordingly, this study demonstrates a modeling test to understand the characteristics of the sedimentary ground using the changing ratio of fraction of the sample passing through a 75-µm sieve. The effect of particle arrangement on hindered settling properties, sedimentation properties, the distribution of water content of sedimentary ground, and physical properties can be determined by the modeling test. The study also suggests the calculation method for the travel distance of the outlet and the volume of input soil based on the experimental results.     

Slope instability near the shelf break,Western Gulf of Alaska*

Abstract

The uppermost continental slope in the western Gulf of Alaska, from southern Albatross Bank to Portlock Bank, includes two broad areas where large submarine landslides occur and one intervening area where they are absent. In the areas containing large slides, seismic reflection records show evidence for active nearsurface folding and consequent slope steepening, which is apparently the ultimate control on this sliding. Evidence is lacking for similar active steepening in the area containing no large slides, where slope gradients are relatively gentle. Relatively small, shallow slides, fundamentally different from the larger ones, occur in all three areas on slopes that are not necessarily actively steepening. These slides are probably stratigraphically controlled, with failure occurring along weak subsurface strata. Strong earthquakes and the related accelerations are probably responsible for the actual triggering of many of the large and small slides. As long as the tectonic setting remains as it is today, future large‐scale sliding should remain confined to the two broad areas in which it now exists. Relatively small‐scale and shallow sliding might occur in any of the three areas.     

Probabilistic analysis for wave‐induced submarine landslides

Abstract

When waves propagate over the ocean floor, they induce a change of hydrodynamic pressure, positive under the crest and negative under the trough. These pressure changes may cause shear failure in soft sediments and lead to submarine landslides. This paper presents a general analytical procedure for evaluating the probability of wave‐induced failure in offshore clay sediments. Both the wave and the un‐drained shear strength of clay sediments are considered random. Numerical results of some analyses are also presented.     

Global circulation from SEASAT altimeter data

Abstract

A set of time‐averaged sea surface heights at 1° intervals, derived from the adjusted SEASAT altimeter data, and the GEML2 gravity field are used to estimate the long‐wavelength stationary sea surface topography. In order to reduce the leakage of energy in the estimated sea surface topography, the GEML2 field is augmented by the Rapp81 gravity field to generate geoidal undulations with wavelengths consistent with the ones of sea surface heights. These undulations are subtracted from the sea surface heights, and the resulting differences are subjected to filtering in order to recover sea surface topography with minimum wavelengths of 6000 km and an estimated accuracy of 20–25 cm. These estimates agree well with oceanographic and other satellite‐derived results.

The direction of current flow can be computed on a global basis using the spherical harmonic expansion of sea surface topography. This is done not only for the SEASAT/GEML2 estimates, but also using the recent dynamic topography estimates of Levitus. The results of the two solutions are very similar and agree well with the major circulation features of the oceans.     

Use of swath data in realtime navigation by pattern recognition

Abstract

A new method is proposed with the aim of reducing dead reckoning error (DRE) during multibeam echosounding survey in deep ocean. Bathymetric data is used in this method to estimate DRE in position fixing. This method can be activated at any desired interval to check the DRE accumulation in addition to available external navigation systems. A pattern recognition algorithm is developed to quantify the shift in position of a selected bathymetric feature that has been observed already once. This difference is used to correct the position fixing and navigation data.     

Effects of lime treatment on the geotechnical properties of dredged mud

Abstract

The preloading method has been widely applied in land reclamation as a green and economical approach to ground improvement. To improve the consolidation behavior, the addition of lime generally improves soil performance. In this article, a modified oedometer equipped with a tactile pressure sensor and a bender–extender element is employed. The effects of lime treatment on the geotechnical properties of dredged mud are investigated; these properties include the coefficient of the earth pressure at rest (K₀), settlement, drainage behavior, and evolution of the elastic wave velocity. The results reveal that the addition of lime not only influences the specimen’s deformation behavior but also influences its shear strength during and after loading. Combined with the evolution of the elastic wave velocity, the microstructural evolution of the specimen could be described and explained. It is concluded that a compressional wave is more sensitive to the change in the specimen’s void ratio and saturation. For K₀, initially (<25?kPa), the higher the lime content is, the greater the K₀ fluctuation is. Afterwards, the higher the lime content is, the smaller the K₀ fluctuation is, and finally K₀ slowly approaches approximately 0.2.     

Mechanical properties of calcareous silts in a hydraulic fill island-reef

Abstract

The mechanical characteristics of calcareous silt interlayers play an important role in the stability of island-reef foundations. Direct shear and consolidation tests were performed to study the relationship between the mechanical properties and the physical parameters of calcareous silt. Based on the consolidation test results and analysis of the settling examples, different calculation methods for soil settling were compared. The results show the following. (1) The relationship between the cohesion and water content of calcareous silt can be represented by an M-shaped curve. The water contents corresponding to the two peaks of the M-type curve increase with increasing dry density. (2) When the dry density is less than 1.33?g/cm³, increasing the density significantly improves the internal friction angle of calcareous silts. When the dry density of the calcareous silt is greater than 1.33?g/cm³, the internal friction angle is affected by both the dry density and the water content. (3) The shear strength decreases when the water content exceeds the optimum level. (4) The compressive modulus of calcareous silt is larger than that of terrigenous silt. Specifically, it decreases with decreasing dry density and increasing water content. (5) The stepwise loading method should be used to estimate the soil settling before fill engineering construction.     

On the response of a Coulomb‐damped poroelastic bed to water waves

Abstract

The problem of forced vibration of a slightly inelastic porous bed by water waves is treated analytically on the basis of a linearized expression of the nonlinear damping term for the grain‐to‐grain friction in bed soils and the linear theory by Biot (1962a [Jour. Appl. Physics, 33:1482–1498]) on the elastic wave propagation in porous media. A dispersion relation of water waves is obtained as a function of wave frequency, water depth, permeability, Poisson's ratio, rigidity, and specific loss of bed soil. Three types of elastic waves are induced in a bed by water waves: a shear wave and a compressional wave in the skeletal frame of soil, and a compressional wave in the pore fluid. The compressional wave, due to the motion of the pore fluid relative to the skeletal frame of soil, is highly damped by the viscosity of pore fluid and only a short range effect near the boundaries of discontinuity, such as a sea‐seabed interface. The seabed response to water waves is characterized by the two Mach numbers, i.e., the ratio of water‐wave speed to shear‐wave speed in soil and the ratio of water‐wave speed to compressional‐wave speed in soil. Most of the water‐wave propagation problems fall into the subsonic flow condition, where elastic waves in the bed travel faster than water waves.

For sandy beds, generally the speeds of compressional and shear waves are much higher than the phase velocity of the water wave. For this case, the solution of the Coulomb‐damped poroelastic bed response presented in this paper approaches the solution of the massless poroelastic bed response in Yamamoto et al. (1978 [Jour. Fluid Mech., 87(1): 193–206]). The damping of water waves due to internal grain‐to‐grain friction is equally or more significant than the damping due to percolation in sand beds.

For clay beds, the speed of the shear wave in soil becomes low and comparable to the phase speed of the water wave. The bed motion for this case is considerably amplified due to the near‐resonance vibration of shear mode of bed vibration. The water wavelength on a clay bed is significantly shortened compared to the water wavelength over a rigid bed. The water wave damping due to internal grain‐to‐grain friction in soil becomes much larger compared to the water wave damping due to percolation in clay beds. Long water waves over a soft clayey bed attenuate within several wavelengths of travel distance.     

A new look at the phenomenon of offshore pile plugging

Abstract

Open‐pipe piles are widely used for offshore structures. During the initial stage of installation, soil enters the pile at a rate equal to the pile penetration. As penetration continues, the inner soil cylinder may develop sufficient frictional resistance to prevent further soil intrusion, causing the pile to become plugged. The open‐ended pile then assumes the penetration characteristics of a closed‐ended pile. The mode of pile penetration significantly alters the soil‐pile interaction during and after installation. This affects the ultimate static bearing capacity (mainly in granular materials), the time‐dependent pile capacity (in clays), and the dynamic behavior and analysis of the piles.

Following a summary demonstrating the effects of pile plugging, a review of the common view of offshore pile plugging is undertaken. The interpretation of plugging by referring to the average plug length has led to the erroneous conclusion that in most piles significant plugging action does not occur.

Establishment of an analogy between soil samplers and open‐ended piles enabled correct identification of plugging by referring to the incremental changes in plug length. Examination of case histories of plugging of offshore piles revealed that beyond a certain penetration depth‐to‐diameter ratio, most piles are plugged.     

Influence of cap size and strength on settlements of TDM-piled embankments over soft ground

Abstract

This study investigates the impact of pile cap size, soft layer thickness and pile strength on load transfer and settlement behaviors of embankments supported by floating and fixed T-shaped deep cement mixing piles and conventional DCM piles under volume control. Preliminary investigation is performed by a series of small-scale physical model tests. The results reveal that the differential settlement can be substantially reduced with an enlarging pile cap as a result of larger embankment load transferred to the piles. The extended numerical analysis results demonstrate that the pile efficacy is related to the individual pile bearing capacity, which, in turn, depends on the pile cap size. The soft layer thickness has an insignificant effect on differential settlement but a significant effect on average settlement, while the pile strength plays an important role in differential settlement only when the cap size is not very large. Shape factor of at least 3.0 is recommended to ensure the reduction in differential settlement and minimize the effect of the change in pile strength.     

Consolidation solution of soil around a permeable pipe pile

Abstract

In practice, how to quickly improve the bearing capacity of piles in a short time is of great significance. In view of this, a technique of setting grooves and installing PVDs (prefabricated vertical drains) at the surface of the pile is proposed in this investigation to accelerate the consolidation of the surrounding soil. A radial and circular consolidation model is established for permeable piles. The finite cosine transform, finite Weber transform, and discretization method are used to obtain a semi-analytical solution for the consolidation model with a mixed drainage boundary condition at the surface of the pile. The sensitivity of the consolidation process to the strip number and the width of PVDs is discussed. The results show that the technique of installing PVDs in piles could potentially improve the bearing capacity. When the area of drainage channels is fixed, the bearing capacity of permeable piles can be more effectively improved by using a higher strip number of PVDs and a smaller PVD width.     

Instrumentation of SEASWAB experiment

Abstract

From September 1975 to April 1976 offshore production Platform V in South Pass, Block 28 (East Bay, Louisiana), was instrumented to measure the effect of storm waves on the soft sediments typical of the Mississippi delta (in a project given the acronym SEASWAB). A portion of this project consisted of four identifiable units of instrumentation (see note): (1) an accelerometer package buried 1 m in the sediment to measure three‐dimensional sediment accelerations and an associated pressure transducer, which measured wave‐induced pressures; (2) an array of instruments that included a wave staff, electromagnetic current meter, and a pressure transducer to examine various relationships between wave properties; (3) a wave‐, current‐, and wind‐measuring station 3.35 km inshore of Platform V to determine the transformation of the waves as they moved over the sediments; and (4) a transponder buried in the mud, the position monitored so that long‐term mudflow could be measured. The direct measurement of seafloor oscillations required the unique instrumentation of the accelerometer system. Three Bruel and Kjaer 8306 accelerometers mounted at right angles to each other made possible the measurement of small oscillations (～0.01 m) at low frequencies (0.1–0.3 Hz). The acoustic method of measuring long‐term mudflow was subject to problems associated with sound propagation in shallow water. The range of the system was found to be 2.74 km, apparently independent of depth. Multiple returns received after single interrogations of the transponder decreased the accuracy of the system.     

The nature and engineering behavior of carbonate soils at Bombay High,India

Abstract

This paper presents the results of a laboratory investigation undertaken to study the nature of two submarine carbonate soils from Bombay High off the west coast of India, as well as to study the shear and plasticity behavior of their sand and silt‐clay fractions, respectively. Scanning electron micrographs reveal that the carbonate content in both soils is comprised primarily of nonskeletal particles of various types. X‐ray diffraction and infrared absorption analyses indicate that in one soil the carbonate fraction consists of calcite and aragonite minerals, whereas in the other soil dolomite is also present. The non‐carbonate fraction of both soils is comprised primarily of quartz and feldspar, and also some clay minerals. The nature of the carbonate fraction of the two soils indicates that they were formed by different depositional processes.

During drained triaxial shear the nonskeletal sand grains of both soils exhibit a lower degree of crushing when compared with that of the skeletal carbonate sands, and thus appear to be stronger foundation material.

Although the carbonate contents of the silt‐clay fractions of the two soils are similar, they exhibit markedly different plasticity characteristics . This is probably because of the microlevel cementation produced by carbonate material in one soil.

This study leads one to the conclusion that carbonate content alone should not be treated as a parameter which controls the engineering behavior of submarine soils; the nature and form of carbonate material must also be identified.     

Simplified methods to analyze geosynthetic mattress resting on deformable foundation soil

A special case of geosynthetic tubes is the geosynthetic mattress characterized by its larger width-to-height ratio and higher lateral-force-resisting capacity. When the geosynthetic mattresses are adopted for dike or breakwater construction in coastal engineering projects, they are often laid on deformable foundation soil. The induced ground settlement will influence the cross-sections and tensile forces of the geosynthetic mattresses. Two analytical methods, called the Winkler foundation method and the e–log p method, are proposed in this paper to analyze the geosynthetic mattresses resting on deformable foundation soil. For the Winkler foundation method, the subgrade soil is assumed as a Winkler elastic foundation represented by the modulus of subgrade reaction, K_f. For the e–log p method, the 1D consolidation relationship (e–log p curve) is adopted to describe the behavior of the subgrade soil within which the distribution of vertical surcharge is calculated using Boussinesq equation. The numerical analyses using FLAC are also conducted to evaluate the accuracy of the proposed methods. The differences between the analytical and numerical solutions are also discussed. It is found from this paper that the proposed e–log p method is more suitable than the Winkler foundation method for the calculation of the geosynthetic mattress resting on deformable foundation soil.     

Shaking table study on liquefaction behaviour of different saturated sands reinforced by stone columns

Abstract

Liquefaction is a phenomenon developed in loose and saturated layers of sands subjected to dynamic or seismic loading, and often leads to excessive settlement and subsequent failures in structures. Several methods have been proposed to improve soil resistance against liquefaction, among which use of stone columns is one of the most applicable methods. In this research, the effect of stone columns with different geometries and arrangements on the liquefaction behaviour of loose and very loose saturated sands subjected to vibration is investigated using shaking table. Results of the experiments show that when using stone columns in sand layers, the level of maximum settlement is significantly reduced. Further, the presence of stone columns significantly reduces pore water pressure ratio. This further indicates that stone columns have a positive effect and reasonable performance, even in relatively strong earthquakes, provided that the number and cross-section of the columns are sufficient. In addition, stone columns reduce the pore water pressure dissipation time. Moreover, by increasing cross-sectional area and the number of columns, both pore water pressure and settlement decrease. Stone columns in loose sand have a greater effect on the reduction of pore water pressure compared to that of very loose sand.     

Relevance of Bathymetry for Tsunami Run‐up Modeling

Abstract

Marine positioning is relevant for several aspects of tsunami research, observation, and prediction. These include accurate positioning of instruments on the ocean bottom for determining the deep‐water signature of the tsunami, seismic observational setups to measure the earthquake parameters, equipment to determine the tsunami characteristics during the propagation phase, and instruments to map the vertical uplift and subsidence that occurs during a dip‐slip earthquake.

In the accurate calculation of coastal tsunami run‐up through numerical models, accurate bathymetry is needed, not only near the coast (for tsunami run‐up) but also in the deep ocean (for tsunami generation and propagation). If the bathymetry is wrong in the source region, errors will accumulate and will render the numerical calculations inaccurate. Without correct and detailed run‐up values on the various coastlines, tsunami prediction for actual events will lead to false alarms and loss of public confidence.     

Steric height trends at Ocean Station Papa in the Northeast Pacific Ocean

Abstract

We estimate secular changes in steric sea level in the northeast Pacific Ocean using the 27‐year time series of monthly hydrographic observations for Station PAPA (50°N, 145°W). Linear trends based on the entire data record suggest that steric heights relative to 1000 db are increasing at a rate of 0.93 mm/yr and that 67% of this increase is due to thermosteric changes at depths below 100 m; the smaller halosteric contribution to the steric trend appears to be confined to the upper 100 m. A trend of 0(1 mm/yr) is consistent with estimates of sea level rise based on coastal tide gauge records. However, a critical examination of the results indicates that sea level changes of such small magnitude would be masked by the large (1–10 cm) interannual variability of open ocean steric height. This is verified by recalculation of trends using abridged versions of the data set. We conclude that our trend estimates are still open to question and that the present 27‐year time series is too short to permit accurate resolution of possible climate‐induced changes in global sea level.