Subsurface Characterization at Ground Failure Sites in Adapazari, Turkey

Ground failure in Adapazari, Turkey during the 1999 Kocaeli earthquake was severe. Hundreds of structures settled, slid, tilted, and collapsed due in part to liquefaction and ground softening. Ground failure was more severe adjacent to and under buildings. The soils that led to severe building damage were generally low plasticity silts. In this paper, the results of a comprehensive investigation of the soils of Adapazari, which included cone penetration test (CPT) profiles followed by borings with standard penetration tests (SPTs) and soil index tests, are presented. The effects of subsurface conditions on the occurrence of ground failure and its resulting effect on building performance are explored through representative case histories. CPT- and SPT-based liquefaction triggering procedures adequately identified soils that liquefied if the clay-size criterion of the Chinese criteria was disregarded. The CPT was able to identify thin seams of loose liquefiable silt, and the SPT (with retrieved samples) allowed for reliable evaluation of the liquefaction susceptibility of fine-grained soils. A well-documented database of in situ and index testing is now available for incorporating in future CPT- and SPT-based liquefaction triggering correlations.     

Practical Model of Frost Heave in Clay

Freezing behavior of clay differs from that of silt. This difference stems primarily from the low permeability or hydraulic conductivity of clay, and the higher water content of saturated clay. Freezing effects include simultaneous heave and consolidation. Six small physical model columns of clay were frozen: one at 1g; and five on a centrifuge at various scales and with corresponding accelerations, to bring self-weight stresses into similarity with a full scale column of clay 4?m in height. The experimental results demonstrated the importance of replicating the prototype stress conditions in a model. They demonstrated the importance of local water content on development of heave in clay, and the relative insensitivity of heave to location of the phreatic surface. Low permeability caused the clay to behave essentially as a closed system with regard to water flow. A simple analytical model was developed to explain observed soil response. Further research is recommended to provide more guidance in selecting input parameters.     

In Situ Measurement of Nonlinear Shear Modulus of Silty Soil

A new field test method to evaluate in situ nonlinear shear modulus of soils was developed. The method utilizes a drilled shaft as a cylindrical, axisymmetric source for shear loading of soil at depth. The applicability of the test method was studied by conducting small-scale, prototype experiments at a “calibration” field site in Austin, Texas. Numerous conventional in situ and laboratory measurements were performed to characterize the soil at the field site. The “small-scale” nature of the tests involved using a 381?mm (15?in.) diameter, 3.7?m (12?ft) long drilled shaft. Experimental results from this field study provided an opportunity to compare laboratory and field measurements of the G?log?γ and G/Gmax?log?γ curves. This comparison was used to investigate the accuracy of common procedures relating field and laboratory modulus reduction curves. Nonlinear modulus measurements were performed at depths of 1.8?to?2.1?m (6?to?7?ft) in a silt (ML). The field G/Gmax?log?γ curve for this soil at low confining pressures are in general agreement with the laboratory curve from an intact specimen as well as empirical curves.     

Shear Strength and Stiffness of Sands Containing Plastic or Nonplastic Fines

This paper presents the results of a systematic laboratory investigation on the static behavior of silica sand containing various amounts of either plastic or nonplastic fines. Specimens were reconstituted using a new technique suitable for element testing of homogeneous specimens of sands containing fines deposited in water (e.g., alluvial deposits, hydraulic fills, tailings dams, and offshore deposits). The fabric of sands containing fines was examined using the environmental scanning electron microscope (ESEM). Static, monotonic, isotropically consolidated, drained triaxial compression tests were performed to evaluate the stress-strain-volumetric response of these soils. Piezoceramic bender element instrumentation was developed and integrated into a conventional triaxial apparatus; shear-wave velocity measurements were made to evaluate the small-strain stiffness of the sands tested at various states. The intrinsic parameters that characterize critical state, dilatancy, and small-strain stiffness of clean, silty, and clayey sands were determined. All aspects of the mechanical behavior investigated in this study (e.g., stress-strain-volumetric response, shear strength, and small-strain stiffness) are affected by both the amount and plasticity of the fines present in the sand. Microstructural evaluation using the ESEM highlighted the importance of soil fabric on the overall soil response.     

Analysis of Factors Influencing Soil Classification Using Normalized Piezocone Tip Resistance and Pore Pressure Parameters

This paper discusses the development of a framework for classifying soil using normalized piezocone test (CPTU) data from the corrected tip resistance (qt) and penetration pore-water pressure at the shoulder (u2). Parametric studies for normalized cone tip resistance (Q = qcnet/σv0′) and normalized excess pressures (Δu2/σv0′) as a function of overconsolidation ratio (OCR = σvy′/σv0′) during undrained penetration are combined with piezocone data from clay sites, as well as results from relatively uniform thick deposits of sands, silts, and varietal clays from around the globe. The study focuses on separating the influence of yield stress ratio from that of partial consolidation on normalized CPTU parameters, which both tend to increase Q and decrease the pore pressure parameter (Bq = Δu2/qcnet). The resulting recommended classification chart is significantly different from existing charts, and implies that assessment of data in Q–Δu2/σv0′ space is superior to Q–Bq space when evaluating piezocone data for a range of soil types. Still, there are zones of overlap for silty soils and heavily overconsolidated clays, thus requiring that supplementary information to Q and Δu2/σv0′ be obtained in unfamiliar geologies, including variable rate penetration tests, dissipation tests, CPT friction ratio, or soil sampling.     

Predicting the Morphodynamic Response of Silt-Laden Rivers to Water and Sediment Release from Reservoirs: Lower Yellow River, China

The high sediment load of the Yellow River results in rapid infilling of its reservoirs when sediment is not regularly flushed. Simultaneously, the downstream reaches of the Yellow River experience extremely high siltation rates, which are reduced when sediment is retained in its reservoirs. To minimize siltation in the reservoirs and the downstream river bed, water and sediment are released from the reservoir in a controlled way through flushing experiments. In this paper, we analyze the effect of such a flushing event on the downstream river bed through data analysis and numerical modeling. Sedimentation may be minimized by relating the amount of sediment released from the reservoir to the sediment available for release through operational monitoring and by releasing relatively clear water after turbid water. Despite this flushing of sediment, the reservoir will eventually fill up, and more sediment released again into the lower Yellow River. The change in discharge magnitude and frequency brought about by the reservoir will then probably result in increased siltation rates in the lower Yellow River compared to the predam situation.     

Experimental Study on the Shearing Behavior of Saturated Silty Soils Based on Ring-Shear Tests

The shearing behavior of saturated silty soils has been examined extensively by performing undrained and partially drained (the upper drainage valve of the shear box was open during shearing) ring-shear tests on mixtures of a sandy silt with different loess contents. By performing tests at different initial void ratios, the shear behavior of these silty soils at different initial void ratios is presented and discussed. Undrained-shear-test results showed that the liquefaction phenomena in ring-shear tests were limited within the shear zone; for a given void ratio or interfine void ratio, both the peak and steady-state shear strengths decreased with increase of loess content. The partially drained shear tests revealed that a great reduction in the shear strength could result after the shear failure, due to the buildup of excess pore-water pressure within the shear zone; the magnitude of reduction in shear strength after failure was affected by the initial void ratio, the shear speed after failure, as well as the loess content in the sample. For a given void ratio or interfine void ratio, with increase of loess content, the drained peak shear strength became smaller, while the brittleness index became greater. It was also found that due to localized shearing, the permeability of the soil within the shear box after drained shearing could be three orders of magnitude smaller than before shearing.     

Maintaining Equity in Surface Irrigation Network Affected by Silt Deposition

Planning maintenance activities in surface irrigation systems is essential for optimal use of the annual credits. In many countries, the equity of the water distribution is largely affected by sediment deposition but budgets do not allow the performance of all the necessary maintenance works and priorities must be defined by the irrigation agencies. A methodology based on numerical modeling is developed and illustrated on a secondary network in South Pakistan. Improvements on the current desilting procedure are proposed, but it is shown as well that the system could be designed differently in order to preserve the equity longer.     

Installation of Suction Caissons in Sand with Silt Layers

Suction caissons are increasingly being used for offshore anchors because of their ease of installation. However, for soil profiles that have sand overlain by layers of silt, there are reasons to believe that it may not be possible to install suction caissons. This may happen because the low permeability of the silt will create a hydraulic blockage, and thus diminish the upward hydraulic gradient required to reduce the penetration resistance in the sand to enable caisson penetration. The tendency of the silt to be sucked up, and its effect on the underlying sand, are not clearly understood. Furthermore, the blockage by the silt layer may be less than if a clay layer is present. This is because scouring of the silt may occur, allowing an upward seepage gradient to develop, and hence causing a reduction in penetration resistance. A series of suction caisson installation tests have been conducted in a geotechnical centrifuge to provide data on the penetration resistance and mechanisms for soil profiles where silt layers are present, either on top, or interbedded between layers of sand. In these tests, the thickness of the silt layer and its depth below the sand surface have also been varied. It has been found that if silt overlies the sand, a larger suction force is required for installation than when installation occurs in homogeneous sand, but that this force is still much smaller than the soil resistance when the caisson is pushed into the ground. When a silt layer is present, the suction pressures are observed to follow a similar trend that is independent of the position and thickness of the silt layer. Scouring of silt was evident in the tests, and significant upward movement of the soil plug inside the caisson was observed in all soil profiles when silt layers were present. The plug height was seen to reduce after the suction was turned off, suggesting that the resulting heave was unstable.     

Evaluation of Cyclic Softening in Silts and Clays

Procedures are presented for evaluating the potential for cyclic softening (i.e., onset of significant strains or strength loss) in saturated silts and clays during earthquakes. The recommended procedures are applicable for fine-grained soils with sufficient plasticity that they would be characterized as behaving more fundamentally like clays in undrained monotonic or cyclic loading. The procedures are presented in a form that is similar to that used in semiempirical liquefaction procedures. Expressions are developed for a static shear stress correction factor and a magnitude scaling factor. Guidelines and empirical relations are presented for determining cyclic resistance ratios based on different approaches to characterizing fine-grained soil deposits. The potential consequences of cyclic softening, and the major variables affecting such consequences, are discussed. Application of these procedures is demonstrated through the analysis of the Carrefour Shopping Center case history from the 1999 Kocaeli earthquake. The proposed procedures, in conjunction with associated liquefaction susceptibility criteria, provide an improved means for distinguishing between the conditions that do and those that do not lead to ground deformations in fine-grained soil deposits during earthquakes.