|Session 3.2 - Liquefaction Mitigation|
It has been known that unsaturated sands have a significantly higher resistance to liquefaction than saturated sands. Liquefaction potential of saturated sand can be reduced if the degree of saturation of the sand is effectively lowered. A centrifuge model test was performed, in which air was injected into a saturated ground model through a vertically penetrated pile, to see how wide the air injected from the tip of the pipe spread in the originally saturated ground.
Keywords: degree of saturation, liquefaction resistance, sampling by ground freezing method, sand conpaction pile
W. Okada, S. Terzaghi, J.Q. Cooper, M.A. Patel and T.P. Adhikary
A case study of seismic design for a highway upgrade in pumice-rich soils is presented. Seismic design in such soils is difficult because of the unique characteristics and lack of existing test information for pumiceous soils. Furthermore it has been demonstrated that conventional measurements of in situ material properties do not adequately characterise the static and dynamic soil properties. Alternative means of determining the actual soil properties are discussed.
Numerical modelling was used to calculate the factor of safety, deformations and excess pore pressure generated during the design earthquake for embankments subjected to seismic loading. The numerical model output is compared with predictions based on Makdisi and Seed’s simplified procedure. Liquefaction potential analysis via conventional methods is also presented, with checks via numerical analysis.
Loadman portable falling weight deflectometer, in situ CBR test, plate bearing test and nuclear densometer proved to be reliable in situ testing for the pumiceous sands while various other tests were found to be unreliable. Particle size analysis and Cone Penetrometer Tests (CPT’s) do not adequately determine the liquefaction potential for a pumice sand, which is strongly dependent on relative density. A rigorous finite element analysis may provide an alternative to laboratory testing. Makdisi and Seed’s simple procedure for predicting permanent deformation of an embankment proved to be applicable to the pumiceous sands.
Keywords: pumiceous sands, in-situ testing, seismic design, embankments, liquefaction, permanent deformation
Liquefaction and cyclic softening of fine-grained soils occurred in Adapazari during the 1999 Kocaeli, Turkey earthquake. The results of a large number of cyclic triaxial tests performed on "undisturbed" specimens of silty and clayey soils obtained from throughout the city are discussed. The tests show that the current state-of-the-art method for the determination of liquefaction susceptibility of fine-grained soils, the Chinese criteria method, is not reliable. Soils that liquefied often met the liquid limit and liquidity index conditions of the Chinese criteria, but did not typically meet the clay-size condition. Test results do not show a marked effect of soil plasticity on cyclic strength of soils with (N1)60 < 10 when PI < 12. However, soils with PI > 20 tested at effective confining stresses < 50 kPa did not generate significant strain after the application of a large number of loading cycles.
Keywords: silt, liquefaction, Adapazari, Chinese criteria
Soil liquefaction is one of the major factors affecting the behaviour of piles founded in seismically active areas. Although methods are available for seismic analysis of pile foundations, in many of them the soil is assumed to be an elastic material. Here a numerical model is presented which takes into account the reduction of soil stiffness and strength due to pore pressure generation and subsequent soil liquefaction, in addition to the material non-linearity. Results obtained from the new method are compared with centrifuge test data and they show excellent agreement with the observed pile behaviour. To investigate effects of soil liquefaction on the internal pile response, a parametric study is carried out for a range of material and geometric properties of the pile and soil. The effect of the nature of the earthquake on pile performance has been studied using 25 earthquake records scaled to different acceleration levels. It is found that the ‘pseudo velocity’ of the earthquake, which is the gross area under the input acceleration record, has a significant influence on the pile performance in liquefying soil.
Keywords: Pile foundations, soil liquefaction, seismic analysis, centrifuge test data, Mindlin's equation