Development of automatic funnel system for cyclic simple shear tests on sandy soils

Liquefaction is one of the most important topics in geotechnical earthquake engineering. Much research has been done and still is being done on the liquefaction of sandy soils under dynamic loads. In 1964, the Alaska Good Friday and the Niigata earthquakes in Japan, the damages caused by liquefaction were noticed and afterwards geotechnical engineers began serious work on the subject. When sands are considered it is not possible to take undisturbed samples (except very special and expensive methods such as ground freezing), hence specimens are deposited with different techniques in the laboratory and need to be saturated before they could be tested. The mentioned specimen preparation process requires quite challenging working period, besides it is very time consuming. Mistakes, either due to lack of experince and time, that are made through specimen preparation process could affect the test results considerably. Determination of the liquefaction resistance from dry sand specimens is explained in order to simplify the mentioned challenging specimen preparation process. Fully dry specimens of clean sand and silty sand, which were deposited by dry funnel deposition, were subjected to drained constant volume cyclic simple shear loading. Calculation of equivalent excess pore pressures for dry specimens is explained based on the constant volume control mechanism of the simple shear testing. Laboratory testing of clean, silty and/or clayey sands are still quite important for investigating dynamic soil behavior including liquefaction. Various laboratory research on geotechnical engineering requires reconsititution of sandy soil specimens in such a away that the influence of several factors including density, stress history, loading conditions, fines content, plasticity mineralogy and morphology of soil grains could be investigated in a controlled environment. Several different specimen preparation methods had been employed in the literature to reconsititute specimens in the laboratory, such as moist tamping, slurry deposition, water sedimentation and air pluviation. In addition to the mentioned methods, dry funnel deposition is also a commonly used specimen preparation technique for reconsitituting clean, silty and clayey sand specimens. Dry funnel deposition is one of the most commonly utilized specimen reconsitution methods for both triaxial and simple shear testing of sandy soils. The basic procedure of the method is simple, where the dry soil is deposited through a funnel, which is raised gently along the axis of symmetry of the specimen allowing the soil to gradually fill the space encapsulated by a split mold or stack of rings. The specimen can be saturated by CO2 flushing and de-aired water percolation. During funnel raising process, experimentalists could loose the control of the raising speed, vertical alignment or even could shake the funnel, which could influence the initial fabric and therefore the dynamic response of specimens. In this study, an automated funnel was designed and developed for an NGI type cyclic simple shear apparatus. The funnel has acontrol unit which allows various computer controlled funnel raising speed. The funnel has six extensions, each having 35 mm length. This study investigates the influence of funnel raising speed and the height of the funnel (by using extensions) on the relative density and the cyclic liquefaction resistance of a clean and silty sand (with %10 fines content). An equation is developed showing the ralationship between the funnel raising speed and the relative density of specimens. The influence of the automatization was discussed by comparing the results of a series of cyclic simple shear tests, in which the specimens were prepared by manual and automatic dry funnel deposition techniques. The experimental program of this study was done in the Soil Mechanics Laboratory at Civil Engineering Department of Yeditepe University. Two soils were used in the experimental program: a clean sand and a silty sand. The clean sand is named as Sile Sand 20/55 and classified as poorly graded sand (SP) in Unified Soil Classification System (USCS). The silty sand was obtained by thoroughly mixing Sile Sand 20/55 with a non-plastic silt on dry weight basis such that the resulting soil had 10% fines content. The non-plastic silt is called IZ silt. In order to investigate the cyclic liquefaction bahavior of sandy soils, an NGI type cyclic simple shear device was used. All specimens were consolidated to the initial vertical effective stress of 50 kPa (i.e. 'vc=50kPa) and stress-controlled cyclic loading was applied at cyclic stress ratio (CSR= 0.12, 0.1, 0.08) with a frequency of 0.1 Hz. Liquefaction was assumed to occur when the vertical effective stress on a specimen was reduced to zero (i.e. v=50kPa). Manual dry funnel deposition is the conventional technique, where the funnel is raised manually. In this study, a stopwatch was used during the manual deposition to record the durations of funnel raising periods in order to calculate the funnel raising speeds. Whereas, in automatic dry funnel deposition, the funnel is raised automatically with the help of a specially designed mechanism. The mechanism involves an aligment section to mount the funnel on the simple shear apparatus, a computer controlled motor, which raises the funnel along the vertical axis of specimen at a constant speed, and a control unit, which allows the experimentalist to choose the funnel raising speed depending on the type of the soil and target relative density value. The technical capacity of the motor used in the design allowed the funnel raising times of 15, 30, 45, 60, 75, 90, 105, 120, 135, 150 seconds for 4 cm stroke. The typical simple shear specimens in this study have a diameter of 64 mm and height of 20 mm. The lateral confinement was provided by aligning several Teflon coated rings around a conventional latex membrane. The fabricated split mold allowed the rings to be located and aligned inside the mold body before the deposition starts. Such an approach prevented the possible disturbance of specimens during the positioning and alignment of the rings. It was found that there is a logarithmic relationship between the funnel raising speed and relative density of the clean sand and silty sand specimens prepared either by the manual or the automatic dry funnel deposition techniques. The change of consolidated relative density of clean sand and silty sand specimens with various funnel raising speeds show that the relative density increases with the funnel raising speed. Different number of funnel extensions were studied to see the influence the of funnel height (pouring height of the dry soil) on the relative density. It was observed that as the number of the extensions on the funnel increased, the achieved relative density also increased. This is an interesting remark which shows that the deposition process in deed starts much earlier than the initial funnel movement, perhaps as soon as the experimentalist starts to deal with the soil. In other words, before any funnel movement, the height from which the dry soil is poured into the funnel could make a difference in the fabric of resulting specimens. Liquefaction resistance of the manual and the automatic dry funnel deposited specimens were compared. It was observed that the number of cycles to liquefaction increases with the relative density in an exponential manner, and trend curves for automatic and manual dry funnel deposited specimens coincide for both clean and silty sands.