LAMINAR BOX TEST

liquefaction

ABSTRACTFor studying dynamic soil Behaviour, laminar box tests

provide advantages over a conventional rigid box.

The ability to allow shear deformation of soil while providing sufficient surrounding confinement is a more realistic representation of the free-field boundary conditions and thus soil-structure interaction.

Despite the use of a laminar box in earthquake research, there are insufficient studies specifically aimed at determining the response of soil in the box.

, a laminar box was developed to examine the response of dry sand subjected to a ground motion recorded from the Christchurch earthquake sequence.

INTRODUCTION Since the two major earthquakes struck Christchurch in

September 2010 and February 2011, extensive investigations have been undertaken to evaluate the damage. The investigations revealed that significant soil deformations occurred throughout the CBD and induced numerous damage to structures. The damaged structures were not limited to residential housing but also to infrastructures founded on shallow foundations and pile foundations, including bridges. These observations confirmed the significance of soil deformation in inflicting structural damage. In addition, Qin et al. (2013) has confirmed that soil deformation can affect the performance of secondary structure.

Small scale laboratory tests are relatively accurate for predicting deformation of a homogeneous soil specimen. However, a small scale soil element is less preferable in earthquake research compared to a large scale soil specimen. Admittedly, a large scale homogeneous soil specimen used in a laminar box test provides a better representation of the effective confining stresses and boundary conditions in free-field soil. A laminar box is a flexible soil container that can be excited using base excitations to replicate earthquake actions on the soil specimen (Fig. 1). The ability to allow approximate constant shear deformation while providing sufficient confinement is a more realistic representation of the free-field boundary conditions

COMPONENTS OF THE CONTAINER

The 3 dimensional layout of the container has been depicted in the Figure. The container consists of the following main components:

Layers and ball bearings;

Base plate and the saturation and drainage system in the floor;

The upper and the side guides;

Internal membrane used as a cut-off and keeping the moving bearings away from dust.

ISOMETRIC SCHEMATIC VIEW OF THE DESIGNED LAMINAR CONTAINER, 1)

ONE OF THE FRAMES; 2) SIDE GUIDE COLUMN; 3) UPPER HORIZONTAL GUIDE

CROSS; 4) BASE PLATE; 5) ENTRANCE AND DRAINAGE PIPES; 6) ELASTIC

MEMBRANE.

FINAL DESIGN SPECIFICATIONS OF SMALL SCALE LAMINAR BOX

Module specification

Box-Internal Base Size 23” x 23”

Box-Height 24”

Box Weight (empty) 150 lbs.

Box-Max Soil Volume 8 ft.3

Support Shake table

Number of laminates 16(15 moving layers)

Laminate thickness 1.5”

Inter-laminate bearings Ball bearings surrounded with teflon

Maximum weight(box & soil) 1000 lbs

Shaking direction Full motion in the X,Y horizontal plane

Inter laminate displacement 25”

METHODOLOGY LAMINAR BOX

It consists of 12 layers of rectangular rigid Aluminiumframes. Aluminium was used because it is relatively light while it has sufficient stiffness to provide the confinement. Each laminar was stacked on top of each other separated by ball bearings to allow relative movement with minimum friction. A volume of soil 800 mm long by 800 mm wide by 700 mm high can be tested. The inner surface of the laminar box was sealed by a rubber membrane. Because of the weight limitation on the shake table, only a 380 mm depth of dry sand was considered in the study. The soil specimen in the laminar box was formed by raining the sand from a fixed height of 1 m above the base of the box. The maximum shear strain allowed in the specimen was approximately 17%.

DESIGN CONSIDERATIONS

A soil layer under a level ground surface is usually in a K0 condition, while, during an earthquake, the soil at different depths may move differently in the horizontal plane following the upward shear wave propagation. To provide such flexible but unyielding side boundaries as in the field, laminar simple shear boxes composed of layers of frames are commonly used in the tests. For horizontal two-dimensional earthquake shaking, every layer of the frames should be able to move freely in every direction, i.e., multi-directionally, in a horizontal X-Y plane that follows the movement of the soil in the container. This can be accomplished if the frames are allowed to move bi-axially in both X and Y axes simultaneously.

ASSEMBLY OF THE LAMINAR BOX

The laminar shear box developed at NCREE is composed of 15 layers of sliding frames as schematically shown in Fig. Each layer consists of two nested frames, an inner frame with inside dimensions of 1880 by 1880 mm and an outer frame with inside dimensions of 1940 mm by 2340 mm. Both frames are made of a special aluminum alloy specific gravity=2.70 with 30 mm in thickness and 80 mm in height, except the uppermost layer which has a height of 100 mm. The aluminum alloy is adopted for its sufficient strength and rigidity to provide unyielding boundaries, and for its light weight to minimize the effect of inertia of the frame on the soil movements. These 15 layers of frames are separately sup-ported on the surrounding rigid steel walls, one above the other, with a vertical gap of 20 mm between the adjacent layers. The 20 -mm gap is provided to avoid rupture of the rubber membrane in-side the box during a large relative deformation between layers. This gap was considered narrow enough to prevent the membrane from excessive bulging and the sideward expansion of soil. Thus, a sand specimen of 1880 mm by 1880 mm by 1520 mm can be placed inside the inner frames. The mass of each layer of frames is about 14 % of the mass of a 100-mm layer of soil enclosed by the inner frame.

CONCLUSIONA large biaxial laminar shear box was developed for shaking table tests on saturated sand under one- and two-dimensional shakings. The shear box is composed of 15 layers of aluminum alloy frames with a specimen size of 1880 by 1880 by 1520 mm. The soil inside the frame at every depth of the laminar box can have a multidirectional movement, without torsion, in the horizontal plane during a shaking test. A specially designed pluviator was used to prepare the sand specimen inside the shear box by the wet sedimentation method. The uniformity and saturation of the specimen were evaluated and found satisfactory according to the thin-walled tube sampling and P-wave velocity measurements. The shaking table tests of the laminar shear box with or without the sand specimen demonstrated that the performance of the biaxial laminar box fulfills the design requirements. The instrumentation installed within the soil and on the frames could also obtain reliable measurements of the soil movements and pore water pressures during shaking tests. Analyses of some of the shaking table test results have been presented elsewhere e.g., Ueng et al. 2004 and further tests are underway to obtain more databases for theoretical and numerical analyses of ground responses, liquefaction, and soil-structure inter-action under earthquake shakings.