Nawawi Chouw, Rolando Orense, Gonzalo Barrios, Tam Larkin, University of Auckland (EQC funded project 14/662)
This experimental research project was carried out to determine the response of saturated sandy soil to strong earthquake shaking. The term “saturated” means all the spaces between the sand grains are filled with water. These conditions exist throughout a large area of Christchurch and wide-spread damage to buildings and other facilities resulted from the Canterbury series of earthquakes. In addition to the investigation into how saturated sandy soil behaves in an earthquake, this study also investigated the nature of the shaking of buildings that have their foundations on saturated sandy soil.
One technique used in civil engineering is to carry out experiments using model buildings that are a scaled down version of real buildings. The model buildings are placed on the surface of sand that has been saturated with water. This models conditions similar to those that occur in Christchurch. A large special box (2m x 2m x 2m) was constructed to contain the saturated sandy soil. The box has the capability of moving in a controlled manner that closely follows that of real earthquake ground movement. Finally, the special box is placed on a “table” that is designed to move with a pattern of motion similar to real earthquake ground shaking. This whole experimental set-up is known as “shake-table testing” and closely reproduces the conditions that exist when earthquakes occur in cities that are built on saturated sandy soil.
Various scientific instruments are placed in the sandy soil that give the investigators information on the response of the soil to earthquake movement. This information is of value in understanding why and how the sandy soil losses strength during earthquake shaking and how this effects the foundations of buildings. Various patterns of building foundations were studied, since it is believed that in circumstances where buildings are close by each other, as occurs in large cities, the way one foundation moves effects the surrounding foundations.
The information that was gathered and studied has helped civil engineers to better understand the effects of earthquakes on saturated sandy soil and how the complex patterns of movements of foundations of adjacent buildings effect each other. Eventually this information will likely appear in design guides to be used by practicing professional civil engineers. As we understand more about earthquakes and the effects on the large cities of the world, facilities that are more earthquake resistant can be built.
This report concerns a series of experiments carried out to investigate the earthquake response of a system involving the foundations and the supporting saturated sand. A large laminar box was designed and constructed to simulate the passage of earthquake waves through the saturated sandy soil. The box was placed on a large shake table which provided the excitations. The response of the soil was closely monitored using a variety of instrumentation and the data analysed to enable an understanding of the predominant features of the response of the system.
The philosophy behind the design and the method of construction of the laminar box are described and the method of preparing and saturating the sand outlined. An initial series of experiments utilising a simple instrumented structure on dry sand was carried out as a verification of the functioning of the laminar box, i.e. if it was suitable for the purpose. The response of the structure was compared with theoretical solutions available in the literature. This work throws light on the role of soil-structure interaction in earthquake response, which is often ignored in usual design practice.
A major part of the work presented here concerns the response of a body of saturated sand to a series of ramped harmonic loadings, all with an amplitude of 0.2g but with 3 different frequencies: 1, 1.5 and 2 Hz. The response of the sand without the presence of footings was first explored. This was followed by studies of a system comprising of a single footing and a more complex system of a cluster of 6 closely adjacent footings. All footings were mounted on the surface of the saturated sand. The sand and footings were extensively instrumented to record excess pore water pressure, acceleration of the sand and the footings, relative lateral displacement at three elevations and vertical displacement of both the sand surface and the footings.
The results are herein discussed and the most important features are presented that lead to an improved understanding of liquefaction effects on shallow foundations including the nature of the soil and footing response to the different types of loading. From this information, conclusions and a series of recommendations are made that will be of use to designers.