Pressures in Sand from Waves and Caisson Motion

In The Netherlands recurring floods have occurred in the southwestern part of the country from infrequent large storm surge from the ocean. The Delta plan was originated to increase the safety against flooding. This resulted in shortening of the coastline by closure of various estuaries, including the Eastern Scheldt estuary. A closure of the large estuary is needed during impending storm surge conditions, which will require a major structure. Gates on the structure will be open to allow for normal tidal action in mild weather. They will be closed when storm surge conditions threaten. A major question for the design of such a large structure is the competency of the sand for providing an adequate foundation. Various methods have been developed for estimating the competency of the sand, including numerical models, which attempt to predict the support conditions. These numerical models have been developed by engineers in The Netherlands. It is customary to question the validity of any numerical model until it has been verified by physical measurements. Therefore, it is desired to provide a validation for the numerical models that predict the dynamic conditions in the sand from waves and structure motion in the Eastern Scheldt region. PURPOSE OF THIS STUDY The purpose of this study is to create a carefully controlled laboratory experiment at large scale which will provide physical measurements to which the numerical models can be compared. A large structure is desired which is subject to large standing waves. The dimensions of the Wave Research Facility at Oregon State University are such that both conditions can be realized. The results of the model study will be compared with predictions from various numerical models by others. The large scale is desired in order to reduce or eliminate scale effects which are known to exist between small model studies and large prototype conditions. SCOPE OF THE WORK A set of thirteen tests were designed for measuring the important parameters which define the support capabilities of the sand. A suitable sand was located near Astoria, Oregon, and a sufficient quantity was trucked to Corvallis. A suitable test sand bed was established on which a caisson was placed. Standing waves were subjected to the caisson and the sand upstream from it and measurements were made of water surface fluctuations, water pressures, caisson motion, pressure fluctuations within the sand, and the porosity of the sand. Details of the experiments are explained later in this report. GENERAL CONCLUSIONS The testing was successful. Three configurations for the caisson and upstream filter were investigated as detailed later in this report. Three separate wave frequencies were studied. The majority of the 36 channels of information were recorded and visual records are included in this report. In addition, analogue and digital records were made on magnetic tape, which are being delivered to the Delft Hydraulics Laboratory with this report.