Development of Buried Sensors for Stiffness Measurements of Soft Clays Using Bender Elements

The use of vertical drains, along with pre-loading, is the most popular method of soil improvement in coastal areas to meet growing demand for diverse uses of limited land space. Piezometers are the key instrument for continuous monitoring throughout the consolidation period. Basically, piezometers have several disadvantages such as incapability of providing any mechanical properties, meticulous installation procedures, misleading results caused by unexpected hydrogeologic boundary condition, and relatively high damage rate. As an alternative instrument for monitoring soil improvement, a piezo-electric sensor was elected to measure stiffness changes during consolidation. The piezo-electric sensor is composed of two bender elements mounted on a frame. Bender elements, composed of piezo-materials and metal shims, have favorable features suitable to field instrumentation such as simple principle of energy conversion, excellent control capability and small physical size. The prototype sensors have been developed through extensive in-house program to optimize in terms of data quality and installation convenience. The first prototype sensor was implemented in coastal mud using SPT(standard penetration test) rods pushed with a routine boring machine and excellent shear wave signals were measured. The sensor is on the further collective refining process.