Design of a Miniature Cone Penetrometer

Abstract

The aim of this project is to elaborate upon the design proposal of a miniature cone penetration test device. The usage of CPTs in the geotechnical centrifuges has proven to be a reliable experimental method. The usage of a CPT in a centrifuge can provide determinant results for many applications of geotechnical experimentation. The usage of cone penetration tests in centrifuge modelling is a significant experimental method in geotechnical engineering. The measurement of cone resistance and pore-pressure during in-flight tests can provide valuable data to correlate with soil properties and material behaviour. Through the years, this method has proven to be reliable and repeatable and has helped in verifying correlations of laboratory measurements and field data. The inexpensive and time-efficient nature of centrifuge testing and the reliability of cone penetration testing complement each other. This method has many relevant applications and acts as an effective tool. A literature study is conducted to obtain practical information on the design process of minia- ture CPTs. The existing designs are studied and their specifics are compared to the boundary conditions of the centrifuge of Delft University of Technology. This apparatus is capable of con- ducting tests at 300 times the gravitational acceleration and can carry samples with widths up to 400 millimeters. Two sets of existing sample containers are considered in this project which define physical dimensional boundaries. These sample containers consist of rectangular and cylindrical boxes with defined dimensions. The existing boundaries are further analyzed with respect to boundary effects derived from previously conducted tests to define design specifications for a potential miniature probe. The definition of such boundaries set the basis of the design proposal. For the rectangular containers a miniature probe with a maximum diameter of 4 millimeters can be used. As for the cylindrical sample containers, miniature probes with maximum diameters of 7.5 and 9.5 millimeters are appropriate to be designed. The analyzed existing designs are then scaled and altered with respect to the determined boundary values and the proposal is further evaluated.
Three miniature CPTs are designed with diameters of 4, 7.5 and 9.5 millimeters. Each design has certain applications and can be used in specific scenarios. All three designs include modular load cells and sub-parts that can be replaced and altered. Each proposed device consists of a modular load cell designed based on required material properties to experience a minimum amount of 500 micro-strain without buckling. The first design, with a cone diameter of 4 millimeters, can be used in any container with a minimum width of 12 centimeters and for soil samples with a maximum average grain size of 200 micrometers. The second design, with a cone diameter of 7.5 millimeters, can be used in containers with a minimum width of 22.5 centimeters and is applicable to soil samples with a maximum average grain size of 270 micrometers. The final design, with a diameter of 9.5 millimeters, is meant to be used in sample containers of widths above 28.5 centimeters and for soil samples with a maximum average grain size of 340 micrometers. The designs are then evaluated with regards to manufacturing costs and feasibility. An estimation is made based on previously designed and patented devices and material catalogues provided by manufacturers. The cost of the first two designs are estimated to amount to 1580 to 2080 Euros, whereas the third design is estimated to cost 3080 to 3580 euros due to temperature compensated pore-pressure sensor that is included in the design. Upon further evaluation, the first design with a diameter of 4 millimeters is chosen as the most feasible and practical concept due to applicability and practicality of the design.