The A63 Castle Street Improvement scheme includes an excavation approximately seven metres deep within a layer of normally consolidated alluvial clay. The collection of good quality undisturbed samples of this soil for laboratory testing proved challenging, therefore great reliance was put on site testing, especially on cone penetration tests. The procedure to calibrate and combine the data gathered from the field CPT and dissipation tests with all the ground investigation data is described. The use of CPTs provided a continuous profile with depth of the geotechnical properties of this extremely variable layer. Additional considerations are given on the effects of the use of a u1 or u2 piezocone position for the measurement of pore water pressure.