The weathering profile in mudstone outcrops is often a gradational transition from weathered clay to unweathered mudstone. The strength and small-strain stiffness of these materials, and of stiff clays and weak rocks in general, is critical to the design of geotechnical structures. Monitoring data and ground investigation data were gathered during the construction of an instrumented embankment on weathered, Jurassic-aged mudstone for the HS2 high-speed railway. These included in-situ downhole geophysical tests and extensometers at the embankment location. These were compared to 18.2 km of ground investigation data from the wider mudstone outcrop [1]. This large dataset included borehole records, undrained shear strength tests and index tests. The embankment loading and the installation of extensometers enabled the measurement of in-situ strains for a range of small and medium strains. It was therefore possible to characterise both the in-situ maximum stiffness and the stiffness reduction with strain of the individual layers within the ground profile. The results showed stiff clays derived from weathered mudstone at shallow depth (<10 mbgl). They were characterised by low undrained shear strength (<300 kPa) and by maximum stiffness values and stiffness reduction behaviour that compared to laboratory measurements from fine-grained soils [2]. However, the unweathered mudstone at greater depth (>12 mbgl) showed higher strength (>300 kPa), higher initial stiffness and a more rapid reduction in stiffness with strain than the overlying clay. Both the strength and stiffness profiles showed a transition zone of weathered mudstone between the weathered clay and the unweathered mudstone. This was in agreement with index test data and strata descriptions from borehole records in the wider mudstone outcrop.