||The marble area in Ma On Shan in Hong Kong is symbolized by highly variable rockhead contours and deep depressions filled with thick layers of weak materials such as debris flow deposits. Piling experience in the area shows that very large pile penetration, often larger than 100 m, is needed to obtain required pile capacity. Verticality becomes a concern for such long and slender piles. A trial pile test program consisting of four very long driven steel H-piles, instrumented with inclinometers and strain gauges, was carried out in Ma On Shan. Pile load tests and verticality monitoring at several penetration depths were carried out to the investigate the pile capacity and performance. In this research, results of verticality monitoring are reported and interpreted with regard to rockhead inclination and special soil conditions. Large lateral pile movements occurred during pile penetration due to sloping rockhead or rock fragments. Both one-side sway and cyclic sway were observed at different penetration depths, leading to loose contact between the soil and the pile shaft. Driveability analysis of long piles by using GRLWEAP is also conducted. The capacity and the load transfer in the very long piles are studied based on the results of static load tests and dynamic tests, and their relations to the pile verticality are discussed. The small pile capacity found from the static and dynamic pile tests can be explained by the verticality effect, which is related to the formation of a void along the pile shaft due to pile sways during driving and the development of a highly remoulded zone of soil surrounding the pile shaft due to sudden changes of penetration direction. Three common static analysis methods for predicting pile capacity, including the API method, the ICP method and the FHWA method, are evaluated with the results of static load tests. A more reasonable and comparable prediction with the results of static load tests is obtained using the ICP method.