Abstract:One of the most popular types of foundations in layered subsoil with very differentiated soil shear strengths are precast piles. One of the reasons is the fact that we can well control the driving process during the installation of these piles. The principles of the assessment of bearing capacity and settlements of the piles given by Eurocode 7, concentrate on two main methods, i.e., Static Pile Load Tests (SPLT) and Dynamic Driving Analysis (PDA). However, the evaluation of real load-settlement curve for piles being driven in layered subsoil, where noncohesive and cohesive soils occur alternately, is neither easy nor straightforward. In the paper, the results of both SPLT and PDA tests for objects on the highways in Poland are presented. Field investigations carried out at various time points since the installation of piles (from 7 to 90 days) revealed an increase of bearing capacity with time. The reason for this may be a change of the soil state near the piles due to their driving (displacement of piles) as well as a change of microstructure at the contact between soil and pile shaft. The results of load tests were referred to the geotechnical parameters of the subsoil, which was recognized by means of traditional borings and CPTU tests. The results of tests allow phenomena occurring with time to be assessed and bearing capacity of precast piles to be predicted.

The paper presents technological changes concerning not only the method of construction, but also the materials used. Another aspect is the methods of calculating the load-bearing capacity and settlement of piles. With acceptance for use the PN-EN:1997-1:2008 standard in 2010, it was necessary to change the approaches applied that based on many years of experience and tradition. The best method in this case is to forecast the full load-settlement characteristics. On the basis of the collected data, a comparative analysis of particular displacement piles technologies was made. The usefulness of individual methods of load-bearing capacity calculation depending on the pile technology was determined.

It is of utmost importance in the design process that a suitable, safe and concurrently economic solution be chosen. Issues connected with foundations require adopting a special approach. Soil is a composite medium, and it is essential to describe as precisely as possible the interaction taking place between the subgrade and the structure. There are many design methods of pile foundation that are based on in-situ test of soils. As a result of incessant development of the pile installation technology, the results obtained with design methods used to date do not assure the required accuracy, and consequently the values of pile load capacity are not sufficiently accurate. A partial solution to this problem may be applying the observation method. Based on assumptions of phenomenology as a science that enable cognition of natural and mathematical phenomena, a procedure has been developed for projecting loading-settlement dependencies for Vibro piles installed in port areas in Poland. Those areas are characterised by similar soil and water conditions, as well as soil genesis and parameters. This procedure enables estimating the limit of load capacity values with much better accuracy than the hitherto used direct methods. The full range of the s(Q) function also enables the possibility of designing structures allowing for stringent criteria of admissible settlements having a specified value.

Vibro piles belong to the group of full displacement piles with an expanded base, characterised by a very high load capacity, especially in non-cohesive soils. The problem is to adopt a reliable method for the determination of full load–settlement (Q–s) curve. A frequent difficulty is the determination of the load capacity limit based on the static load test because the course of the load–settlement curve is of a linear nature. This publication presents the empirical method. It allows direct prediction of a full axially loaded pile settlement curve based on the values of qc cone resistance obtained in cone penetration test (CPT). The advantage offered by this procedure is the accuracy of the obtained limit values in relation to the actual load-bearing capacity as compared to other methods based on soil parameters obtained in in situ testing. An additional advantage is the Q–s characteristics, which enable designing for intermediate values, allowing for the criterion of minimal or equal settlements. The shape of analytical curves was compared with static pile load test (SPLT) curves. This comparison showed large convergences between the analytical and measured curves.