Nowadays, the most efficient geoengineering methods of soil strengthening under the construction of civil structures are accompanied by shakings which induce vibrations in soil. The vibrations propagate in all directions, having destructive influence on neighbouring buildings or other infrastructure elements. In some cases, the vibrations may result in excessive effort of construction elements, which may result in cracking, decrease of bearing capacity and stiffness. In extreme cases, the construction may even be destroyed. Vibrations accompany e. g. the dynamic replacement method, dynamic compaction, prefabricated piles driving and sheet piling driving. Similar phenomena also occur in soil strengthening with a heavy vibratory soil compactor. It is obvious that the range of influence mostly depends on the vibration’s frequency. However, other factors such as soil type (more precisely its deformation and attenuation characteristics), the presence of ground water, the depth of rock or applied energy also play an important role. These latter factors mostly concern the use of vibratory soil compactors. Also, the impact on the people in the influenced buildings should be taken into account, especially when conducting works close to office and residential buildings. In practice, special monitoring is used to assess the real influence of the vibrations on the neighbourhood. It is based on in-situ measurements of amplitude and frequency of vibrations occurring on some elements of the observed structures. On this basis and with the use of standards  and , the harmful impact of the conducted works is assessed and the appropriate measures are undertaken to prevent the negative influence. In extreme cases, it may result in changing the strengthening method. Moreover, the monitoring may be expensive. Therefore it is crucial to undertake research (both theoretical and experimental) which will facilitate the design by simulating the phenomenon of vibrations propagation and their influence on its surroundings. Field tests will be necessary for the calibration and verification of the adopted model and its parameters. Dynamic replacement (DR) is one of the most popular techniques often applied in order to strengthen weak soil under road embankments. The method owes its popularity to a large number of road construction projects being currently realized in Poland. DR column is formed by dropping a pounder (rammer) of a specific shape and weigh of 15 – 30 tonnes. This is usually conducted on a working platform, which makes the use of an 80-tonne crane possible. In the first stage, the pounder is dropped from the height of up to 25 m to form a crater, which is refilled with coarsegrained material. The following drops of the pounder form the column. In Poland, the diameter of the columns varies between 1.6 – 3.0 m and their length is up to 6m . This simple and rapid method allows strengthening of weak soil up to the depth of 6 m. It increases the strength of both cohesive soils (clays, silts), as well as organic and anthropogenic soils. Column formation process is accompanied by vibrations perceptible in the surroundings. Therefore, in most cases, the structures located close to the construction sites are monitored so that a safe drop height (and thus the proper energy) might be selected.
This paper features the results of investigations carried out during stone column formation on G1 section of highway DTŚ. Over 30 000 m2 of soil under road embankment have been strengthened. Soil investigations performed before the beginning of the strengthening (CPTs and boreholes) show that the upper part of the soil consists mainly of soft silty clays and medium sands, and the lower part – the bearing layer – is made of firm clays. In some places anthropogenic soils were encountered from the soil surface up to the depth of 1 m. Columns were formed with a 15-tonne pounder. At least 20 drops were performed from the maximal height of 15 m to form a column. The crater was refilled 7 times during column formation. In this case, the columns of burnt (red) shale measured 2.0 – 2.5 m in diameter and 3.5 – 4.5 m in
length. The spacing was variable, ranging from 4.5 m x 4.5 m to 6.0 m x 6.0 m depending on location. Partially excavated columns on the construction site of DTŚ are shown in Fig. 1. During the construction of embankments, columns settlements were monitored. The observed values were closed to the predicted ones .