Ground Improvement By Dynamic Replacement Columns On The Basis Of The Segment Of Siekierkowska Highway In Warsaw

An essential and urgent work when building the Siekierkow-ska Highway in Warsaw was a realization of the segment from Bora-Komorowskiego Street to the intersection of Ostrobramska, Płowiecka, and Marsa Streets. On account of ground conditions (organic ground) and the deadline, which is the end of 2005, it has been decided to apply a Dynamic Replacement method by making dynamic replacement columns. It concerned a segment where the thickness of weak ground exceeded 3-4m. There, where the occurrence of organic soil was shallow, it has been de-cided to apply soil replacement by so called: gredging. Executing the dynamic reinforcement mainly aimed at accelerating organic ground consolidation. The organic ground was loaded with a road embankment in order to meet the bearing capacity and set-tlement conditions as fast as possible, taking into consideration operational load of embankment and road. The reinforcement was executed by Freyssinet Polska company under geotechnical su-pervision of the Building Research Institute.

The planned site is located on the overflow area of the Vistula river. On the whole new-built segment, there occurred river de-posit such as: organic clay (silty clay) and mud. The mud (organ-ic ground, stratum II) occurred approximately 1m under the or-ganic clay stratum or partially directly under the surface of the site. In the primary report, their condition was defined as soft – IC=0.60, with a natural humidity of approximately 70%. In stra-tum III (silty clay) medium ones, IC=0.25-0.49 – yielding state. The maximum depth of organic ground deposition was approxi-mately 4 meters beneath the surface of the site. Beneath the or-ganic ground stratum there occurs moderately compacted medi-um and coarse sand – (IC=0.50-0.65). The first water table occurred periodically on the surface (on mud and organic clay stratum). The second water table is forming under the organic ground stratum, in sand on the depth of approximately 4-6 m be-neath the surface of the site and has a tense character (Fig. 1).

On account of a short lead time, it has been decided that a part of road embankments will be founded on the ground improved by Dynamic Replacement method. The Dynamic Replacement method is a consistent expansion of the Dynamic Consolidation method applied by Louis Menard. By using the same equipment and a similar technology, it is possible to improve the soil which did not improve by applying the Dynamic Consolidation method.
For ages, the Dynamic Replacement method is successfully applied in organic ground, mud and peat, which are the ground similar to the ones that deposit on the area of the building high-way. The Dynamic Replacement method consists in making grain material large diameter pillars in the cohesive soil (figure 2).
It has been decided that in this case, the pillars will be formed by sticking in specially selected aggregate with a 12 tons pounder dropped at the height of 20meters. In order to lift and drop the pounder, a crawler crane with bearing capacity of 70 tons, equipped with a free-falling mechanism has been used (Fig. 3).
The beginning of the compacting process was performed on the surface of the formed working platform, which made up of non-condensed layer derived from non-cohesive ground (with thickness of approximately 1m). It also enabled the heavy equip-ment to move on the platform.
A single column execution was preceded by few series of blows. The first series of blows was executed on the surface of the working platform which resulted in creating a crater with the depth of approximately 2m. In most cases the crater’s diameter in its upper part amounted to approximately 2.5m, while nearby its bottom it amounted to approx. 1.5m. Next, a batch of aggre-gate was poured into the crater interior and some blows were ex-ecuted until the column’s bottom displaced. Successive stages such as: pouring the aggregate into the excavation and perform-ing the blows, were repeated until a distinct reduction of the pounder penetration in comparison with the previous stages. In many cases, the end of column forming was signalized by a dead sound combined with a sudden decrease of pounder penetration value, which meant that any further displacement of the column is impossible.
The completion of the column forming amounted to a total crater filling and concentration of the upper part of the column by a series of blows. The densification of the column at the final stage was completing when the ground started to heave visibly. An average ground heave in the column surroundings amounted to approximately 15cm (Fig. 4.)
It has been noticed that when performing further blows it is possible to drive other rations of aggregate into the core of the column. It involved a massive heave (even higher than 50cm) around the pillar which betoken a significant diameter increase of the column in its upper part.
The columns were made on a square grid with a displacement of every second row. The site was divided into two parts: In the first one, loaded with embankment at the height of over 4.5m a column grid measuring 5 x 5m was applied, and 5.5 x 5.5m in the second one.
The material used for forming the columns was an aggregate prepared on the building site. There have been used some demo-lition materials such as crushed concrete debris in the ratio of 1 to 3 with the medium sand.
The final stage was aimed at densification of the working plat-form and the surface of the ground with an approximate volume of 2m. It took place as a result of surface pounding, a so-called “Ironing Phase”. The process involved a usage of a flat-shaped pounder and a square base. Single blows were performed on a two times more dense grid so that 50% of the reinforced area could adjoin to the pounder base. After the procedure, the surface stratum of the ground (approximately 50cm) or the surface of the working platform was still slightly loose. Performing the classic concentration by heavy vibrating rolls was essential. There have been applied 4 one-track roll passages, taking an optimum hu-midity (spraying) into consideration.