Application of dynamic compaction for soil strengthening under a large hall floors - Application of dynamic compaction for soil strengthening under a large hall floors -

Kanty Piotr, Warchał Tomasz, Malicka Agnieszka, Krejczy Radim

In this paper is presented soil strengthening using DC technology. The soil improvement works of 28 000m2 floor area were performed in Havirov, in 2015/2016. Dynamic compaction was used there as an alternative to stone columns. Apart of this type of soil improvement, also another two types of technologies were executed: dynamic replacement and bi modulus columns. Both are also shortly described in present paper. Key words: dynamic compaction, soil strengthening, soil improvement.

Designing structures’ foundation and floors on anthropogenic soils is one of the most common problems during design process of warehouses localized on old industrial areas. According to obligatory standards, this type of existing soils conditions cannot be used to directly foundation. Therefore, many of designers use piling systems to foundation building structures on that type of soils. Piling is, of course, technically proper solution, but economically it increases costs of whole budget of investment. On the other hand, piling solution works only as a second-foundation in layers of well-capacity soils, which transfers whole reactions from structure to deeper part of ground. Piling solution works only punctual, instead of soil improvement propositions, which can improve conditions under whole area of warehouse and/or structures pillars. Geotechnical market can offer huge range of soil improvement solutions. On areas with industrial soils, one of typical design proposition is executing dynamic compaction, especially under slabs, dynamic replacement as a soil improvement for footing or plates, where also the bi-modulus columns also can be used with success. In the article those three technologies will be shown based on realization in Havirov, on investment as follow: Nová Továrna na Výrobu Zdravotních Setů – Procedurepak, executed by Skanska CZ.

Three techniques were used as soil improvement in the investment as in above. Firstly, the dynamic compaction (DC) and dynamic replacement (DR) were executed. After dynamic technologies execution, bi-modulus columns (BMC) were realized. DC technology is a simple dynamic technology, used for improve loose non cohesive soil or anthropogenic soils with lack of silt and organic parts. Invented in 60s of 20th century, patented in Menard company, is still very popular technique for reinforcement those types of grounds. Main assume of this technique it to improve weak subsoil by high energy transmission during dropping the pounder. As a result, compaction of subsoil depending on its basic condition, structure and depth is achieved. Dropping energy is transferred to  subsoil  by multiple impacts with properly shaped steel pounder, with weight ranging from 10 up to 40 tons, freely falling from height between 5 to 40 m.

To perform an effective Dynamic Compaction executing, the lattice - boom cranes are used, obtaining sufficiently high impact energy. Dynamic Compaction method consist of two pounding phases: in first phase the deep layers are compacted, in the second stage intermediate ones. After those two stages execution, surface compaction (so-called “ironing”), is carried out within whole improved area. To confirm designing assumption, before executing DC technology the test plot is normally preceded. The grid spacing and impact energy (weight, shape of pounder and height of its drop) needed to achieve designed parameters is checked there. DR (Dynamic Replacement) technology consist on executing big diameter aggregate columns in cohesive soil. It is an amplification of DC technology, with using aggregate fraction till 125 mm. The columns are formed by a heavy pounder 15 up to 30 tons weight, dropped from height ranging from 10 up to 30 m. A single column is formed by a few series of pounding. The pounding process is commenced in a shallow excavation filled in with mineral or recycle aggregate. In first series of pounding, crater in subsoil is formed, and then filled in with a backfill material. Subsequent stages of aggregate adding to the excavation and of pounding are repeated till the moment when DR column is formed according to the previously elaborated design. Often the end of the column forming is indicated by a thud combined with a sudden reduction of the pounder penetration value. Large diameter columns (ranging from 1.6 m up to 3.0 m) are driven to a depth ranging from 2.0 m up to 5.0 m. More information about this technique can be found in [1] and [2]. The BMC (Bi-Modulus Columns) soil improvement technology consists of several stages. The BMC core is made in the same way as the CMC (Controlled Modulus Column) column. A specially designed displacement auger installed on a machine equipped with a high torque and static vertical thrust head displaces the soil horizontally towards the hole centerline. When the displacement auger reaches the required depth the injection grout based on a concrete mixture is pumped under pressure to the hole. The pumped concrete flows through the auger pipe. The concreting process is performed under a pressure which does not cause any damage to the hole walls and prevents from mixing the soil with the injection grout.

A BMC head is formed applying the SC technology in a point of construction of the CMC core. By a specially designed downhole vibroprobe installed on the equipment assembly the BMC head is formed in three basic stages: vibroprobe driving, aggregate backfill and compaction.

Project in those three – DC, DR and BMC –technologies, were executed in November and December 2015 in Havirov - Nová Továrna na Výrobu Zdravotních Setů Procedurepak. General Contractor was Skanska. DC technology were used to improve subsoil under the slab, except 10 m width area nearby sewage system, where BMC columns were executed. DR columns were performed under tanks, nearby to main hall (figure 2).

Geologically, the subsoil is built as anthropogec soil till about 4-6 m under the terrain. Below some clays and silts, and silty sands are formed till the end of investigation – 25 m below the terrain level. Details are shown in figure 3.

As a design solution, the following assumption were made:
1. Technologies:
a. DC – under floors;
b. BMC – along sewage system (φ700);
c. DR – under tanks.
2. DC technology
a. Average relative density to the depth of 5m: ID ≥ 0,50; Edef ≥ 45 MPa;
b. Average relative density to the depth of 8m: ID ≥ 0,45; Edef ≥ 40 MPa;
c. Drop points spacing: 6,0 m x 6,0 m – square net
Before starts of executing whole works for flooring test area was planned and executed.

The test field in dimensions 30,0 m x 30,0 m was done to verify the design assumption and to make eventually corrects in designed solution. The scheme of test field with phases section is shown below on figure 4a. On “part A” the orange points indicates first phase drop point, the green one the second phase. On the other site (part B) violet dots indicates drop points for phase 1 and 2 (drops of second phase in the same points as phase 1). Location of dynamic probing DPM points is shown in drawing 4b. During the test the pounder was dropped from the height 10-15m, 8 times on each point. Only during heave test the drops were different: 4x10m, 4x12m and 4x15m. To perform the soil strengthening pounder which weight was 9 and 14 tones was used. Dimension of the pounder base was 2x2m and 1,5x1,5m respectively.

The test field was carried out to check main aspects of work:
- Soil compaction after works (dynamic probing DPM)
- Ground lowering after the works (survey measurements)
- Working parameters (no of drops, no of cycles, drop height checked by “heave test”)

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