The article describes the experience of achieving the necessary deformability indicators for areas of soft foundation soils during the construction of a residential complex with underground parking.
During design a residential complex “White Island” in St. Petersburg, the engineers faced a problem. In general, the geology of the construction site made it possible to build houses on shallow foundations. However, this solution could not be fully implemented because of the existence of areas of soft soils. To avoid relative settlements, most of the building needed to be placed on a pile foundation. GEOIZOL Project engineers proposed an alternative – to modify soft soil areas using the “crushed stone pile” technology, which made it possible to implement the original design solutions and achieve saving of about 40% compared to the pile foundation.
Features of crushed stone columns
The deep vibrocompaction technology with the construction of pillars made of inert materials, the so-called “crushed stone piles”, is an effective way to strengthen soft soils. It has found application in a number of industrial, civil and infrastructure construction projects, including the M-11 Neva and the Central Ring Road, Crimean Bridge, Moscow residential complex Lyubertsy Park, Estel plant in Kolpino (St. Petersburg) and others.
As a result of deep vibrocompaction bearing capacity of soil mass is increased. Time of consolidation is reduced and subsidence properties of soils are eliminated. “Crushed stone piles” work as drains through which water is squeezed out of the surrounding soil, because of what the base stabilizes already during the work.
When installing “crushed stone piles”, a vibroflot is used, which, due to horizontal oscillations and high-pressure air supply (dry method), is immersed in soil, forming a bore. Through the connecting pipe (bottom feed method), which is a part of the vibroflot construction, soil filler is fed to the bottom of the bore: crushed stone, sand, concrete recycling, etc. As the material is fed, the vibroflot compresses it, increasing the pile to the surface of the earth.
When improving the characteristics of the soil, a field of “crushed stone piles” is carried out, arranged with a calculated step. For each “crushed stone pile” the on-board vibroflot system automatically generates a passport, which reflects the characteristics of both the finished “crushed stone pile” and the technological processes during its construction. These data allow to determine the depth of the pile, its geometry and the degree of compaction.
Quality control of the work performed also provides for plate-bearing tests of the pile body and testing of soils in the inter-pile space by static probing. This allows you to determine the deformation properties of the soil after its modification and issue a conclusion on the bearing capacity of the base.
Residential complex “White Island” is a apartment residential building, consisting of 12 sections (10-12 floors each) and a two-level parking for 435 cars. Large building area, which exceeds 79 thousand square meters, determined the heterogeneity of geological conditions.
The soils in the construction area of the residential complex are represented by dense sands, in which it is possible to counstruct shallow foundations. However, the existence of local sites of outcropping of soft soils, mainly soft loams, significantly limited the use of such a technical solution
“Soil conditions dictated to the designer the use of deep foundations. The use of bored piles was considered, because the use of precast piles in this area is unacceptable, and due to the existence of dense sands, the indentation method was not suitable. Due to the risk of relative settlement, the pile foundation had to be provided for almost every building of the complex, – says Ivan Bogdanov, chief designer of GEOIZOL Project. “We offered an alternative – to strengthen the soft areas with” crushed stone piles “in order to achieve the required deformability of soils throughout the entire construction area.”
The installation of crushed stone piles was carried out from the day surface, before the excavation of the pit. Their depths varied from 12 to 14 meters. At the same time, the builders immersed the sheet pile, which was the enclosing structure of the pit. After that they began excavation and erection of the underground parking.
“In addition to the identified areas requiring reinforcement, already during the work an area with loose soil was discovered on the site of the dismantled building . It was modified using the same technology. According to calculations, it was necessary to increase the depth of the “crushed stone piles” to 18 meters, ” – said Ivan Bogdanov.
In total, the project for strengthening the basement soils included the installation of 1606 “crushed stone piles” with a diameter of 800 mm and a length of 4.5 to 8 meters, arranged under the foundation slab of the residential complex.
Local reinforcements with “crushed stone piles” made it possible to achieve the required deformability of the soil mass throughout the entire construction site.
According to geotechnical modeling, the installation of “crushed stone piles” reduced the design settlement from 25 cm (without reinforcement) to 14 cm. Actual deformations revealed as a result of geodetic monitoring remained within 5 cm.
The technical solution proposed by the engineers of “GEOIZOL Project” made it possible to reduce the costs of constructing the foundation of the residential complex “White Island”. The soil modification was approximately 30-40% more economical compared with the installation of the pile foundation.
Сhief designer of GEOIZOL Project
The article was published in the journal “Underground Horizons” No. 27