Modern construction conditions are characterized by a shortage of sites with sufficient bearing capacity, which necessitates the development of foundations with weak structurally unstable soils. Such soils include water-saturated clayey, silty, peaty and subsiding loess when soaked.
Given the specifics of such foundations to the attention, construction on these sites is impossible without special engineering measures aimed at improving the characteristics of soils, reducing their compressibility and strengthening the structure. Domestic and foreign engineering experience is based on the use of technologies aimed at preparing foundations for further construction and the use of pile foundations, which are based on strong soil layers.
Technologies aimed at improving the construction properties of soils can be divided into two main groups:
1) those related to the introduction of additional components or elements into the soil base;
2) those that are limited to certain technological techniques without the use of additional devices or soil additives. These technologies are mainly associated with compaction of the base, including heavy rammers, which allow creating a compacted soil layer to a depth of 12 meters, the modulus of deformation exceeds 25 MPa.
The first group of measures includes:
- complete or partial replacement of the weak layer with the arrangement of soil cushions, which are laid in layers with mandatory compaction of each layer and have high mechanical characteristics;
- injection of solutions that change the structure of soils (cementation, silicification, clay formation), as well as electroosmosis, which is used for water-saturated clay soils;
- jet-grouting of soils, in which cement mortar and soil are mixed, resulting in the formation of so-called "soil-cement piles”. Jet - grouting of soils is a technology for securing weak or heavily waterlogged soils in geotechnical construction, which consists in pumping cement mortar under high pressure (up to 40-50 MPa) into a well (diameter 100-130 mm) to destroy and simultaneously mix soil particles with cement mortar. The technology has become widespread since the late 1990s - early 2000s. During such mixing, after the cement mortar hardens, soil cement with high deformation-strength and anti-filtration characteristics is formed. The volume of soil reinforced in this way is called a "soil-cement element" or "soil-cement pile" and has a diameter of 600 to 2000 mm.
Cementation begins with drilling leader wells with a drilling tool, the lower part of which has nozzles; when the drill string is lifted, the solution is injected through the nozzles with simultaneous mixing of the soil; as a result, a soil-cement element is formed.
- vertical reinforcement of the base with soil or concrete elements.
Reinforcement of foundations is a set of measures and technologies designed to strengthen soil masses by including in their composition special elements that are in close interaction with the soil, but are not structurally connected to the foundation. Such elements improve the deformation properties of the foundation, interacting with the soil on the lateral surface and at the level of the tip. The transfer of loads from the structure to the foundation is carried out through an intermediate soil cushion made of low-compression material, which is poured and compacted in layers. This technology is a more economical alternative to pile foundations, which require additional costs in the zero construction cycle [1].
In the early 1990s, France was the first in the world to industrially implement an innovative concept called Controlled Modulus Columns (CMCs), a technology for vertical reinforcement of weak foundation soil by rolling out a well using a special auger and forming small-diameter unreinforced concrete columns [2]. The technology for reinforcing the foundation with controlled modulus columns consists of arranging a grid of rigid (semi-rigid) vertical reinforcing elements that cut through weak soils and rest on layers with sufficient bearing capacity, while there is no structural connection of the controlled modulus columns with the foundations. The load is transferred from the building using a sand (redistribution) cushion, which allows the use of such a solution allowed the use of the joint work of vertical reinforcing elements and weak soil under load from the building. Reinforcement of the foundation with columns of a given strength reduces the settlement of the foundation of buildings and structures in the range from two to 10 times, increases the bearing capacity of the foundation soil and ensures its stability.
- vertical drainage of the base in combination with a temporary loading embankment, which starts the process of squeezing out pore water from water-saturated clay soil, which significantly speeds up the process of base consolidation [3].
Sand drains can be drainage elements or newer elements consisting of a core of special flat plastic and a geotextile shell, which serves to protect the drain from clogging. First, the vertical drain is immersed in the soil by pressing the guide pipe (needle). Then the guide pipe (needle) is pulled out in reverse. In this case, the drain remains in the soil and is cut off above the working surface of the soil.
- Menard Vacuum TM soil vacuuming, which is based on the effect of reverse pressure from pumping stations on the thickness of consolidated soil. This technology was developed in the late 1980s and is often used on unique and critical sites [4].
Vacuum compaction is similar in effect to vertical drainage, while it allows you to significantly reduce or eliminate the need for additional loading and ensure good controllability of the consolidation process, and accordingly, more accurate prediction of settlement values.
The main advantages of vacuum sealing technology are:
− reduction of the base consolidation period compared to classic vertical drainage;
− significant reduction in the need for inert material for the loading embankment;
− eliminating the risk of loss of foundation stability during pre-construction consolidation, including the possibility of installing a berm of consolidated soil around the perimeter of the site for further zero-cycle work.
Deep vacuuming allows you to perform the following tasks:
− acceleration of filtration consolidation before the start of operation of the facilities;
− ensuring the stability of the base;
− limiting secondary consolidation (skeleton creep) to permissible values.
Thus, in world practice, technologies for strengthening and stabilizing foundations are widely and successfully used, which are an alternative to the use of pile foundations. Artificial foundations are a full-fledged and equivalent alternative to deep foundations and soil replacement measures, while often winning in terms and cost. At the moment, strengthening and stabilizing soils allow bringing to the required physical and mechanical characteristics almost any soils: water-saturated clay, sandy, technogenic, subsiding, swelling soils and soils prone to thixotropy.
List of literature:
1. Hamidi B., Massé F., Racinais J., Varaksin S.The boundary between deep foundations and ground improvement // Geotechnical Engineering. 2016. Vol. 169 GE2. P. 201-213.
2. Racinais J., Thomas B., Ong R.Twenty years of CMC successful application // Retrieved from 19th SEAGC & 2nd AGSSEA. Kuala Lumpur, 2016.
3. Racins J., Plomteux C.Design of slab-on-grades supported with soil reinforced by rigid inclusion // Retrieved from EYGEC 2011. Rotterdam, 2011.
4. Masse F., Spaulding CC, Ihm CW, Varak-sin S.Vacuum consolidation: Review of 12 years of successful development // Proceedings of the Geo-Od-yssey Conference, ASCE, Blacksburg VA, June 9–13, 2001. P. 6.
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