Material replacement under foundation
What is material replacement?
Material replacement is the process of removing the existing and installing materials with better properties which are achieved by controlled installation. This is one of the most common foundation soil improvement processes applied when low soil physical and mechanical properties are present under the foundation. The purpose of the material replacement is to improve the bearing capacity and / or reduce total settlement of foundation soil.
Material replacement under foundation
Generally, increasing the bearing capacity and decreasing settlement is achieved by replacing the low soil layer just below the foundation.
If a low load-bearing layer is at a depth which can be replaced, it can be removed completely and replaced by material of appropriate granulometric curve and compactness.
Picture 1. Material Replacement – excavation
Installing the required quality material ensures increased compactness of the installed layers in relation to the existing foundation soil and higher calculation factors of bearing capacity are achieved. The installation and assembly of new material must be performed in layers, and the compactness must be checked by testing.
Picture 2. Material Replacement – filling in layers
In determining the design of replacement layer compressibility modules, it is necessary to know the compressibility module of the foundation soil on which the first alternate layer is installed. The foundation soil compressibility module can be tested with a static or dynamic plate.
Picture 3. Material Replacement – compactness testing
When a low load layer is thicker, it is not necessary to replace it entirely, but only to a certain depth determined by the calculation. The required depth is determined by calculating the load capacity of the low layer and the distribution of additional strains per depth based on the design. Replacement is carried out to the depth at which the corresponding load of the low layer appears.
When the first replacement layer has been installed, it is necessary to select a granulometric composition that will not mix with the base (tampon material) or to install a layer of geotextile between the base and the replacement layer.
Picture 4. Material Replacement – Installation of geotextiles on the base
When the foundation of one part of the structure is located above a rock base and the foundation of the second part above the soil, material replacement is also applied. There is a danger of differential settlement and structural damage due to unequal stiffness of the base in such case, and the material is replaced under the foundation in soil (stiffness) and in rock mass.
Material replacement under foundation – example from practice
Landscaping of the foundation soil under the foundation structure was executed for the purpose of production construction. The foundation of the structure was intended as a shallow foundation on the foundation spread footings (dimensions 3.0×3.0 m and 0.50 m height).
Due to varying physical and mechanical characteristics of the foundation soil and digging depth, the design of material replacement under foundation was planned in thickness of 0.50 m with the previously installed geotextile on the base. The conditions of foundation were equalized by the conducted works and the possibility of potential damage on structure was eliminated.
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