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Building a House Foundation with FBS Blocks

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FBS blocks are foundation wall blocks used for constructing load-bearing structures of buildings of various purposes, operating within a temperature range of -70 °C to +50 °C. FBS blocks are primarily used in constructing basement walls, technical underground spaces (e.g., gas distribution points or boiler rooms), and for laying strip foundations. These blocks can be used for building foundations on almost any type of soil and in all climatic regions.

If the goal is to increase the load-bearing capacity of FBS blocks, they are used in combination with foundation pads, placed directly under the blocks. Foundation pads FLS extend the contact area (sole) of the foundation with the soil, thus increasing the bearing area of the building. Heavy concrete grades B12.5 M150 and B15 M200 are used in the production of FLS slabs and FBS blocks.

FBS Block Size Range

Since FBS blocks are the most common type of foundation blocks, they are manufactured in several sizes for use across a wide range of applications:

  • block height - 69 cm;
  • length - 90, 120, and 240 cm (designations FBS-9, FBS-12, FBS-24);
  • cross-sections - 30, 40, 50, and 60 cm.

Foundation Preparation

The first stage in foundation construction is axis layout. This begins with transferring foundation axes to the base (bottom of the trench), previously prepared. Axial strings are stretched, and intersection points are transferred to the trench bottom using plumb lines or laser instruments. These points serve as reference marks for measuring project dimensions and are secured with metal pins (e.g., cut rebar) so that the string-tie extends 2 mm beyond the external edge of the strip foundation.

Foundation pads are most commonly made from leveled and compacted sand. However, in some cases, alternative preparation may be specified in the design. For sandy soils, FBS blocks are installed directly on the leveled base; for other soil types, preliminary sand preparation is performed.

The sand preparation layer should be 5–10 cm thick. Loose, unconsolidated soil must not be present under the foundation sole. The sand base must extend 200–300 mm beyond the actual foundation dimensions in both width and length.

Block Foundation Installation

Before lifting the first block, ensure that the crane’s support legs are positioned far enough from the trench or excavation edge to prevent soil slippage and edge collapse. Installation of foundation blocks is not permitted on snow-covered or water-logged surfaces.

First, installation of guide blocks (masts) begins. They are placed at corners and intersections of the foundation and lintels. The accuracy of their installation determines whether actual foundation dimensions and alignment match the project specifications. The string-tie is gradually moved upward as more FBS blocks are laid. Small deviations from the string-tie can be corrected by shifting blocks with a crowbar.

Alignment and correct placement of guide blocks are verified using a leveling instrument. If a block is laid with a deviation exceeding allowable limits, it is removed by crane, repositioned, and reinstalled.

The most common installation pattern resembles bricklaying with alternating rows every other or every second row, and staggered joints. The masonry is carried out using cement mortar, with a maximum height of up to five blocks.

If the wall’s project length is not a multiple of the foundation block length, gaps formed during laying are filled with special auxiliary blocks using cement mortar, or so-called plugs — monolithic inserts made of reinforced concrete.

Small gaps between foundation blocks are also allowed to accommodate underground utilities such as sewage, water supply, and electrical lines. All joints between FBS blocks must be fully filled with mortar — this applies to both horizontal and vertical joints.