Foundations

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    Foundations - Basic Concepts

    COURSE 1 / MODULE 2 - FOUNDATIONS / LESSON 1 - BASIC CONCEPTS

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    The foundation is a summary of all the structures on which the rest of the structure is built. Its function is to carry all the loads of the building and transfer them to the load-bearing soil. 

    Further requirements for the foundation:

    • allow the building to sink as little as possible;
    • prevent uneven subsidence, avoiding destructive stresses;
    • resist water pressure and frost effects.

    To achieve this, the following five rules are very important when designing and constructing the foundation:

    1. The foundation plane must be below the load-bearing plane of the soil - if feasible;
    2. The foundation plane should be above the water level to avoid problems from water pressure;
    3. The foundations should be below the frost line to avoid the risk of ice floes lifting the foundations and causing stress-induced subsidence;
    4. The footings should be spaced sufficiently apart so that they do not load on each other but on the subsoil;
    5. When built adjacent to existing structures, foundation planes should be brought together.

    It may not be possible to fully achieve the above objectives in all cases.
    Therefore, more complex and material-intensive designs should be used instead of the simpler forms of shallow foundations, while deep foundations should be used in extremely difficult situations regarding the load-bearing abilities of the soil.

    1. Shallow foundations
    
    The types of shallow foundations are:

    • Strip Foundation - A typical foundation for a masonry building is a strip foundation under the walls of the building. Typically, it is cast in the excavated foundation pit, but it can also be made with formwork. 
    • Spread Footing - The typical solution for pillared buildings is spread footing, which may be pad, stepped or sloped footing. There is also a well-established prefabricated solution: on monolithic pad footings, precast pocket foundations hold the prefabricated columns.
    • Rigid Grid Foundation - Similarly to strip foundations, it may be justified to join the point foundations into a single beam, but it is more common for static reasons to join in both directions, creating a rigid grid foundation.
    • Raft or Mat Foundation - In the case of high groundwater levels, a continuous monolithic reinforced concrete raft or mat foundation may be required, which can be further strengthened by a grid of ribs.

    2. Deep foundations

    Deep foundation solutions are needed when there is insufficient bearing soil near the surface, or when there is a risk of slippage due to water run-off.
    In summary, foundations deeper than 3 metres are called deep foundations.
    
    They are of the following types:

    • Well foundation - Well foundations are typically used for 3 to 6-metre foundation depths.
    • Pile foundations - Pile foundations can extend up to 20 metres deep. Structural calculations typically take into account the transfer of load through the casing wall, even if this is above the level of the load-bearing soil. 
    • Slurry wall - The reinforced concrete slabs of the slurry walls can extend up to 40 m, forming a continuous boundary around the building, thus providing a drainage method and allowing for the creation of multi-level utility spaces (e.g. underground car parks) below the surface.

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  I.2 - LESSON 2
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    Shallow Foundations

    COURSE 1 / MODULE 2 - FOUNDATIONS / LESSON 2 - SHALLOW FOUNDATIONS

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    Strip Foundation

    The strip footing is the most common foundation for masonry structures, and is wider than the masonry above it.
    It is typically made of concrete or floating concrete, poured between the foundation trench walls.
    If the soil is not of sufficient strength, formwork is required for its construction.
    If structural reinforcement is required, scaled reinforcement is also placed in the concrete.

    The implementation process is as follows:

    1. Site clearing to remove plants and humus, the top layer of soil.
    2. The batter boards are installed, fixing all the corner points and the boundary lines, which can be marked out on the surface of the soil, for example with lime powder.
    3. Dig the foundation trenches, either by machine or by hand, taking care to ensure that the levels are at the correct height.
    4. Formwork - only if necessary. In this case the foundation trench should be made wide enough to accommodate the formwork.
    5. A blinding layer is prepared for the steel fixing.
    6. Concreting takes place using a rotating drum mixer or by pouring ready-mixed concrete, in layers of approximately 25 centimetres.
       The layers must be compacted with a rod vibrator.
       During solidification, the concrete must be watered to avoid rapid loss of moisture, which could lead to the concrete overheating. 
    7. Removal of formwork.
    8. Backfilling in layers of 15 cm and compaction around the foundation.

    The foundations of the partition walls are made in the same way as the main walls, but with a smaller cross-section.
    The cross-section can be further reduced by using reinforced beams.
    For light loads, it may be sufficient to reinforce the substrate with reinforcing steel.
    
    In the case of sloping terrain and the use of different foundation levels, the strip foundation must be stepped.
    The maximum level difference (step height) is 50 cm, and the maximum angle of inclination for the stepping is 30 degrees.

    Spread Footings 

    Spread footings support columns or pillars and are typically made of monolithic reinforced concrete.
    Spread footing can also be a prefabricated reinforced concrete structure, a so-called pocket foundation.

    Constructing monolithic reinforced concrete is the same as for strip footings, except that the footings are isolated and not continuous.
    When they are constructed, the longitudinal rebars of the pillar typically continues into the foundation to create a fixed joint.
    Waterproofing can be also accomplished by placing metal sheeting under the pillar, or by insulating around the entire spread foundation.
    
    For pocket foundations, the typical solution is to first construct the monolithic concrete spread foundations and then place the pocket foundations on top of these.
    The placement requires 8 to 10 cm of blinding concrete on top of the pad footing and then placing the pocket foundation in the bedding mortar laid on its surface. 
    The pillar is then lifted into the pocket, initially fixing it with wedges and finally concrete.
    Waterproofing can be achieved with a pillar made of watertight concrete.

    Strap and Grid Foundation

    This may be necessary if the load is too heavy, resulting in point bases that are too large or too close together.
    Also, it may be necessary where there is a risk of subsidence or slipping. 
    The construction is the same as for a strip foundation.

    Mat Foundation

    A mat foundation is particularly needed in poor bearing soils, but most of all in high groundwater.
    The mat foundation resists the water pressure through its weight and its uniform design, which provides protection against flotation.

    Construction of the mat foundation:

    • After site clearing and setting out, drainage follows.
    • After the excavation work make a min. 10 cm thick insulation protector concrete substrate, on which the waterproofing layers are laid.
      The substrate is wider than the mat in all directions and is provided with a perimeter beam to receive the insulation retaining wall, which can be built from small bricks, for example.
    • Complete the ground waterproofing arouind the entire mat foundation.
    • Lay 5 cm of blinding concrete, and follow with any necessary formwork and ironwork.
    • Complete the concreting, preferably in one session.
    • Finally, the earth is backfilled.

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    Deep Foundations

    COURSE 1 / MODULE 2 - FOUNDATIONS / LESSON 3 - DEEP FOUNDATIONS

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    Well Foundation

    Installation of a prefabricated well foundation:

    1. The cutting edge is placed at the planned location of the foundation.
    2. Excavation of the soil begins within the area of the cutting edge.
    3. The first reinforced concrete casing segment is placed on it and earthwork continues, causing the segment to start sinking together with the cutting edge.
    4. Subsequent casing segments are then placed until the planned depth is reached.
    5. The bottom of the well must be sealed with a waterproof concrete layer; this is the bottom plug.
    6. The inside of the well is filled with lean concrete or sandy gravel.
    7. Finally, a load-bearing head plug is made from a reinforced concrete slab on top.
       
       

    Installation of a steel-cased well foundation:

     

    1. The steel casing wall is sunk.
    2. The soil is excavated using a well-digger bucket machine.
    3. Concreting begins, filling the well in stages.
    4. Simultaneously with the concreting, the steel casing wall is continuously pulled out.
       
       

    The well foundation bodies are typically connected by a grid footing.

    Pile Foundations

     

    Precast piles can be used in loose soil layers and for maximum foundation depths of 10-12 meters. These piles have various cross-sections with side widths of 30-40 cm and a steel-tipped point. The precast reinforced concrete piles are driven into the ground using pile driving, pile vibration, soil flushing, or a combination of these methods.

     

    Drilled Piling, Without Casing (e.g., Soil-Mec): As part of the Soil-Mec process, a guide pipe is installed in the upper section (3-5 m) of a 80-150 cm diameter bore made with a bucket auger to prevent collapse. Bentonite slurry performs the same function in the lower part of the bore. The pre-prepared reinforcement cage is placed into the finished bore, followed by concreting.

     

    Drilled Piling with Recovered Casing (Benoto): In the Benoto process, a casing pipe with a minimum diameter of 80 cm is driven into the soil using rotation and up-and-down movements. Simultaneously, soil is continuously excavated from inside the pipe. This is followed by the placement of the reinforcement and the concreting. The casing pipe is continuously recovered as the concreting proceeds.

     

    Driven Piling with Recovered Casing (Franki): As part of the Franki process, a casing pipe is placed at the planned location of the pile and filled with concrete to a height of 1 meter. The hardened concrete plug is compacted (rammed) to drive the casing pipe into the ground to the desired depth. At the end of the process, the concrete plug is driven out of the bottom. The reinforcement is then placed, and layered concreting begins. Simultaneously, the casing pipe is continuously recovered using constant up-and-down movements.

     

    Diaphragm Wall

     

    A diaphragm wall is typically 40-120 cm wide, 6-40 meters deep - usually extended below the load-bearing soil level, which ideally is also a waterproof layer.
    Better cooperation with the supported soil can be increased by anchoring it back.

     

    Diaphragm wall construction process:
    

    1. Construction of the guide beam;
    2. Excavation of soil from a single panel section in the width of the wall;
    3. Simultaneously, diaphragm wall slurry (grout) is pumped into the place of the excavated soil;
    4. Reinforcement cage is lowered into place;
    5. Concrete is poured from the bottom up, continuously displacing the slurry.

    
    
    

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