Category : Home Renovation
– Context of compaction
– Compaction events
– Sources of compaction
– Preventing compaction to avoid possible disorders
Compaction is the deformation of the ground under the action of the weight of the structures it supports. Buildings behave differently depending on their structure and foundations in relation to the ground: this is known as “uniform compaction” and “differential compaction”.
Context of compaction
A building rests on the ground and transmits a set of loads to it. The foundations play a role in the good distribution of these load constraints. These foundations are adapted both to the nature of the soil and to the structure of the building.
In general, under the action of the building loads, the compaction of the ground remains uniform and of small amplitude: the construction, endowed with a good rigidity, does not suffer any damage.
When the compaction is heterogeneous (differential), with differences in level between different foundation points, disorders may occur. For a very rigid structure, the failure of one support simply results in the transfer to neighbouring supports: no damage is suffered as long as the compaction remains of low amplitude.
For a flexible structure that adapts to the movements of the foundations, disorders may appear at the level of the elements of the finishing work: uncoupling joints must be provided to prevent them.
Compaction can therefore be allowed as long as the building structure or its components can absorb it. There are orders of magnitude of the permissible compactions given by specialists in soil mechanics or residential developers. They recommend values for different construction elements for total or differential compaction:
– 5 mm to 1 cm for the differential compaction of a large brick wall;
– 3 to 6 mm for a conventional residential building between two points 10 metres apart;
– 5 to 10 cm for the total compaction of a beam and a differential compaction of 2.5 to 4 cm;
– 2 to 5 cm for the total compaction of a masonry wall;
– 8 to 30 cm for the total compaction of an invert.
Manifestations of compaction
Disorders related to uncontrolled differential compactions are manifested in the form of cracks:
– at 45° from the angle of an opening or a lintel for a fragile structure;
– by blocks at the joints for rigid structures such as concrete walls;
– in stairs for structures made of small masonry elements such as block walls;
– diagonally (shear cracks) for differential compaction of a gable in relation to the rest of the building;
– horizontal in the case of differential compaction of the gable by tension or swelling;
– stepped by compaction or swelling at the corner or centre of the building;
– in a paving connected to the foundations.
Compaction disorders can also be manifested by outcropping of prefabricated facade panels.
Note: Compaction disorders also occur on ancillary structures: at the level of networks such as sewage or rainwater drains cut off at the compaction level, at the level of the manholes of these networks, which subside due to compaction and cause fractures in the pipes, and at the level of peripheral structures, such as a terrace or embankment around the construction causing subsidence and cracks.
Sources of compaction
The sources of compaction can be of different kinds:
– a variable bedding level for the same building, such as a house with only a partial basement;
– a heterogeneous type of subgrade: one side on agile soil and another side on limestone or presence of hard spots (rocks) and soft spots (compressible such as uncompacted fill);
– variation in the moisture content of the bedrock, such as clayey soil that shrinks in times of drought and swells when the rains return;
– Unusual water infiltration and accumulation due to weathering or drilling of a pipe;
– the proximity of roots, the presence of an underground source, the topographical configuration (unevenness, steep slope);
– unstable soil consisting of fill, such as a former rubble deposit, the filling of a former underpass or quarry;
– the presence underground of natural voids and mining subsidence;
– work in the surrounding area, such as excavation for the foundations of a new building adjoining or located nearby;
– vibrations caused by nearby work (such as pile driving) or the passage of heavy machinery in the vicinity;
– the weight of new construction loads on adjacent land;
– the deterioration of concrete or steel embedded in concrete due to the aggressiveness of the environment;
– design and execution errors due to an under-evaluation of the efforts or to not taking into account the soil study: this case is encountered especially in the case of deep foundations.
Prevention of compactions in order to avoid possible disorders
Soil investigation (geotechnics) and foundations are closely linked and are the essential basis for preventing compaction. Soil investigation and the study of the structure of the building to be constructed make it possible to assess the pressures exerted (load diffusion bulb) and to determine the shape of the foundations accordingly.
Adaptation of the foundations
In a building, the foundations ensure the transfer of forces: vertical compression, the building’s own weight, overloads, and they respond to the reaction forces of the ground (its bearing capacity).
The type and dimensions of the foundations are thus determined. They imply knowledge of the nature of the soil: a soil study (geotechnical) is therefore strongly recommended. Ignorance of the geotechnical properties of the soil can lead to the construction of foundations that are not deep enough, or that are built on unstable fill or on heterogeneous soils of different bearing capacity.
Foundations take various forms depending on the geotechnical study: superficial when the hard ground is close to the surface, deep when the resistant layer of foundation is at a great depth, special to meet particular criteria such as in marshes.
Good to know: Depending on the region of construction, earthquake-resistant building regulations must also be complied with.
Points to watch out for
The depth of the hard ground for the foundations should be determined, the cleanliness of the excavation bottoms should be checked to ensure that the footings are properly poured on the hard ground (clean soil and fill falling to the bottom) and the pouring of the foundations should be postponed in the event of heavy rain (stagnation of water at the bottom of the excavation).
It is also necessary to highlight the heterogeneities of a soil requiring specific adaptations (such as the construction of a foundation in wells and longlines to pass an area of unstable embankments).
Good to know: when you build on a subdivision, a soil study linked to the development permit for this subdivision is available and can provide you with information.
Expansion and Compaction Joints
As soon as the masonry walls reach large surfaces, it is necessary to intersect them with joints that respond to the expansion or shrinkage of the materials, to the difference in load brought by the construction on the foundations, to the presence of heterogeneous ground and therefore different foundations, to the antiseismic protection (antiseismic rules).
The spacing between the joints is determined according to several criteria.
The first is the location of the building: in dry regions on shores, the joints must be provided every 20 metres, whereas in temperate wet regions, it is every 35 metres in the case of load-bearing masonry.
Then come the general design of the building, the nature of the masonry materials, the function of the masonry (load-bearing or not) and the role of the joint.
Good to know: the joint must be regular over the entire height (without contact points) and at most 2 cm wide. On the facade, it must be protected against infiltration by a watertight bead and a joint cover. On the inside wall, the joint can be made with a flexible product with good fire resistance.