Drying out of the ground surface layers.
Long periods of drought result in the evaporation of water normally present in the foundation ground, in some cases causing a reduction in its volume. This contraction is never uniform, either because of lack if homogeneity of the foundation ground or because the structure itself prevents the heat of the sun's rays from directly and uniformly reaching the foundation ground.
This causes a loss of tensional equilibrium in the building above, with the appearance of more or less pronounced fissures (1).
The presence in the immediate vicinity of the building of plants with a very extended root system which can remove water from the foundation ground (weeping willows for example) can cause similar results as can variations in the level of the water table in the area (2).
In many cases, the effects of these factors occur over very long periods of time and this makes it difficult to establish their exact cause.
Leaking water pipes, sewers and drains, etc.
One of the commonest causes of subsidence is the washing out of the foundation ground caused by leakage from buried pipes (3).
The flow of liquids in zones not even immediately adjacent to the building, acts on the foundation ground in saturating it and reducing its mechanical resistance: the foundation material is no longer able to uniformly support the load above and this causes differentiated subsidence. In the same way, infiltrations of uncontrolled surface water, even in small quantities, can cause plasticization of the foundation ground, with an inevitable lowering of its resistance.
Excavations near to the building.
Excavating adjacent to buildings, especially in non-cohesive ground - sand and gravel - without taking proper precautions, can cause dangerous subsidence (4).
The preventive analysis of all the potential causes of subsidence - from the nature of the ground to the presence of a water bearing stratum, to the type and depth of the foundations of the buildings - is fundamental to the establishment of an adequate solution which can ensure the stability of the excavation front.
Inadequacy of the foundation structure.
An inadequate relationship between the pressure imposed on the foundation soil and the acceptable ground load is a frequent cause of structural subsidence. Another cause is the slow consolidation of cohesive type ground. Not rarely in fact, cohesive ground reacts quite slowly to the imposed pressure in reaching a state of equilibrium in periods which can be longer than 10 years. In determining the cause of a subsidence it is therefore necessary to be very careful not to underestimate slow ground movements and not to overestimate the elements which produce effects in a short time.
Replaced soil.
When ground is tampered with during the construction of a building - that is temporarily removed and then put back - it can lose its supporting properties; the differentiated settlement which derives from this can cause fractures which are sometimes quite consequent, (above all in the medium and long term, even longer than ten years).
There are numerous other causes which can cause impairment of buildings.
Here is a brief list of some of them:
· landslide movements to be checked over a considerable area;
· changes in the volume and direction of water flows due, for example, to the carrying out of works near to the affected area;
· differences in the dimensions and depths of the foundation in different parts of the building, which strictly depend on the design criteria employed (5);
· the absence of an organized foundation system above all in very old buildings or in buildings with parts built in different periods;
· overloading caused by ground adjacent to the building when, for example, ornamental gardens are made (6);
· vibrations caused by heavy road traffic or by machinery;
· differences in densification of the foundation ground due to different levels of consolidation (7);
· grounds with different lithological compositions (8).
Long periods of drought result in the evaporation of water normally present in the foundation ground, in some cases causing a reduction in its volume. This contraction is never uniform, either because of lack if homogeneity of the foundation ground or because the structure itself prevents the heat of the sun's rays from directly and uniformly reaching the foundation ground.
This causes a loss of tensional equilibrium in the building above, with the appearance of more or less pronounced fissures (1).
The presence in the immediate vicinity of the building of plants with a very extended root system which can remove water from the foundation ground (weeping willows for example) can cause similar results as can variations in the level of the water table in the area (2).
In many cases, the effects of these factors occur over very long periods of time and this makes it difficult to establish their exact cause.
Leaking water pipes, sewers and drains, etc.
One of the commonest causes of subsidence is the washing out of the foundation ground caused by leakage from buried pipes (3).
The flow of liquids in zones not even immediately adjacent to the building, acts on the foundation ground in saturating it and reducing its mechanical resistance: the foundation material is no longer able to uniformly support the load above and this causes differentiated subsidence. In the same way, infiltrations of uncontrolled surface water, even in small quantities, can cause plasticization of the foundation ground, with an inevitable lowering of its resistance.
Excavations near to the building.
Excavating adjacent to buildings, especially in non-cohesive ground - sand and gravel - without taking proper precautions, can cause dangerous subsidence (4).
The preventive analysis of all the potential causes of subsidence - from the nature of the ground to the presence of a water bearing stratum, to the type and depth of the foundations of the buildings - is fundamental to the establishment of an adequate solution which can ensure the stability of the excavation front.
Inadequacy of the foundation structure.
An inadequate relationship between the pressure imposed on the foundation soil and the acceptable ground load is a frequent cause of structural subsidence. Another cause is the slow consolidation of cohesive type ground. Not rarely in fact, cohesive ground reacts quite slowly to the imposed pressure in reaching a state of equilibrium in periods which can be longer than 10 years. In determining the cause of a subsidence it is therefore necessary to be very careful not to underestimate slow ground movements and not to overestimate the elements which produce effects in a short time.
Replaced soil.
When ground is tampered with during the construction of a building - that is temporarily removed and then put back - it can lose its supporting properties; the differentiated settlement which derives from this can cause fractures which are sometimes quite consequent, (above all in the medium and long term, even longer than ten years).
There are numerous other causes which can cause impairment of buildings.
Here is a brief list of some of them:
· landslide movements to be checked over a considerable area;
· changes in the volume and direction of water flows due, for example, to the carrying out of works near to the affected area;
· differences in the dimensions and depths of the foundation in different parts of the building, which strictly depend on the design criteria employed (5);
· the absence of an organized foundation system above all in very old buildings or in buildings with parts built in different periods;
· overloading caused by ground adjacent to the building when, for example, ornamental gardens are made (6);
· vibrations caused by heavy road traffic or by machinery;
· differences in densification of the foundation ground due to different levels of consolidation (7);
· grounds with different lithological compositions (8).
