SURFACE FOUNDATION PATHOLOGY

Report

The pavements on the median of individual houses sometimes suffer subsidence at the foot of peripheral walls. They usually result in the appearance of a void between the flooring and the base of the skirting boards. More or less important cracks appear in partitions and floor coverings.

Tears of pipes passing in or under the paving can also occur.

Diagnostic

A paving on grade is a horizontal concrete structure of large horizontal dimensions in relation to its thickness (minimum of 12 cm for single-family houses), cast on a form of selected materials and implemented to constitute a stable base. The shape is made on the ground in place after stripping the topsoil, and the paving is in continuous support on this shape, with a possible interface (layer of sand, film, insulation, etc.). As soon as the ground and / or the subgrade show weaknesses, the concrete paving follows these movements.

The main causes of sagging of pavements on the median are:

Land unsuitable for paving on a median

Heterogeneous land (or sloping land with backfill and cuttings), which can lead to differential settlement of the shape, and therefore of the paving; loose soils, frequent in France, which may include rocky kidneys, constituting hard points; gypsum or limestone soils, in which large cavities can appear, by dissolution of the rock.

Compressible land comprising soft strata (peat, mud) below strata of better resistance, resulting in tipping or sagging of the paving; land with unstabilized embankments or foundations in the process of consolidation.

Clay soils sensitive to shrinkage-swelling phenomena linked to changes in the water content of the soil, leading to subsidence of slabs, which are generally non-uniform (see sheet A.2).

Land subjected to the action of water: variations in the level of the water table, leading to cycles of soil compaction and swelling; lands subject to flooding during floods from nearby rivers, causing erosion or hydraulic compaction of the base of the pavement, resulting in rapid subsidence of the latter; land in a basin or with a clay layer, not very permeable, just below the level of the foundations, resulting in saturation of water in the soil, loss of mechanical resistance and differential settlement.

Poor construction of the subgrade

Insufficient stripping of the platform, resulting in migration of aggregates from the subgrade to the underlying soil, which is too soft, and settlement of the form leading to sagging of the paving; pouring of the paving directly on top of the topsoil, without making any shape, resulting in subsequent decomposition of organic matter and instability of the ground.

Nature and composition of the form layer unsuitable for the applied loads, form made from materials containing rubble or clay impurities weakening its resistance; so-called “run-of-the-mill” or friable materials, comprising a high percentage of fine elements, which generate settlements.

Insufficient compaction of the form, in particular on the peripheral banks, the main factor of the slab subsidence; insufficient tightening of the granular skeleton considerably reduces the load-bearing capacity, resulting in a sharp decrease in resistance to deformation; compaction on layers that are too thick (do not exceed 20 cm in thickness per layer to be compacted), too wet or using unsuitable compactors; absence or insufficiency of compaction along load-bearing walls.

Other potential causes:

Rupture of pipelines buried under paving, resulting in leaks causing soil scouring and leading to subsidence of paving.

Presence of an insulation only at the periphery and not under the entire surface of the paving, resulting in differential settlement depending on the compressibility of the insulation.

Insulation compressibility not suitable.

Compaction of the soil under the effect of water suction by the roots of trees implanted too close (distance to be respected from 1 to 1.5 times the height of the tree at its adult size).

Good practices

At the design level

obtain local information on the level of the water table, on possible floods and on the existence or not of paving in neighboring buildings;

establish a rigorous diagnosis of the risks, in order to assess the feasibility or not of paving on an embankment: consultation of the departmental PPR Argiles, if there is one, soil reconnaissance (geotechnical study obligatory in certain cases) and drilling allowing to identify the layers of ground and to evaluate their mechanical characteristics;

in case of clay soils, prefer a floor on crawl space to the solution of a paving on an embankment, sensitive to the effects of shrinkage-swelling.

At the implementation level

carry out sufficient stripping of the topsoil (to be evacuated over a thickness of approximately 30 cm);

realize the shape with suitable materials;

make a careful compaction of this form, in layers of 20 cm maximum, with suitable material, including, essential point, on the periphery and in line with the facades and splits;

check the compaction results (DTU 13.3);

check the quality of construction of the pipes buried under paving.

The essential

Do a paving on the median only on a ground suitable for receiving it (according to the study of the ground).

Pay great attention to the implementation: composition of the subgrade, quality of compaction, control of the result.

To consult

DTU 13.3: Pavings - Design, calculation and execution.

DTU 21: Execution of concrete structures.

NF P94-093: Soils: recognition and testing.

Determination of the compaction references of a material

Normal Proctor Test, Modified Proctor Test.