Pathology foundations

SURFACE FOUNDATION PATHOLOGY

Subsidence of paving of single-family houses

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.

Movements of foundations of single-family houses - Part 1: current settlements

Report
Individual houses are usually superficially founded with reinforced concrete footings.
In the presence of a deformable ground, these foundations can undergo movements which could cause significant damage.
The deformability of the ground under the effect of the loads applied can thus promote the appearance of cracks or cracks on the walls. Interior fittings, especially if they are hard (plaster tile partitions, tiling), can also be affected by these movements.
A differential settlement of the foundations of the order of a centimeter may be sufficient to induce these phenomena.
Diagnostic
The individual houses concerned were generally built in masonry (bricks or concrete blocks), without a complete basement, on reinforced concrete foundations of shallow depth (generally between 0.50 m and 1 m).
The function of these foundations is to diffuse the loads of the building (weight of materials and operating loads) in the ground to a depth which does not, in principle, exceed 3 to 4 m below their base level.
If, within these limits, the ground is composed of rocky elements in a homogeneous way, no serious incident is to be feared. The pavilion, on the other hand, risks suffering the consequences of ground movements if the latter randomly contains rocky inclusions or has a deformable layer (clayey, silty, sandy).
Note: the specific risk of clay shrinkage / swelling is discussed in sheet A2.
Three conditions must be met to trigger cracking.
A compressible soil below the foundation level. The presence of compressible soils within this limit of influence of the foundations can cause the soil to deform too strongly under the weight of the structures. The deformation of the soil is not uniform.
Irregularly distributed loads to the foundations. This imbalance of forces on the ground causes differential settlement. The ground will therefore deform more or less depending on the points of support.
(see diagram below). This is also the case when major excavations are made in the immediate vicinity of existing foundations.
The fragility of the superstructure. The differential deformations of the ground are for the superstructure of the imposed displacements, which generate tensile and shear stresses in the superstructure. The weakest points and in particular the masonry joints will be the first not to resist these stresses.
The other causes of disorders.
The location of the building on a heterogeneous soil, containing rocky inclusions which constitute as many hard points or, on the contrary, plant debris (peat) or decomposing organic matter (mud) which form compressible zones.
The installation of the house directly on top of the topsoil or at such a shallow depth that the ground is not protected from frost (refer to FD P18-326 Concrete - Frost zones in France).
The juxtaposed presence of two embankments of different ages under the foundations: an old one, not very compressible, and a recent one, poorly compacted during the works.
The subsequent creation of a platform against a facade of the pavilion. It creates an excessive parasitic load in line with the sole of this wall.
The rupture of a buried pipe or a rainwater manhole, or even simply the fall of precipitation in the excavation bases during the execution. It causes an inflow of water which locally changes the consistency of the ground at the foot of the wall.
The close presence of some trees.
Good practices
Carry out a soil study which must take into account the planned construction and indicate the predictable settlement values. It is from these values that the BET structures will be able to size its work. Compressible or clayey soils require special vigilance and the adoption of specific measures (see sheet A2 already cited).
Do not use fiber concrete processes in foundations, which are covered by a Technical Assessment, without having informed the insurer.
Thoroughly strip and clean the bottom of the excavation.
Postpone the pouring of the foundations in the event of too much rain.
Respect the other conditions for installing shallow foundations set by DTU 13.11.
In the case of sloping constructions, respect the maximum slope between two successive footings, which is limited to a slope of 3 to 1.
The essential
Do a soil study.
Install in accordance with the BET structures specifications (reinforcement, dimensioning, depth).
To consult
NF P 94-500: Geotechnical engineering missions - Classification and specification.
XP P94-011: Soils: reconnaissance and testing.
DTU 13.11: Shallow foundations.
DTU 13.12: Rules for the calculation of shallow foundations.
NF DTU 20.1: Masonry structures in small elements

Movements of foundations of detached houses - Part 2

Report
So-called "swelling" clays are dangerous for foundations: settling in times of drought, lifting when the clays rehydrate.
This alternation of settlements and heaving causes damage in the walls, because they occur in a heterogeneous way under the foundations. In severe cases, the cracks can reach a width of 30 to 40 mm (these are called cracks) and may require
the total deconstruction of the work.
They can also impact outdoor facilities (sidewalks, stairs, etc.), access roads, underground water networks, etc.
Diagnostic
In normal seasonal alternation, variations in water content disturb the water balance over less than one meter of so-called swelling soils (predominantly montmorillonite clays).
Likewise, a prolonged drought causes a strong evaporation of water between the surface and the clay bank to a depth of 2 to 4 m (see diagram). This phenomenon causes a significant decrease in the volume of the clay soil, which results in a shrinkage of up to ten centimeters.
These ground movements are not uniform under buildings because they form a screen against evaporation. Significant differential forces therefore appear between the center of the pavilion and its periphery, hence the appearance of cracks or cracks.
This process is aggravated by the presence, near the pavilion, of certain vegetation whose water needs are important: oaks, poplars, ash trees, etc.
The very nature of the clays concerned can give rise, during a subsequent very rainy period, to an opposite phenomenon of swelling which tends to close the cracks.
The pavilions installed on this type of clay, which is predominantly montmorillonite, are usually subjected to cycles of shrinkage and swelling, in particular at the level of the most directly exposed parts, located at the periphery. While the central part of the pinna may appear more stable in the short term, it may be subject to the effects of progressive swelling of a large amplitude in the longer term.
This may be due to a continuous supply of water during the rainy period under the effect of rising water tables, for example, followed by the impossibility of evacuating this excess water in the drier season, or to a rupture of a pipeline.
Good practices
Consult upstream geological maps on: http://www.argiles.fr/.
Identify whether the ground is in an area affected by the phenomenon of swelling clays.
Carry out a soil study to know the main geotechnical characteristics and send it to the BET structures.
Laboratory tests (Atterberg limits, oedometer test) are essential to accurately recognize the type of soil encountered and the swelling potential of any questionable clay layer.
Check the local existence of a specific PPR (Risk Prevention Plan), and take the necessary precautions:
move construction away from trees, or use anti-root screens;
anchor the construction more deeply and evenly: minimum depth of - 0.80 m in low to medium hazard, -1.20 m in high hazard;
stiffen the structure (vertical and horizontal chaining);
provide a rupture joint between the structures (the house and the garage, for example);
limit evaporation on the ground near houses (terrace or geomembrane).
The essential
Visit the website www. argiles.fr.
Have a geotechnical study carried out by a specialized BET and the concrete study it induces.
Lower the excavation bottom (-0.80 m to -1.20 m depending on the hazard).
Keep vegetation away.
To consult
DTU 13.11: Shallow foundations.
DTU 13.12: Rules for the calculation of shallow foundations.
NF DTU 20.1: Masonry works in small elements.
DTU 21: Execution of concrete structures.
NF P94-500: Geotechnical engineering missions - Classification and specifications.
XP P94-011: Soils: reconnaissance and testing.
Although withdrawn, DTU 11.1 sometimes appears in certain written documents.

Lack of waterproofing on a buried wall causing the development of dry rot

Disorder
Significant humidity is detected on the gable wall of an old house. In addition to rotting of woodwork and floor structures, the development of dry rot is detected not only on the affected wall but also on other walls.
Diagnostic
Since the demolition of a neighboring building located behind the gable concerned, rainwater has trickled down the wall and infiltrated the grassy area. The water passes behind the plastic protection not held in mind. They are also blocked by the old foundation not demolished. All of this water eventually seeps into the brick wall of the house.
Recommendation
A real waterproofing of the buried walls including the old foundation of the demolished building and a drainage system should have been put in place at the foot of the brick wall. In addition, a rapid reaction during the first manifestations of the disorders would have avoided the development of dry rot throughout the house.

Lack of drainage from a buried wall

Disorder
Significant humidification of an old buried wall separating a private garden from a wine building located 3 m below
Diagnostic
Before the creation of a private garden upstream from the dividing wall, there was a shed at the same height as the current garden.
The removal of this shed, which previously protected the land from runoff, led to water migrations in the ground, water then migrating towards the old buried separating wall, then appearing in the lower premises located on the other side. side of the wall.
Recommendation
Replace the water protection of the upstream land (initially provided by a shed) by providing effective drainage against the old wall, or even lower this land to allow the buried part of the wall to be sealed.

Collapse of a building following a broken EP pipe

Disorder
Claim declaration: "Folding of the cladding".
The building actually squeezed more than 20 cm on the first frame of gable posts.
Diagnostic
Cause of damage: differential settlement
Origin: rupture of an EP pipe embedded under the paving which had entrained the fines from the ground.
Recommendation
Maintenance of the building has not been done.
At the first signs of the EP descent leak (loading), the owner should have been worried about the possible consequences and not wait for the building to collapse.

Sagging of a paving under the weight of the chimney

Disorder
The installation of a chimney in a single-family house, one year after the reception of the pavilion, caused the slab to sag on the bank, under the influence of this chimney.
The settlement measured to the right of the plinth is of the order of 0.5 to 1 cm. This deformation of the support caused the chimney to crack at various points, making it unusable.
Diagnostic
The embankments made along the base wall were poorly compacted during the preparation of the support platform for the paving body.
The one-time overload of 800 kg brought to the edge of the paving by the chimney, significantly higher than the operating loads for which the pavements for residential use are designed, caused a settlement of the free edge of the concrete paving along the structural wall. .
Recommendation
During the construction of the pavilion, the structural work company should have checked the quality and the implementation by successive layers of the backfill materials, in particular along the base walls, and possibly carried out monitoring tests. Westergaard plate test or Proctor test.
The location of the chimney being also known during the construction of the house, any adaptation of the foundations should have been studied according to the overloads induced by this structure.

Subsidence of an extension

Disorder
The first manifestation was a sagging of the paving of the extension part compared to the floor on crawl space of the existing part. Damage then appeared on the interior plaster plates: cracks, cracks.
During the appraisal operations, no particular cracking was visible in the exterior masonry. On the other hand, false plumbs of these masonry were visible, as well as a rupture of alignment of the roof ridge. The damage was progressive.
Diagnostic
As regards the paving, the latter, carried out on poor quality new backfill, produced its classic and differential settlement compared to the old part with a floor carried on Sanitary Vacuum.
The other disorders were not directly linked to this slump in the pavement. The cracks observed were in fact the consequences of the extension tilting outwards.
After soundings, we were able to observe that the level of the foundations of the extension part was well above that of the old part, in a black and organic horizon. Roots were present under the foundation of these foundations. These roots came from good-sized willows nearby.
We are therefore in the presence of subsidence of shallow foundations set too high, in a soil that is still organic. Compaction worsened during dry periods as in 2003 by the greediness of the old and powerful willows nearby.
Recommendation
Given the high thickness of organic soil and the presence of willows nearby, a preliminary geotechnical study was essential to determine the level of foundation, also taking into account the level of the existing foundations. This study would also have made it possible to determine the level of influence of the willows in the vicinity and to provide anti-root felts or any other type of equivalent protection.

Sloping slabs and partitions

Disorder
After the start of a settlement of about 5 mm on the edge of the paving, sudden aggravation of the disorder two years later to lead to a cracking reaching 2 cm of opening in the ceiling, and compromising the stability of the counter partitions.
The paving / tiling assembly has not suffered any obvious disorder or lack of flatness, despite this settlement of 2 cm.
Diagnostic
The technique of paving on the ground, separated from the base, requires perfect compaction of the base, which is known to be difficult on the edge of the paving due to the overhangs of the footings.
Many builders, to avoid this problem, have tips for supporting paving in these sensitive areas.
In this case, the mason had placed two 0.10 chipboards vertically on the overhang of the sole; Badly wedged against the base, these agglomerations suddenly collapsed, dragging the paving.
Recommendation
To avoid settling of the paving, the solution of the floor on crawl space or of the slab supported on the base, with the necessary bonding steels, is preferable.
However, if one wants to achieve a detached paving, the compaction of the foundation must be perfect.

Paving slump for insufficient compaction

Disorder
The paving gradually sinks from a corner of the pavilion. The gap under the plinth reaches 2 cm. At the level of the thresholds of French doors, the tiles have peeled off, cracked and chipped. Apart from these singular points, the tiling is intact.
Diagnostic
The masonry walls of the house show no deformation or cracking. The underground networks are not leaky. During construction, the leveling of the platform required the implementation of an embankment at least 40 cm high. The compaction was carried out all at once without a machine adapted to such a thickness. Under these conditions, the platform was insufficiently compacted and settled down.
Recommendation
- Reduce the height of the backfill layer, or
- Have suitable compaction equipment on site, or
- Replace the paving on the median with a supported slab or a floor on a crawl space.

Insufficiently supported shallow foundation subsidence

Disorder
The garage gable has a continuous, through horizontal crack with a 2 mm opening at 30 cm from the ground, as well as a vertical crack located exactly in the middle of the gable.
Diagnostic
The seating level of the house is located approximately 1.50 m higher than the natural terrain immediately behind the house.
The soil study carried out during the appraisal revealed the fact that the foundation of the house is lowered to 60 cm deep (frost protection) while the schistous substatum is located at about 1.60 m deep in a silty sand backfill with insufficient bearing capacity
Recommendation
The simple examination of the configuration of the premises with a level difference of nearly 1.50 m between the two adjacent sites justified, if not a soil study, at least a serious search for "good soil".
A solution using wells and stringers could have been considered.

Subsidence of foundations

Disorder
We observed a movement of the walls of the construction on the front and rear facades which results in cracking following the joints between the cement blocks harped with dislocation of the blocks and partial fall of these.
Diagnostic
A ground survey allowed us to observe that the entire pavilion was founded on insufficiently compacted embankments and that the altitude at which the foundation was constructed did not comply with frost protection.
Recommendation
It was necessary to compact the backfill to obtain an acceptable tillage rate and lower the foundation to a depth allowing it to be protected from freezing and, in this respect, comply with DTU 13.12

Conservatory sag founded on embankment

Disorder
A significant settlement of the veranda soil occurred gradually.
It takes the form of a gap of about 5 mm under the baseboards along the wall of the house.
No tiling element cracked except at the connection with the house at the threshold. Here the tiles sound hollow and start to crack.
We find the same space outside between the bricks of the base of the veranda and the masonry of the house.
Diagnostic
The veranda is founded partly in the embankments of the construction along the house, and partly on the natural ground for the external part of the veranda.
The whole structure, the tiled paving, and even the veranda itself gradually rotated: the external part founded on the good soil (stripped natural ground) did not sink in while the internal part of the veranda based on insufficiently compacted embankment descended.
Recommendation
In the basement fill area alongside the house, the veranda foundation should have been lowered to the same level as the basement so that it could be based on the natural terrain and not the backfill.

Trees and individual houses

Disorder
After episodes of drought in the region, the masonry and the libages of foundation cracks in the area of tree plantations.
The land is clayey. Cyclical opening and closing of cracks according to the seasons and rainfall.
The foundations are however seated at -1.20 m from the TN.
Diagnostic
By making a trench between the vegetation and the house, roots are present at the bottom of the excavations. These have developed from the angle of the house because here, the water content of the clays varies less quickly than in the garden.
By suction effect, the roots pump the water under the foundation which causes the punctual withdrawal of the clays under the angle of the house, then a differential settlement of the angle.
Recommendation
During construction:
If the tree belongs to the owner of the house, the builder and the owner should have cut and stumped the tree,
If the tree belongs to the neighbor, the builder and the contracting authority should have moved the construction away from the property line, respecting the safety distance indicated below or creating an anti-root screen lowered to a minimum depth of 2.50 m .

Insufficient casing

Disorder
New flooding of the basement and the exterior courtyard under 95 cm of water. Pumping, put in place as a precaution, prevented the water from rising to a height of 1.20 m.
Diagnostic
The waterproofing covering of the casing, a PVC membrane, was raised only 50 cm against the vertical walls. When the level of the water table rose, during the winter of 2000-2001, the water passed over these readings of very insufficient height.
Recommendation
The designer did not take into account the conclusion of the report of the geotechnician who had installed piezometers after the first flood. This report noted a water depth of 27 cm on the day of the survey, but the conclusion took into account the foreseeable fluctuations in the water table and recommended a minimum casing height of 1.50 m.

Collapse of a neighboring gable before pouring foundation footings

Disorder
At the end of the earthworks on a platform, a neighboring house partially collapsed
Diagnostic
The construction operation began with a general earthwork of the platform and a bench of insufficient size was temporarily left in the vicinity of an existing house.
No recognition survey of existing foundations has been undertaken. As the house has no foundation and the level of the platform is located about 30cm below the first stones of the gable, the earth has crept under the house.
This phenomenon was accentuated with the rains, causing the sudden fall of the pinion.
Recommendation
Comply with article 1.33 of DTU 12 (extraction in several phases of the cuttings and resumption of underpinning of the foundations).
Carry out a reconnaissance of the existing foundations by drilling before earthworks.

Collapse of marnière

Disorder
A subsidence of the underground structures was observed, with:
• cracks following the joints of the exposed masonry on the outside of the pavilion,
• collapse of structures visible inside the basement at one of the corners of the pavilion.
The foundations were shallow with a partially buried basement.
Diagnostic
This disorder testifies to a very insufficient bearing capacity of the ground supporting the foundations at this location.
This collapse, however, remains localized and results from a one-off terrain accident.
This is a classic case of collapse above a marl-type cavity backfilled with compressible or insufficiently compacted materials.
These cavities may have been only partially backfilled after operation.
Recommendation
A soil study could have made it possible to directly detect this cavity in the best case.
But without direct detection of this cavity, which remains punctual, a soil study would have made it possible to observe an anomaly in the neighboring soils in the event of the collapse of this marl by decompression of the latter.
Likewise, a preliminary neighborhood survey could perhaps have made it possible to find out about the existence of this old marnière exploitation.

Cracking under the effect of shrinkage-swelling of clays

Disorder
The rear part of the house, housing the garage off the hook from the living area, is the site of a very significant cracking of up to 2 cm of opening, horizontal and stepped, appearing in a period of "drought", and attesting to differential settlement; the masonry is totally dislocated in the corner of the garage door.
Diagnostic
The 50x25 section strip footings are anchored at 0.70 m in clays; soil reconnaissance showed that these clays were excessively sensitive to water (methylene blue tests).
This sensitivity to water is twice as high on the rear facade than on the front facade, which explains the differential settlement.
On the other hand, the presence of an adult oak at a distance less than the height of the tree greatly aggravated the desiccation of the soils.
Recommendation
In the absence of a soil study, take into account the nature of the soil, by anchoring the footings more deeply, at depths at which the clays are less sensitive to desiccation (according to PPR Withdrawal Swelling if there is, at a depth of 0.80 m, or even 1.20 depending on zone B1 or B2).
Likewise, the presence of the tree had to be analyzed: by cutting it and desiccating it, or by setting up an anti-root screen.

Pavilion foundation at the edge of the neighbour's basement

Disorder
Appearance of significant cracks on the front and rear facades of an individual pavilion with a single ground floor in the adjoining area of the neighboring pavilion.
The cracking on the rear facade is reflected inside the construction with dislocation of the masonry.
Diagnostic
The neighboring pavilion, built prior to the damaged structure, is built on a basement, on the property line.
The foundations of the damaged building consist of continuous reinforced concrete footings lowered over the entire periphery of the structure at approximately -0.70m / TN.
To the right of the neighbor, the foundations are thus seated in the external backfill of the masonry wall during the construction of the basement and therefore present a significant and progressive settlement.
Recommendation
The foundations of the damaged structure should have been lowered to the ground not altered by the work of the neighboring pavilion, that is to say at the base of the wall of its basement.
The foundations of the return facades then had to be stepped in order to catch up with the current base level of the sole in accordance with DTU 13.12 "Calculation rules for shallow foundations".

Foundations destabilized by EP manhole leak

Disorder
Several years after the completion of the construction work, the client observed the appearance of cracks, mostly horizontal, at an angle of the extension opposite to the existing construction. The maximum opening is 1.8mm.
Strangely, no cracks are visible on the linings glued on the inside of these concrete block walls. There is no noticeable trace of water infiltration in the house.
The other angle of the extension is not affected.
Diagnostic
An EP gaze is located at the relevant angle. His visual examination revealed that the mortar seal between the PVC pipe and the concrete manhole was imperfect. The look is probably elusive.
The water which infiltrates very frequently, and always in the same place, in the clayey loam soil entrains the finest particles. And over the years the soil is decompressed to the level of the foundation bed. These are then destabilized and move.
The diagnosis made, a gauge is put in place on the main crack and the manhole is sealed by the client. 9 months later a new reading of the gauge shows that the crack has only worsened by 0.3mm, while the bedrock dries up. The diagnosis is therefore confirmed.
Recommendation
It is always preferable to have the work carried out by a competent and properly insured company.
EP manholes and buried networks installed on or in excavation embankments move as they naturally compact over the years. In fact, these embankments are only very rarely compacted with suitable mechanical means, and in thin layers. As a result, the mortar seals between pipes and manholes crack and leaks appear. This can happen even if the manholes are fixed on consoles secured to the construction.
It is therefore advisable to use manholes fitted with rubber gaskets for the penetration of the pipes. Thus, even if the pipes move a little, the tightness of the manholes is preserved.

Shallow foundations

Disorder
Vertical lizard at the junction between the existing building and its extension
The opening of the crack is more pronounced in the upper part.
Diagnostic
The vertical crack reflects the absence of an expansion joint between the two constructions
The particular shape of its opening indicates a slight tilting of the extension linked to the settlement of the foundations located at the end.
The foundation of these foundations was located less than 40cm deep.
Recommendation
Create an expansion joint between the two buildings
Lower the foundation base in the right ground, respecting the frost protection.

Infiltration through a buried wall made of polystyrene shuttering blocks

Disorder
The interior walls of the basement were clad with BA13 plates to protect the polystyrene. Water infiltration resulted in complete degradation of these plates.
Diagnostic
The exterior plaster has been reinforced with a metal mesh. This provision does not comply with the general conditions of use and implementation of coatings applicable to walls made using processes based on expanded polystyrene formwork blocks which provide for a system of reinforced fiberglass reinforcements.
The buried external chaining is not protected by any waterproofing system: the polystyrene is visible when the earth is removed.
The vertical facing ensuring the vertical drainage of the wall does not go up to the top. This arrangement not only renders the drainage inefficient but also aggravates the disorder by promoting the accumulation of water behind the drainage facing.
Recommendation
Strictly respect the prescriptions of the Technical Assessment and the CSTB documents for the type of coating.
Carry out a careful implementation of waterproofing and drainage.

Infiltration in the basement by piercing exterior plaster

Disorder
Significant infiltration in the basement through exterior concrete block walls.
Diagnostic
During the construction of the sidewalk around the pavilion, the concrete paving was poured in contact with the facade plasters. In order to prevent the sidewalk from settling on recent embankments, this paving was firmly attached to the facade by means of horizontal bonding steels piercing the exterior plaster approximately every 30 cm.
Recommendation
Completely prohibit the anchoring of the sidewalk in the facade by bonding steels, in order to avoid penetrations of the plaster and infiltration by these openings.
Interpose a polystyrene separating sheet between the concrete paving of the sidewalk and the facade, in order to allow differential expansion between these two structures and thus avoid the appearance of injuries on the plaster.

The consequences of a lack of waterproofing in a rainwater manhole

Disorder
Outside:
Linear horizontal crack of 5 to 8 mm opening in whistle, located in the angle near the rainwater manhole.
Interior: 5 mm gap under the tiled plinths.
Diagnostic
Cause: differential settlement.
Origin: leakage of the rainwater manhole.
As shown in the diagram, rainwater is diffused along the foundations.
Saturated with moisture, clays lose their bearing capacity.
Recommendation
Implement prefabricated watertight manholes.

The risks of poorly controlled stumping

Disorder
Exterior:
Appearance of cracks of 2 to 3mm in stairs on the exterior walls.
Interior:
Voids under plinths of almost 2cm.
Diagnostic
Cause: differential settlement.
Origin: The land was leveled after stumping and before the arrival of the mason.
The mason has set the foundations on ground that has been altered indiscriminately.
Recommendation
Stumping should have been coordinated by the mason and the builder.
The mason and the builder should have foreseen:
-A deepening of the earthworks at the bottom of stumps,
-A large concrete retrofit,
-a reinforcement of the reinforcement of the foundation footings.

Slant movement of a terrace on a median

Disorder
During the construction of a single-family house, a terrace on the ground floor was created along a gable as an extension of a terrace on a supported slab located on the front facade.
More than 2 years after acceptance of the work, a spill from the end of the terrace onto the median was observed with the appearance of cracks.
At the angle formed by this gable and the facade is a pluvial waterfall foot look. This look is fractured.
Diagnostic
This masonry manhole is fractured due to its positioning on the one hand on the supported terrace (hard point) and on the other hand on the backfill of the neighboring terrace. The water collected in this manhole flows into the backfill causing it to move and the slab founded on it to tilt.
Recommendation
A more judicious positioning of this manhole, either at the level of the supported slab, or at the level of the terrace on the ground, would have prevented the breakage of this one and therefore any presence of water in the backfill.

Opening of excavations in clay

Disorder
Cracks appear in the corners of the house in the masonry in elevation.
Diagnostic
The client shows us photos of the construction and tells us that the excavation of the footings took place during the rainy period, on a Friday. During the weekend, the bottom of the excavations remained filled with water. The soles were only poured on Monday.
The clays, sensitive to water, rehydrated during the waiting period for the foundations to be poured.
Recommendation
- Open the excavations of the soles,
- make 1 hole deeper at each angle outside the right-of-way of the house to avoid the accumulation of runoff at the bottom of the excavations and above all,
- concrete immediately after the opening of the excavations.

Pollution of a pavement by organic matter

Disorder
After several months of operation and routine maintenance, uniformity defects appear on industrial paving in the form of small holes of different sizes with the inclusion of splinters of pieces of wood.
The presence of roots and rootlets in the upper part of the paving is incompatible with the use of the building for packaging food products.
Diagnostic
The organic materials present in the ready-to-use concrete constituting the body of the paving have their origin in the aggregates used by the concrete batching plant, the source of which of these materials is known to contain plant impurities in significant quantities.
Recommendation
Require ready-mixed concrete conforming to standard NFP 18-305. The concrete producer is required, in particular, to apply a quality plan which guarantees the regularity of the operations to verify the conformity of the constituents.

Land thrust on buried wall

Disorder
The back wall of the garage has a "belly" of several centimeters.
Significant cracks start from the four corners of the wall and meet in the middle of the panel.
The stability of the whole is threatened.
Diagnostic
The masonry of hollow agglos of 20 is centered on a BA flange 50 cm wide.
The panel has only 3 vertical chains incorporated into the masonry (1 in each corner and 1 in the middle)
The thrust of the land over 3.60 m high was totally underestimated. The structure was designed as a classic pavilion, the basement of which is buried to a height of 2.20 m.
Recommendation
Going beyond the dimensions of ordinary pavilions, the contractor should have sought the advice of a reinforced concrete design office. The study would have led to the construction of a real retaining wall made of reinforced concrete or, at the very least, mounted in banchor blocks with a sufficient number of stiffeners.

Presence of marnière under a flag

Disorder
To date no disorder is noticeable at the level of the pavilion, the marnière having remained in relatively good general condition. However, the risk of more or less long-term collapse cannot be excluded. Indeed, after visiting this marnière, a few pockets of collapse were observed. This marnière was exploited from around 1850 until the years 1900 to 1930.
Its existence was no longer known, and it is therefore without special precaution that the pavilion was established on this marnel whose roof is located about 20 m deep underground.
Diagnostic
The origin of this exploitation is probably agricultural, the type of material extracted (chalk) generally used to amend agricultural soils. Its operation was listed in the departmental archives of the department concerned but the town had not kept any traces. The building permit was therefore issued in ignorance of this underground cavity.
The presence of this marl was accidentally detected by the owner of the pavilion during a partial collapse of the land, in fact localized collapse at the level of the access shaft to this former exploitation.
These collapses are the consequence of the pushes of upper moist clays.
The roof of the cavity consists of a bed of flint naturally linked by chalk.
This binder has many cracks saturated with clay, which facilitates sliding between the elements constituting the vaults. The collapses can therefore be more or less important depending on the density and importance of these cracks.
Recommendation
In this specific case, a deep soil study would have made it possible to detect the marlière given its large size. However, a more superficial study would not have allowed this detection, the marnière having remained complete and without major collapse, that is to say without decompression of the ground around. As for the access shaft, it is too punctual for one to be able to hope to carry out a correct boring in its alignment without its knowledge.
On the other hand, a neighborhood survey or research in the archives, would have allowed the identification of land at risk. These surveys are important in particular in regions already reputed to have this type of old farm.

Paving lifting

Disorder
Between the load-bearing walls of the basement, the concrete paving presents a curvature of the order of 4 cm with cracks parallel to the facades. The longitudinal cross wall of the basement has a vertical crack. This one put under observation during 18 months worsens regularly.
Diagnostic
The photo taken during the work by the client suggested that the foundations are unsuitable for the peaty nature of the soil. The reconnaissance survey and the soil study carried out as part of the appraisal confirm the narrowness of the foundation footings, and the presence of peat and groundwater 50cm below the foundations. In fact, unsuitable foundations are gradually sinking into the ground.
The presence of large blocks of chalk in the bedding layer of the foundations is the cause of the sole rupture under the crack of the slit.
Incidentally, because unrelated to the disorders, it is noted that the foundations are not "frost-free" (70cm deep in the region) to the right of the boring, at the entrance to the basement.
Recommendation
The site is notoriously boggy, moreover the map mentions it in full. Peat is very compressible. The local builder should therefore have had a prior geotechnical study carried out in order to determine the appropriate foundation method and have them dimensioned by the geotechnician or a BET.
Some savings are very expensive!

Raft settlement

Disorder
These are vertical cracks between two parts of the pavilion.
Diagnostic
pavilion, located at the bottom of the valley and at the edge of one of the slopes of this valley, was built on a peaty horizon with a raft-type foundation to distribute the loads over this horizon recognized as heterogeneous at the opening of the excavations.
The part closest to the slope (garages), which is also the least loaded, ended up on more resistant soil, while the other part was carried out on peat at the bottom of the valley.
This results in an inevitable differential settlement between the two parts, the more distant and more loaded part settling more significantly.
Recommendation
As soon as the dubious peaty horizon was recognized at the opening of the excavations, a soil study to assess the homogeneity and the bearing capacity of the soil should have been requested.
The realization of a raft cannot avoid the compaction and heterogeneous behavior of the soils.
It only allows a better distribution of the loads to limit the punctual forces on a ground.
This implies that the distribution of the superstructures themselves remains as homogeneous as possible from one end of the building to the other for real efficiency of the raft system.

Differential settlement between an old building and its extension

Disorder
An extension in concrete structure was built as an extension of a house from the 1960s. The foundation system selected is a set of strip footings and point footings.
The foundations of the new gable undergo a slight settlement and the whole of the new construction rotates slightly. The expansion joint does its job but the gap reaches almost two centimeters at the level of the 1st floor.
Diagnostic
The land slopes significantly towards a river located below. Along the old gable, the new construction is founded at the same depth as the existing one. At the other end of the extension, the strip footings and the point footing are no longer based on the same layer of land as that of the old foundations. The inevitable differential settlement is aggravated by the difference in the bearing capacity of the soils.
Recommendation
No soil study (even simplified) has been carried out. Such a study would have led to lowering the level of the foundations to a deeper layer of "good soil". The "normal" differential settlement would have been much smaller and the opening of the expansion joint would have been reduced.
The configuration of the land should have alerted the builders to the need to carry out preliminary investigations.
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