Building ground expertise - the stable foundation for every house construction

Subsoil expertise Subsoil investigation

It has become clear in recent years, and not just from numerous television documentaries, that not every building is built on a stable foundation. Particularly in times of cost and time savings, a ground survey is often seen as superfluous. This may work well for ideal plots of land. For all other plots, however, unfavourable soil structures can cause significantly more costs and time than the builder would like. So play it safe and have a subsoil survey carried out!

A subsoil survey is also referred to as a soil survey, geological survey, foundation survey or subsoil investigation. In general, the geological condition of a property is examined more closely.

Important for the ground investigation are
- the structure of the soil with its layers,
- the determination of the soil group and soil class,
- the groundwater situation with the height of the groundwater table,
- the possibility of water infiltration (infiltration capacity) and
- the identification of possible contamination (mineral oil, unexploded ordnance).

Following the drilling samples, it is possible to clarify how the subsoil will behave with the building to be erected on it. The experts have various mechanical values available for this purpose, which are determined and described in documentation.

These are among others:
- Weight (ratio of weight force to volume),
- Shear strength (load-bearing capacity of the floor),
- Friction angle (angle of possible load on a surface without causing slipping) and
- Cohesion (cohesion of the individual particles of the soil).

Interactions with neighbouring buildings are also taken into account.

In a nutshell: The subsoil report provides information on how your dream home can be built on your property. It shows what effort is required to excavate the building pit, whether stabilisation measures are necessary and what the foundation or basement should look like.

When is the right time for a ground survey?

If you have found a plot of land that you like as a building site, you can make initial enquiries about the soil. The building authority, the water management authority and the environmental authority can provide information on this. They often have some basic information on the local soil situation. Insight, for example into the Contaminated site and suspected contaminated site registers of the individual federal states (here Baden-Württemberg), you can apply online.

A little tip: If you discover sagging or raised areas on or around the desired property (e.g. pavement slabs), clear cracks in the façade of old buildings, leaning parts of buildings, heaped-up areas, street names that indicate the soil conditions or former uses (e.g. sand path, gravel pit path), then you should be careful. Old maps or development plans also show former industrial sites, which indicate whether the soil may be contaminated (e.g. oil).

If your own initial analysis sounds very promising, you should still have a ground survey carried out before finalising the purchase of the property. This is much more detailed than the information provided by the authorities and protects you from unpleasant surprises, as soil conditions can vary greatly even in the smallest of spaces.

Of course, the soil survey can also be ordered from an expert after the purchase. However, you then run the risk of incurring significant additional costs and construction delays if the terrain is difficult. If the soil survey is carried out before the purchase, the amount of earthworks can be precisely quantified and thus provides planning security.

There is no validity period for subsoil reports, but local changes may cause the groundwater level to change, for example. If you have an older soil survey, consult an expert to determine whether a new investigation is necessary.

In a nutshell: The best time for a ground investigation is before you buy a property.

What is a subsoil expertise needed for?

As already described in the section "What is a subsoil report?", this document is an important tool for construction planning.

Conclusions for house construction can be derived from the values determined:
- Will the floor support the planned house or will changes have to be made?
- Is settlement to be expected? Can the construction window be shifted accordingly or will compaction of the soil help?
- How deep can you build? Is a basement possible?
- What must the foundation or cellar look like (masonry, waterproof reinforced concrete)?
- Is the excavation pit suitable for backfilling?
- How much work will be involved in the earthworks (time, costs)?

The development of a basement, for example, depends crucially on the groundwater level. All areas that are constantly in contact with the water-bearing layer must be adequately sealed, for example with a reinforced concrete trough. Poorly draining seepage water can also damage the building in the long term if it is built improperly. Both aspects can be derived from the subsoil report, as can the load-bearing capacity of the soil. How easily the soil can be worked is just as important for construction. The Soil class Information. The construction costs, in turn, are calculated according to the soil class.

A little tip: If you are building with a house construction company, you cannot assume that a soil survey has been carried out. The favourable offers are only worthwhile if the ground conditions are ideal. Without a ground investigation, it can happen that subsidence leads to the house leaning or water penetrates the foundation because the cheapest but least favourable method was used.

Last but not least, the subsoil expertise can also protect against the purchase of a contaminated property. Unexploded ordnance from the Second World War can still be found in city centres today, the disposal of which incurs high costs. These are borne by the owner. Contaminated properties, such as former petrol stations or garages, can also be expensive for developers. For these two reasons, too, it is advisable to carry out a ground investigation.

In a nutshell: A subsoil survey is necessary for every new building or changes to an existing building to ensure that the house is built on a secure foundation.

Is a ground survey mandatory?

The Building law has required such a report since 2008 if you want to build a new building on the property. This gives you and the individual trades security in all planning matters and during the construction phase. If there is already a building on the site, a ground investigation is not absolutely necessary. However, it is still advisable. As the owner of the property, you are generally responsible for all matters relating to the property. If, for example, it is discovered during the laying of pipes that there are aerial bombs or contaminated sites in your private ground, you will be solely responsible for their disposal.

A little tip: If you have ordered a ground investigation, the liability risk is transferred to the commissioned surveyor.

What information does a subsoil report contain?

The subsoil report is a comprehensive analysis of the soil with its composition and soil mechanical properties. The interactions between the building (to be constructed), the groundwater, the soil and the buildings in the neighbourhood are specifically examined and evaluated. The soil samples are also analysed for contaminated material using a geochemical preliminary investigation. The subsoil report provides precise details of the various measured values and parameters required by the structural engineer. As a rule, a foundation recommendation (e.g. foundation slab, trough) is included in the soil expertise.

The following information is documented in the subsoil expertise:
- General information (owner, building and location),
- Structure and layers of the subsoil,
- Soil type with characteristic values,
- Soil mechanics (settlement behaviour, load-bearing capacity),
- Groundwater level and infiltration capacity,
- Bottom resistance,
- Frost resistance,
- Guidelines for earthworks and sealing the building (e.g. watertight basement made of a reinforced concrete trough) and
- Results of the preliminary geochemical investigation (if requested).

If the soil survey identifies chemical contamination, a further survey will be required to check for pollutants.

Who needs the subsoil expertise?

The structural engineer uses the ground survey to carry out his calculations. Using various key figures, he can plan your dream home with all its walls, ceilings and foundations or basement.

Once construction begins, the earthworks are also carried out on the basis of the soil report. Any existing problems are documented in the report and appropriate measures are advised. If, for example, the excavation has to be disposed of, information is provided on this, as well as on any necessary compaction of the soil.

For the excavation of the construction pit and thus also for the expense and Cost consideration the statement regarding the soil class is important. Soil that is too hard or too soft is less ideal and requires more work. If contaminated material is found in the ground investigation, it must be disposed of. The soil report also contains characteristic values from the geotechnical evaluation that are important for the earthworks engineers. If the excavated material is unsuitable for further construction, it must be removed and other filling material organised.

Important information for the shell builder is the infiltration capacity and the groundwater level. A decision is then made as to how a basement must be sealed to protect the building from water ingress.

Construction costs can also be calculated on the basis of the ground investigation.

In a nutshell: The soil survey is required by the structural engineer, the cost centre and the earthworks and shell construction trades.

What risks can be ruled out by a subsoil survey?

As a first risk, you can minimise or completely exclude your own personal liability risk. For this reason, it is advisable to obtain an expert opinion before you buy. If you have commissioned an expert to examine the soil, you can start building your house with planning certainty.

The following risks are guaranteed to be uncovered with a ground survey:
- Difficulties and complications during the earthworks,
- uneven soil profile in various places (subsidence is likely),
- groundwater table too high and poor infiltration properties (to be taken into account when building a basement),
- Insufficient load-bearing capacity of the soil (compaction of the soil necessary),
- Contaminated sites and pollutants in the soil and
- Bomb find.

You have already learnt from the information provided so far that buying a plot of land without a soil investigation is like a lottery. The risks listed also show very clearly that building a house without a soil investigation can cause consequential damage, especially of a financial nature. For example, if the groundwater presses strongly against the foundation from below, waterproofing can be damaged. Likewise, construction delays cannot be ruled out if the ground is too hard or if contaminated sites have to be removed first.

A little tip: If problematic substrates are not assessed in detail, the house construction may not run smoothly either. Leaking cellars and walls, cracks in the walls or a deformed roof truss can be the result of incorrect construction planning and execution. Therefore, commission a subsoil survey before signing a contract!

What should be taken into account in a ground survey?

Firstly, you should get an independent expert on board who is authorised to draw up a subsoil report. These include geologists, geotechnicians and civil engineers. Ideally, you should find an expert from the region who is familiar with the local area.

The scope of a soil survey can vary, but should essentially contain all the information listed in the section "What information does a soil survey contain?". The expert needs around two to four weeks to prepare the report. This involves taking samples on site, which consists of a percussion core probe in at least two areas of the property and small pile-driven boreholes. During the percussion coring, a hollow body is driven into the ground, which contains the various soil layers when removed. These are then analysed in the laboratory using various soil mechanics tests and measurements.

As a rule, the subsoil investigation does not include a geochemical analysis with regard to possible pollutants and contaminated sites in the soil. This must be ordered separately when the contract is awarded.

In a nutshell: Commission an expert in geosciences or civil engineering to carry out a professional analysis of all physical and soil-mechanical, and possibly also geochemical, parameters from the ground soundings.

Can a subsoil expertise be used for disposal?

The general subsoil investigation consists of analysing the physical and mechanical properties of the soil. As mentioned in the last section, an analysis for pollutants must also be commissioned. Only this geochemical declaration analysis can determine whether there are substances in the soil that need to be disposed of separately.

Sometimes a preliminary geochemical investigation is carried out as part of the ground investigation, which already indicates whether there are pollutants in the soil and whether further analysis is necessary.

A little tip: If you purchase a property that is located on a former industrial site or commercial area, there is a high probability of contaminated sites. You should definitely arrange for a geochemical investigation.

What costs should be budgeted for a ground survey?

Some builders like to be put off by the cost of a soil survey and prefer to forego this investigation. However, always bear in mind that without such a report, complications can quickly arise, resulting in additional work and costs.

A ground investigation usually costs between 1,000 and 2,000 euros. This mainly depends on the cost of the survey. If, for example, a basement is planned when building a house, the price increases. Additional investigations, such as chemical contamination, also lead to additional costs.

A little tip: You can have the soil survey item included in your financing as ancillary building costs. This is estimated at around 5 euros per square metre of land area.

Conclusion

If you want to build without unpleasant surprises, you should definitely not do without a ground investigation. The cost of such a survey can be offset by planning security, the minimisation of construction difficulties and the exclusion of static risks. Last but not least, nerves and wallets are spared, as liability is transferred to the expert when a subsoil survey is carried out.

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