Why Geotechnical and Topographic Surveys Matter

Knowing what lies beneath the ground and how the plot is shaped is the basis for sound decisions on foundations, drainage and positioning the house.

Surveys are among the items people most want to save on in a building project. The plot looks flat, the neighbours have built houses, so the soil must be fine — you hear this reasoning often. Yet two plots on the same street can have quite different ground conditions. Geotechnical and topographic surveys give the designer measured data rather than guesses, and every later decision is built on that data.

What a topographic survey is

A topographic survey is an accurately measured map of the plot. It shows the boundaries, the terrain and spot levels, existing buildings, trees, fences, roads, and underground and overhead services. Because levels are given in a consistent reference system, the designer knows exactly how high each point on the site is.

This information is the basis for positioning the house on the plot. The floor level of the building, the gradient of the driveway, where rainwater will run and the volume of excavation are all calculated from the topographic plan. Without one, these decisions are made by eye and need correcting during construction.

The topographic plan is useful from a legal point of view too. It shows whether the actual boundaries of the plot match the dimensions in the documents and whether a neighbour's fence encroaches on it. Spotting such a mismatch before construction prevents later disputes.

What a geotechnical investigation measures

In a geotechnical investigation, boreholes are drilled or probing is carried out on the site, soil samples are taken and tested in a laboratory. A report is then prepared. It usually covers:

• the type, thickness and sequence of soil layers;
• the bearing capacity and physical and mechanical properties of the soil;
• the groundwater level and its seasonal variation;
• how aggressive the soil is towards concrete and steel;
• risks of settlement, swelling or landslip.

The number and depth of boreholes depend on the size of the building, the number of storeys and the complexity of the site. The specialist carrying out the investigation sets these parameters together with the designer.

The client can read the report too. It usually ends with a section of conclusions and recommendations: which soil lies at which depth, which foundation type is appropriate and what to watch out for during construction. Discussing this section with the designer helps you understand the reasons behind the decisions.

How the results shape the foundation

The type, depth and dimensions of the foundation depend directly on the geotechnical data. On firm ground a strip foundation may be enough; on weak or variable ground a raft or piled foundation is needed. Without data, the designer either makes the foundation stronger than necessary to be safe, or takes a risk.

Neither outcome is desirable. An over-designed foundation means extra concrete and reinforcement. An under-designed one shows itself years later as cracks in the walls and distorted doors and windows, problems that are very hard to fix in a finished building.

Groundwater, drainage and basements

The groundwater level directly limits plans for a basement, an underground garage or a pool. A basement can be built where the water table is high, but it requires robust waterproofing, a drainage system and sometimes pumps. Knowing this before construction lets both the design and the budget be set correctly.

The topographic plan, meanwhile, shows how surface water moves. Building a house at the lowest point of the plot can send rainwater towards the building. Drainage and gradient decisions are based on this information.

The terrain also decides what happens to excavated soil. The topographic plan is used to calculate where soil will be cut and where it will be filled. A balanced solution reduces the need to remove surplus soil from the site or bring more in.

When the surveys are done

The best time is before design begins and, if possible, before the plot is bought. The topographic plan is needed for the architect's first sketches, and the geotechnical report for the structural design. The survey results become part of the project documents and may also be required in the building permit process.

If an existing house is being extended or a storey added, a survey is still needed. In that case both the ground and the condition of the existing foundation are examined.

For Zaferoğlu İnşaat, topographic and geological maps are the first step of a build: foundation and excavation decisions rest on these documents. Tell us how many storeys the house will have and whether there is a basement, because the scope of surveys depends on it.