Soil Investigation: What It Is and Why It's Mandatory

A soil investigation tells you what the ground under your house is made of, how much load it can carry and how it will behave in an earthquake. The soil and foundation investigation report that comes out of it is the input to the structural design. Foundation type and depth, whether a basement is sensible, drainage needs and the earthquake parameters used in the calculations all come from that report.

Speaking as a civil engineer: a foundation chosen without a report is a guess. Sometimes the guess works. When it doesn't, the foundation is the most expensive part of a building to put right.

The legal side: no report, no permit

A soil and foundation investigation report is part of every building permit application in Turkey. Its scope and method are set by regulations of the Ministry of Environment, Urbanisation and Climate Change and by the Turkish Building Earthquake Code (TBDY 2018), in force since 1 January 2019. The building inspection firm and the municipality both review it. A single-storey detached house, a light gauge steel home or a prefab building is not exempt.

The report is prepared and signed by a team of geological, geophysical and geotechnical (civil) engineers. Copying the neighbour's report isn't accepted; each plot needs its own study.

What happens on site

Boreholes and samples

A villa plot usually gets a few boreholes. Depth depends on the ground and the building, commonly 10-20 m for a house, less if sound rock turns up early. Samples are taken at intervals, and a borehole log records which layer was met at which depth.

SPT (Standard Penetration Test)

At set intervals during drilling, a standard hammer drives a sampler into the ground and the crew counts how many blows it takes to push it 30 cm. That number, the N value, gives a direct read on how dense a sand is or how stiff a clay is, and it feeds into liquefaction analysis.

Geophysical measurements

Seismic refraction, MASW or microtremor surveys measure the shear wave velocity of the ground. The average over the top 30 metres, Vs30, is one of the main quantities used to assign the local soil class under TBDY 2018.

Groundwater

The water level is measured in the borehole. A single reading reflects that season: a plot that looks dry in August can be very different in February. Always check the date of the measurement in the report.

Laboratory tests

• Sieve and hydrometer analysis: grain size, and whether you're dealing with sand or clay
• Atterberg limits: how plastic a clay is and how much it tends to swell and shrink
• Unconfined and triaxial compression: shear strength
• Consolidation: how far the ground will settle under load over time
• Chemical analysis: sulphate and chloride content, which affects concrete grade and cement type

TBDY 2018 soil classes

Class	In short
ZA	Sound rock
ZB	Slightly weathered, moderately strong rock
ZC	Very dense sand and gravel, hard clay or weathered rock
ZD	Medium dense to dense sand, stiff clay
ZE	Loose sand, soft clay
ZF	Ground needing site-specific study (liquefiable soils, very soft or organic clays and similar)

Moving from ZA towards ZE, earthquake waves are amplified more by the ground and the design seismic load rises. The same house on ZE ground needs noticeably more reinforcement or steel than on ZB. On a ZF site, design can't start until a site-specific analysis has been done.

How the report shapes the foundation

Ground condition	Usual solution
Firm ground, high bearing capacity	Strip footing or raft (shallow foundation)
Medium dense ground with differential settlement risk	Raft foundation
Soft, fill or organic ground	Ground improvement or piles
High groundwater	Drainage and a waterproofed foundation; dropping the basement is also an option
Swelling clay	Deeper foundations and perimeter drainage

Light gauge steel and prefab houses are light, so bearing capacity is rarely the problem. Differential settlement, swelling clay and water don't care about weight, though. A light house will still crack on soft fill.

What we look for around Şile and Ağva

There is no single ground type here. In the same village, one plot hits rock a few metres down while the plot below it on the slope sits on a thick layer of weathered clay. The situations we meet most often:

• Clay soils that swell in wet winters and shrink in summer. Foundation depth and perimeter drainage decide the outcome.
• Sloping plots, where cut slopes need stability checks and often a retaining wall. Excavate the slope the way the report recommends.
• Plots near streams: around the Göksu and Yeşilçay in Ağva, groundwater can be high and the ground alluvial. Flood risk needs a separate look.
• Sandy ground near the shore: loose sand plus a high water table means a liquefaction analysis is required.

Old fill is another regular surprise. On a plot that was levelled years ago, the top few metres may be imported soil. You can't tell from the surface; a borehole shows it.

The report isn't the end of it

A report is a model: the ground between a handful of boreholes is filled in by assumption. When the foundation is dug, the base should be compared with the layers the report describes. If a pocket of fill, a soft clay lens or water turns up, the geotechnical team and the structural engineer need to hear about it before any concrete goes in. The mistake I see most on site is pouring blinding over something unexpected and carrying on. Photograph the formation and get it into the building inspection record, so there's evidence if questions come up later.

Time and cost

For a single villa plot, fieldwork usually takes 1-3 days and the lab work and report 1-3 weeks. As of early 2026 the approximate cost is ₺25,000-₺60,000 depending on the number and depth of boreholes, and more on plots that are hard to reach, hit rock or need deep drilling. It's a small line in the overall budget; fixing a foundation afterwards usually costs many times more.

Common questions

Can I use my neighbour's soil report?

You can read it for a general idea, but the permit needs a report for your own plot. Ground conditions can change within a few metres.

Can I get a soil survey done before buying the land?

If the seller agrees, yes, and I'd recommend it. On a sloping plot or one near a stream, an early assessment shows you the likely cost of piles, drainage or retaining walls before you commit.

What should I look for in the report?

The soil class, the recommended foundation type and depth, the groundwater level and when it was measured, and the assessment of liquefaction and swelling. If the report says the foundation excavation should be inspected once it's open, write that into the building contract.

Talking about house type, basements and budget without knowing what's under the plot leaves the conversation half-finished. For villa construction and a single-storey steel house alike, make the soil investigation the first step of the project.