Reusing Old CPT Soundings: When Is It Justified and Where to Find Them?
For an extension, an added storey or an internal renovation, new site investigation is by no means always necessary: often the ground near the plot has already been tested. The only question is whether that old CPT sounding is usable for your design — and how to turn it from a yellowed scan into something your software can calculate with.
Where do you find existing CPT soundings?
- DINOloket / BROloket — the Dutch public subsurface archive, maintained by TNO, contains hundreds of thousands of CPTs, often as GEF or as a scanned chart. Since the introduction of the Dutch Key Register of the Subsurface (BRO, 2018), new CPTs must be submitted here.
- The municipal building archive — the original building permit almost always includes a foundation report with CPTs, right at the location of the building.
- Your own project archive or the client — earlier renovations, neighboring buildings, or the original specifications.
When is reuse justified?
In most places the soil profile hardly changes on a timescale of decades; a CPT from 1975 usually still describes the sand stratum just fine. Still, age is not the only criterion. Assess case by case:
- Distance to the new load. Is the CPT located on or right next to the construction site, or tens of meters away? Particularly in areas with channel deposits, the layering can vary strongly over short distances.
- Depth. Does the CPT extend well beyond the intended pile tip level? A sounding that stops just inside the bearing layer says little about possible deeper soft interlayers.
- Changes since then. Has the site been excavated or raised, or has the groundwater level changed structurally (for example due to a lowered polder water level)? Then levels and negative skin friction may no longer be correct.
- Consistency. Several CPTs around the location that show the same picture are together stronger than one isolated test.
In doubt, or dealing with a large or high-risk structure? Then new investigation is the only right choice. For a light extension with a limited increase in load, however, a good existing CPT is often a sufficient basis — provided you record its origin and your considerations in your report.
From scan to calculation file
Old CPTs from archives are almost always images: a scanned chart without underlying data. To calculate with them, you digitize the cone resistance line and export to GEF. Pay attention to:
- The reference level. Old sheets sometimes put the depth axis in m NAP (the Dutch vertical datum), sometimes in m below ground level. Check what it says and carry the ground level over into your export.
- The unit of qc. Sheets from before roughly 1980 sometimes use kgf/cm² instead of MPa (a factor of ~10).
- Verification afterwards. Plot the GEF file next to the original scan and check the layer boundaries.
The digitizing itself doesn't have to take all afternoon: in this step-by-step guide we show how the CPT Converter gets you from scan to GEF in a few minutes. Why the qc line is what matters most is covered in the article on cone resistance and pile design.