Before a building's structure can be designed, the engineer needs to know what kind of ground will carry it. In Indonesia, the two field tests requested most often are the cone penetration test (CPT, locally called sondir) and the standard penetration test (SPT), which is carried out inside a borehole. Both push into the soil, but they tell you different things. Choosing one without thinking it through can leave the foundation design short of data.
What is a CPT (sondir)?
In a CPT, a rod with a cone at its tip is pushed into the ground in stages. The equipment records two main values:
- Cone tip resistance (qc), the resistance of the soil against the cone.
- Sleeve friction (fs), the friction along the sleeve behind the cone.
The ratio between the two, the friction ratio, helps estimate the soil type, for example clay or sand. In Indonesia the procedure is set out in SNI 2827:2008.
A CPT is fast, relatively economical and gives an almost continuous profile with depth, which makes it well suited to mapping how the ground varies across a site. Its limits are just as clear. It takes no soil samples, and it stops when it meets very stiff layers, gravel or rock. So a CPT may refuse before it reaches the depth that a deep foundation needs.
What are boreholes and the SPT?
In a borehole investigation, a hole is drilled and, at set intervals, an SPT is carried out. Under SNI 4153:2008, a 63.5 kg hammer is dropped repeatedly from about 0.76 m to drive a split barrel sampler into the soil. Penetration is divided into three 150 mm stages. The first stage is treated as disturbed by drilling, so the N value is the number of blows for the last two stages.
A borehole gives you three things a CPT cannot:
- Soil samples that can be seen, described and tested in the laboratory.
- SPT N values at different depths, widely used in bearing capacity correlations.
- The ability to get through hard layers, so the investigation can reach the depth needed for bored piles or long driven piles.
Boreholes also allow undisturbed samples to be taken for laboratory tests such as shear strength and consolidation, and the groundwater level to be recorded. On the downside, a borehole takes longer and costs more per point, and SPT results depend quite a lot on the equipment and how the test is done, for example the hammer's energy efficiency.
CPT and SPT compared
| Aspect | CPT (sondir) | Borehole + SPT |
|---|---|---|
| Main data | qc and fs, continuous profile | N value per interval, borehole log |
| Soil samples | None | Yes (disturbed and undisturbed) |
| Hard layers or gravel | Often refuses | Can drill through |
| Speed per point | Fast | Slower |
| Laboratory testing | Not possible from CPT alone | Possible from samples |
| Typical use | Mapping site variation, light buildings | Deep foundations, basements, multi-storey buildings |
When is a CPT enough, and when do you need a borehole?
There is no single rule for every project, but the usual pattern is:
- CPT only can be enough for light buildings on sites where the ground is already well known, provided the geotechnical engineer judges the data sufficient.
- Boreholes with SPT are almost always needed for multi-storey buildings, projects with basements and deep foundations. The engineer needs to know the soil type, the depth of the bearing layer and the groundwater level with confidence.
- A combination is the most common choice for buildings. A few boreholes give the "key" to the soil layers, while CPTs at more points show how those layers change across the site.
The number of points, their depth and the laboratory tests should be set by a geotechnical engineer based on the planned building, following geotechnical design requirements such as SNI 8460:2017. An investigation that is too thin often leads to a conservative, expensive foundation, or to surprises on site when piles do not reach the expected layer.
Tip: Investigate the ground before foundation design starts, not afterwards. On large sites or where the soil is likely to vary, ask your engineer whether extra CPTs are needed between boreholes.
What to ask for in a soil investigation report
The soil report will be used again and again by the structural designer, the piling contractor and later technical assessors. Make sure it contains:
- A layout of test points, with coordinates or dimensions from the site boundary.
- CPT plots (qc, fs, friction ratio) for every point.
- Complete borehole logs: soil descriptions, SPT N values by depth and the groundwater level during drilling.
- Laboratory results from soil samples, where they are part of the scope.
- Interpretation and recommendations from the geotechnical engineer, including foundation options and risks such as soft soil or potential settlement.
Raw data without interpretation is hard for an owner to use. Recommendations without raw data are hard to check.
How this links to foundation choice
CPT and SPT results drive the next decision, such as choosing between bored piles and driven piles, which we compare in Bored Piles vs Driven Piles. For tall buildings with basements, borehole data is also used to design the excavation and retaining walls, as on the government and office buildings whose structures Stadin has designed.
Conclusion
CPT and SPT are not rivals; they complement each other. A CPT maps ground variation quickly, while boreholes with SPT provide samples, N values and the depth reach that building foundations need. For most multi-storey buildings, using both is the safest choice.
Need help planning a soil investigation or reading the report? Stadin's geotechnical and foundation team can help.
This article is general information. Every project still needs to be analysed and calculated by a competent structural or geotechnical engineer, in line with the applicable SNI codes and regulations.
