How many boreholes, and how deep, the NSR-10 requires
Puts a number on the «get a soil survey» that usually comes without one. And flags what pushes up items that are almost never in the budget for a country house: the slope, the 10 % gradient and the retaining walls.
Minimum ground investigation
3 boreholes, 6 m deep
low-category construction unit
| By number of levels · 2 | Low |
| By maximum column load | No figure yet |
| The more onerous of the two governs | Low |
| Total drilling, as an order of magnitude | 18 m |
Without the column load this classifies by levels alone, and that is a provisional floor: once there is a calculated structure it has to be looked at again, because the more onerous of the two governs.
How it is counted
- Counting storeys includes every floor, basement, terrace and plant level.
- At least half the boreholes must fall within the footprint of the building on the site.
- At least half the boreholes must reach the minimum depth; the engineer may require more depending on the foundation.
- With basements or cuts, depth is measured from the bottom of the excavation; where fill is placed, from the original ground level.
Meeting the minimum does not release the engineer from making whatever additional boreholes their judgement calls for, and if they do not make them, they must set the recommendation down in writing in the report.
The study must be directed and signed off by qualified civil engineers holding a current professional licence, and those engineers must be able to show more than five years of experience in the geotechnical design of foundations (counted from the licence) or postgraduate study in geotechnics.
Where it comes from
The tables
NSR-10 (the Colombian seismic code) tables H.3.1-1 and H.3.2-1. The construction unit is classified into four categories by the total number of levels and the maximum service loads on columns, always taking the more onerous of the two. Low: up to 3 levels or under 800 kN, 3 boreholes of 6 m. Medium: 4 of 15. High: 4 of 25. Special: 5 of 30. The absolute minimum for any investigation is three boreholes.
When it is mandatory
NSR-10, H.1.1.2: definitive geotechnical investigations are mandatory for all urban and suburban buildings of any use group, and for buildings on land unfit for urban use in use groups II, III and IV. That second half drops out every time someone summarises the sentence, and the cut turns a rule with an edge into a blanket prohibition that does not exist.
What adds items to the budget
H.5.2.1: on, at the top of or at the foot of a slope requires a stability analysis advised by a geologist, and the stabilisation works have to be designed. H.7.1.3 (b): where the ground slopes more than 10 %, topographic effects call for 2D or 3D numerical models. H.6.5: retaining walls shall always be provided with filters and drainage. «Always» is a word the code barely uses.
What this tool does not claim
A tool that answers what its source does not say is worse than no tool at all. This is what is deliberately left unanswered here, and why.
- It is not an investigation, it is its minimum size. The investigation is carried out and signed by an engineer, and the chapter itself makes clear that meeting the minimum does not remove the need for any additional boreholes the professional’s judgement calls for.
- It does not say what it costs. The price of a borehole depends on access, on the rig, on the ground and on the city, and there is no published national figure that can be cited.
- It does not classify the building’s use group, which is a separate question in the same chapter and which changes when the investigation is mandatory on land unfit for urban use.
- It does not resolve what type of foundation you need. That comes out of the investigation, not out of the category.
- Without the column load the result is provisional: it classifies by levels alone. Once there is a calculated structure it has to be looked at again.
I have a plot and I want to build on it.
We check what the planning rules allow on your plot, what can be built and what it costs, before drawing anything. Then we develop the full project through to the drawings and the permit application with the curaduría (the licensed private office that issues building permits in Cali and other large Colombian municipalities; in smaller ones, the municipal planning office).