How Many Boreholes Are Required for a Building? An Example as per IS 1892:2021

One of the most common questions we receive during soil investigation is: “How many boreholes should we do for our building?”

Consider this example:

Plot Area: 4,000 m² (approximately 0.4 hectare)

Proposed Building: G+10 + 1 Basement

Approx. Building Height: 35 m

Basement Depth: approximately 3.5–4 m

As per IS 1892:2021, Clause 5.6.2, Table 2, for building(s) on a site covering about 0.4 hectare, the guideline specifies at least one borehole at each corner and one at the centre. For a typical rectangular site, this generally means 5 investigation points.

For a multistorey building up to 50 m height, IS 1892:2021 also provides for boreholes in a grid pattern with investigation points not more than 50 m apart, subject to a minimum of two boreholes and covering the built-up area. Therefore, the final borehole layout should always consider the actual building footprint and structural requirements.

Can the client do only 2 or 3 boreholes?

Sometimes fewer boreholes may be proposed based on the project footprint, available previous data, uniform ground conditions or the consultant’s technical assessment. However, reducing the investigation simply to save cost is not advisable.

Soil conditions can change even within the same plot. One location may contain dense soil while another may encounter loose soil, clay, fill material, groundwater or weaker strata. IS 1892:2021 itself states that additional boreholes/tests may be carried out when significant variation in stratification is observed.

What about borehole depth?

The drilling depth should not be decided only from the number of floors.

IS 1892:2021 requires the investigation depth to be adequate for evaluating safe bearing capacity and settlement. For shallow foundations, it gives guidance related to the estimated foundation width; raft and pile foundations have different depth criteria.

So before deciding “5 boreholes × 10 m” or “3 boreholes × 15 m”, the proposed foundation system, loads, building footprint and expected ground conditions should be considered.

Planning a building, basement, industrial structure or redevelopment project?

Send us your plot dimensions, number of floors and basement depth, and our team can help you understand the likely soil investigation requirement.

GTSM Testing Labs LLP

NABL Accredited Laboratory – TC-6629

Soil Investigation | Borehole Drilling | SPT | SBC | Geotechnical Testing

Ahmedabad, Gujarat

98240 49466

Reference:

IS 1892:2021 – Subsurface Investigation for Foundations — Code of Practice (Second Revision)

Official Bureau of Indian Standards page:

[IS 1892:2021 – BIS Standard Details](https://standards.bis.gov.in/website/standard-details?encryptedId=eyJpdiI6InI0NVNSL1pVL1JpS21FSXQxd0pKUUE9PSIsInZhbHVlIjoicDhia3ZzMkZFSUFFbWUySGN3TzVuZz09IiwibWFjIjoiYjc4NTI1NzgzN2EwMDNmNzg5OGVlOTBjMjJhMjMwMmI3MmZmY2MxZWU4NzViNDU3MWFmYzRlNTkyNjAzYWI1MCIsInRhZyI6IiJ9&utm_source=chatgpt.com)