The determination of adequate number and sufficient depth of boreholes is critical to gather enough data for the design and construction of geostructures such as foundations, bridges, retaining walls, cut slopes, and embankments.

There is no specific rule to determine borehole number and depth. Therefore, several factors, such as structural importance, the geological character of the ground, and equipment availability, can play an important role.

One should take full advantage of engineering experience and judgment, and follow guidelines of applicable codes to determine the minimum number and depth of boreholes. The quantity and depth of boreholes should cover the soil zone that is affected by the load of the geostructure.

**How to determine Number and Depths of Boreholes for Geostructures?**

- Borehole numbers should be enough to find out soil-layer variations at the construction site.
- If the location of loads is within the boundary of the structure, then at least one borehole should be drilled at the position of the heaviest load.
- Table-1 shows a minimum number of boreholes based on the area of the proposed buildings and due diligence in subdivisions.
- Except in the case of very dense materials or bedrock, the minimum depth of borehole is 6m.
- Drill a borehole at least 3m into bedrock.
- For dense, coarse-grained soil and very stiff clays, extend boring depth up to a range of 5-6m to corroborate that soil layer thickness is sufficient.
- Boreholes must penetrate very soft deposits or fills below the proposed geostructure.
- For compressible soils like clay, boring depth is the greater of foundation width times a value between 1 and 3, or the depth at which the stress increment caused by heaviest foundation load is smaller than 10%.
- Table-2 shows a minimum number and depth for common geostructures.

Building area, m^{2} | Number of boreholes | Subdivision area, m^{2} | Number of boreholes |

100 | 2 | 4000 | 2 |

250 | 3 | 8000 | 3 |

500 | 4 | 20000 | 4 |

1000 | 5 | 40000 | 5 |

2000 | 6 | 80000 | 7 |

5000 | 7 | 40000 | 15 |

6000 | 8 | – | – |

8000 | 9 | – | – |

10000 | 10 | – | – |

**Table-1: Minimum Number of Boreholes for Buildings and Subdivisions Based on Area**

Geostructure types | Minimum borehole depth | Minimum number of boreholes |

Shallow foundation | 5m, or foundation width time a value ranging from 1 to 3 | One but place boreholes at node points along grids of sizes from 15mX15m to 40mX40m |

Deep foundation | 25-30m except for bedrock which is 3m | One but place boreholes at node points along grids of sizes from 15mX15m to 40mX40m |

Retaining wall | 1 to 2 times wall height, 3m for walls on bedrock | One borehole for wall-length smaller than 30m, For wall-length longer than 30m; 1 for every 30m or wall height times a value between 1 to 2 |

Bridge | 25-30m except for bedrock which is 3m | Two boreholes for piers, Two boreholes for abutments |

Embankments including roadway | Either 6m or two times embankment height, whichever is greater | 1 for every 60m If soil does not vary crucially, otherwise 1 for every 120m |

Cut slopes | 6m below the bottom of the cut slope | On slope: 3 boreholes Along the length of the slope: 1 for every 60m If soil does not vary crucially, otherwise 1 for every 120m |

**Table-2: Minimum Number and Depth of Boreholes for Various Geostructures Based on Soil Type**

## FAQs

**What is the purpose of borehole?**

Boreholes are used to take soil samples which are then tested to get enough information about the construction site to enable engineers to set safe designs and execute construction processes properly.

**What should be the depth of borehole in bedrock?**

The depth of borehole in bedrock should be 3m.

**How many boreholes are required for bridge construction?**

Minimum two boreholes for bridge abutment and two boreholes for piers are needed.

**How many boreholes are needed for adequate soil report?**

There is no specific rule to determine borehole number and depth. Therefore, several factors, such as structural importance, the geological character of the ground, and equipment availability, can play an important role.

**What is the minimum depth of boreholes for shallow foundation?**

The minimum depth of borehole for shallow foundation is 5m, or foundation width times a value ranging from 1 to 3.

**Read more:**

How to Setup Borehole Layout for Site Investigation of Different Foundations?

What are the Errors in Borehole Data Interpretation for Site Investigation?

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