It is crucial to determine the soil bearing capacity of soil before designing and constructing the foundation of any building, particularly in a seismically active country like Bhutan, which is generally considered to lie within Seismic Zones IV and V. In such high-seismic regions, the performance of foundations is critical to the overall stability and safety of structures, as inadequate soil strength or improper foundation design can significantly amplify seismic effects and lead to excessive settlement or structural failure. Therefore, conducting proper soil investigation and bearing capacity testing is an essential prerequisite for safe, economical, and code-compliant foundation design.
This case study article focuses on the bearing capacity of soil in Thimphu, the capital city of Bhutan, where rapid urban development and varying geological conditions necessitate careful geotechnical evaluation. Thimphu’s subsurface conditions are influenced by local geology, slope variations, and groundwater conditions, resulting in significant variation in soil properties from one location to another. Understanding the typical range of soil bearing capacities in Thimphu, while emphasising the importance of site-specific testing, helps designers, engineers, and authorities make informed decisions in accordance with the Bhutan Building Rules and Regulations (BBR).
Definition of Soil Bearing Capacity
Soil bearing capacity, or bearing capacity of soil, is defined as the capacity of soil to resist external load applied to it. In simple terms, it is expressed as the capacity of soil to support the load without failure.
In other words, the SBC of the soil is defined as the maximum average coverage contact pressure between the foundation and the soil, which should not reduce shear failure in the soil.
Also, Read: Bearing Capacity of Soil: The Foundation of Geotechnical Excellence
Method of Determining SBC
There are several methods of determining the SBC, such as the Standard penetration test (SPT), plate load test, borehole investigation, etc. Here, only the standard penetration test is discussed. The test conducted is for the school building construction, as it is demanded by the designer and the Engineering Division (Infrastructure Division now) of Thimphu Thromde.
Test Location in Thimphu
The soil-bearing capacity was carried out at various schools located within Thimphu before the construction of the school building. These school buildings are designed by the School Planning and Building Division (SPBD), assuming a bearing capacity of 150kN/m2 and for the accommodation of 420 students.
Before constructing a building, it is crucial to determine the bearing capacity of the soil. The Safe Bearing Capacity (SBC) was evaluated at two different schools: Changangkha Middle Secondary School and Lungtenphu Lower Secondary School.
SBC Test Conducted by:
The contractor is required to conduct the bearing capacity test before the construction of the school building. Therefore, the bearing capacity was conducted in collaboration with the Bhutan Standard Bureau (BSB). The test was conducted for two pits at the same location of construction for both schools. The practical test was conducted, and a sample of the soil was collected for the laboratory test.
A standard penetration test was adopted for checking the bearing capacity at the construction site.
Report for SBC:
The readings are recorded by the technician of BSB and computed by the executive engineer of the same organisation. The test report reading was 240 and 384kN/m2, which is greater than 150kN/m2 as adopted by SPBD for designing the school building foundation.
Soil Bearing Capacity in Thimphu
The bearing capacity test results exceed the adopted allowable bearing capacity of 150 kN/m² used in the structural design, confirming that the in-situ soil conditions in Thimphu are generally competent, with allowable bearing capacities typically ranging from 180 kN/m² to 300 kN/m².
However, as stipulated under the Bhutan Building and Regulations (BBR), subsurface conditions are inherently variable due to local geological formations, slope effects, groundwater conditions, and construction-induced disturbances. Therefore, reliance solely on generalised or regional soil parameters is not considered adequate for foundation design. It is essential that the soil bearing capacity be verified through site-specific geotechnical investigation before finalising the foundation design for buildings and other structures, particularly where unfavourable soil conditions or variable strata are anticipated.
Also, read: How to Prepare Structure Drawings
Why verify soil bearing capacity in Thimphu?
Yes, it is important for the contractor, designer, and engineer to verify the soil bearing capacity (SBC) in Thimphu before designing or constructing foundations. Although regional data indicate that the local soil generally has competent bearing capacity, site-specific conditions can vary significantly due to local geology, slope, groundwater, and construction-related disturbances. Verifying the SBC ensures that the foundation design is safe, economical, and compliant with the Bhutan Building Regulations (BBR), particularly in areas prone to seismic activity.
Also, Read: Types of Bearing Capacity Failures and Their Characteristics: Terzaghi’s Analysis
FAQs:
Q: What is the Soil Bearing Capacity in Thimphu?
Answer: The soil bearing capacity of Thimphu usually comes between 180kN/m2 and 300kN/m2 when tested for the pit depth of 1.5m. However, it may vary depending on the types of soil and the site location. Therefore, it is always advisable to conduct a bearing capacity test before designing and constructing the building.
Q: At what depth were these bearing capacity values obtained?
Answer: The values are obtained on soil conditions at approximately 1.5 meters below the ground level, which is a common depth for shallow foundations.
Q: What type of foundations are suitable for these bearing capacities?
Answer: For bearing capacities between 180–300 kN/m², commonly used foundations include: Isolated (pad) footing, Strip footing and Combined footing. Whereas, heavier loads or weak soil, Raft foundations and Pile foundations (if needed) are suggested by code.
Q: Is this value an ultimate or safe bearing capacity?
Answer: The values obtained are safe/allowable bearing capacities, which means a factor of safety has already been considered.
References:
- Ministry of Information and Communications. (2023). Bhutan Building Regulations 2023. https://www.moit.gov.bt/wp-content/uploads/2023/10/Bhutan-Building-Regulation-2023.pdf
- Thimphu Thromde. (2025). Thimphu Thromde: Evolution of the capital city. https://thimphucity.bt/wp-content/uploads/2025/03/Thimphu-Thromde-Evolution-of-the-Capital-City.pdf
- Department of Engineering Services, Ministry of Infrastructure and Transport. (2014). Guideline for soil bearing capacity and geotechnical investigation of buildings 2014. https://www.moit.gov.bt/wp-content/uploads/2010/11/Guideline-for-Soil-bearingcapacity-and-Geotechnical-Investigation-of-Buildings-2014.pdf
