What is load bearing capacity of soil?
Understanding soil’s load-bearing capacity is a critical aspect of structural design and construction. Historically, builders relied on experience and observation to construct structures. The science of understanding soil behaviour and capacity was unknown. With sheer observations and requirements to understand soil behaviour, scientists coined the term ‘load bearing capacity of soil’. Karl von Terzaghi, an Austrian civil engineer, pioneered the scientific theory of soil’s ultimate bearing capacity. His theory defines that if the foundation depth is less than or equal to its width, it is called a shallow foundation. He also developed a method to determine the bearing capacity of soil through general shear failure cases. In 1951, Meyerhof expanded Terzaghi’s theory by incorporating considerations for rough, shallow, and deep foundations. His theory could also be applied to rough, shallow, and deep foundations. It was mostly the same as Terzaghi’s. However, it included three main factors: shape, depth and inclination.
To appreciate its importance, we must first define soil’s load-bearing capacity. In this blog, you will be able to understand the depth of this concept.
Classification of load-bearing capacity of soil
The load-bearing capacity of soil refers to its ability to support applied loads, representing the maximum pressure it can sustain without failure when a foundation is laid. The load bearing capacity is further classified into 2 categories:
- Ultimate bearing capacity: Theoretically, it is the maximum pressure soil can support without collapsing in the absence of any safety measure.
- Allowable bearing capacity: It states the practical load bearing capacity of the soil. It can be calculated as:
Allowable bearing capacity = ultimate bearing capacity/safety factor. It states the soil’s acceptability to handle the weight without failure and with applied safety measures.
Factors influencing the load bearing capacity of soil
Every region has different soil with varying textures and load-bearing capacities. However, the factors that affect the load-bearing capacity of soil are consistent. This includes the following factors:
- Soil strength: The load-bearing capacity is influenced by the soil type, with cohesionless soils relying on friction angles to determine load distribution. This factor determines the ability of the cohesive soil to bear load. Soil with loose particles is less capable of bearing weight than cohesive soil.
- Foundation width: The width of foundations directly impacts the soil’s load-bearing capacity, particularly in cohesionless soil. It is directly proportional to the soil’s shear strength, which determines the level of internal friction. On the contrary, the load bearing capacity of cohesive soil is not related to its shear strength.
- Foundation depth: The foundation depth is directly proportionate to the load bearing capacity of the soil. To understand this better, let’s say the deeper the foundation, the greater the bearing capacity of the soil. However, this rule applies primarily to cohesionless soils.
- Spacing between foundation: For adjacent foundations on cohesionless soil, a spacing of approximately 1.5 times the foundation width is recommended to prevent overlapping stress zones and to maintain soil bearing capacity. For example, if the foundation width is 10 meters, the distance between adjacent foundations should be around 15 meters.
How to determine soil load capacity?
Before understanding the formula to derive load bearing capacity, let’s first understand the three main components:
- Ultimate bearing capacity: The ultimate bearing capacity, also known as (qu), is the maximum pressure the soil can bear.
Safe bearing capacity: This is denoted as (qs), the maximum amount of pressure soil can bear with all the safety factors considered.
Net bearing capacity: Net bearing capacity, also represented as (qn), is the difference between ultimate bearing capacity and overburden pressure.
Ultimate bearing capacity can be calculated using Terzghi’s theory and formula, which is as follows:
qu = CNc + γDfNq + 0.5γBNγ
Where,
- qu: Ultimate bearing capacity
- C: Cohesion
- Nc, Nq, Nγ: Bearing Capacity Factors, depend on the angle of internal friction of soil
- Df: Foundation depth
- B: Foundation width
- γ: Unit weight of soil
Let’s understand this further with the help of an example:
Let’s assume,
- Cohesion = 0.2 kg/cm2
- Unit weight = 1.70 gm/cm3
- Foundation depth = 1.0 m
- Foundation width = 2.0 m
- The Bearing Capacity Factors for dense sand =
- Nc = 57.75
- Nq = 41.4
- Nγ = 33.3
Using Terzaghi’s formula, the ultimate bearing capacity of soil is = 26,515.9 kg/m2
Methods to test load bearing capacity of soil
Each soil type varies in composition, texture, density, and load-bearing capacity, requiring different established methods to suit each type, which are as follows:
- The plate bearing test: The plate bearing test is an in-situ test performed directly on the construction site. In this test, a certain amount of weight is imposed on the sample plates and the result induced is measured. This data is then used to derive load settlement curves, determining the bearing capacity.
Standard Penetration Test: The standard penetration test, Also known as SPT, is exceptionally helpful in cohesionless soils. In this test, the bearing capacity is determined by measuring the resistance of the soil.
Cone Penetration Test: The cone penetration test (CPT) uses cone-tipped equipment to measure soil resistance, differing from SPT in methodology. The only difference between the two is that, unlike SPT, CPT uses cone-tipped equipment to impose weight on the soil.
- Pressuremeter Test: This is also an in-situ test like the plate-bearing test. It measures the deformability and strength of the soil by blowing an inflatable cylindrical tube inside the borewell. The changes in pressure and volume are recorded, and bearing capacity is calculated.
How to increase soil load capacity?
When soil’s bearing capacity is insufficient, specific techniques can be employed to enhance its bearing strength.
- Increase in the depth of the foundation: Increasing the foundation depth is a simple and effective way to enhance soil’s load-bearing capacity. The deeper we go in the soil, the higher the bearing capacity. However, this method is highly recommended for cohesionless soil. It cannot be applied when soil moisture is relatively high.
Soil drainage: Soil moisture content and load bearing capacity are inversely proportional to each other. The higher the moisture in the soil, the lesser the load bearing capacity. To overcome this challenge, engineers often use the technique of draining the soil moisture, excess water is drained out using pipes.
Soil compaction: It is the most common method to enhance the load bearing capacity of soil. It reduces the gaps and holes between the soil particles, allowing it to become dense and enhancing the load bearing capacity. Soil compaction also removes the chances of soil over-moisturisation.
- Soil confinement: This method is useful in shallow foundations. It encloses the soil around the foundation, protecting it from over-moisturisation and excess water.
- Usage of geogrids: Geogrids are an effective technique to increase load bearing capacity due to its core functionality of tensile improvement.
How can you increase the load bearing capacity of soil?
The bearing capacity of the soil can be remarkably improved using highly efficient geosynthetic materials like geogrids. Here’s how:
- Geogrids enhance soil capacity by compensating for its low tensile strength and providing reinforcement.
- The geogrids also enhance the technical strength of soil as they work as protective shields between the foundation and soil.
In moisture-heavy soils, geogrids act as barriers, preventing seepage and improving soil stability. They act as a guard and avoid unwanted growth of moss and other fungal substances which can weaken the foundation.
It also gives excellent stability to the structure during seismic movements below the surface.
Strata Geosystems' role in strengthening soil load capacity
StrataGrid is a cutting-edge product made by interweaving high-tenacity polyester yarn. It has exceptional tensile strength and provides vertical and horizontal soil reinforcement. What stands out in this product is its UV-stabilized coating that ensures longevity and resistance against adverse weather conditions. With these remarkable features, StrataGrid is a successful soil reinforcement technology and provides exceptional load-bearing capacity to the soil.
- The tensile strength of soil is quite low. To improve and to provide extensive support to soil, engineers use geogrid.
Strata Geosystems took on the challenge of elevating a pair of side-by-side railroad tracks through a bustling downtown traffic corridor, collaborating with KS Union Pacific Railroad and Burlington Northern Santa Fe Railroad. Our team of experts found a way to significantly improve the load-bearing capacity of the soil, ensuring the structure could withstand heavy loads. Our experts proposed a solution of installing horizontal layers of StrataGrid at 18″ intervals (one layer per wire basket) all the way to the top of the wall. To further bolster the structural stability, the team backfilled the facing of temporary wire baskets with fine-grain sand and wrapped it in a blanket of microgrid face wrap. The project’s impact was substantial, resulting in the creation of new grade separations that connected five new bridges and one rehabilitated bridge along a 1.8-mile strip. This seamless integration allowed both trains and vehicles to flow unimpeded through eight major thoroughfares. Our commitment to excellence was evident in the extensive monitoring conducted over several months, which validated our solution. Even under heavy loads, the structure remained rock-solid, moving no more than half an inch in any direction, including vertical settlement.
- Strata Geosystems provided a reinforced earth solution for a steep slope platform on a mountain edge in Ireland, Western Europe, using StrataGrid uniaxial geogrid and StrataSlope systems.
Strata proposed an innovative and sustainable solution to build a working platform on the edge of a steep slope while minimizing the amount of fill material used. In the design calculations, we considered a surcharge of 45 kN/m² and a required bearing capacity of 200 kPa on the finished surface of the equipment used for constructing 75-meter-tall turbines. Our geosynthetics analysis determined that a combination of on-site granular material and StrataGrid 35 kN and 55 kN with 600mm spacing could make 10-meter-high, 70-degree angled walls feasible for the contractor. Our team of experts designed the reinforced soil wall following the guidelines of SD8006:2010 for internal stability and Eurocode EC7 for global stability. By introducing this reinforced earth solution for the steep slope, we significantly reduced the overall height of the structure from approximately 30 m to a more practical 10 m. This approach not only reduced the overall carbon footprint of the structure but also ensured the platform could support construction traffic and heavy lifting equipment, effectively improving the load-bearing capacity.
To learn more about this product, speak to our experts.