This article discusses the different types of soil investigations that are performed to gain an understanding of the properties of the soil and determine the suitable types of foundations that can be used based on these properties.
The reports generated from soil investigations are also examined, along with the various types of foundations that are recommended for different types of soils. This information can be useful in construction projects where it is essential to ensure that the foundation is strong and stable.
By conducting soil investigations, engineers can determine the strength and stability of the soil, which is crucial for selecting the appropriate foundation type. The type of foundation selected is dependent on the characteristics of the soil, such as its bearing capacity, settlement characteristics, and drainage properties.
The reports generated from soil investigations provide valuable information on the soil’s properties, including its composition, density, and moisture content. This information is used to determine the suitable types of foundations that can be used for the specific soil type.
Overall, soil investigations are an essential part of any construction project. They provide critical information that can be used to select the most appropriate foundation type, ensuring the safety and stability of the structure.
Types of Soil Investigations for Foundation Selection
Subsurface Soil Investigations
Soil engineers conduct subsurface soil investigations by examining subsoil conditions through test borings. The number and location of borings depend on the type of building and site conditions. For uniform soil conditions, borings are spaced 100-150′ apart, while for more detailed work, where soil footings are closely spaced and conditions are not even, borings are spaced 50′ apart. Larger open warehouse type spaces, where fewer columns are present, require fewer boring samples.
Borings must reach firm strata, pass through unsuitable foundation soil, and extend at least 20 feet further into bearable soil. Borings are not taken directly under proposed columns, and their location is indicated on the engineer’s plan. They provide information about depth, soil classification, moisture content, and sometimes groundwater level. Physical properties such as particle size, moisture content, and density are also recorded.
The subsurface soil investigation report should include recommendations based on testing of materials obtained from on-site borings. It should cover bearing capacity of soil, foundation design recommendations, paving design recommendations, compaction of soil, lateral strength (active, passive, and coefficient of friction), permeability, and frost depth.
Surface Soil Investigations
Surface soil investigations are necessary for construction in various cases. These include instances where the water table is high or where there are problematic soils like soft clay, peat, loose silt, or fine water-bearing sands. Another reason for such investigations is when rock is close to the surface, which would require blasting for excavations. Similarly, sites with dumps or fills or those that show evidence of subsidence or slides require surface soil investigations.
Above-ground indicators also provide insights into soil conditions. For example, buildings near a construction site may need shoring or earth, and existing foundations may require reinforcement. Rock outcroppings indicate bedrock and are suitable for bearing and frost resistance, but they can pose challenges during excavations. Water bodies like lakes indicate a high water table and may necessitate waterproofing of foundations.
The terrain of the construction site is also a significant factor in surface soil investigations. Level terrain is easy to work with and has fair bearing but poor drainage. Gentle slopes are also easy to work with and have excellent drainage. Convex terrain, like a ridge, is a dry and stable place to build, while concave terrain, like a valley, is a wet and soft place to build. Steep terrain is expensive to excavate, and there is a risk of erosion and sliding soils.
The presence of foliage can also indicate soil conditions. Some trees indicate moist soil, while large trees signify solid ground. Therefore, surface soil investigations are necessary to assess the soil conditions accurately and take appropriate measures to ensure the safety and stability of the construction project.
Soil Classifications
Soil mechanics engineers have developed a classification system that enables them to determine the characteristics of a soil. This system, known as the unified soil classification system, relies on the identification of soils based on their textural and plasticity qualities, as well as their behavior grouping. Soil mixtures found in nature typically contain varying proportions of particles of different sizes, and each of these components contributes to the overall makeup of the soil.
The unified soil classification system is a useful tool for engineers dealing with soil mechanics. By analyzing a soil’s textural and plasticity qualities, engineers can identify its properties and behavior. The system also helps engineers group soils based on their behavior, allowing them to better understand how a given soil will behave under different conditions.
When analyzing soil, engineers must take into account the different components that make up the mixture. Soil mixtures are composed of particles of varying sizes, and each of these components contributes to the overall makeup of the soil. By understanding the properties of each component, engineers can gain a better understanding of the overall characteristics of the soil.
Overall, the unified soil classification system provides engineers with a valuable tool for analyzing and understanding soil. By identifying the properties of a soil based on its textural and plasticity qualities, and grouping it with respect to behavior, engineers can make more informed decisions about how to work with a given soil. By taking into account the different components of a soil mixture, engineers can gain a deeper understanding of the soil’s properties and behavior.
Soil is classified on the basis of:
- Percentage of gravel, sand, and fines.
- Shape of grain.
Plasticity and compressibility characteristics of soil
The Unified Soil Classification System (USCS) utilizes both descriptive names and letter symbols to categorize soil according to its primary characteristics. This categorization process involves both visual inspection and laboratory testing to determine the appropriate group for the soil. The USCS classifies soil particle sizes into four categories: cobbles, gravel, sand, and fines, which comprise either silt or clay.
The particle size of soil is categorized from largest to smallest as cobbles, followed by gravel, and then sand, which is further divided into three subcategories: coarse, medium, and fine. Lastly, fines refer to soil particles composed of silt or clay.
Soil shear strength is determined by its cohesion, which is influenced by water content and how sticky it is, as well as its internal friction, which is based on the size of its grains. To assess soil shear strength, triaxial compression testing is conducted.
Soil Groups:
Soil can be categorized into three groups: coarse-grained, fine-grained, and highly organic. Coarse-grained soils are composed of gravel and sand particles, which make them suitable for building foundations if well-drained and confined. The G series (GW, GP, GM, GC) is particularly good for bearing weight, identified by the percentage of gravel and sand present. Fine-grained soils, on the other hand, consist of smaller silt and clay particles, and are suitable for foundations but require compaction. The most favorable of the fine-grained series is the CL, identified by cohesive properties and permeability. Highly organic soils, including peat, humus, and swamp soil, contain decomposed organic material and are not suitable for construction due to their high compressibility and moisture content. Soil names in the unified soil classification system are based on grain size and textural properties for coarse-grained soils, and plasticity properties for fine-grained soils.
When taking soil samples for geotechnical engineering, relevant information for determining foundation suitability includes particle size, mineralogical composition, grain shape, and binder characteristics for coarse-grained soils, and strength, moisture, and plasticity for fine-grained soils. Visual inspection during the preliminary stages can provide some insight into the soil’s behavior as a building component, and it can be classified according to the unified soil classification system. However, laboratory testing may be necessary for more accurate results. Ultimately, a soil’s strength and consolidation, which contribute to its compaction characteristics, are essential factors for determining its suitability as a building foundation.
Soil Problems
The problem of uplift pressures in soil can be mitigated by utilizing well-drained and free-draining gravels (GW, GP). This is particularly important for fine-grained soils consisting of silts and clays, which are prone to heaving of foundations and formation of boils due to potential frost action. However, frost action only becomes a concern when two conditions are present simultaneously: a source of water during the freezing period and a sufficient period of freezing temperatures to penetrate the ground.
In general, silts and clays (ML, CL, OL) are more susceptible to freezing due to their high moisture content. In contrast, well-drained granular soils are less susceptible to freezing and creating foundation problems. The drainage characteristics of soils directly reflect their permeability, and the presence of moisture in base, sub-base, and sub-grade materials may cause the development of pore water pressure and loss of strength.
Soils with little or no fines (GW, GP, SW, SP), such as gravelly and sandy soils, have excellent drainage characteristics. On the other hand, fine-grained soils and highly organic soils have poor drainage characteristics, which can lead to the development of pore water pressure and subsequent loss of strength. Overall, selecting soils with good drainage characteristics can help mitigate uplift pressures and ensure stable foundations.
Compaction of Soil
The sheepsfoot and rubber tired rollers are both frequently used equipment for soil compaction. One advantage of the sheepsfoot roller is that it leaves a rough surface, which can lead to better bonding between layers. On the other hand, granular soils that are composed of well-graded materials, such as GW and SW, tend to yield better results in terms of compaction compared to poorly-graded soils like GP and SP.
Fine grained soils can also be compacted
The investigation of soil compaction through field tests is highly desirable for most construction projects. The suitability of soil as a foundation material depends on its strength, cohesion, and consolidation characteristics. The type of structure, load, and usage will dictate whether a soil is appropriate for a given construction project. A soil may be suitable for one type of construction but require special treatment for another.
Gravel and gravelly soils (GW, GP, GM, GC) typically have good bearing capacity and experience minimal consolidation when subjected to a load. Well-graded sands (SW) also tend to have good bearing capacity. However, poorly graded sands and silty sands (SP, SM) have variable capacity based on their density.
Some soils that contain silts and clays (ML, CL, OL) are prone to liquefaction, resulting in poor bearing capacity and significant settlements under loads. CL is likely the better fine-grained soil for foundations.
Organic soils (OL and OH) as well as highly organic soils (Pt) generally have poor bearing capacity and often display significant settlement under loads.
Types of Foundations Based on Soil Investigation
The choice of foundation for fine-grained soils that contain silt and clay depends on the load’s magnitude and the foundation’s location relative to the soil. Spread footings may suffice in some cases, but when the soil is weak, and the load is relatively heavy, alternative methods such as pile foundations may be necessary. It may also be possible to remove and replace unsuitable soils economically by over-excavating and importing engineered soil or compacting and filling back. Geotechnical engineers, based on soil borings, recommend appropriate foundation systems and determine minimum depths, bearing capacity, and special design and construction procedures. The safe bearing capacity of soil is the ultimate bearing capacity divided by a safety factor, typically 2-4. The ultimate bearing capacity is the maximum unit pressure a soil can sustain without allowing significant settlement. Bedrock has the highest safe bearing capacity, while well-graded, confined, and drained gravel and sand have a safe bearing capacity of 3,000-12,000 PSF. Conversely, silts and clays have a lower safe bearing capacity of 1,000-4,000 PSF.
Role of Foundations
In order to properly support a building, it is necessary to transfer the load to the ground. Additionally, the building must be anchored against wind and seismic activity to prevent damage. One important consideration is to isolate the building from frost heaving and expansive soils, as these can cause movement in the foundation that can lead to structural damage over time. A foundation must also hold the building up from moisture to prevent water damage.
Beyond these functional requirements, foundations can also provide living spaces, such as a basement or storage area. Mechanical systems may also be housed in the foundation. Three common foundation configurations are Slab on Grade, Crawl Space, and Basement. Each configuration has its own set of advantages and disadvantages, and the choice depends on various factors such as climate, soil conditions, and the intended use of the building.
Foundation Types
Spread Footings
Spread footings are a common choice for buildings with light loads or weak, shallow soils. These types of footings consist of square pads at column locations, with reinforced walls to accommodate elongation forms of bearing walls. Spread footings are designed to directly deliver the load to the supporting soils, and the area of the footing spread is determined by dividing the applied force by the soil’s safe bearing capacity (f=P/A). Spread footings are typically suitable for low-rise buildings ranging from 1-4 stories, as long as the soil is firm enough to support the building on the footing’s spread area.
For increased lateral stability during earthquakes, footings at column locations can be interconnected with grade beams. Spread footings are a popular choice due to their cost-effectiveness. To ensure durability, the depth of footings should be below the topsoil and frost line and installed on compacted fill or firm native soil. It is crucial to place the footing above the water table, and concrete spread footings should be at least as thick as the width of the stem.
As the building’s weight increases concerning the bearing capacity or depth of good bearing soil, the footing needs to expand in size or alternative systems must be used. Overall, spread footings are an economical and practical solution for buildings with light loads and firm soil conditions capable of supporting the building on the spread footing area.
Drilled Piers or Caissons
Drilled caissons (piers) and grade beams can be used as a foundation solution for structures built on expansive soils with low to medium loads or high loads with rock at a reasonable depth. Caissons can be straight or belled out at the bottom to spread the load. The function of the caissons is to deliver the load to the stronger soil capacity located not too far down, while the grade beam spans across the piers and transfers the loads to a column foundation.
This method is particularly useful when dealing with expansive soils, as these types of soils can cause significant damage to foundations over time due to their ability to expand and contract with changes in moisture content. By utilizing caissons and grade beams, the load of the structure is transferred to a layer of soil that can withstand the forces placed upon it, reducing the risk of foundation failure. Furthermore, this method can be used in areas with high loads and where rock is present at a depth that is not too far down.
Pile Foundations
Expansive soils and compressive soils with heavy loads can pose a challenge for building foundations, especially when the underlying soil lacks the capacity to bear the load. In such cases, two types of piles can be used: friction piles and end bearing piles. Friction piles rely on the resistance between the skin of the pile and the surrounding soil when a reasonable bearing stratum is absent. End bearing piles, on the other hand, transfer the load directly to the soil of good bearing capacity.
The bearing capacity of the piles is determined by the strength of the pile structure itself or the strength of the soil, whichever is less. Piles can be made of various materials such as wood, steel, reinforced concrete, or cast-in-place concrete. Cast-in-place piles are created by drilling a hole in the earth and filling it with concrete. This type of pile is suitable for light loads on soft ground where drilling will not cause a collapse. Friction piles obtain their capacity from the perimeter of the shaft and the surrounding earth.
Mat Foundations
Reinforced concrete rafts or mats are an excellent solution for constructing small, lightweight buildings on weak or expansive soils, such as clays. These structures are typically made of post-tensioned concrete, which enables the building to float on or in the soil like a raft. They can even be used for buildings as tall as 10-20 stories, providing essential resistance against overturning.
One of the significant advantages of using a reinforced concrete raft is that it allows for a much larger bearing area, which is necessary when dealing with weak or expansive soils. In situations where the footing would need to be spread out significantly, it may be more cost-effective to pour a single, thick slab instead. This approach can be more economical and require fewer forms.
Reinforced concrete rafts are often used instead of driving piles, which can be more expensive and disruptive to the surrounding area. They are especially useful in areas with expansive clays or silts, as they enable the foundation to settle without significant differences. Additionally, these rafts can be less obtrusive than other foundation solutions, resulting in less of an impact on the surrounding environment.
General Summary of Soil Investigation and Types of Foundation
Soil is an important factor to consider when building foundations. In terms of suitability for foundations, sand and gravel are considered the best option, followed by medium and hard clays, while silts and soft clays are ranked as poor. Organic silt and clays are considered undesirable, and peat is completely unsuitable for foundation construction.
The potential for shrinkage and swelling, which is usually characteristic of clay-like soils, increases with the plasticity index (PI) and cohesiveness of the soil. Granular soils consisting of gravel and sands are classified as non-cohesive soils, while silts, clays, and organic soils are considered cohesive soils.
To limit differential settlements in concrete foundations, the maximum settlement should be between ¼ to ½ inch. Typically, the cost of foundations represents around 5% of the total construction cost. Spread footings are the most economical option, especially when the safe bearing capacity is at least 3000 pounds per square foot (PSF). Piles, on the other hand, are the most expensive foundation option, with a price tag that can be 2 or 3 times that of spread footings.