Improper soil properties can lead to issues like excessive settlement, cracking, or structural damage down the road. That’s why it is critical to conduct thorough soil investigations before deciding on a foundation system. Too often, builders overlook the need for detailed subsurface exploration and instead make assumptions based solely on a visual inspection of the surface. In this article, we are going to look at different soil types used for foundations and the tests conducted to choose the best soil type for a specific foundation.
Why Soil Testing For Foundation Holds So Much Importance?
What appears stable on the exterior could conceal unfavorable conditions just beneath the surface like compressible soils, zones of poor drainage, or unexpected layering that weren’t accounted for in the foundation design. Such oversight can then require expensive repairs, remedial work, or even complete replacement of improperly designed foundations.
In order to select the most suitable foundation for your project that will provide long-term structural support, you need to start with proper testing and analysis of the on-site soil conditions. This involves hiring a geotechnical engineering firm to conduct subsurface investigations using methods like test borings, laboratory testing, and environmental evaluations tailored to the specific needs of your project.
The resulting soil report provides critical information about soil composition, bearing capacities, compaction potentials, settlement characteristics, and other geotechnical Soil Properties across your site. It is the single most important document to guide determination of appropriate foundation types that will withstand the loads from your building without excessive movement over time.
Testing The Soil
Testing the Soil Properties for a foundation involves hiring a professional soil testing company to extract core samples from your land. These samples will be examined in a lab to determine critical details like:
- Soil classification and layering
- Bearing capacity and compressibility
- Moisture content and drainage
- Presence of rocks or organic matter
- Depth to water table
A few common soil testing methods include:
- Visual examination – Analyzing texture, consistency, color of undisturbed soil cores
- Grain size analysis – Determining composition of sand, silt, clay and gravel
- Water content tests – Measuring moisture levels in soil
- Compaction tests – Evaluating ability of soil to be dense and strong when compacted
- Consolidation tests – Knowing time for soil to settle under applied pressure
Common Soil Properties Types Used For Foundation
Clay Soils
Clay soils composed of very small particles are prone to shrinking and swelling as they absorb and release water depending on weather conditions. This changing soil volume can exert pressures on foundations over time, risking cracks or structural damages. Clay is also slow to drain and compressible under heavy loads.
When building on clay grounds, options like deep foundation systems such as piles are suitable to bypass the unstable upper clay layers and transfer loads directly to more competent soil at greater depths. You can also improve poor clay by mixing stabilizers to increase its load-bearing capacities.
Sandy Soils
Although sand offers good drainage, loose sandy soils have low cohesion and can settle significantly under foundation loads. During storms, water percolation may even cause localized voids and collapse beneath footings.
For such granular soils with low bearing capacities, options like helical pier foundations or reinforced concrete mat slabs spread loads over a larger area to reduce pressures. Compacting sand around the foundation perimeter also prevents future failures due to erosion or scouring.
Organic Soils
Areas with high organic content like topsoils, mud or decomposed plant material are notoriously weak. Organic soils are very compressible and don’t provide reliable bearing capacities for foundations regardless of system used.
The only way to build safely on these soils is by completely removing all organic-rich layers and replacing them with structural fill before installation of any foundation type. Otherwise, continued settlements will damage your structure over time.
Rocky Sites
While unweathered rock presents few concerns being inherently strong, shallow bedrock requires special considerations. Rocky sites demand deep excavations that are expensive and risky for basement construction. Shallower systems like strip-type foundations with tie-beams spanning across footings can be better suited for these conditions.
After reviewing your soil report, the foundation professional will be able to recommend an appropriate system matching the bearing capacities and drainage characteristics of the most substantial soil stratum present at your site. Some common foundation alternatives are:
Conventional Spread Footings
These pad footings underneath load-bearing walls and columns are a standard economical option for sites with adequate load-bearing sandy or silty soils, bedrock at shallow depths, or where organic layers have been properly removed and replaced with compacted structural fill. Footings need to be sized as per calculated bearing pressures.
Pile Foundations
When bearing capacities of surface soils are insufficient, transferring loads from deep piles embedded into stronger subsurface layers becomes necessary. Different pile types include driven concrete, precast concrete, helical or screw piles depending on specific job requirements. Pile foundations are suitable for sites with clay soils, loose sands, organic soils and shallow bedrock.
Mat (Raft) Foundations
This monolithic slab foundation acts like a rigid mat to spread loads evenly over a large contact area. It’s used for buildings on weak, compressible, or uneven terrain to minimize differential settlements. Reinforced concrete mat slabs can work well with variable fills or clay soils experiencing swelling/shrinkage cycles that standard individual footings may not withstand.
Combination Foundations
Some projects utilize dual systems, such as pile caps supporting grade beams and mat slab or beam-and-slab combination to address complex subsoil conditions with varying properties near the surface and at greater depths.
Specialized tests that may be needed for certain Soil Properties or project types
Bearing Capacity Tests
For determining the maximum pressure a given soil can accommodate before failing or settling excessively under static loading conditions. Common field tests involve load cells fitted to precast concrete blocks or plates loaded incrementally with calibrated weights or hydraulic jacks. Laboratory versions include the Standard Proctor, CBR and triaxial tests on sampled soils.
Consolidation Tests
When constructing on highly compressible saturated clay deposits, this test measures amounts of settlement over time as pore water pressures dissipate under applied loads. Results help assess post-construction settlements to limit tolerable levels for different structures based on intended use.
Shear Strength Tests
Critical for designing deep excavations, retaining walls or slopes since shear strength Soil Properties like cohesion and internal angle of friction influence stability. Vane shear apparatus or triaxial compression testing fit this purpose by simulating shear planes in homogeneous soil elements.
Permeability Tests
Important if dewatering will be required during construction, or to assess drainage performances of backfill soils. Constant or falling head permeability tests are conducted on relatively undisturbed soil samples to determine their hydraulic conductivity coefficients.
Special Considerations for Problem Soils
Some soil types and conditions require extra precautions or special foundation designs beyond standard systems. Here are a few examples:
Expansive Clays
These clay soils prone to shrinking and swelling can cause significant structural issues over time due to changes in moisture content. Foundation types suited for expansive clays include reinforced concrete slabs or stiffened mat slabs designed to resist upward and downward pressures. Chemical or mechanical soil stabilization treatments may also help reduce volume changes. Proper drainage and backfilling are necessary to keep these moisture-sensitive soils fairly dry.
Collapsible/Hydrocompactive Soils
Loose, dry soils like loess that compact greatly on becoming wetted pose serious threats. Surface loads can suddenly drop infrastructure into voids. Presoaking or moisture conditioning should precede construction to encourage collapse settlements outside loaded areas. Then deep pile or pier foundations bypass collapse zones, isolating structures from such weak soils.
Backfilled or Fill Soils
Soils compacted for development are often weaker than undisturbed natural ground. Settlement monitoring allows fills to consolidate fully before building. Geogrid soil reinforcement or vibro-concrete stone columns installed beforehand can accelerate consolidation under live loads. Raft slabs distribute concentrated loads to safe bearing pressures.
Soluble Bedrock
Layers of limestone prone to dissolution that risk creating voids or sinkholes beneath structures need special care. Solutions involve placement of controlled low-strength material encased in reinforced concrete below foundations as a cavity-bridging fill. Measures preventing ingress of water like drainage diversions are equally important.
Coastal/Riparian Sites
Buildups along coastlines or riverbanks subject to flooding and erosion require deeper, more robust foundations than those for inland constructions. Pile designs should consider scour protection as well as lateral loads from water, wind, waves and currents. Knowledge of minimum expected and historic flood elevations guides minimum finished floor levels.
Seismic Zones
Where seismic activity occurs regularly, geotechnical engineers model subsoil conditions and structure-soil-structure interaction to design earthquake resistant foundations. These may incorporate seismic isolators at foundation-superstructure interfaces, shear keys in piled footings, reinforced retaining structures or collectively behaving mats spreading energy through inertial interaction.
In summary, the type of ground beneath your home or building will have a big effect on how strong and sturdy the foundations need to be. Different soils like clay or sand act in different ways when holding up heavy structures.That’s why it’s important to test the dirt on your property before starting any foundation work. The tests will show what your soil is made of and how well it will support foundations. Then you’ll know the best foundation style to use without problems in the future.