When it comes to constructing a building, bridge, or road, there are many components involved, but the foundation is undoubtedly the most crucial. A strong foundation is paramount to the project’s longevity and resilience. However, it’s essential to understand that the installed foundation is only as good as the supporting subgrade on which it is constructed. Therefore, assessing the subsurface conditions beneath the project is critical to ensure the stability of the project’s structures. Our teams specializing in Drilling, Geotechnical, and Construction Materials testing help analyze the soil to ensure that the foundation can support the project’s structures. They provide the necessary expertise to ensure that your project is built safely and sustainably.

Soil has numerous properties that Summit’s Geotechnical Engineers evaluate to provide clients and construction team members with foundation design criteria.  Utilizing our in-house drill rig, Summit investigates the soils at a site to assess the soil’s engineering properties (i.e., classification,  gradation, unit weight, angle of internal friction, cohesion, moisture, etc.) to obtain information on an existing subgrade’s strength and consistency (i.e., Standard Penetration Testing (SPT) N-Values), and to assess the presence of groundwater and shallow rock.  All of these criteria influence a soil’s allowable bearing capacity. In addition, other measures (or factors) to consider concerning bearing capacity are anticipated structural loading, footing width, and embedment depth if this information is not readily available.  

Summit’s engineers also evaluate the soil’s elastic (immediate) settlement and consolidation settlement under sustained loading. Settlement can be defined as the downward vertical movement of soil due to an applied load, increasing the soil’s effective vertical stress and strain. The majority of the short-term, elastic settlement will be experienced relatively quickly during construction. Post-construction consolidation settlement is a longer process that occurs over the years as increased vertical stresses are applied to the ground on or above soft or loose soil strata.  

Another type of settlement that should be considered and evaluated is the differential settlement. Differential settlement occurs when part of a structure is constructed on relatively strong soil or a more rigid base, such as rock or weathered rock, and the other part of the structure is on softer grounds or fill material. Other factors, besides the ones already mentioned, to consider when performing our evaluation include existing and proposed grade elevation, finished floor elevation, and foundation-bearing height. In the end, any bearing or settlement failure is costly to remediate; therefore, our engineers collaborate with design team members to ensure we have current and accurate grade information and structural loading and dimensions.  

In addition to bearing capacity and settlement, the classification of the soils. (gravel, sand, silt, or clay) In general, sands, silts (ML), and clays (CL) are considered suitable for use as subgrade material or as controlled fill to build up structural areas of a site during grading activities.  However, fat clays (CH) and elastic silts (MH) cannot be utilized as structural fill material. They must be removed from beneath footings due to their tendency to lose strength at increased moisture levels, causing foundation problems. CH clay also tends to shrink and swell with moisture variation, resulting in foundation issues.  These behaviors result in many geotechnical engineers recommending a grade separation between these soils and the bottom of the foundation system of the development.   

Understanding all these soil properties is imperative to properly evaluate foundation design criteria and ensure a project’s resiliency.   

ASTM D-2487 Soil Classification Chart

Reference: Wikipedia, https://en.wikipedia.org/wiki/Unified_Soil_Classification_System

Site Selection Testing

Before purchasing a property for development, Summit‘s Geotechnical & Drilling departments will collaborate with a project owner to ensure the property is suitable for the proposed development. We then perform a preliminary geotechnical investigation to evaluate subsurface conditions at a property and determine overall suitability, with significant factors influencing suitability being the presence of extensive uncontrolled fill, unsuitable soils, or shallow bedrock.  Uncontrolled fill is fill soil that was placed during prior grading activities that contain organics or construction debris or material that was placed without being properly compacted.  These soils typically need significant undercut or ground improvements to prevent settlement issues after the construction of the structure.  Significant amounts of unsuitable clays and silts that lie beneath a site can result in the need for large quantities of imported suitable soil or aggregate to properly grade the site to ensure the stability of the subgrade and its ability to support the structure‘s foundation.  Extensive shallow bedrock across a site can result in significant blasting to achieve the final grade for the project. All of these undesirable subsurface conditions and others can significantly increase construction costs, so we check for these during our due diligence phase investigation and inform the owner of these factors before purchasing the property.  This information tells the owner about special considerations that must be assessed to develop the site and evaluate the potential costs to complete the project. 

Once the final site plans have been created, a higher level of investigation occurs within established building footprints and stormwater pond areas to provide more accurate foundation design parameters to the design team. With the subsurface information provided from the additional soil test borings, our engineers will sharpen their pencils on bearing capacity and settlement calculations and provide recommendations for foundation types, pavement, seismic site classification, site preparation, controlled fill, slopes, excavations, groundwater, construction specifications, and any construction allowances to include in contractors’ bids.  These parameters are essential for foundation designers to design the building foundations. Like previously mentioned, soil borings will also indicate the depth to groundwater, which influences these foundation design parameters and, if shallow enough, can cause problems during construction.  Shallow groundwater can reduce the overall bearing capacity of the subsurface and can cause difficulties for the contractor to make necessary undercuts of existing soils if ground improvements are required.  Shallow groundwater can also affect the stability of slopes and retaining walls.  This additional investigation also provides an opportunity to understand the bedrock profile better if shallow bedrock was encountered during the due diligence phase investigation.   

The information provided in our final geotechnical report allows for more competitive bids by contractors to the project owner by reducing the amount of unknown or unforeseen subsurface conditions that may be encountered during the construction process.  Before groundbreaking, the contractor’s knowledge of subsurface conditions increases preparedness to prevent delays and reduce the number of and size of change orders during the construction phase. 

During construction, Summit‘s Construction Materials Testing team tests soil more thoroughly than possible during a geotechnical investigation. While a geotechnical investigation involves borings typically spaced hundreds of feet apart to obtain general subsurface properties of a site, CMT personnel can evaluate soil properties during mass grading and fill placement, or on a footing-by-footing basis within the building pad, or along the length of a proposed roadway embankment to allow for specific remediation recommendations. 

• The soils in use are tested for classification again to ensure that the material will not have excessive swell, settlement, or low bearing capacity.  
• The bottom of the footings is tested using a Dynamic Cone Penetrometer (DCP) to ensure that the geotechnical engineer’s soil bearing capacity is met. 
• Fill soils in footings, embankments, retaining walls, or other areas of structural fill are tested for density. The value achieved in the field is compared to a laboratory–determined value known as the Maximum Dry Density. The field density must meet a percentage of that determined laboratory number, typically 95%, 98%, or 100% in structural areas. This compaction percentage is reduced in most instances to 85% in non-structural areas. Compaction testing can be performed with a nuclear gauge, drive-tube method, or sand cone method for determining compaction.  
• At various points during earthwork and grading operations, Proofrolls can be a really useful observation in order to assess subgrade stability. Surficial soils could be marginally compacted, but if deeper soils are unstable, a proofroll (loaded dump truck rolling over an area at 2-3 mph) will reveal the instability. Having a stable working subgrade is the first step in achieving the required compaction requirements for a job.  

Soil testing is also imperative for transportation projects and is conducted in the pre-construction and construction phases. Our Geotechnical and Drilling teams perform the pre-construction investigations to help characterize the soils within the proposed project corridor and determine where shallow rock and groundwater might be present. Characterization of the soils is critical for determining the overall earthwork balance of a proposed roadway project. Roadway design engineers need to know how much of the anticipated borrow obtained from proposed cut areas will be used as borrow for proposed embankment areas or as backfill. In addition, general areas with poor subgrade soils need to be identified for removal during construction before embankment placement or the construction of the pavement section. Critical areas, often areas with large amounts of anticipated earthworks, will be analyzed by Summit’s Geotechnical engineers for slope stability, embankment stability, and settlement. Similarly, soil characteristics are important parameters for bridge foundations design and the settlement calculation at bridge end bents. 

All of these factors greatly influence the cost of the construction of a roadway project. Understanding the subsurface characteristics within a proposed project corridor is vital to allow for a more accurate bid by contractors and cut down on unforeseen change orders and cost overruns that may occur during construction. Soil testing does not stop at the pre-construction phase for transportation projects. Like private sector projects, soil testing is extensively conducted during construction, and subgrade soils are constantly evaluated.