While numerous components help construct a building, bridge, or road, one of the most critical aspects is the foundation. A strong foundation is paramount to a project’s resiliency. However, an installed foundation is only as good as the supporting subgrade on which it is constructed. Therefore, understanding the subsurface conditions beneath a project is critical to assessing the longevity of the project‘s structures. Our Drilling, Geotechnical, and Construction Materials Testing teams get their hands dirty to analyze the soil and ensure it can support the project‘s foundation.
Soil Properties and Foundation Design Criteria
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 to this, other criteria (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 in a relatively short period, 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 taken into account 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 soils 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 elevation. In the end, any sort of bearing failure or settlement failure is very expensive to remediate; therefore, our engineers rely on collaboration with design team members to ensure that 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 and 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, again 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
So when is the right time to test your project’s soil?
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. Summit will 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.
Construction of this project was completed in March of 2012 (Summit was not involved in the project’s original Geotech/CMT phases). By July of 2017, the building owner noticed cracks in their foundation and contacted Summit‘s Geotechnical department to investigate the potential causes for the foundation failure. Through evaluation and testing, Summit determined that the soil beneath the foundation consisted of Highly Plastic Clay, a soil type as you know, that shrinks and swells with varying moisture conditions. A combination of field density testing of in-place material and moisture-density laboratory testing also showed that the soil was not properly compacted during earthwork and grading prior to foundation installation. The deleterious soil type and poorly compaction effort of the soil beneath the foundation both contributed to the foundation‘s failure and subsequent costs to the owner for repairs. Testing soil both before and during construction activities would have gone a long way to mitigate this issue and avoid costly repairs.
During a recent project Summit was involved in, our site evaluation determined that the on-site soils were of low strength and unsuitable (CH/MH) to be constructed on as-is. Based on our findings, and if no ground improvements were considered, the resulting allowable bearing capacity was only 1,500 psf. By considering a structural load applied to a spread footing, the footing size is inversely proportional to bearing capacity. For example, if your bearing is low, your footing size will increase to carry the same load. Conversely, if you have a higher bearing, your footing size can decrease.
Understanding the cost implications, Summit implemented a more collaborative approach and worked directly with the owner, designer, and contractor to evaluate ground improvements that would increase the bearing capacity of soils. This iterative process of evaluating a multi-variable approach of balancing the cost of the ground improvement with the material savings of smaller footings was truly a game-changer for this client. We evaluated the cost of material and time savings for each option to select the best site-specific solution for the team that met the specifications and minimized the impact on the budget and overall construction schedule.
These examples illustrate why our experienced teams test subgrade soils both before and during construction to prevent costly issues from arising both during construction and long after construction is completed. Our passion is providing our engineering expertise to clients in impactful ways that address their specific needs and goals without blowing the budget. Our engineers and field staff is the best in the business, and we would love to serve you on your project. Our attention to detail and quality, experienced staff and expertise, responsiveness and communication, and focus on your needs set us apart from other firms.