Before ground breaks on any new construction project, the site in question must be investigated to determine whether the soil has the bearing capacity to support the proposed building. Typically, this investigation is done by a third-party geotechnical engineering consultant, and the results are compiled into the geotechnical soil report.
Using a variety of field-observed and lab-tested data, the geotech will describe the site and subsurface conditions that will have an impact on design and construction of the project. From that data, they’ll make recommendations about whether ground improvement is necessary to reach the required bearing pressure, or if deep foundations should be considered.
But with no universal standard governing how much data must be collected during the investigation and written into the final report, the geotechnical engineer and owner are left to negotiate what should be included.
The geotechnical engineer wants the most thorough investigation possible and the owner wants to keep the cost of all geotechnical work as low as possible. These two requirements are more aligned than they appear on the surface. There are opportunities to save significant sums of money on ground improvement design and construction by getting the clearest data at the report stage.
No matter how thorough the investigation, the geotechnical contractor has to design and install a ground improvement system — or deep foundation if ground improvement isn’t feasible — that meets the bearing pressure and settlement criteria of the building.
The subcontractor’s bid will be based on the amount of information provided. If they only have minimal strength and settlement information about the soil, the contractor will be forced to overdesign their ground improvement plan to meet these parameters. A lack of data also increases the risk of the contractor encountering soil conditions not specified in the report, resulting in change orders and a rise in cost.
The initial savings of a less thorough investigation will quickly be negated by a more conservative, and thus more expensive, aggregate pier design. Allowing the geotechnical consultant to conduct a detailed investigation of the site helps the owner achieve the goal of creating the most cost-effective ground improvement design.
Conducting site investigations and writing geotechnical reports are a big part of a geotechnical consultant’s job, so this won’t be a 101 description of how to write a report. Rather, it’ll explain which aspects geotechnical subcontractors find most valuable in developing the highest quality, lowest cost quote for the owner.
At absolute minimum, the geotechnical subcontractor’s ground improvement design has to meet the required bearing pressure to support the building. This requires the engineer designing the ground improvement to have a clear idea of the strength and settlement parameters of the soil throughout the site.
To design the most efficient aggregate pier system, they need as much data about the soil as possible. This means more soil borings, more soil samples taken at each boring site, more field-observed strength tests and more lab analysis done on the samples to determine the strength of the soil.
Specifically, though, the subcontractor is looking for tests and analysis that give a good idea of soil strength. The more strength and settlement information available to the geotechnical contractor, the more they can refine their ground improvement plan, identifying which sections of a site require a more involved design and which don’t, resulting in significant cost savings.
Here are three ways to test a site’s soil strength and settlement parameters:
Standard penetration tests (SPT): A routine soil testing and sampling procedure conducted in the field within a soil boring. A small, hollow tube is hammered into the ground and the number of blows required to advance is measured, giving a rough idea of soil strength. The soil collected within the hollow tube can then be retrieved and used to visually identify the samples, or taken to the lab for further strength tests.
While the data collected from an SPT isn’t as robust as that collected from the next two tests, SPTs are an inexpensive way to visually and physically corroborate the information collected in shear strength, consolidation and cone penetrometer testing.
Shelby tubes: These cylindrical sampling containers are used to collect undisturbed samples in cohesive soils for visual and lab inspection. Samples are typically collected at five-foot elevation intervals within a boring.
While a myriad of tests can be run on a Shelby tube samples — plasticity, moisture, visual inspections — the most useful pieces of information are results from the shear strength testing and consolidation testing. Oftentimes, you see nearly continual Shelby tube samples in a boring; however no testing of these samples has taken place, so there is no strength information on the soil.
Consolidation tests give the settlement parameters of the soil and shear strength tests give a more accurate picture of soil strength than SPT blow counts. The data collected from these two tests gives a good idea of the column strength a subcontractor can achieve with ground improvement at different points throughout the site.
Undisturbed sampling with Shelby tubes allows for more accurate strength and consolidation testing within the lab compared to samples obtained through traditional SPT methods.
Cone penetrometer testing (CPT): Both the Shelby tube and SPT testing are done at intervals, testing only one strata at a time within the soil boring. Cone penetrometer testing (CPT) can provide more reliable data than either, with a probe continuously measuring the soil shear strength throughout the elevation profile. It gives the best information on the actual, in-situ strength of the soil.
CPT is generally more expensive than SPT or Shelby tubes, but it provides better strength data than either. Still, SPTs and Shelby tubes are important to visually corroborate the data gathered from a CPT.
While CPT equipment isn’t widely available throughout the country, the insights gleaned from CPT tests are so valuable that it’s worth seeking out for large-scale projects. The initial cost of running the tests is quickly justified through the significant savings achieved with a more efficient ground improvement design.
In addition to a thorough subsurface exploration program, laboratory testing of soils is critical to developing a safe and efficient ground improvement design. Ground improvement design relies upon accurate determination of the soil strength/stiffness and its compressibility.
Gradation and Atterberg Limits testing help the geotechnical engineer accurately classify soils and verify visual classifications. Additionally, unconfined compression testing and various consolidation tests give insight into how the soils will react when loaded, and allow for the most efficient and appropriate ground improvement design.
By investing in thorough field explorations and soil sampling, as well as appropriate laboratory testing, the owner puts their design and construction teams in a position to successfully support their project.
The strength and settlement data gathered in the tests above help the geotechnical subcontractor design and construct the most cost-effective ground improvement plan. But a geotechnical report can’t be discussed without mentioning the importance of the foundation recommendations section.
A lot hinges on the way a geotech words their recommendation for aggregate piers. Recommending a propriety product such as Rammed Aggregate Piers* — which can only be installed by a small subset of contractors — greatly limits the pool of contractors who can bid on the ground improvement project. Additionally, construction specifications should be written to include non-proprietary terminology.
Using broader language, like “aggregate piers” or “vibro stone columns,” opens up the bid to more subcontractors and more value. When geotechs and structural engineers word their recommendations and specifications for aggregate pier ground improvement this way, it ultimately helps the geotech provider offer greater value to the owner.
* Rammed Aggregate Piers is a registered trademark.