Re-stating the importance of geotechnical investigations for designs and constructions
Doyin (Adesokan) James-Crutchlow Ph.D., P.Eng.
One of the things that is common is that people call soils “dirt”, which leads to soils being treated dismissively, in a way that other construction materials such as concrete or steel are not treated. Soils are construction materials, and must be treated, not as dirt, but as important engineering materials that can make or mar the performance and operational lives of constructions founded on or within them. Thus, the engineering properties of soils and the implications of those properties for designs and construction must be properly determined and correctly applied.
A revered professor in the field of unsaturated soil mechanics (Dr. L. S. Barbour) once said to our class during a lecture, “soils do not lie, it is humans who can possibly lie”. An interpretation of that statement is that when the type and properties of soils are not properly determined, or when these are determined but the information obtained is not effectively and appropriately utilized, unwanted issues can happen. Such unwanted issues can make it seem as though the soils have lied about their real behaviour under the required conditions.
Another recognized professor of geotechnical engineering described soils as “not CHILE” – that is, “not Continuous, Homogenous, Isotropic, and Linearly Elastic”. This implies that soils do not behave like most typical engineering and construction materials do, and, soil properties and behaviours can change quickly or vary widely, even within a relatively small area. Factors such as geological and depositional history, previous or current site use, and several others, influence the stratification and types of soils found at a site. For example, alluvial and lacustrine soils are known to have varying horizontal stratifications that reflect their geological and depositional history.
Individual soil types such as peats, organics, clays, silts, sands, and the various combinations of these soil types behave differently under various conditions. In addition, different site conditions, such as variations in applied loads, saturated and unsaturated conditions, chemicals and other fluids, heat, frost, freezing and thawing cycles, wetting and drying cycles, cyclical, dynamic, irregular or random loading, vibrations, earthquakes or similar disturbances, and more, cause different responses in individual soil types.
Peats, muskeg, and organic soils are highly compressible, making them poor and undesirable foundation or load bearing soils. Depending on the water content, clay soils can be highly compressible, or they can desiccate and crack – in response to cyclical weather and climatic conditions. Also, some clays are susceptible to high swelling or shrinkage (e.g. bentonite or montmorillonite clay) or can exhibit thixotropic or liquefaction behaviour (e.g. Leda clay or other similar glaciomarine clays). All these and other significant and unique behaviours of clay soils influence how clays are designed for in various applications. Silts and loess soils can exhibit collapsible, or piping behaviours, saturated loose sands or silty sands can exhibit liquefaction behaviours, sands can hold water as an aquifer or be free draining, and so many other distinctive soil behaviours.
Each specific behaviour of soil types can have different implications for designs, constructions, and other applications. Nevertheless, if thorough geotechnical investigations are completed to correctly identify and characterize the soil types at a site and if the soil properties are correctly determined and all that information is well utilized in the design, construction, or application, then soils do not and will not lie. Figure 2 shows a sample subsurface profile from a geotechnical site investigation. The profile shows the soil stratigraphy and the groundwater elevations at various locations across the site.
In the Figure 2, the soil stratigraphy consists of continuous and discontinuous soil layers, as well as pockets of different soils at various depths. The profile shows pockets of sandy silt, clayey silt, and silty clay within an otherwise continuous layer of glacial till. Also, above the glacial till layer is a layer of backfill material from the previous land use.
In the sample profile presented in Figure 2, the geotechnical investigations helped to properly identify and characterize the soil types, subsurface stratification, and groundwater elevations across the site, which in turn helped to obtain the soil parameters that were needed to correctly design for the proposed site application. However, what is seen sometimes for constructions and applications is that a site is merely stripped and cleared, and then the proposed construction is completed (or structure or storage vessel is placed) on a compacted backfill, without knowing in detail what lies beneath within the foundation soil layers. Hence, in such situations, if there happens to be layers or areas of undesirable soils within the foundation soil layers or if there are groundwater issues or aquifers (Figure 2) not accounted for, problems can occur during or after the constructions. Such problems can include excessive or differential settlements, drainage issues, structure or building serviceability issues, cracks, tilting, collapse, and others.
Sometimes, however, it is not a case of not knowing what lies beneath, but a case of knowing – from having completed geotechnical investigations, but totally ignoring what the geotechnical report has stated or recommended. Thus, resulting in defective or unsafe constructions.
When the data obtained from geotechnical investigations reveal the presence of undesirable soils or site conditions, evaluations are completed to determine the most suitable ground improvement techniques that can be deployed to enhance the site to achieve the desired ground conditions. Such ground improvement techniques can include the use of chemical stabilization and amendments, or the use of mechanical ground improvements such as excavating the weak layers and replacing with higher shear strength materials, or the use of geosynthetics.
Geosynthetics such as geogrids, geotextiles, or mechanical geocomposites (e.g. geogrid and geotextile combinations – Figure 3), can be used for soil reinforcement, separation, or stabilization to improve and fortify poor ground conditions. Likewise, geosynthetics such as geotextile tubes can be used for constructing or repairing foundation columns in soft soils, prefabricated vertical drains (PFDs) can be used to increase soil drainage and consolidation for constructions and applications, and several others.
Typically, geotechnical investigations are completed in two phases: Phase I and Phase II. Phase I involves completing desk-based studies where the geological, depositional, historical site use, and other site data, are collated. Phase II involves going to the site to complete site reconnaissance involving in-situ and ex-situ field investigations, drilling, and sampling of investigative boreholes for further investigations in the field or in the laboratory. The data collected from the two phases of the geotechnical investigation are used to characterize the soils at the site and to prepare the geotechnical report which would contain specific recommendations, relative to the proposed construction or site use.
Geotechnical investigations can also include geophysical surveys. Geophysical surveys provide continuous lateral and vertical imagery and measurements of the subsurface over a relatively larger area. Geophysical surveys, when completed in tandem with the Phase I desk-based studies in the geotechnical investigation, can be used to plan and direct the location, depth, and number of drilling and sampling locations for the Phase II geotechnical investigations.
Ultimately, geotechnical investigations and the thorough use of the results and reports from the investigations are essential for designing for different soil types and site conditions, and for various constructions and site applications. Hence, geotechnical investigations must be appropriately completed and the site-specific recommendations from the investigations must be duly implemented to ensure safe and sound constructions that perform as needed for the required durations and service lives.
References
- R. D. Holtz, W. D. Kovacs, T. C. Sheahan, An Introduction to Geotechnical Engineering, 2nd Edition, Pearson Prentice Hall, (2011)
- D. P. Coduto, M. R. Yeung, W. A. Kitch, Geotechnical Engineering Principles and Practice, 2nd Edition, New Jersey, Pearson Prentice Hall, Upper Saddle River, (2011)
- S. Arya, M. O’Neill, G. Pincus, Design of Structures and Foundations for Vibrating Machines, Gulf Publishing Company, Houston, Texas (1979)
- B. Caicedo: Geotechnics of Roads: Fundamentals, Taylor and Francis Group, London UK, (2019)
- R. M. Koerner, J. P. Welsh, Construction and Geotechnical Engineering using Synthetic Fabrics, John Wiley and Sons, Inc. (1980)
- R. M. Koerner, Designing with Geosynthetics, 6th Edition, New Jersey, Pearson Prentice Hall, Upper Saddle River (2012)
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