Subsurface Investigation

In order to design the foundation of structures, we must have good understanding of the soil conditions. The first step to this is to perform subsurface investigation. The most reliable and preferred way for this, is to see the actual soil, by taking samples.

This can be done by drilling boreholes in the ground. The samples taken are then tested for various properties, many of which we already talked about, to understand the soil better. The minimum borehole depth, for the smallest structures should be around 30 feet and this value will go to much higher numbers depending on the size of the structure. 

Minimum number of holes and their spacing and depth are determined by geotechnical engineer based on factors such as visual inspection, existing geotechnical and geological data from previous projects in the area or the ones obatined from local agencies, adjacent structures, depth of groundwater, and type and size of intended project.

By drilling holes, a great deal of information can be obtained about the soil. First and most importantly, the soil profile can diectly be seen at various depths for each location where the holes are opened. Methods vary for opening holes in different soil types for different purposes of testing and for different water table elevations.

These methods include hand augers, bucket augers, rotating wash, hollow stem augers, but we will not discuss further details about those here. Among them only the hand auger can be operated by hand, and for others, machines are needed. Undisturbed sample recovery is the important part of drilling bore holes. Sampling methods include split barrel, shelby tubes, continuous soil augers, pitcher tubes, which we will also not discuss in further detail, but detailed procedures were established for each method, in order to take the samples as undisturbed as possible.

In case of rocks, coring is performed. A rock’s quality directly depends on how intact a certain length of core can remain without breaking into segments. After obtaining rock cores, tests can be done on them as well. 

The information gathered from boreholes can be represented on boring logs, which are the most fundamental part of a geotechnical report made for a project. These logs will also give information about subsurface conditions encountered during drilling and sampling. The logs must be completed in the field and not the lab, and it is important to indicate whether something is observed in the field or in the lab. These logs are then included in the geotechnical report for the entire site. A typical boring log is organized generally as below. The information that is included is typically as below but the format and what exact items are included will definitely vary from log to log, including some items that are not shown here.

Sample boring log

Some of the items that are seen in the example boring log above need no further clarification as they are obvious. Now let’s clarify the remaining ones that may not be so obvious at first:

  • Drilling and Sampling Method: We have just talked mentioned most widely used ones in the previous page.
  • Caving Depth: During drilling processes, soil may cave in sometimes and this needs to be noted if applicable.
  • Hole Size: The diameter of the drilled hole
  • USCS Soil Classification: USCS stands for Unified Soil Classification System, which is the most widely used system for classifying soils, although there are other ones. We have already talked about soil classification in a previous section of this book.
  • Graphic Log: Every soil in geotechnical engineering, has commonly accepted graphical symbols, for engineers to recognize them at once. And this can be illustrated graphically in this column.
  • % Recovery: During sampling process, not necessarily 100% of the material can be recovered all the time due to various reasons and this % can be estimated here.
  • Material Description: One of the most important parts of a boring log, this column includes an engineering description of the material by the engineer.
  • SPT Blow Count: SPT stands for Standard Penetration Test, which we will talk about in the following pages. It is a very widely used soil testing method and done by advancing a rod with hammer blows into the soil. The more blows required to advance the rod, the stronger the soil. Many other properties of soil are now correlated with SPT results, thanks to extensive prior research, and that is why SPT count is one of the most important things to know and indicate in a soil boring log. An engineer who sees the blow count immediately will have some idea about the soil.
  • Data that are measured in laboratory: Under this category, we can see a few things. Water content and dry unit weight, we already talked about under Phase Relationships section. Fines content means, the portion of fine soil in the whole soil sample, such as clays and silts. Liquid Limit and Plastic limit are related to how plastic a soil is, which we will talk about in the coming pages soon.

Various things that are observed during drilling operation, even before the holes are complete or any tests are performed, will also give valuable information to an experienced geotechnical engineer. These include the appearance of the material for rock and soil debris, how much drilling mud (a high consistency liquid that is pumped into the hole to prevent it from caving in) is required at what stage and how much of that mud is returning back from the hole, the torque and pushing energy required to advance the borehole for a certain distance and many more details. All of these can show a good deal of information, even before preparation of logs. Apart from these visual observations at the ground level, in some rare cases even more detailed work can be done, such as lowering electronic sensors or cameras to borehole.

It is also important that before any of all this boring work is done, geotechnical engineer seeks to obtain existing data from prior historical and geological studies. These can be in the form of topographical, geological, agricultural maps, site development histories, aerial photos and even information obtained from local people. Combined with visual inspection during site visit, even before starting subsurface investigation, the geotechnical engineer can begin to understand the site, and identify potentially problematic areas such as soft or wet areas or sudden potential changes in soil conditions, and can then use this information for determining boring locations and depths. 

The next subsection, “soil testing and measurement” is also closely related and can be considered as a direct continuation of this subsection.

In the next post of this series, we will discuss “Soil Testing and Measurement”

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