Investigations
Now that we have examined various failure mechanisms, one can better decide what type of investigation to conduct. I will touch on several different investigations here, but they are by no means the only possible methods.
Borings and Test Pits
As with most geotechnical jobs, borings and test pits are the bread and butter of a side-hill wall investigation. The exact number and spacing are dependent on the specifics of the project and any applicable design guidelines or codes required by the owner or by law. But here are a few things to consider. (Figure 9: Limited access coring rig perched on the side of an existing embankment slope. Photo courtesy of NCS Consultants.)
It’s not advisable to let the investigation be limited by access and cost. Only drilling borings that are truck-accessible will not give you a complete picture of what is going on at the wall line and below it. The latter is important from a global stability perspective to nail down the stratigraphy. A mix of borings at the wall line, and above and below it is advisable for that reason. The cost associated with a global stability failure or with major contractor claims during construction would likely be much higher than the cost for some additional investigations. The tricky part is selling that idea to the project managers and owners who control the money.
Figure 10: Crux Subsurface, Inc. difficult access rig setting up on a side-hill slope. Photo courtesy of Bharat Khandel, ADOT via NCS Consultants, LLC.
Geophysics
In areas where bedrock is shallow, strategically located seismic refraction lines can be tremendously useful for generating your subsurface profiles for global stability and settlement analyses. These are especially useful when constrained and validated by borings. There may be other geophysical methods that are useful in this application, but seismic lines are probably the most common and the only ones with which I’m familiar.
Figure 11: Example of seismic refraction survey for determining depth to bedrock. Photo courtesy of NCS Consultants, LLC.
Piezometers
Water pressure behind retaining walls is ALWAYS a bad thing…very, very bad! Determining if and at what depth groundwater is encountered is very useful to incorporate into your global stability modeling. If you are using coring methods to collect samples, they frequently drill with water and polymer slurry, so determining the depth to groundwater is not very reliable. So why not install at least some temporary piezometers while you’re out there and maybe some semi-permanent ones as well.
Geologic Mapping
The terrain where you frequently find side-hill retaining walls is often close to mountains and hills and sometimes some complex geology. Performing a regional geological mapping exercise can help you constrain the geologic units you are encountering which can help give your boring and lab testing program some focus. Outcrop scale mapping can help to determine if there are joint or bedding orientations that might cause problems from a global stability perspective. (Figure 12: Geologist mapping a fault in the field. Photo by Hitchster.)
In the Laboratory
Of course no geotechnical field investigation would be complete without a laboratory investigation. As mentioned previously, global stability is the name of the game, so in addition to N-Values, direct shear samples, dry and moist unit weights, proctor densities, water contents and index properties are probably most important. In areas of potentially swelling soils, testing to establish their presence and swell characteristics are important as well.
For rock, unconfined compressive strength tests are critical to help prevent claims from contractors on the type of equipment needed for excavation and drilling or driving. Direct shear tests on bedding planes or joints may be required in some cases as well.
Chemical testing (sulfates, chlorides, pH, resistivity, etc.) will help to tell if the material from cuts or the existing fills will be suitable for wall backfill or if a borrow source will be needed. This can be a HUGE line item on the engineer’s estimate and should not be overlooked!
