Digging Deeper into In Situ Stabilization

In situ stabilization (ISS) is a technology commonly associated with geotechnical stabilization of loose soils, but it can also function as a remediation treatment. In a previous post, we explained the basics of ISS, and later dug a little deeper in a webinar presentation ISS 101: What You Need to Know When Considering In Situ Stabilization, you can now watch the webinar on demand.

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Our live webinar audience posed a lot of insightful questions—questions that you may have, too. In this blog post, we present some of that conversation, covering considerations when deciding to use (or when using) ISS.

WHAT ARE REASONABLE DEPTHS FOR ISS? WHAT IS THE DEEPEST YOU’D CONSIDER USING ISS FOR?

When we talk about depth, we should talk about the methods and equipment we are going to use. Depths of up to thirty feet can be achievable and appropriate when bucket mixing or using a mixing tool on the end of an excavator. With benching and/or shallow pre-excavation, we can typically go a little deeper than that; potentially up to 40 feet with a larger excavator with sufficient breakout force, depending on the lithology encountered.

Reaching 100 feet is achievable with the auger method. Remember, maximum depth is greatly dependent upon lithology, diameter of the auger, and foot pounds of torque. You also must balance depth with cost effectiveness. The larger the diameter of your auger, the more material you can treat with a single mixing pass of the tool, which means you don’t have to move and set up the ISS equipment as many times. Smaller diameter augers, when mixing at greater depths, may be less cost effective due to the increased number of borings and additional number of setups necessary over a given volume.

WHAT ARE THE EFFECTS OF ISS ON ADJACENT STRUCTURES?

Inherent to any type of intrusive subsurface work near structures, care should be taken prior to implementation of any subsurface soil disturbance to evaluate three distinct possible effects:

1. Direct impact by contact to structures,
2. Impacts causes by vibration to structures particularly if structures are older or in disrepair
3. Impacts caused by differential settlement either during or following ISS operations

Direct impact to subsurface spread footers or other features can be accessed via structural drawing reviews and test pitting measures, if deemed appropriate. Vibration monitoring can be achieved via completion of a vibration plan and use of seismic meters around the area of the work to assure that subsurface vibration does not exceed levels that may cause damage to structures present and within close distances from the work. Lastly, settlement plates, strain gauges and/or tilt-meters are routinely used to assess possible settlement conditions that may occur while implementing the ISS remedy to ensure that differential settlement or liquefaction conditions are not observed. The benefit of ISS is that compliance sampling is typically performed real time, with daily samples collected to evaluate bearing capacity and compressive strengths to assure future buildability and less risk of differential settlement from occurring.

 
IF MIXING WITH PORTLAND CEMENT, HOW LONG DO THE CEMENTITIOUS AGENTS REMAIN COMPETENT IN PLACE IN THE SUBSURFACE?

There’s an expectation of complete containment for 30 - 40 years, depending on groundwater geochemistry. Change in geotechnical conditions of the subsurface groundwater system or physical altercation of the matrix itself may reduce that timeframe.

ARE THERE ANY CHALLENGES TO WORKING NEAR TIDAL RIVERS?

Not necessarily. However, the salinity of the water is something to keep in mind, when using surface water or a groundwater source for reagent mixing or slurry production. The impact is in the mixed water, not necessarily in the groundwater.

A bench study is recommended for evaluating reagents when the site is in a brackish or saline environment adjacent to a saltwater body. That way, we can determine if we need a type 1 or type 2 Portland cement, or something that’s going to be more appropriate for a high chloride content. This is also true of sodium bentonite, as it doesn’t mix well with saltwater.

 
IS IN SITU STABILIZATION OF ACID TAR POSSIBLE?

It depends. Acid tars were mostly managed in pits and lagoons where sulfuric acid was used to treat oils. Typically, most of those materials would be treated ex situ or removed entirely, then the impacted soils beneath the pit or lagoon would be treated. In one example of stabilizing acid tars, preliminary bench scale testing had been conducted with good results by using primarily quick lime. Dealing with acid tar as a source material is challenging. The use of ISS in these cases really depends on the goals for the site, the material, and if it’s intended to remain in the impoundment and capped.

DO REGULATORS FIND AN ISS APPROACH ACCEPTABLE?

ISS is successfully implemented all over the country. You should check with your local regulatory agency to confirm its use for your specific project site.

ARE THERE ANY METHODS TO STABILIZE PARTICULAR DEPTH INTERVALS?

Absolutely. We have the ability to tweak your injection rate at a specific depth to selectively treat a specific zone using an electronic data recording system. It’s very easy to do with the equipment available today. As an example, this may be done in the CCR world to treat beneath an impoundment.

We have the ability to adjust amendment ratios on nearly a per-foot basis by changing our fluid-to-reagent ratios or even our overall dosing in discrete zones. We like to plan for those intervals in advance  and optimize treatment from a cost and performance standpoint. That way, the lowest passing criteria complies with the project goals.

WHAT IS THE CURRENT UNDERSTANDING OF LONG-TERM EFFECTIVENESS FOR ISS?

The expectation is that the solidified matrix and monolith lasts as long as any other subsurface concrete material that could be affected by geophysical parameters or chemical alteration. In the case of metals, you’ve literally fixated the contaminant—or rendered it insoluble. Organics may degrade a bit in the matrix, but they shouldn’t be released because the permeability of the matrix is very low.

If you’d like to learn more about ISS, watch ISS 101: What You Need to Know When Considering In Situ Stabilization.

If you have an upcoming project and think ISS might be a good fit, let us know.

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