Treatment of soil generated by tunnel construction①

Using the adsorption layer construction method to solve the problem of contaminated soil at a tunnel excavation site where construction must resume promptly

Emergence of a problem

During the construction of an expressway, arsenic, selenium, and boron levels exceeding the standard levels were detected from the excavated soil at a tunnel section.
Although an exploratory pollution study was conducted for this large-scale construction, the tunnel excavation work was halted after seeing the test results.

Diagnosis

Large-scale pollution countermeasures were anticipated, and a Measures Study Committee comprising learned individuals was convened. As it would be difficult to handle this issue as a single company, the countermeasure methods were reviewed by a team, and the following was confirmed.

First priority was that excavation work resume promptly.
Therefore, a short-term and effective countermeasure was required.
A temporary storage site would be required because contaminated soil would be continually produced while the tunnel was being excavated. However, the on-site conditions would make it impossible to secure a temporary storage site.
Countermeasures should be performed on time in parallel with the excavation work.

Validation

In this situation, the Measures Study Committee and consulting companies discussed various measures, one of which was the adsorption layer construction method. The Committee contacted AMEC as a manufacturer of the material used in the adsorption layer construction method. We made a presentation on the most conceivably effective method for this task.

Planning

In the case of these measures, the most important conditions were the “completion of the countermeasures in a short period of time” and that the “countermeasures be implemented in parallel with the excavation work.”

The adsorption layer construction method was highly evaluated because it satisfied these conditions and was less expensive than other methods.
Once the adsorption layer is placed, it becomes possible to continually transport the excavated contaminated soil.

Furthermore, the adsorption layer can be placed on any ground area (uneven or sloped), and has a relatively high degree of flexibility with regard to site specifications.
Another merit of this method is that contaminated soil is not directly processed and only dissolved toxic heavy metals are processed; therefore, the risk of secondary pollution by the addition of chemical agents is very low.
The response to the requirements and the merits of this method were comprehensively evaluated, and as a result, the “adsorption layer construction method” was highly regarded and adopted as the first choice as a countermeasure for contaminated soil.

Using our CAPA-CT samples, various evaluations and validations were then performed by the client.
As a result, CAPA-CT was adopted as the material for the adsorption layer construction method.

The key to the adoption was that, predictably, many materials solely adsorbed boron and did not adsorb a small amount of arsenic and selenium, whereas CAPA-CT was able to adsorb not only boron but also a small amount of arsenic and selenium.
Additionally, it was later reported that an inspection into the compatibility with the construction equipment indicated that the material was very suitable.

Solution to the problem

After the material was adopted, test constructions and a main construction were conducted. In the meantime, we were sure that, in the event of any issues, AMEC’s follow-up system was one of the best. The main construction was completed, and the monitoring study showed no problems at which point we knew that we had achieved our goal.

Comments from the person in charge

AMEC does not regard the adoption and sales of our materials as the goal.
Our real goal is achieved when we successfully complete the implementation of countermeasures, starting from the proposal of a construction method and materials to the confirmation of the effect after construction, followed by a monitoring study.