ranite and gneiss are widely distributed in the Aichi prefecture part of the Shin Tomei Expressway. Its 973m-long Gakudozan Tunnel and its surrounding ground is dominated by sandy gneiss and pelitic genesis. According to the pre-excavation survey, it was found that the gneiss-dominant area contained naturally-occurring heavy metals, including lead, cadmium, arsenic, and selenium. Since heavy metals can cause ill effects on human health, even if they are naturally occurring, they should be handled in accordance with the Soil Contamination Countermeasures Act (SCCA), which regulates environmental standards for handling of land with heavy metals in Japan.
The levels of heavy metals found in the gneiss were higher than the standards defined in the SCCA. It should be noted that although arsenic is not classified as a heavy metal, this article includes it in heavy metals since it is one of the hazardous substances.
Thus the tunnel was excavated along with the heavy metal survey. When excavated soil contained heavy metals, it was used for embankments within the main road of the expressway after necessary measures were taken. This article describes the plans for the presurvey of heavy metal-containing soil and the testing method, its production situation during construction and responses during construction when the plan required changes.
Original plan
From the pre-survey, the amount of the tunnel-excavated soil that contains heavy metals and the amount that requires countermeasures were predicted, and the construction plan was made based on this.
Soil with arsenic that exceeded the Environmental Standards was expected from 3 per cent of the length of Gakudozan Tunnel and the adjacent Nukata Tunnel. The heavy metal-containing soil was planned to be contained within embankments of the expressway, after the measures that were specified in Japan’s SCCA were taken. Due to the small amount of expected heavy metal-containing soil, anything that exceeded the environmental standards, and that would be produced prior to the completion of the embankment for heavy metals containment, was to be transported to paid concrete recycling facilities.
Survey method for heavy metals
Due to the necessity for making a tentative decision on the survey method of heavy metals in the soil before the test results were obtained, a test method that promptly provided results was desired. The voltammetric method was chosen after some consideration. To ensure the transparency of the adoption of this method, it was discussed with the environmental bureau, and then deliberated in a review session by a third-party.
In the environmental standards, the amount of heavy metals in soil and the elution amount, which represents the degree of elution in water, are specified.
The heavy metal-containing soil in this area was a small amount, but was expected to exceed the environmental standards in terms of the elution amount. Four types of substances were expected in this area (i.e., lead, cadmium, arsenic and selenium), and they were the subject of the survey. In order to judge the treatment necessity immediately after excavation, the survey frequency was set to once a day for total excavated soil, since blasting was used for tunnel excavation and all of the rock waste within the profile was mixed, and hence separation among lithological characters was not possible.
The specified test method in the Soil Contamination Countermeasures Act takes two weeks to get the elution amount test results. Therefore, the voltammetric method was used for testing elution amounts and contents immediately after excavation in order to obtain results faster. The voltammetric method is a qualitative and quantitative analysis of dissolved substances by the electric potential and electric current that are measured with the electrode submerged in the solution. This method enables results to be obtained in about 10 hours, and therefore it can reflect construction progress more effectively than the SCCA’s specified method. In the course of adopting this official method, the standards for the voltammetric method were set based on correlation between the measured values of the SCCA’s method and the voltammetric method for each substance. Figure 1 shows the relationship between the two methods, in terms of arsenic as an example. The correlation coefficient was high and had a sufficient correlation.
The results and countermeasures were discussed in the aforementioned review session and it was confirmed that they were the proper method and countermeasures.
Although the voltammetric method provided results in a short period of time, a temporary storage yard with capacity for a one-day supply of tunnel rock waste was made for tentative storage at the construction site. A large tent was installed in the temporary storage yard to prevent the possible heavy metal elution from the tunnel rock waste in case of rain during temporary storage.
Heavy Metal-Containing Soil ProduCtion
In Gakudozan Tunnel, arsenic that exceeded the environmental standards was found in about 70 per cent of the excavated length, which greatly surpassed the original expectation. The maximum concentration was 6.90mg/l, while the elution standard in the Environmental Standards was 0.01mg/l. From the adjacent Nukata Tunnel (1,800m long), a high concentration of arsenic appeared. At the end, more than 400,000m3 of total heavy metal-containing soil was produced (70 per cent of excavated soil) from tunnel excavation for a total of two ‘up’ and two ‘down’ lines.
In Japan, a 30 times higher elution amount than the SCCA’s environmental standards (arsenic : 0.01mg/l×30 : 0.3mg/l) is defined as the Second Elution Standards, and more strict countermeasures (more than two) are required. For this construction site, soil with a higher level than the environmental standards but with less than the Second Elution Standards was used as embankment material.
Figure 2 shows the tunnel length direction and the arsenic elution amount from the tunnel rock waste of ‘up’ and ‘down’ lines of Gakudozan Tunnel. Little arsenic elution was found at the location about 100m from the tunnel’s mouth, but an arsenic level higher than the environmental standards was found in most of the deeper rocks. There was no trend in the detected arsenic elution amounts. For Gakudozan Tunnel, the arsenic elution amounts were completely different between the up and down lines that were only 30m apart. In both Gakudozan Tunnel and Nukata Tunnel, heavy metals that exceeded the environmental standards, other than arsenic, were not detected.
In order to handle the unexpectedly large amount of heavy metal-containing soil, a countermeasure to mainly contain heavy metals (the embankment) was adopted.
Due to the soil allocation scheme, Gakudozan Tunnel passage was needed in the early stages to use it for soil transportation. Excavations of both Gakudozan Tunnel and the adjacent Nukata Tunnel began at the same time.
However, unexpected amounts of heavy metal-containing soil were produced from both tunnels from the early stages when the preparation for embankments was not yet ready.
Therefore, halting the originally planned, costly transportation of heavy metal-containing soil outside of the construction site was considered. Since the countermeasure embankment did not start using the soil until a year later, Nukata Tunnel excavation was paused and it was used as a temporary storage for heavy metal-containing soil in order to continue excavation of Gakudozan Tunnel.
Furthermore, a temporary storage was set up in the area near the tunnel where cutting was completed, during the period prior to when the embankment area would be ready for acceptance. The bottom of the temporary storage was paved with asphalt to provide a waterproofing layer, and the soil was covered with waterproof sheets during temporary storage.
As the heavy metal-containing soil increased, the embankment area to accept it became insufficient, and utilisation of mountainous narrow grounds as countermeasure embankment areas was discussed. Since the narrow grounds were sloped, a solidification and insolubilisation method was considered.
Prior to the countermeasure embankment construction, explanatory meetings were repeatedly held for a half year to obtain the consent of neighbourhood residents, who were concerned about health hazards. At the explanatory meetings, the following items were explained: the reasons for choosing the location of the countermeasure embankment, characteristics of heavy metals that would be buried, and the testing method for water quality in the surrounding area. In the end, with understanding and cooperation of the environmental bureau, the residents agreed with the conditions that the water quality survey in the surrounding area would be continued, even after the construction was completed.
Since heavy metal-containing soil production widely varies depending on location, it is important to perform detailed research and examinations in advance, and to make a plan based on the premise that the original expectation would be exceeded during construction.
Seepage Control and Containment
The seepage control and containment method is a construction method to prevent heavy metal elution by blocking rainwater from heavy metals, using clay and liners with low water permeability. This method has the following advantages: the construction progresses at almost the same rate as the common embankment, once the bottom impermeable layer is built, and its maintenance is relatively easy since rainwater penetration is blocked when it is completed.
Overview of Seepage Control and Containment
Figure 3 shows the outline of the embankment structure. A double layer of impermeable synthetic rubber sheeting (t=1.5mm) with moderate elasticity was used as the top impermeable layer. The bottom impermeable layer consisted of a combination of a clay layer using bentonite mixed soil and a single layer of impermeable sheeting. To avoid effects on ground water as much as possible, the countermeasure embankment was planned at a higher level than the ground water level. Instead of temporarily covering the embankment with liners, a drain pipe was installed as a rainfall countermeasure during the heavy-metal embankment construction (Figure 4).
During construction, it promptly discharged rainwater out of the embankment that fell on the heavy metal-containing soil. When it rained, drainage water from this drainpipe was checked for arsenic elution. A neutralisation plant for arsenic was built next to the construction site, in case arsenic elution was found from drainage. For the bottom impermeable layer, the bentonite mixed soil was installed and then impermeable sheets were placed.
In a laboratory test, the bentonite mixed soil was first pretested for the relationship between the amounts of added bentonite and the dry density, to meet the specified hydraulic conductivity (K ? 1×10-8m/s). Then the number of passes of compaction that provided the specified dry density was specified by a field trial. Wide impermeable sheets were used to reduce numbers required for adhesion at the construction site.
Unwoven fabric and a protective sand layer (t=500mm) were used as protective materials for the impermeable sheet.
Since the tunnel rock waste was masses of rock, the embankment with the heavy metal-containing soil was built by the specified construction method where the adequate number of passes and the finish thickness of each layer were decided by testing. The official method analysis was conducted for every 900m3 of the countermeasure embankment with the heavy metalcontaining soil, as specified in the SCCA, and the characteristics of the heavy metal-containing soil were monitored. Once the heavy metal-containing soil was embanked to a predefined level, the upper impermeable layer was promptly constructed and the containment of the heavy metal-containing soil was completed. According to the aftermentioned water quality monitoring results, heavy metal elution due to rain did not occur.
SolidifiCation and inSolubiliSation method
As the amount of heavy-metal containing soil largely increased, the countermeasure embankment was required even for narrow slopes. Countermeasures that used liners were not sufficient to maintain embankment stability, so solidification and insolubilisation was considered rather than the seepage control method.
In this method, cement is used as an insolubilisation agent. Solidification of the cement provides the insolubilisation effect. There were examples of relatively small-scale constructions using soil with cement as an insolubilisation agent, but no precedent existed for masses of rock.
Samples were taken from masses of rock that contained heavy metals that were broken into less than 100mm pieces, and the insolubilisation effect was checked by a laboratory test. They were then used for the expressway embankment and water quality was monitored.
Confirmation test for solidifiCation and insolubilisation
Effectiveness of the solidification and insolubilisation method using the Portland blast-furnace cement was verified first by a laboratory test. For this construction, arsenic was checked since it was the only heavy metal found from the tunnels.
For the verification test, cement solidification test pieces were made with soil that had a higher arsenic level than the elution standards. The relationship between the curing period of the solidification test pieces and the arsenic elution amount was investigated. The test was conducted for four original soils with different arsenic elution levels, and the stabiliser additive rate was tested for three levels (Table 1).
Required test pieces were created using a 100mm mold. They were isothermally cured for the predetermined period in atmosphere at 20oC. After curing, the samples were broken into less than 2mm particle size and the official method analysis was conducted. Since the elution amount was steady for 60 days, and even for 180 days of curing, the construction method was determined based on these results. The test results are described in Figure 5. A verification test for long-term stability is not required by law, but a twoyear ground water test is mandated.
For the original soil sample with an arsenic elution amount of 0.37mg/l, 5 per cent cement addition lowered the arsenic elution amount to about 0.001mg/l, which was less than the elution amount standards for soil, 0.01mg/l. For the original soil sample with the arsenic elution amount of 1.7mg/l, 5 per cent cement addition lowered the arsenic elution amount to around 0.3mg/l, i.e., the Second Elution Standards. The solidification and insolubilisation effect by cement was effective even after 180 days.
Long term stability of the solidified and insolubilised soil against pH changes
In order to examine the long-term stability, the solidification and insolubilisation effectiveness was tested in strong acid and alkaline conditions.
In general, it is said that there is a possibility of elution of heavy metals. when the insolubilised soil is exposed to acid or alkaline conditions. Causal factors include the oxidisation effect of sulfuric acid that is eluted from pyrite in heavy-metal containing soil, and pH changes due to exposure to an alkali.
As an evaluation of the long-term stability of the soil, which was solidified and insolubilized by cement, and of the stability of the insolubilisation technology, elution tests were conducted for the 60-day cured test pieces. For the test, sulfuric acid or lime hydrate was added. The test results are shown in Table 2.
For the original soil that had less than 0.013mg/l arsenic elution amount, re-elution of arsenic was not observed in acidic and alkaline conditions after solidification and insolubilisation, and therefore, they were stabilised.
For the sample with the elution amount of 1.70mg/l, the cement additive rate of 5 per cent decreased the elution amount less than 0.3mg/l, the Second Elution Standards. Thus, the cement additive rate was set to five per cent of the dry weight of the base material (100kg/m3) at this construction.
ConstruCtion of the solidified and insolubilised embankment
For the tunnel rock waste that was judged as heavy metalcontaining soil, a screen mesh size of 40mm was used in the crushing machine. However, due to its flatness, actual maximum grain size was over 40mm and less than 100mm after crushing. To produce the embankment material, the grain size was adjusted, and then the soil was mixed with cement and water by the portable soil improvement machine.
The embankment material was handled as stabilised soil, and quality control standards were set according to company standards: the degree of compaction should be higher than 97 per cent in terms of density ratio. Testing frequency specified in the law is every 900m3 for countermeasure embankment. Arsenic elution was also checked at the final stage by the SCCA’s 0fficial method and the voltammetric method.
Due to the difficulty of sampling from the stabilised-cement embankment, samples were made with the embankment material from the construction site immediately after mixing. They were tested for the arsenic elution amount after seven days of curing.
As a result, the voltammetric method showed that arsenic elution amount was lower than the environmental standards. In addition, the arsenic elution amount measured by the SCCA’s official method did not exceed the lower limit (0.001mg/l).
Water Quality Monitoring
The construction within the office was undertaken according to the following plan, which included countermeasures for muddy water in addition to heavy metals.
- In order to prevent discharge of material to rivers during construction, rainwater in the all construction areas was directed to settling basins where fine-grain fractions were separated, and then water was discharged to rivers.
- For water quality monitoring during construction, turbidity, pH, and electrical conductivity were measured twice a day at the inlet of the settling basin, and the outlet to the rivers.
- Monthly water quality surveys (including heavy metals) were conducted both upstream and downstream of the sites
- In order to monitor the effects on ground water, observation wells were installed up and downstream of the embankment
- During the construction of the heavy-metal countermeasure embankment, arsenic concentration in drainage water was measured at the outlets of the drain pipes that discharged rainwater from the embankment.
Results and remarks From the results of the water quality monitoring for the seepage control method, pH and arsenic concentration at each observation point and measurement did not exceed the environmental standards during and after the construction. The results of the ground water quality monitoring for the solidification and insolubilization area also did not find any abnormal value in pH and arsenic concentration.
Drainage water from the solidified and insolubilised embankment was alkaline. This water was directed to the muddy water treatment plant, which was used at the time of tunnel excavation, and then discharged to rivers after the water pH was adjusted.
The risks of construction works that produce heavy metal-containing soil often appear unexpectedly.
Therefore, it is important to include the following: a detailed pre-study, a plan that takes into consideration the elution characteristics of heavy metals and testing methods learned from case studies, preparation of possible countermeasures for changes, and advance discussions with local communities and relevant ministries. Also, proper adoption of countermeasures for changes is important during construction.
