IMPACT ACCELERATION TEST METHOD FOR QUALITY CONTROL OF CEMENT STABILIZED SOIL

Transcription

1 IMPACT ACCELERATION TEST METHOD FOR QUALITY CONTROL OF CEMENT STABILIZED SOIL A Sato, Civil Engineering Research Institute of Hokkaido, Japan S Nishimoto, Civil Engineering Research Institute of Hokkaido, Japan T Suzuki, Department of Civil Engineering, Kitami Institute of Technology, Japan Abstract For use as banking material, soil and sand generated as construction byproducts that have insufficient bearing strength without treatment are being stabilized more frequently with cement or cementitious solidifying agents. To perform quality control for stabilized banking soil, the unconfined compressive strength of the samples is used, rather than the density. However, it is rather difficult to directly measure the unconfined compressive strength of an embankment, because it is difficult handle specimens made from samples taken from the embankment, as they break easily because of the low target improvement value of solidified soils, and also it is more expensive as the samples have to be transported to the laboratory. Consequently, a new method for quality control of embankments was developed in which the unconfined compressive strength of the site is estimated based on the strong correlation between unconfined compressive strength and impact. As a result, the quality of embankments stabilized by cementitious solidifying agents can be controlled more conveniently, economically and rapidly. Keyword: impact, quality control, unconfined compressive strength, cost reduction 1. Introduction In the past, soil and sand from construction sites that lacks enough strength for use as bearing ground material were disposed of as waste. However, toward realizing a more sustainable society through the efficient use of resources, it is becoming more common to stabilize such soil with cement or cementitious solidifying agents for use as banking material. Usually, the quality of such stabilized material is controlled according to their strength rather than density, and the unconfined compressive strength is used in the process. Measuring the unconfined compression strength of the stabilized soil requires laboratory tests on sample test pieces taken from inside the embankment. However, considerable care is needed in handling specimens, as they are breakable because of the low target value for stabilization. Also, transportation of the samples to the laboratory makes this method much more expensive than a density control method. Therefore it is quite difficult to directly measure the unconfined compressive strength of embankments. Consequently, it was decided to control the quality of embankments by estimating the unconfined compressive strength at the site, with a method that uses impact, which correlates closely to unconfined compressive strength. The relationship between impact and unconfined compressive strength differs according to soil properties, and not all types of soil can be fully controlled with a single standard control value. Therefore, we made a laboratory test for soil of each

2 type and applied the results to strength measurement of soil at the construction site. This allows the quality control of materials that are stabilized by cementitious solidifying agents through impact, and the convenience of this method reduces the cost to about one-fifth of the cost of conventional methods. 2. Use of Cement to Stabilize Unsuitable Soil As a measure to address the high water content and poor work efficiency of soil discharged from construction work, soil stabilization with cement is becoming increasingly used at construction sites as it allows shorter work periods and reliable stabilization results 1), although its cost is slightly higher. The strength of embankments made from materials without solidifying agents, such as cement, is normally managed by adjusting the degree of compaction, because of the correlation between strength and degree of compaction. For materials made with the same mixing rate of cement, the strength is affected by compaction density. For materials made with different mixing rates of cement, the mixing rate has more effect on the strength than does the compaction density. Therefore, direct measurement of the strength is used for construction management, and it is normally adjusted according to the unconfined compressive strength. Samples taken at job sites are made into test pieces at laboratories to perform unconfined compression tests, but test pieces often break during the process because of the low target improvement value of solidified soils of the embankment. In an alternative method, the cementstabilized soil is mixed onsite, compacted to the same density as the embankment, cured onsite or indoors, and then tested for unconfined compression strength after the required number of curing days. It is difficult to say whether embankment strength can be accurately measured with this method, because curing conditions differ from those of the actual embankment. In Hokkaido, which is in northern Japan, the quality of Decide the target strength. embankments made from cement-stabilized soil is controlled by controlling the impact, which correlates closely with unconfined compressive strength, Estimate the cement addition rate. and the number of sites where the method is used is increasing. This paper will report on a method that uses impact to control the quality of embankments Test construction made from cement-stabilized soil. 2.1 Impact The process of stabilizing unsuitable soil with cement is shown in Figure 1. Quality control of embankments by impact testing is performed at the construction management stage of the process shown in this figure. When a rammer equipped with an accelerometer free-falls onto the target ground, the impact is the negative from the point of impact to the point when the rammer becomes stationary (Figure 2). The test shows large values for hard ground and small values for soft ground. The velocity of the rammer on impact with the ground is constant, regardless of the hardness of ground. However, the time it takes the rammer to become stationary after impact is short on hard ground, which means that the is large. That time is longer on soft ground, which means the is small. Realizing that there is also a strong correlation between impact and embankment density, the Geotechnical Division of the Civil Engineering Research Institute of Hokkaido has developed a device for the convenient density control of embankments 2). The device is readily portable, measuring 8 cm in height and 7.9 kg in weight. The hemispherical rammer weighs 4.5 kg and is equipped with an accelerometer. The rammer free-falls from a height of 4 cm, and the device gives immediate digital display and printout of the impact. Determine the cement addition rate necessary for construction. Main construction Construction management Figure 1 Procedures for cement stabilization of soil Velocity V Velocity V Time Time Stationary Impact Impact = V/t Figure 2 Outline of impact

3 2.2. Target strength of the embankment Cement-stabilized soil is expected to be strong enough 1) for the trafficability of construction machines, and 2) for the stability of embankments. The former is expressed as the cone index value that allows trafficability of the construction machines that are to be used. The latter is the strength that will keep the embankment from failing, which is the unconfined compressive strength of qu 7 = 1 kn/m 2 measured after 7-day curing, according to The Manual for Measures Against Unsuitable Soil in Hokkaido 3). Construction of embankments should use the mixing rate of cement that satisfies both of these requirements. Furthermore, soil stabilization with cement requires confirmation that the measured elution of hexavalent chromium meets the standard. 2.3 Standard control value for embankments The target strength of an embankment is the cone index that allows trafficability of construction machines and the unconfined compressive strength of qu 7 = 1 kn/m 2. The relationship between qu 7 and impact is obtained beforehand through indoor tests as shown in Figure 3, and the standard control value (I 7 ) for the embankment is decided from the impact that corresponds to qu 7 = 1 kn/m 2. The impact from the indoor test is the average value of 4 points measured at the upper surface of the test piece made in a 15-cm mold, compacted in 3 layers with 55 blows per layer, and cured for 7 days. The impact for the site is obtained by measuring the values at 1 points within an area of about 1 m 1 m. On account of fluctuations in mixing accuracy, the two highest and lowest values are excluded, and the average of the 6 remaining values is regarded as the impact for this construction location. If the impact is not measured immediately after construction, then the surface should be covered with plastic sheets while being cured to avoid effects of weather, or measured at a surface by scarping about 5 cm off the surface and making that new surface flat just before measuring. 2.4 Test construction The first layer of the studied embankment is tested (Figure 4). The test embankment is constructed with about 3 different mixing rates of cement, based on indoor tests that show what rates will produce the target strength of qu 7 = 1 kn/m 2. Roller compaction is performed using the machines that are to be used in the actual construction, with about 3 variations in the number of roller compactions. The length of the embankment needs to be about 15 m for spreading and compaction Changes in impact according to elapsed time at the test embankment are obtained up to 7 days after construction. The relationship between the mixing rate of cement and impact after 7 days according to different numbers of roller compactions is obtained (Figure 5) in deciding the optimum mixing rate of cement and number of roller compactions that achieve the target impact. Figure 5 shows that the target strength is satisfied at the cement mixing rate of B or greater under the number of roller compactions b and c. In main Unconfind compressive strength; qu 7 (kn/m 2 ) Figure 3 3mm Figure 4 Impact of embankment after 7days qu 7 =1kN/m 2 Target impact onsite I 7 Impact ; I 7 (G) Relationship between impact and unconfined compressive strength Test embankment Cross section of test construction Numbers of roller compactions a Numbers of roller compactions b Numbers of roller compactions c C B A Target impact onsite Mixing rate of cement(%) Figure 5 Relationship between mixing rate of cement and impact under different numbers of roller compactions

4 embankment construction, the lesser of b or c is chosen. Impact is measured at the test embankment from immediately after roller compaction to 7 days thereafter, and the relationship between the days elapsed and the impact according to each number of roller compactions is obtained (Figure 6). From this figure, the control value for the impact immediately after compaction (I ) that satisfies the target impact is determined. 2.5 Construction of the main embankment The embankment is constructed with standard control values I and I 7 that are obtained from the test construction. The embankment is constructed by confirming that the impact s measured immediately after compaction and after 7 days satisfy the values I and I 7, respectively. Since the lower layer of the embankment is wider than upper layers and there are 7 days before the next layer is placed, the value I 7 can be measured. However, with small-scale embankment construction, there may be less than 7 days available before the next layer is placed, when the construction advances to the upper layer of the embankment. In this case, the method is to estimate the strength after 7 days, by using the strength from a shorter curing period of less than 7 days. Therefore, the impact s after 3 days (I 3 ) and after 5 days (I 5 ) that satisfy I 7 are read from Figure 6, and the embankment is controlled according to I 3 and I 5. However, the strengths of materials stabilized by cement tend to be greatly affected by the curing temperature, when the mixing rate of solidifying agent is small (less than 5%), and at the early curing stage (3 days) 4). Therefore, the value I 7 should not be estimated from impact that is measured before 3 days has elapsed. Since the value I 7 can be obtained from the final embankment, it should be verified that I 7 satisfies the target value. Figure 6 Relationship between days elapsed and impact Table 1 Basic properties of the unsuitable soil Density of soil particle ρs (g/cm 3 ) Natural water content wn (%) 36. Over 2mm(%) 25. Particle size 75µm-2mm(%) Onsite Verification Test Less than 75µm(%) 4.7 In actual soil stabilization, cementitious Liquid limit w P (%) 55.6 solidifying agent was used for stabilization, and the impact method was used to control the quality of the embankment. Basic properties of the tested unsuitable soil Plastic limit w L (%) Classification symbols for ground materials 29.9 are shown in Table 1. The material has relatively good grain size distribution, with an Maximum dry Compaction density ρdmax appropriate mix of gravel, sand and finegrained fraction. The natural water content is properties (g/cm 3 ) Optimum moisture approximately 1.5 times the optimum water 3.9 content, and the cone index is 2 kn/m 2 content wopt(%) for Cone index qc(kn/m 2 ) 2 soil with natural water content. Therefore, this is a weak soil that cannot support the trafficability of a swamp track (wide track) bulldozer. Fine-grained fraction Conglomerat e sand 3.1 Selecting the solidifying agent for stabilization The target strength for the unsuitable soil is the strength that allows trafficability of construction machines and that secures embankment stability. A swamp track bulldozer is used for roller compacting; therefore, the strength after soil is mixed with cement should allow the trafficability of a swamp track bulldozer, i.e., the cone index should be qc of at least 3 kn/m 2. Furthermore, the target strength that will secure the stability of the soil is an unconfined compressive strength of qu 7 >= 1 kn/m 2 after curing for 7 days. There are 4 types of solidifying agents that are mixed: cementitious solidifying agents, B-type blast furnace slag cement, quicklime, and quicklime solidifying agents. Each

5 Table 2 Mixing rate of solidifying agent to achieve target strength, and cost comparison Solidifying agent Cementitious solidifying agent B-type blast furnace slag cement Mixing rate of solidifying agent to achieve target strength (kg/m 3 ) qc=3kn/m 2 qu 7 =1kN/m 2 Satisfies both of these is mixed in one of 4 different mix ratios. Then, immediately after mixing the solidifying agent into the soil, qc values are obtained from test pieces made in a 15-cm mold, compacted in 3 layers with 55 compactions per layer with a 2.5 kg rammer. Furthermore, the qu 7 value is obtained from test pieces that are made with this density and cured for 7 days and made according to the method in Producing Test Pieces by Static Compaction of Stabilized Soil by the Japanese Geotechnical Society. The mixing rate of solidifying agent that achieves the target strength is obtained from the test results (Table 2). For all solidifying agents, the mixing rate required to satisfy the target unconfined compressive strength is greater than that required to satisfy the target cone index. In the case of this unsuitable soil, stabilization was performed with a cementitious solidifying agent, because this agent is economical as it requires the lowest mixing rate. The quality of the embankment was managed by controlling the unconfined compressive strength after curing for 7 days, that is, the impact I 7 after 7-day curing. Furthermore, it was confirmed that the measured elution of hexavalent chromium is within the standard. 3.2 Determining the standard control value I 7 To determine the standard control value for the embankment, the relationship between impact and unconfined compressive strength is obtained by mixing cementitious solidifying agents into the unsuitable soil and measuring both (Figure 7). From this figure, the impact that will achieve qu 7 = 1 kn/m 2 is I 7 = 59 G. Therefore, embankments that are stabilized with cementitious solidifying agents should be constructed such that the impact exceeds 59 G 7 days after roller compaction. 3.3 Determining the mixing ratio of cementitious solidifying agents through test construction The indoor test results show that achieving the target strength requires 46 kg/m 3 of cementitious solidifying agent to be mixed with the unsuitable soil. For test construction onsite, the cementitious solidifying agent is mixed at the rates of 3, 4, and kg/m 3, and the numbers of roller compactions are 3, 5, and 7, considering the difference in seasonal conditions. The relationship between the mixing rate of cementitious solidifying agents and the impact of the test embankment 7 days after roller compaction are shown in Figure 8. For the mixing rates of 3, 4, and kg/m 3, the rate of cementitious solidifying agent that satisfies the target Unit price of solidifying agent (yen / ton) Material cost (yen / m 3 ) , , 1,175 Quicklime , 2,52 Quicklime solidifying agent , 1,462 Unconfind compressive strength; qu7 (kn/m 2 ) qu 7 =1kN/m 2 Target impact onsite I Impact ; I 7 (G) Figure 7 Relationship between impact and unconfined compressive strength Impact ; I G) Target impact onsite I 7 3 roller compactions 5 roller compactions 7 roller compactions Mixing rate (kg/m 3 ) Figure 8 Relationship between the mixing rate of the cementitious solidifying agent and impact under different numbers of roller compaction

6 value in the test construction is kg/m 3, and the number of embankment compactions is 5. With this material, the test construction shows that for all mixing rates, the strength is greater when the number of roller compactions is 5 than when it is 3 or 7, and that the material shows overcompaction and reduced strength with greater numbers of roller compactions. Therefore, sufficient care should be taken in the number of roller compactions during construction. If overcompaction occurs during construction from excessive roller compaction, one can achieve the target strength by increasing the mixing rate of solidifying agent with reference to Figure Determining the control value I for immediately after roller compaction The relationship between impact and the number of days after embankment construction is shown in Figure 9. For the embankment made by material with cementitious solidifying agent mixed at a rate of kg/m 3, and roller compacted 5 times, the standard control value for immediately after roller compaction is obtained from Figure 9. The impact immediately after embankment construction (I ) is read from the figure as 33.8 G. Then, in constructing the embankment the impact immediately after roller compaction is kept to more than I. 3.5 Impact of the main embankment The impact during construction of the main structure of the embankment is shown in Figure 1. All values satisfy the standard value of I = 33.8 G measured immediately after roller compaction. After 3 days, some are slightly lower than those obtained from test construction. However, after 7 days, all values satisfy the target impact of 59 G. In constructing the main embankment, it is required to secure the necessary strength immediately after roller compaction. Moreover, it is required that the change in strength indicated by the relationship between days elapsed and impact obtained for the constructed embankment be approximately the same as the change in strength indicated by that relationship obtained for the test construction. We were able to secure the strength of the embankment because the materials used in the construction of the embankment satisfied both requirements. Quality control is performed every 1 m 3 of the embankment, which is the same rate as for embankments that do not use solidifying agents such as cement. Impact ; I (G) 4. Summary The studies can be summarized as follows: 1) Measuring the impact is a convenient method of controlling the quality of embankments made using materials that have been stabilized by cementitious solidifying agents. 2) The increase in embankment strength onsite shows a similar trend to that in the test construction. By measuring and controlling the impact immediately after roller compaction and the same after 7 days, the strength of the embankment can be managed. 5. Conclusion It takes very little time to measure impact ; therefore, if the value is not the target value, it can still be used to correct shortcomings in construction, such as by indicating the need for immediate recompaction by roller. Furthermore, the measurement method is simple and produces the I =33.8G Time elapsed (days) Figure 9 Relationship between days elapsed and impact Figure 1 Impact of the embankment

7 same result regardless of the person performing the measurement. Quality control of stabilized-soil embankments constructed by the Hokkaido Regional Development Bureau is beginning to be performed by the method introduced in this paper. It is anticipated that this method will be used at other construction sites in the future. Furthermore, because the impact s of materials treated for stabilization differ even when they have the same unconfined compressive strength, we think it is necessary to conduct further studies of the factors that affect impact. References [1] Matsuo, S. (1972); Handbook for Soil Stabilization [2] Soil Research Laboratory, No. 3 Research Department, Testing Laboratory of the Civil Engineering Department, Hokkaido Development Bureau (1985); Proposed Manual for Measures against Unsuitable Soil in Hokkaido [3] Hokkaido Development Bureau; Specifications for Road and River Construction Work [4] Japan Cement Association (23); Manual for Ground Stabilization by Cementitious Solidifying Agents Third Edition, Gihodo Shuppan Co., Ltd. [5]The Japanese Geotechnical Society (2); Methods and Descriptions of Soil Tests