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Mechanical and load transmission properties of soil-concrete block

In order to recycle dredged soil, our research group developed interlocking blocks from dredged soil. Dredged soil interlocking blocks (350 mm in width, 350 mm in depth and 100 mm-150 mm in height) were made from soft clay dredged at the Kanmon run. They were produced with the constant dehydration pressure 5 MPa and cement content of 15%, 20% and 25% per the dry weight of clay. The unconfined compression test and bending test were carried out to investigate material characteristics of dredged soil interlocking blocks in the water for six months. On the other hand, in order to check the effect of interlocking blocks, a road was constructed by using interlocking blocks on the surface and, as a roadbed material, dredged and dehydrated soil and crushed stone for mechanical stabilization were used. Characteristics of dispersion of traffic load and settlement were evaluated by running experiments. Stress was measured by four earth pressure gauges and settlement was measured by surveying. The following conclusions are obtained: (1) There was close relationship between unconfined compressive strength and water-cement ratio. Therefore, it is concluded that unconfined compressive strength of dredged soil blocks are controlled by water-cement ratio. (2) Dredged and dehydrated soil can disperse traffic load effectively. (3) Soil-concrete blocks can’t disperse traffic load effectively, but have sinking suppressing effect when using dredged and dehydrated soil as roadbed material.


INTRODUCTION
At the Kanmon run, an important international seaway, dredging work is carries out for deepening seabed every year. Dredged soil is transported to Shinmojioki disposal site, but the disposal capacity is little remained. New disposal sites are needed for dredged soil which will be generated in the future, but construction takes long time.
In order to recycle dredged soil, our research group has conducted basic study and Yamashita et al. (2011) succeeded making soil block (Φ53.4 cm×H50 cm) that has strength equals to concrete.
In this study, we developed interlocking blocks from dredged soil. Cement was mixed with soft clay dredged at the Kanmon run and dehydrated. To investigate mechanical properties, the unconfined compression test and bending test were carried out on interlocking blocks in the water for six months. A road was constructed by using interlocking blocks on the surface and characteristics of dispersion of traffic load and settlement were evaluated by running experiments.

Material experiment
The material used in this study is soft clay dredged at the Kanmon run (called Kanmon clay). The physical properties of Kanmon clay is shown in Table 1. Japanese Geotechnical Society Special Publication The 6th Japan-China Geotechnical Symposium Kanmon clay was prepared with initial water content of 300% and mixed with cement. The mixture was poured into the dehydration equipment (W350 mm×D350 mm×H1000 mm) shown in Fig. 1. The mixture was dehydrated with dehydration pressure of 2 MPa for 30 min, followed by constant dehydration pressure of 5MPa. In order to investigate material characteristics, the unconfined compression test (JIS A 1216) and bending test (JIS A 1106-2006) were carried out.

Running experiment
The sectional view of road is shown in Figure 2 and the horizontal projection of road is shown in Figure 3. The earth pressure gauges were set in base course for measuring vertical earth pressure. After spreading each layer (20 cm), a pressure was given 8 times by rolling machine (4 t.). These works were continued 5 times and roadbed of 1 meter in height was constructed. Dredged and dehydrated soil (called "dehydrated soil) and crushed stone for mechanical stabilization (called "crushed stone") were used as roadbed materials. On the surface of the road, soil blocks were used as interlocking blocks. The earth pressure gauges were set in four points to investigate differences of roadbed materials and to check the effect of interlocking blocks. Loaded cars and empty cars were used as traffic loads. The settlement was measured by surveying. Figure 4 shows the relationships between unconfined compressive strength and cement content and between bending strength and cement content. Average unconfined compressive strength for cement content of 25% was 9.2 times than that of 15%. This is because of hydration reaction. Unconfined compressive strength varied greatly with the specimen cement content of 20%. The possible factor is water content dispersion. The dispersion occurred because the drainage time was not sufficient when making blocks. It can be seen that average bending strength doubles when cement content is increased by 10%.  Figure 5 shows the relationships between unconfined compressive strength and water content and between bending strength and water content. Unconfined compressive strength decreased when water content increased. The reason is considered that water in specimen works as a gap, therefore, when water content is higher, specimen becomes frailer.  Figure 6 shows the relationships between unconfined compressive strength and dry density and bending strength and dry density. Unconfined compressive strength and bending strength increased when dry density increased. Unconfined compressive strength differed due to cement content with almost same dry density. Therefore, it can be suggested that increase of strength is related to the density increase and to the chemical reaction. The relationships between unconfined compressive strength and water-cement ratio and between bending strength and water-cement ratio are shown in Figure 7. In this paper, the water-cement ratio is defined as weight ratio of water and cement. It can be seen that unconfined compressive strength decreases with increasing water-cement ratio. Following equation between unconfined compressive strength and water-cement ratio was obtained:

STRENGTH PROPERTY
q u is the unconfined compressive strength and W/C is the water-cement ratio. The coefficient of correlation R in equation is 0.96. So, it is considered that there is close relationship between unconfined compressive strength and water-cement ratio. Also bending strength increased with decreasing water-cement ratio. It is found that, for high strength of soil blocks, it is necessary to decrease water-cement ratio.   rates of stress were 27.96 kPa/m (crushed stone) and 3.91 kPa/m (dehydrated soil). The increasing rate of crushed stone was 7 times higher than that of dehydrated soil. The increasing rates of stress calculated by wet unit weight were 17.36 kPa/m (crushed stone) and 11.23 kPa/m (dehydrated soil). Therefore, it is considered that dehydrated soil can disperse traffic load effectively. The relationship between vertical stress and running frequency is shown in Figure 10. Vertical stress was about 30 kPa smaller when dehydrated soil was used as roadbed than crushed stone was used. And the vertical stress became large when using soil block on the surface of the road.  Figure 11 shows the relationship between traffic stress amplitude and running frequency. Traffic stress amplitude is defined as the gap of maximum stress and minimum stress measured by traffic load. It shows that traffic stress amplitude increases setting blocks on the surface. And traffic stress amplitude tended to decrease due to increase of traffic frequency. The traffic stress amplitude was smaller when dehydrated soil was used as roadbed than crushed stone was used. It should be mentioned that dehydrated soil can disperse traffic load effectively. The relationship between settlement and running frequency is shown in Figure 12. The settlement increased with the increase of running frequency. Especially, the settlement was large when dehydrated soil was used as roadbed. The effect of soil block could not be confirmed when using crushed stone as roadbed material, but when using dehydrated soil, soil block reduced settlement more than 1 cm.

CONCLUSIONS
In order to investigate mechanical and load properties of soil interlocking block, the unconfined compressive strength and bending strength were examined and running experiments were carried out. The following conclusions are obtained: 1) There was close relationship between unconfined compressive strength and water-cement ratio. Therefore, it is suggested that unconfined compressive strength of dredged soil blocks are controlled by water-cement ratio. 2) Dehydrated soil can disperse traffic load effectively. 3) Soil-concrete blocks can't disperse traffic load effectively, but have sinking suppressing effect when using dehydrated soil as roadbed material.

ACKNOWLEDGEMENTS
We appreciate the generous support received from SCOPE for research development of this study. We gratefully acknowledge the "Shin-Mojioki disposal site Dehydrated soil + Block construction project III technology investigative commission" for their professional advice. We also express our sincere thanks to all who were involved in this study for their helpful discussions and help.