Beneficial use of marine dredged sand and sediments in road construction

Transcription

1 Beneficial use of marine dredged sand and sediments in road construction Kamali Siham *, Bernard Fabrice, Dubois Vincent, Abriak Nor Edine Civil Engineering Department of Ecole des Mines de Douai 941, rue Charles Bourseul, B.P.838, 598 Douai, France Abstract A huge quantity of marine sand and sediments are dredged each year in Europe. Until recently, sediments were considered as a waste product of the dredging process and were a serious concern for the harbour managers. Today, they are seen as a valuable and natural resource that provides opportunities for a number of beneficial uses. This paper investigates the potential uses of Dunkirk marine dredged sediment as new material in road construction projects. Different mixtures with different water, Portland cement and calcium hydroxide contents are experimentally studied. The tests conducted on these mixtures are on one hand compaction characterization and bearing capacity and on the other hand compression tests. Since sediments contain a high water amount, their reuse requires a method to dewater them. Two different processes are investigated. The mechanical strength results depend significantly on the treatment process. One of them shows that marine dredged sand and sediments can be successfully used as a new material in road construction. The other one presents some complications due to the presence of salt and organic matter in the treated sediments. Indeed, these ones have harmful effects on the binder mechanical strength development. A complementary investigations using scanning electron microscope have been made for a better understanding of this phenomenon. Keywords: dredged sediment, road construction, cement, lime 1. Introduction Dredging activities are necessary for maintenance of channels and harbours. In Europe, very large amounts of material are dredged each year; most of them, 185 millions (Oslo and Paris convention, 1995), remains in the aquatic system. The large part of the dredged material is sediments. Until recently, sediments were considered as a waste product of the dredging process and were a serious concern for the harbour managers. Today, they are seen as a valuable and natural resource that provides opportunities for a number of environmental, economic and aesthetic beneficial uses. Several categories of beneficial uses have been identified, among them: aquaculture, agriculture, forestry, erosion control, shoreline stabilization, construction and industrial uses. The beneficial use of dredged material in road construction is one of the main opportunities considered by French harbour managers. The purpose of this study is to determine if sediments and sands typically dredged from Dunkirk harbour in France are structurally suitable as formulated soils for road construction and at cost that is competitive with other alternatives. Recently, different mixtures using Dunkirk dredged sand have been identified for a suitable use in road construction (Bernard et al, 2) (Abriak et al, 3). This paper focuses on the fine fraction of dredged material which is sediment. Reuse of dredged sediment in road construction purposes requires checking specific geotechnical and environmental criteria. The environmental considerations are in progress and will not be presented in this study, only geotechnical considerations will be developed. The road section is composed of different layers with different thicknesses. The structural material of each layer should check different criteria specified in French standards. The main recommended tests to evaluate the suitability of a specific material are Proctor and tensile tests. Proctor test evaluates the material compaction (French standard NF P94-93,1993). The measured engineering characteristic is the water content value which insure an optimal compaction of the material, w OPN. Directly after this test, the immediate bearing capacity, IPI index, which determines the capacity of a material to support under construction the circulation of the building machines (French standard NF P94-78,1992) is measured. * kamali@ensm-douai.fr - Tel : 33 () Fax : 33 ()

2 Tensile test characterizes the mechanical strength of a material. Two engineering characteristics are measured during this test, maximum tensile strength and elastic modulus. These values are then reported in a specific abacus in order to determine the structural class of a material (see figure 1). Figure 2 provides the recommended characteristics for the different road layers as specified in (assises de chaussées : guide d application des normes pour le réseau routier national, 1998). Direct tensile strength (MPa) 1 S5 S4 S3 S2 S1 S,1 Figure 1. Structural classification of hydraulic binder treated sand according to the mechanical performances at 36 days (French standard, NF P98-113, 1994) Bituminous layer 1 Longitudinal elasticity modulus ( 3 MPa) base layer foundation layer IPI IPI 35 structural class S2 structural class S2 form layer IPI 25 structural class S1 Figure 2. Scheme of road section and recommended engineering properties specified in (assises de chaussées : guide d application des normes pour le réseau routier national; 1998) 2. Sediments Sediments are dredged from west zone of Dunkirk harbour located in the north of France. They are mostly composed by quartz, calcite, organic matter and salt. They have a high water content, more than 1%. 3. Dewatering sediment process The reuse of dredged sediments, whether in road construction or in the most of other beneficial uses, requires a method to dewater them. In this paper, two different processes are investigated: Dewatering process 1: it consists on getting the sediment totally dried using a thermal treatment at 5 C. Two main disadvantages of this process are identified. The first one is that the process is energy intensive and costly. The second one is due to the formation of big blocs of sediment after thermal treatment as shown in figure 3. (a) (b) Figure 3. Sediment aspect after thermal treatment at C (a) and after crushing operation (b)

3 In order to reuse the blocs, a crushing operation is necessary which will involve an additional cost. This operation consists to lead the sediment to a non friable material. However, the main advantage of this process is that all the water is evaporated. Consequently, the problem of the remaining water management involved in mechanical dewatering process is avoided. Dewatering process 2: The water content is reduced using Extract s process, then lime and slightly thermal treatment at 4 C. Extract s process consists on four main steps. 1- the dilution of sediment with water 2- the separation of sediment particles with diameter more than 63 µm from the remained sediment, 3- the deflocculation of sediment to improve its workability, 4- The elimination of big part of water using a mechanical press which may decrease significantly the salt content of sediment. This process provides sediments with water content equal to % which is a very high value for road construction purposes. Thus, a complementary treatment is necessary. The addition of lime mineral is usually used for the soil treatment in order to reduce its water content. The use of this mineral has another benefit effect which is stabilisation of organic matter. Different lime contents are tested experimentally to identify its effect on the water content reduction ratio. Figure 4 shows that 4% of lime allows to significantly reduce the water content of sediments. A thermal treatment at 4 C is used to accelerate the drying operation. In practice, the sediments will be dried naturally at ambient temperature. 1% % water content 8% 6% 4% % % % 2% 4% 6% 8% % 12% 14% Lime content (lime weight -to-wet sediment weight ratio) Figure 4. Lime addition effect on water content reduction of sediments 4. Tests and results on dewatering sediment using process Modified Proctor tests on sediments Modified Proctor tests are hold on crushed sediments at different water contents. The results show that the maximum dry density reached is 1.65 g/cm 3 and the optimum water content is 19.3 %. The material will be considered suitable for a road layer if the part of water content for which the dry density is greater than 95% of 1.65 (optimum dry density) and for which the IPI index checks the required value is large. Figure 5 shows that IPI index values check the criteria for reuse in foundation layer. However the previous interesting water content interval has to be increased especially for high values of water content. Recently, Bernard et al (2) have improved the IPI index of dredged sand by adding suitable amounts of Boulonnais sand and cement. The same methodology is used in this study for dredged sediments. 1,7 dry density g/cm 3 6 IPI ,45 3 1,4 1,35 1,3 1,25 1, Figure 5. Evolution of IPI index and density according to water content of sediment ( process 1 )

4 4.2. Modified Proctor tests on mixtures In order to improve the IPI index values, dredged sand, Boulonnais sand (/4) and Portland cement are considered. The cement is used to provide mechanical strength of the mixtures. First, cement content is fixed to a reasonable value of 6%. Then the content of the other components are identified according to the particle size distribution of the mixture. In order to have a suitable IPI index, the compaction should be high. It is known that the compaction of a granular material depends on its particle size distribution. More spread out it is, better the compaction is. Two mixtures (M1, M2) checking this last point are identified and described in table 1. The particle size distribution of sediment before and after dewatering process 1 are totally different. The particle size distribution of crushed sediment is considered for getting a spread out particle size distribution of the mixtures (see figure 6). Weight of passing % M2 formulation mixture 1 3 Sand sable (/4) Cement ciment Treated sédiment sediment,1,1 1 Sieve size (mm) Figure 6. Particle size distribution of M2 mixture and its different components. Table 1. composition of M1 and M2 mixtures. M1 M2 Dry content Dry content Dredged sediment 4% 64% Dredged sand 34% % Boulonnais sand (/4) % 3% Portland cement CEM I % 6% Figures 7 and 8 present the evolution of dry density and IPI index according to water content of M1 and M2 mixtures respectively. The results show that the two mixtures are suitable for a use in foundation and base road layers if their structural classes are at least S2. However, M2 mixture seems to be more interesting than M1 mixture because of the less abrupt decrease of IPI curve. 2 1,9 1,8 1,7 dry density g/cm 3 7 IPI Figure 7. Evolution of IPI index and dry density according to water content of M1 mixture

5 2 dry density g/cm 3 IPI 7 1,9 6 1,8 1, ,4 1, Figure 8. Evolution of IPI index and dry density according to water content of M2 mixture 4.3. Tensile strength and elastic modulus of M1 and M2 mixtures Direct tensile test is a complex experiment. French standard allows the use of the more simple Brazilian test. It allows to get maximum tensile strength value of a material by using a specific formula. The elastic modulus is obtained using stain - stress curve of compression test. The tests are led on mm diameter and mm height and 28 days aging cylinders. The values at 36 days aging are calculated from those at 28 days using a specific formula given by standards (French standard, NF P98-113, 1994). The results show a very week values of mechanical properties. The mixtures are classified in S structural class which is not sufficient for a beneficial uses on any of the considered road layers. The observation of the surface of the tested cylinders highlights the presence of a white product (see figure 9). This product is then observed and analysed using environmental scanning electronic microscope. The elementary analysis of this matter shows that it is composed of Na and Cl elements. The product may be salt. Generally, salt has a cubic form, the strange form obtained on the samples is probably due to the presence of organic matter (see figure ). Figure 9. Observation of M1 sample surface : presence of white product A A A B B Figure. Scanning electronic microscope image and elementary analysis of the white product The week mechanical performances obtained can be explained by the presence of a both organic matters and high quantity of salt. Several studied have previously shown the negative effect of theses matters on the mechanical strength development of cement based materials (Kamon et al, 1989) (Kaushik and Islam, 1995).

6 The dewatering process 1 as described before and without any complementary treatment is not suitable for the considered road layers. 5. Tests and results on dewatering sediment using process 2 Previously, Bernard et al (2) have shown that a mixture of 52% of dredged sand, 8% of cement and 4% of Boulonnais sand has an interesting compaction and mechanical properties. According to this result, two new mixtures, M3 and M4, using sediment are proposed. The composition of each mixture is given in table 2. The used sediment content is limited because of the high water content. The mixture of sediment and lime is then maintained at 4 C until the water content desired is reached Table 2. Composition of M3 and M4 mixtures. M3 M4 Dry content Dry content e Dredged sediment + 4% of lime (wet sediment) 26.4% 26.4% Dredged sand 37.1% 39.1% Boulonnais sand 28.5% 29.5% Portland cement CEM I % 6% The more the sediment is dried, the more blocs are formed. These blocs are easily destroyed during mixing when the water content of the mixture is higher than 8% (see figure 11). (a) (b) Figure 11. Photos showing the aspects of M3 mixture at 11% water content before (a) and after(b) mixing Modified Proctor tests on new mixtures Compaction tests indicate that the M3 mixture have an optimum water content of 11.6% and an optimum dry density of 2.4 g/cm3 which is a significantly high value (see figure 12). The IPI-water content curve presents suitable values for foundation and base road layers when the water content varies between 7.8% and 11.1%. M4 mixture which contains less cement content, have a very flat shape dry density-water content curve. The dry density is always superior to 95% of the optimum. The IPI curve for M4 mixture is more spread out and consequently the M4 mixture is more stable even if the maximum IPI value reached is only comparing to 7 for M3 mixture. For a water content value varying from almost 6% to 11.2% which is a large interval, the IPI value is higher than that required for foundation road layer. 2,1 2 1,9 1,8 1,7 1,4 1,3 dry density g/cm 3 1, Figure 12. Evolution of IPI index and dry density according to water content of M3 mixture IPI

7 2,1 2 1,9 1,8 1,7 1,4 1,3 1,2 dry density g/cm Figure 13. Evolution of IPI index and dry density according to water content of M4 mixture IPI 5.2. Tensile strength and elastic modulus of M3 and M4 mixtures Several cylinders of M3 and M4 mixtures at the optimum water content are prepared and then tested to determine the classification of the mixtures. The experimental results indicate that M3 mixture at 11% water content is classified in S4 class, and M4 mixture at 9.9% water content in S3 class. Because of the sensibility of the mixture to water content variation, complementary tests are conducted on M3 mixture at weaker water content equal to 7.8%. The results classify this mixture in S2 class. This difference in classification is due to the difference of water content between M3 mixture with 7.8% and M3 mixture with 11%. In fact, at 11% of water content the degree of cement hydration is higher than that at 7.8% which involves a better strength., NFP S5 Direct tensile strength (MPa) 1, S4 S3 S2 S1 S M3-11% water content M4-9.9% water content M3-7.8% water content, 1 Elastic modulus (GPa) Figure 14. Structural classification of M3 and M4 mixtures The high strength results indicate that cement grains has correctly reacted with the available water. This is particularly due to two points: the dewatering treatment process 2 which eliminates a high amount of salt and the addition of lime which stabilizes some organic matters. Complementary analysis using Scanning electron microscope are conducted to check for the absence of salt at the surface of samples and at different points in the sample as shown in figure 15. Other chemical analysis are in progress to exactly quantify the amount of salt remained after the dewatering treatment process 2. From geotechnical point of view, M3 and M4 mixtures are suitable as material for different road layers. M3 mixture can be used for a both foundation and base layers if the water content is well controlled. M4 mixture can be used for foundation layer.

8 A A Figure 15. Observation of M3 sample surface: no white product is observed (a); elementary analysis of M3 sample using SEM : no salt is found (b) 7. Conclusion The reuse of marine dredged sediments, as a new material for road layers requires a method to dewater them. In this paper, two different dewatering processes are investigated: the first one is a thermal treatment at C and its use gives a suitable compaction characteristics but a very weak mechanical strengths because of the presence of high amount of salt. The second process is a mechanical and chemical treatment. The results obtained in using this process are very satisfying and show that Dunkirk marine dredged sand and sediments can be used successfully as a new material for road layers. Two suitable mixtures, M3 and M4, are identified. M3 mixture can be used successfully as road layer material for a both foundation and base layers if the water content is well controlled. M4 mixture can be used for foundation layer. An experimental platform is planned in few weeks to validate the laboratory results. The environmental tests required to reuse the sediments are in progress. 8. Acknowledgments The authors gratefully acknowledge the financial and technical support of this study from the FEDER. They also knowledge Extract company for its technical support. 9. References French standard, NF P 94-78, 1992, Indice Portant Immédiat - Mesure sur échantillon compacté dans le moule CBR. French standard, NF P 94-93, 1993, Détermination des caractéristiques de compactage d un sol - Essai Proctor normal - Essai Proctor modifié (in French). French standard, NF P98-113, 1994, Sables traités aux liants hydrauliques et pouzzolaniques, AFNOR (in French). French recommandations, 1998, Assises de chaussées en graves non traitées et matériaux traités aux liants hydrauliques et pouzzolaniques, guide d'application des normes pour le réseau routier national, SETRA (Service d'etudes Techniques des Routes et Autoroutes) (in French). Bernard F. Abriak N.E., Damidot D., 2, Recycling of sea sands in the Civil Engineering field Dredging symposium, Dunkirk, France, October 9 th. Abriak N.E, Gregoire P., Bernard F., 3, Valorisation du sable de dragage: Etude d'une grave routière à base de sable de dragage", 2nd International Symposium on Contaminated Sediments, mai. Kaushik S.K., Islam S., 1995, Suitability of sea water for mixing structural concrete exposed to marine environment, Cement and Concrete Composites 17, Kamon M., Tomoshisa S. and Sawa K., 1989, On the stabilization of Hedoro by using cement group hardening materials, Journal of the society of Materials Science, Japon, Vol. 38, No. 432, pp (in Japanese).