Specific Problems in using Organic Harbor Mud as Construction Material

J. Metzen, B. Schlue, S. Kreiter, G. v. Halem, M. Elvert, C. Hensen, T. Moerz , D. Lesemann, K. Petereit, C. Tarras

In many harbors at shallow tidal estuarine systems problems arise with tide driven accumulation of highly organic mud. Dredging operations for maintenance of maneuverable harbor water depth lead to vast amounts of organic rich often contaminated sediment. A GER federal regulation restricts the “ocean dumping” of contaminated material and requires to consider alternatives such as re-use or disposal on specially equipped on-shore disposal sites. Disposal capacities are limited in northern GER and costs rose of more than 35 Euro per cubic meter. Therefore on-site disposal, e.g. using the harbor mud as filling and construction material, is a favorable option.

Currently, the second largest harbor construction project in Germany is designed as a pilot study in using dredged harbor mud as construction material. Situated in Bremerhaven at the mouth of the river Weser. A shallow harbor basin is refitted with 550 m of new RoRo (roll on, roll off) piers for car-carrier using sheet piling. A harbor basin water depth of 11 m is maintained by dredging. The main project goals are: increase of RoRo pier length, fast creation of highly needed 60,000 square meters of parking space for automobile export, maintenance of the harbor basin water depth to allow full maneuverability for deep sea car-carrier and the on-site disposal of the dredged material. The project is accompanied by an intense geotechnical monitoring program and it’s successful completion will lead to a reduction of disposal material in equivalent of 2 years of disposal capacity of the federal Bremen facility.

During the construction phase 7 m of dredged harbor mud (TOC: 3-5 w%) was transferred behind the sheet piling leading to local thickness of up to 16 m of soft soils. Special care was taken to reduce water intake of the mud during dredging and relocation. A geotextile, which prevents density induced mixing, capped and separated the mud layer from a following 4 m thick sand layer. An approximately 0.8 m spaced vertical vacuum drainage system with a total drainage length of more than 1200 km was installed to accelerate dewatering and consolidation. Despite of the preventive measures, intense vertical ground movements of the mud layer occurred. The unwanted ground movements are in part explained by the semi viscose and thixotropic properties of the material. Considerable amounts of in-situ methane generation was observed from the mud layer. After six months of vacuum drainage, the overall consolidation state of the relocated mud had considerably improved. Initial shear strength values of 0.2 – 0.6 kPa increased to 1.5 - 2.5 kPa. Only the uppermost 1 to 2 m of mud, that did not experience vacuum drainage, remained under-consolidated with low shear strength values close to the starting values after the relocation.

The aim of our research project is to establish a relationship between shear strength, permeability and methane degassing rate to the mud‘s pore number e. Free and dissolved methane in the mud were determined over depth profiles. Conducting flow rate experiments, we found the in-situ generated gas having a substantial effect on the permeability of the used geotextile when combined with the organic mud. A laboratory one-dimensional compression cell with 22 cm diameter was designed and used for continuous measurement of permeability and methane degassing rates during the mud consolidation process. Predictions of mud settlement and mud creep rates were carried out by means of a two dimensional Finite Element Model across a representative cross section of the filled part of the harbor basin.

Preliminary results indicate that:
1) Commonly applied soil models are incapable and therefore inadequate to incorporate creep subsequent to primary consolidation,
2) Degassing of the mud substantially affects shear strength and consolidation,
3) Vacuum drainage successfully accelerates the consolidation process.

Problems and phenomena encountered during this pilot study stresses the need for interdisciplinary research and collaboration bringing together civil engineering, soil mechanics, modeling and organic geochemistry.