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The occurrence of methane in recent deltaic sediments and its effect on soil stability

  • Whelan T. III
  • Coleman J.M.
  • Roberts H. H. 
  • Suhayda J. N. 
Section 13 Engineering Geology

Summary

River-mouth depositional pattern are modified by sediment-deforming processes of sufficient magnitude to severely endanger bottom-supported structures. Several types of deformations are present, including (a) peripheral slumping, (b) differential weighting and diapirism, (c) radial tensional faulting, (d) mass wasting and flowage induced by wave motion and degassing, and (e) deepseated clay flowage. The processes of bacterial methane production and the resulting effects on sediment deformation have been investigated in four cores taken in the Recent deltaic sediments of the Mississippi River. Dissolved methane in the interstitial waters ranged in concentration from 2 × 10−3 to 1.7 ml/l, whereas total CH4 (dissolved plus bubble phase) ranged from 5 × 10−3 to over 300 ml 1. High concentrations of methane corresponded to zones of low shear strength and were observed where dissolved sulfate was depleted. Calculations of maximum in situ methane concentrations, based on chemical reduction of excess total CO2, indicate that methane could be present above saturation levels (bubble phase). Classical anaerobic geochemical gradients were observed in sediment profiles where no movement had previously occurred. The pore water geochemistry of sediment profiles within peripheral mudflows suggest that coexistence of methane and sulfate indicate convective mixing of sediments and bottom seawater. Calculations using the equilibrium slopes of wave-induced mudflows indicate that shear strengths during movement must be less than the values measured before or after the flow. An improved model of mass movement is presented which relates the physical and geochemical properties of unstable sediments.

Keywords

Shear Strength Methane Production Methane Concentration Interstitial Water Offshore Technology 

La Présence De Méthane Dans Des Sédiments Deltaiques Récents Et Son Influence Sur La Stabilité Du Sol

Résumé

Les structures des sédiments à l’embouchure d’un fleuve sont modifiées par des processus déformateurs d’une ampleur suffisante pour mettre gravement en danger les constructions qui s’appuient sur le fond. On peut distinguer plusieurs types de déformations: a) enfoncement périphérique; b) tassement irrégulier et diapirisme; c) formation de failles de tension rayonnantes; d) perte de masse et écoulement provoqués par le mouvement des vagues et le dégazage; et e) écoulement d’argiles profondes. Les processus de production du méthane par les bactéries et leurs conséquences sur la déformation des sédiments ont été étudies sur quatre carottes prélevées dans les sédiments deltaïques récents du Mississipi. Les concentrations de méthane dissous dans les eaux intersticielles sont comprises entre 0,002 et 1,7 ml/l, cependant que le méthane total (bulles et gaz dissous) est compris entre 0,005 et plus de 300 ml/l. Les fortes concentrations en méthane correspondent aux zones de faible résistance au cisaillement; elles s’observent aux endroits où le sulfate dissous est épuisé. Les calculs de concentrations maximales de méthane in situ, basés sur la réduction chimique du CO2 total en excès, montrent que le méthane pourrait être présent au-dessus des niveaux de saturation (phase bulles). Les gradients géochimiques anaérobiques classiques furent observés sur des profils sédimentaires où aucun mouvement n’était apparu précédemment. La géochimie des eaux intersticielles des profils sedimentaires des coulées de boues périphériques permet de supposer que la coexistence de méthane et de sulfate indique un mélange de sédiments et d’eau du fond de la mer, par convexion. Les calculs utilisant les profils d’équilibre de coulées de boues produites par les vagues montrent que la résistance au cisaillement pendant le mouvement peut être moindre qu’avant ou après l’écoulement. On présente un modéle amélioré de mouvement de masse, tenant compte des propriétés physiques et géochimique des sédiments instables.

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Copyright information

© International Association of Engineering Geology 1976

Authors and Affiliations

  • Whelan T. III
    • 1
  • Coleman J.M.
    • 1
  • Roberts H. H. 
    • 1
  • Suhayda J. N. 
    • 1
  1. 1.Coastal Studies InstituteLouisiana State UniversityBaton RougeU.S.A.

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