Excess pore pressure in Mississippi delta front sediments: Initial report

Abstract

In September 1975, a differential piezometer probe was successfully implanted in the soft seafloor sediments of Block 28, South Pass, Mississippi Delta. The probe sensor is located approximately 6.4 m below the mudline in a water depth of 19 m, and has essentially continuously monitored excess pore pressure (the difference between sediment pore pressure and hydrostatic pressure at that depth) since installation. Excess pore pressure will be monitored until March 1976, when the probe will be recovered.

Immediately after deployment, an excess pore pressure of 54 kPa was recorded. An ambient excess pore pressure of approximately 32 kPa remained after dissipation of that developed during probe installation. Because of the possible presence of gas in the sediments in this area, it is not known with certainty whether the measured excess pressure is pore water pressure, pore gas pressure, or some combination of the two. An excess pore pressure of about 32 ±4 kPa was monitored during Hurricane Eloise and subsequent storms. The exact magnitude and time distribution of these pressure fluctuations is presently being evaluated.     

Ambient and dynamic pore pressures in fine-grained submarine sediments: Mississippi Delta

A multisensor piezometer probe has measured pore pressures in fine-grained submarine sediments of the Mississippi Delta over a period of approximately eight months. Data presented here cover the initial 2650 hours of the experiment. Dynamic and ambient pore pressures were recorded. Analogue data collected from the time of probe insertion include decay characteristics, steady-state (ambient) excess pore pressures, and the response of pore pressures to surface wave activity. The probe was installed in 43–44 ft of water near an offshore platform in the East Bay area of the Delta. Sensors were located at 21, 41 and 51 ft below the mudline. Ambient excess pore pressures were determined to be 0.7, 2.8 and 6.6 psi (lb/in²) at the respective depths. The relatively high excess pressures and the measured laboratory wet unit weights of the soil result in a significantly low effective stress. Pressure fluctuations due to tidal and surface wave activity were observed to produce significant pore pressure response in these soils. Preliminary data obtained using high-airentry and corundum stones indicate that considerably more research is necessary in order to fully understand the peculiarities observed in the data and to assess the role of dissolved and free gas on the pore pressures in submarine sediments.     

The SEASWAB experiment

Abstract

SEASWAB is one element of the Delta Project of the U.S. Geological Survey, a cooperative effort with several universities and other governmental agencies to investigate the processes that cause marine‐sediment instability. The basic purpose of the SEASWAB experiment was to obtain field measurements of sediment motion and pore‐pressure variations in soft sediment affected by wave‐pressure perturbations. This article serves as an introduction to the six papers that follow and that together make up a report on the results of SEASWAB.     

Pore‐water pressure measurements: Mississippi delta submarine sediments

Abstract

A pore‐water pressure probe (piezometer) was implanted in Mississippi delta sediments at a preselected site (Block 28, South Pass area, 29°00´N, 89°15´W) 145 m from an offshore production platform (water depth approx. 19 m) in September 1975. Total pore‐water pressures (u_w ) were monitored for extended periods of time at depths of approximately 15 and 8 m below the mudline concurrently with hydrostatic pressures (u₈ ) measured at depths of 15 m and approximately 1 m below the mudline. Relatively high excess pore‐water pressures, u_e = (u_w ‐u₈ ), were recorded at the time of probe insertion measuring 99 kPa (14.4 psi) at 15 m and 50 kPa (7.3 psi) at 8 m. Six hours after the probe was implanted, excess pore pressures were still high at 81 kPa (11.8 psi, 15 m) and 37 kPa (5.4 psi, 8 m). Pore pressures appeared to become relatively constant at the 8‐m depth after 7 h had elapsed, and at the 15 m depth after 10–12 h. Excess pore‐water pressures averaged 72 kPa (10.4 psi, 15 m) and 32 kPa (4.6 psi, 8 m) prior to the initial effects of Hurricane Eloise, which passed in close proximity to the probe site. Significant variations in pressures were recorded during storm activity. As the effects of the storm subsided, excess pore‐water pressures began to decline slightly at the 15‐m depth; however, concurrently at the 8‐m depth, pore pressures began to increase gradually. During the period of 21–25 days after the probe was implanted, excess pore pressures appeared to become more constant, averaging 24 kPa (3.5 psi) at 15 m and 43 kPa (6.2 psi) at the 8‐m depth. The presence of methane, a common occurrence in these delta muds, may have influenced, or contributed to, the total pore‐water pressures measured during this experiment.     

Discussion of “formation mechanism of large pockmarks in the subaqueous Yellow River Delta”

Abstract

The excess pore pressure accumulation is a key factor when estimating the formation mechanism of large pockmarks, as it determines the liquefaction potential of marine sediments due to water waves. The governing equations for excess pore pressure may have different forms for various types of sediments and then shall reflect the cyclic plasticity of the soil. For water waves propagating over a porous seabed, the liquefaction area induced by waves is generally progressive, which indicates that the liquefaction area will move forward following the wave train. Therefore, the excess pore pressure accumulation can be used to explain the occurrence of the large pockmarks, but the dimension of the pockmark may be related to the heterogeneity of sediment or the wave properties affected by the topography in the subaqueous Yellow River Delta.     

In situ pore‐pressure measurement in Mississippi delta front sediments

Abstract

A differential piezometer was used to monitor excess pore pressure in the soft clayey seafloor sediments of Block 28, South Pass, Mississippi delta, from September 1975 to March 1976. An ambient excess pore pressure of about 32 kPa was measured at a depth of 6.4 m below the mudline in a water depth of 19 m. Storm‐wave‐generated cyclic fluctuations of ± 4 kPa about the ambient were measured during Hurricane Eloise. Irregular, long‐period, small‐amplitude fluctuations in excess pore pressures persisted for 4 days following the storm. An effective stress analysis was made by using excess pore pressures; in situ field vane‐shear strength, t _fv, measurements; and laboratory wet unit weights measured by Lehigh and NOAA. The effective stress of the SEA‐SWAB site soil was calculated to be zero to a depth of about 6 m, below which it increased to 3.5 kPa at a depth of 15 m. Values of c´ = 4.6 kPa, ?´ = 56°, and T _FV/σ_vo(c/p) =0.1–0.2 were calculated, and it was concluded that these data do not represent the in situ condition of the soil because of the probability that the measured soil properties were affected by the presence of gas. However, it is clear that the soil is significantly underconsolidated.     

Formation mechanism of large pockmarks in the subaqueous Yellow River Delta

Abstract

We have identified large pockmarks in an area of approximately 0.3?km² in the subaqueous Yellow River Delta in the Chengdao Sea. Gas eruption channels not been identified in the sediment layers in this area, and the formation mechanism of these large pockmarks remains unknown. To study the formation mechanism of these large pockmarks, we constructed a layered silty sediment model composed of appropriate geological materials. Then, we calculated the stress, displacement, and excess pore pressure in the layered silty sediment from the surface to a depth of 10?m using the Biot theory. A comparative analysis of the calculated results and the data measured in the field was then performed. Based on these results, we established a new formation mechanism for the large pockmarks. With the occurrence of storm waves, two extreme areas of displacement and excess pore pressure appeared in the layered silty sediment. These extreme values increased quickly in the seabed during the continuous action of storm waves. When the excess pore pressure surpassed the effective stress, the top silty layer instantly liquefied and then reconsolidated. Then, when the pore pressure of the interface position exceeded the effective stress produced by the overlying sediment, the sediments experienced “sand boil” damage. With the repeated action of strong waves, the boundary of the pockmark continued to expand, forming a large and stable pockmark. This work is of great value for further understanding and mitigating marine geologic hazards, such as coastal erosion, silt deposition, and unstable sediment, in the subaqueous Yellow River Delta.     

Excess pore pressure observation in marine sediment based on Fiber Bragg Grating pressure sensor

Abstract

Fiber Bragg Grating (FBG) technology has emerged as a relatively new sensing technology for engineering applications because of lots of advantages. In this study, a large diameter probe instrumented with FBG pressure sensors to monitor excess pore pressure in marine sediment is proposed. The principle of FBG differential pressure sensor was introduced. Laboratory tests were carried out to check the workability and stability of the FBG pressure sensor. Offshore field test was also conducted in a wharf in Qingdao of China to evaluate the feasibility of the proposed probe. The installation procedure of the probe was introduced in detail. The excess pore pressure in dissipation test, after installation and pulling process were reported. The permeability coefficient of marine sediment was calculated based on the measured data. The field data show that the proposed probe based on FBG pressure sensor has good feasibility and accuracy in monitoring the excess pore pressure of marine sediment. The generation and dissipation of excess pore pressure is closely related to the degree of soil disturbance. The variation of excess pore pressure after installation can reflect the tide well in the site.     

Pore pressures in marine sediments: An overview of measurement techniques and some geological and engineering applications

Pore pressures in the seabed are extremely sensitive to any imposed stress because of the low permeabilities commonly exhibited by marine sediments. Consequently, the measurement of sediment pore pressures can be used to infer either the nature of the imposed stress (if the sediment properties are known) or the physical properties of the sediment (if the imposed stresses are known). Stresses of many different types may be exerted on the seabed either through hydrostatic forces (e.g. tidal and wave effects), or directly by lithospheric forces (e.g. tectonic and thermal forces). Several techniques for measuring in situ pore pressures in the upper few metres of sediments have been developed, and one instrument, the PUPPI, will operate autonomously in water depths up to 6000 m. Basic sediment properties and processes can already be inferred from pore pressure responses using this technique. However, further application and development could greatly enhance its capability, especially for long-term monitoring of sediment conditions. In this Chapter, pore pressure measurement techniques are briefly reviewed and problems are highlighted. An outline is given of some of the many ways in which pore pressure measurements could be used to gain further insight into geological processes and to determine some of the pertinent sediment properties more accurately for engineering applications.     

Model test of stratum failure and pore pressure variation induced by THF hydrate dissociation

ABSTRACT

A model test program for studying soil stratum failure and pore pressure variation during tetrahydrofuran (THF) hydrate dissociation considering the effects of heating and drainage conditions is presented in this paper. The temperature and pore pressures are recorded during heating. Test results show that the THF hydrate would dissociate to be liquid and then gas when heating. Once pore pressure generated by the flow of released gas/water exceeded the strength of over layer, the layered fractures and soil-gas/water mixture outburst would occur. The high heating temperature and low permeability of over layer both cause excess pore pressure generation and more serious stratum failures.     

Effect of Groundwater Fluctuations on Pore Pressures and Earth Pressures on Coastal Excavation Retaining Walls

Pore water and earth pressures acting on retaining structures are investigated using an efficient coastal double-layered excavation model to determine offshore excavation responses to groundwater fluctuations outside foundation pits. Total pore water pressure includes excess pore water pressure (due to groundwater fluctuations) and steady pore water pressure (due to steady seepage) determined using one-dimensional consolidation theory of double-layered soil and one-dimensional steady-state flow theory, respectively. Rankine's active and passive earth pressures are obtained from pore water pressure. This method is applicable to arbitrary groundwater fluctuation conditions. How physical parameters affect pore water pressure is numerically investigated using examples, demonstrating the method's practicality for calculating pore water and earth pressures.     

Instrumentation of SEASWAB experiment

Abstract

From September 1975 to April 1976 offshore production Platform V in South Pass, Block 28 (East Bay, Louisiana), was instrumented to measure the effect of storm waves on the soft sediments typical of the Mississippi delta (in a project given the acronym SEASWAB). A portion of this project consisted of four identifiable units of instrumentation (see note): (1) an accelerometer package buried 1 m in the sediment to measure three‐dimensional sediment accelerations and an associated pressure transducer, which measured wave‐induced pressures; (2) an array of instruments that included a wave staff, electromagnetic current meter, and a pressure transducer to examine various relationships between wave properties; (3) a wave‐, current‐, and wind‐measuring station 3.35 km inshore of Platform V to determine the transformation of the waves as they moved over the sediments; and (4) a transponder buried in the mud, the position monitored so that long‐term mudflow could be measured. The direct measurement of seafloor oscillations required the unique instrumentation of the accelerometer system. Three Bruel and Kjaer 8306 accelerometers mounted at right angles to each other made possible the measurement of small oscillations (～0.01 m) at low frequencies (0.1–0.3 Hz). The acoustic method of measuring long‐term mudflow was subject to problems associated with sound propagation in shallow water. The range of the system was found to be 2.74 km, apparently independent of depth. Multiple returns received after single interrogations of the transponder decreased the accuracy of the system.     

孔压探杆贯入及潮汐作用下超孔压响应规律研究

Excess pore water pressure is an important parameter that can be used to analyze certain physical characteristics of sediment. In this paper, the excess pore water pressure of subseafloor sediment and its variation with tidal movement was measured following the installation of a wharf in Qingdao, China by using a fiber Bragg grating(FBG) piezometer. The results indicated that this FBG piezometer is effective in the field. The measured variation of excess pore water pressure after installation is largely explained by the dissipation of excess pore water pressure. The dissipation rate can be used to estimate the horizontal consolidation coefficient, which ranged from1.3×10~(–6) m~2/s to 8.1×10~(–6) m~2/s. The measured values during tidal phases are associated with the variability of tidal pressure on the seafloor and can be used to estimate the compressibility and the permeability of the sediment during tidal movement. The volume compression coefficient estimated from tidal oscillation was approximately2.0×10~(–11) Pa~(–1), which was consistent with the data from the laboratory test. The findings of this paper can provide useful information for in situ investigations of subseafloor sediment.     

风暴浪导致的黄河口水下土体破坏试验研究

本文试验利用取自黄河水下三角洲的样品 ,重塑后铺设水槽底床进行水槽试验 ,并利用原状土进行动三轴试验 ,2种试验均测定土体内的孔隙水压力。根据各种情况下孔隙水压力的变化记录 ,表明土体破坏同时其孔隙水压力产生骤变。将本文试验结果与在黄河水下三角洲不稳定区的原位沉积动力学试验孔隙水压力测试结果对照 ,说明黄河三角洲水下斜坡某些土体的破坏 ,未出现波浪循环荷载作用下孔隙水压力积累升高所导致的土体液化破坏 ,而是风暴浪对海底的强切应力作用致使土体产生剪切破坏     

黄河水下三角洲沉积物在循环荷载作用下土体中孔压变化实验研究

利用室内周期循环加载试验 ,对黄河水下三角洲土体中孔隙水压力的变化加以测定 ,通过对波浪水槽试验和动三轴试验 2种方案所获数据分析认为黄河水下三角洲土体 (粉土、粘质粉土、粉质粘土 )存在一破坏的循环极限荷载。在小于此极限循环荷载作用情况下 ,土体中孔隙水压力总体呈现下降趋势 ,没有积累升高的过程 ,不同于砂土在循环荷载作用下孔隙水压力升高导致液化的情况。这一现象对判别黄河水下三角洲土体破坏机制的研究有重要意义。     

Resurveys of active mudslides,Mississippi Delta

A survey procedure using digitally acquired, scale-corrected sidescan sonar has been applied to document active submarine landslides in the Mississippi Delta region. Periodic resurveys have been completed for a 70-km² area in water depths of 10 to 50 m. Sufficient resurvey precision allowed short-term changes in seafloor mapping to be observed. Mudslides showed enlargement by retrogression and downslope surging within a single 5-month period.     

Interstitial water chemistry of Jamaican reef sediment: sulfate reduction and submarine cementation

Chemical and isotopic analyses of pore waters from Jamaican reef sediment suggest the importance of microbial sulfate reduction as a major control upon the origin, distribution, and composition of submarine cements in this fringing reef setting. Fore-reef sediment pore waters exhibit active sulfate reduction and enrichment in ¹⁸O which is consistent with associated active magnesian calcite cementation, alkalinity consumption, and cement enrichment in ¹⁸O. Conversely, lack of widespread submarine cementation in the back-reef corresponds to the diminished resupply of sulfate coupled with input of CO₂-charged meteoric water from a nearby unconfined aquifer into the more stagnant pore waters which lower pore-water magnesian calcite saturation states and preclude active submarine cementation.     

Acoustical penetration and shear strength in gas‐charged sediment

Abstract

Methane concentrations and sediment shear strengths were measured in three foundation borings taken from areas of variable acoustical penetration in the Mississippi river delta front. Acoustically impenetrable or “turbid”; zones were associated with sedimentary methane concentrations above about 30 ml/liter, measured at atmospheric pressure. Sediments in the high‐gas, acoustically turbid zones demonstrated a smaller percentage increase in shear strength with depth than in zones of low gas concentration. The results indicate that a 3.5‐kHz system used for sub‐bottom profiles is unable to determine the thickness of gas‐charged sediments.     

Pore pressure in ocean‐floor sands under random waves

Abstract

The wave‐induced cyclic shear stresses in ocean‐floor sands may cause a progressive buildup of pore water pressure, leading to instability of the bed. This instability may constitute an important consideration in the analysis and design of various offshore engineering facilities. This paper presents a general procedure for the analysis of pore water pressure in the ocean‐floor sands under the action of random waves. Using a simple linear model for the generation of pore water pressure and incorporating the effect of its simultaneous dissipation, a formulation for the expected damage associated with buildup of pore pressure is developed in a stochastic framework. Numerical results of an example analysis are also presented.     

Wetland loss and the subdelta life cycle

The rapid deterioration of marsh habitat observed during recent years in the modern Mississippi River Delta is a consequence, at least in part, of the natural life cycle of subdeltas. With life spans typically less than 200 years, subdeltas or bay-fill deposits are scaled-down versions of major delta lobes, yet provide, through pulses of sediment, nearly all the subaerial land in an active delta. Using maps, charts, and aerial photographs, curves were constructed for rates of change in land area, sediment volume, and linear progradation in the four subdeltas that have formed on the modern Mississippi River Delta since the first accurate survey in 1838.Results indicate that each subdelta (1) lasted for approximately 115–175 years, (2) included both periods of growth and deterioration, (3) was initiated by a crevasse or break in the natural levee system, (4) showed linear advancement and volumetric growth during subaerial deterioration, and (5) displayed a new pulse of subaerial growth during the high discharge decade of the 1970s. Contrary to popular accounts, demise of the Mississippi River Delta through deterioration of its subdeltas is not a result of the construction of artificial levees upstream or discharge of sediment off the continental shelf edge. Rather, it is attributable to a substantial decrease and fining of sediments being transported downstream to depositional sites within a delta that has developed, through natural processes, a complex and inefficient channel network for delivering these sediments.