Showing papers on "Pore water pressure published in 1984"

Smectite Dehydration--Its Relation to Structural Development and Hydrocarbon Accumulation in Northern Gulf of Mexico Basin

Abstract: A comparison of clay diagenesis data obtained from a study of Tertiary shales from the Brazos-Colorado River system of Texas, the Mississippi River system of Louisiana, and the Niger River system of Nigeria illustrates significant differences in temperature intervals over which smectite diagenesis occurs. The threshold temperature required to initiate diagenesis ranges from about 160°F (71°C) in Mississippi River sediments to more than 300°F (150°C) in the Niger delta. Water expelled from smectite into the pore system of the host shale during the process of diagenesis may migrate out of the shale early or may be totally or partially trapped and released slowly through time. In either situation, the water can act as a vehicle for hydrocarbon migration p ovided hydrocarbons are present in a form and in sufficient quantities to be transported. Observations from the northern Gulf of Mexico basin indicate a close relation between buildup of high fluid pressure and the smectite-illite transformation process. Abnormal pressures exert partial control on the type and quantity of hydrocarbons accumulated because pressure potential determines the direction of fluid flow, and overpressuring partly controls the geometry of growth faults and related folds in basins where shale structures are the dominant type formed. The depths to which growth faults can penetrate and the angle of dip that these faults assume at depth are largely dependent on fluid pressure in the sedimentary section at the time of faulting. Dips of some faults in Texas have been observed to change abruptly within the interval of smectite diagenesis, and some faults formed in the overpressured Miocene and younger sections become beddingplane types at depths where the temperature is near that required for thermal generation of petroleum. Although these faults may be important for fluid redistribution in shallow sandstone-shale sections, they are a minor factor in moving hydrocarbons out of shale below the faults in much of the Texas offshore area. Fluid movement upward through microfracture systems in the deeply buried overpressured section overlying and extending upward from fault trends in the sub-Tertiary section is proposed as a mechanism for flushing hydrocarbons from the deeper portion of the northern Gulf of Mexico basin. This flushing process would be enhanced by smectite diagenesis because water derived from smectite that was trapped during basin subsidence would cause the flushing process to continue for longer periods of time and to extend to greater depths than could be attained if only remnants of original pore water were present. Shale tectonism is also the primary mechanism for structural development in the Tertiary section of the Niger delta; however, seismic data indicate that the rate of dip change of seaward-dipping listric growth faults is commonly less than that observed in Texas where dips as low as 10°-15° can occur at depths as shallow as 10,000-15,000 ft (3,048-4,572 m). Syndepositional faults in Nigeria are formed in sandstone-shale sections where the clay composition of shale is primarily kaolinite and where little water of smectite diagenesis has been added to the pore system of the host sedimentary section. Subtle differences in structural styles in the Tertiary sections of Texas and Nigeria are probably the result of differences in clay composition of the shaly sections being deformed

Permeability of fault gouge under confining pressure and shear stress

Abstract: The permeability of both clay-rich and non-clay gouges, as well as several pure clays, was studied as a function of confining pressures from 5 to 200 MPa and shear strain to 10 Permeability ranged over four orders of magnitude, from around 10−22 to 10−18 m2 (1 darcy = 0987 × 10−12 m2) The lowest values were characteristic of the montmorillonite-rich and finer grained non-clay gouges Illite, kaolinite, and chlorite had intermediate permeabilities, while the highest values were typical of the serpentine and coarser grained non-clay gouges Grain size was an important factor in determining permeability, particularly for the clay-rich samples The coarse grained gouges were the most permeable and decreased in permeability after shearing Conversely, the fine grained gouges had characteristically lower permeabilities that did not vary significantly after various amounts of shearing The permeabilities of the non-clay samples were not significantly different than those of the clays Therefore, comminuted rock flours can be equally as effective in reducing the flow of water as the characteristically low permeability clay gouges The strengths of the samples were quite variable The non-clay gouges were consistently the strongest, with yield points (beginning of nonelastic behavior) around 850 MPa, while montmorillonite had an anomalously low strength in relation to all the other gouges at 250 MPa Strength of the saturated samples under drained (low pore pressure) conditions did not correlate with high or low permeability However, the low permeabilities of these gouges could be a factor in the measured low shear stresses along fault regions if excess pore pressures were created as a result of shearing or compaction, and this pressure was unable to dissipate through a thick section of the material

Porosity reduction and crustal pore pressure development

Abstract: The influence of porosity reduction processes on the hydrologic characteristics of the earth's crust is examined. We present a simple mathematical model that shows the effect of porosity reduction on pore pressure development in the crust. Pore pressure in excess of hydrostatic can be generated if porosity reduction rates are sufficiently high. Elevated pore pressure could in turn affect properties such as strength and seismic reflectivity. Indirect evidence from laboratory, theoretical, and field studies indicates that porosity reduction processes are active, or very likely to be so, in large portions of the crust. Porosity, permeability, and pore pressure in the earth's crust may well be time-dependent, even in regions not subjected to large deviatoric stresses.

Solute Transfer Through Columns of Glass Beads

Abstract: Miscible displacement experiments were conducted in a 30-cm-long column filled with glass beads that had diameters in the range of 74–125 µm. Breakthrough curves were determined at various leaching rates for saturated or unsaturated conditions. The breakthrough curves of the unsaturated experiments showed early breakthrough and tailing. These could be described with the classical dispersion equation, provided dispersion coefficients were used, which were about 20 times larger than for saturated columns leached at comparable pore water velocities. A better description of these breakthrough curves was obtained with a model accounting for mobile and immobile water. With this model the dispersion coefficients, as obtained from the saturated experiments, could be used directly. The immobile water content was found to increase linearly with the total water content, while the transfer coefficient for solute transport into the immobile water increased proportionally with the pore water velocity. It is shown, based on the mobile and immobile water and solute transport model, that breakthrough curves of long unsaturated columns should exhibit no early breakthrough and tailing but that the presence of immobile water will result in predictably larger dispersion coefficients than those expected on the basis of saturated flow experiments alone.

Pore pressure and oceanic crustal seismic structure

Abstract: Summary Marine refraction studies during the past decade have found considerable lateral variability in the seismic properties of the upper basaltic regions of the oceanic crust. In many localities, compressional and shear-wave velocities are quite low at the top of the basalt section and velocities increase rapidly with depth. It is concluded that pore pressure may be at least in part responsible for the low upper crustal velocities and contribute significantly to the lateral variability. This is supported by compressional and shear-wave velocity measurements as functions of confining pressure and pore pressure for basalt from the Juan de Fuca ridge and dolerite from the Samail ophiolite, Oman. Within the oceanic crust, regions of overpressure and underpressure will possess anomalous velocities, the magnitude of which will depend upon the porosity and the deviation of the pore pressure from hydrostatic. The influence of pore pressure on velocities is expected to diminish with depth and is unlikely to be significant at lower crustal depths where porosity is extremely low. Of significance, Poisson's ratio is shown to be dependent on pore pressure as well as confining pressure. At constant confining pressure, Poisson's ratio increases with increasing pore pressure. Thus, overpressured regions within the upper oceanic crust are likely to have relatively high Poisson's ratios as well as low compressional- and shear-wave velocities.

The strength of unsaturated mixtures of sand and kaolin and the concept of effective stress

Abstract: SUMMARY Pastes containing sand and 0, 5, 20 and 80 g kaolin kg−1 mixture were equilibrated at matric potentials of – 2, – 10, – 100 and – 1000 kN m−2 or allowed to air-dry. The strength of cylindrical samples was determined in unconfined compression and by the indirect tensile strength test. Measured strengths are explained using the Coulomb-Mohr theory and the concept of effective stress. At pore water tensions greater than 10 kN m−2 effective stress was the dominant factor in determining compressive and tensile strength. The contrast between this behaviour and that of friable topsoils is discussed.

Cobalt in pore waters of marine sediments

Abstract: During early diagenesis on the sea floor, metal-recycling processes exert important controls on the preservation rates and distributions of metals in marine sediments. The elucidation of internal recycling processes, therefore, is important in understanding trace-metal mass balances in the oceans. Recent measurements of cobalt in seawater1 suggest that cobalt is scavenged from the deep sea and hence follows biogeochemical pathways similar to manganese. We present here measurements of cobalt in pore waters of marine sediments and find that cobalt was released into pore waters in the sub-oxic sediment zone during dissolution of sedimentary manganese oxides (Table 1, Fig. 1). Solid-phase cobalt concentrations in surface (oxic) sediments, above the pore water remobilization horizon, are almost double those measured in the deep (anoxic) sediment zone (Table 2, Fig. 1). The sediment and pore water distributions indicate that a large fraction of sedimentary cobalt is recycled with manganese between oxic and sub-oxic pore waters and redeposited in surface (oxic) sediments.

The effects of high pressure and high temperature on some physical properties of ocean sediments

Abstract: A series of laboratory experiments was conducted with four ocean sediments, two biogenic oozes and two clays. Permeability and thermal conductivity were directly measured as a function of porosity, and the testing program was designed to identify any dependence of these physical properties upon hydrostatic pressure and temperature. The results show no discernible effects of pressure, within the range of 2–60 MPa, upon the permeability of any of the samples. Temperature effects, from 22° to 220°C, upon this property are accounted for by applying a viscosity correction to the permeating seawater. Previous investigations have suggested the existence of a pressure-induced and/or a temperature-induced breakdown of the absorbed water which surrounds clay particles, thereby promoting an increase in sediment permeability. Our experimental findings cannot confirm this phenomenon and fail to provide a satisfactory solution to the conflicting data which now exist between the pore water velocities inferred from nonlinear thermal profiles of ocean sediments and those fluid velocities derived from Darcy's law and laboratory permeability data. The effects of sizeable variations in pressure and temperature upon sediment thermal conductivity are found to reflect closely the behavior of the conductivity of the liquid phase alone under these same changes in environmental conditions. This is not surprising due to the relatively narrow range of high porosities encountered in this study. Empirical equations are developed which allow sediment thermal conductivity to be calculated as a function of temperature and void ratio. A hydrostatic pressure correction term is also presented.

Biological enhancement of solute exchange between sediments and bottom water on the Washington continental shelf

Abstract: Solute exchange between the interstitial waters and overlying waters on the Washington continental shelf was investigated based on measurements of the pore-water sulfate distribution and sulfate reduction rates as well as through models describing the distribution of sulfate in anaerobic pore waters. The depth-integrated sulfate reduction rate greatly exceeded the influx of sulfate attributable to molecular diffusion and sediment accumulation acting on the measured vertical sulfate gradients, and indicated that additional transport mechanisms must have been operating. Sediment mixing was probably not the primary mechanism since high eddy diffusivities would be required to depths of 30 cm to maintain the observed sulfate distribution, whereas previously measured210Pb distributions indicated sediment mixing is primarily restricted to depths <7 cm. Irrigation of bottom water through animal burrows was the most likely mechanism. To describe this process, a general diffusive irrigation coefficient, B, was formulated. Vertical profiles of B showed the main irrigation zone occurred between 2 and 10 cm with reduced irrigation rates occurring below this. These coefficients calculated from the sulfate distribution were similar to ones calculated from previously published radon measurements at the same stations, indicating that this formulation of irrigation exchange may be useful in modelling the exchange of dissolved solutes between the pore water and the bottom water.

Introduction and digest to the Special Issue on Chemical Effects of Water on the Deformation and Strengths of Rocks

Abstract: The important role of pore pressure in promoting such brittle processes as cataclasis, hydraulic fracturing, large-scale faulting, and earthquakes within the crust is widely accepted in geology and geophysics [Hubbert and Willis, 1957; Hubbert and Rubey, 1959; Handin, 1958; Handin et al., 1963; Brace and Martin, 1968; Healy et al., 1968; Raleigh et al., 1976; Sibson, 1973, 1980; Raleigh and Evernden, 1981]. Provided that fluid pressure is fully communicated with rock pore space, the effective normal stresses that control crack growth, macroscopic fracture, and friction are reduced by the magnitude of the fluid pressui'e. Beyond this physical effect of pore fluids, there are chemical effects of water on the strength of rocks that are also important in governing differential stresseg and flow in the continental crust. Some of these chemical effects of water on rock deformation have long been recognized.

Formation of Secondary Porosity: How Important Is It?: Part 2. Aspects of Porosity Modification

Abstract: Secondary porosity in sandstones is created by subsurface dissolution of grains or cement by pore water that is undersaturated with respect to one or more of the major mineral phases. Such undersaturated pore water may be derived from: (1) meteoric water driven by a hydrostatic head; (2) compactional pore water containing CO2 released from maturing kerogen; (3) clay minerals reactions including the transformation of kaolinite and smectite to illite; and (4) reactions between clay minerals and carbonate releasing CO2. Calculations of the CO2 generated from different types of kerogen suggest that few basins will generate enough CO2 to produce large-scale leaching in thicker sandstones. Dissolution of minerals and removal of alumin m and silica in solution requires that very large volumes of pore water flow through the sandstone. Because leaching often enlarges primary pore space, it is very difficult to estimate the percentage of the pore space that is secondary. Leaching and formation of secondary pore space may also be accompanied by reprecipitation of other minerals so that the net gain in porosity is less than the observed secondary pore space.

A New System for Ground Water Monitoring

Abstract: This paper describes a new system for ground water monitoring, “the BAT System,” which includes the following functions: (a) sampling of ground water in most types of soils, (b) measurement of pore water pressure, and (c) in situ measurement of hydraulic conductivity. The system can also be used for tracer tests. The system utilizes a permanently installed filter tip attached to a steel or PVC pipe. Installation is normally performed by pushing the tip down to the desired depth. The filter tip can also be buried beneath a landfill. The primary feature of the new system is that the filter tip contains a self-sealing quick coupling unit, which makes it possible to temporarily connect the filter tip to adapters for various functions, e.g. water sampling and for measurement of pore pressure and hydraulic conductivity. The new technique makes sampling of both pressurized water and gas possible. Samples are obtained directly in hermetically sealed, pre-sterilized sample cylinders. Sampling of ground water and measurement of pore pressure can be repeated over a long period of time with undiminished accuracy. This technique is also well-adapted for taking water samples from different strata in a soil profile, in both the saturated and unsaturated zones. Actual installations range from 0.5 to 60m depth.

Modelling water flow through undisturbed soil cores using a transfer function model derived from 3HOH and Cl transport

Abstract: SUMMARY Large undisturbed soil cores (20 cm diam. × 25–30 cm long) were irrigated at rates of 0.5–4 cm h−1 with 0.005 M CaCl2 solution labelled with 3HOH. The cores were used at varying initial water contents and flow in all cases was unsaturated. Breakthrough curves for Cl and 3HOH were markedly asymmetric and unlike those reported for columns of packed aggregates. The data could be satisfactorily described using a density distribution function of the logarithm of cumulative drainage D. The mean and standard deviation of In D were estimated by a curve-fitting procedure from Cl and 3HOH effluent concentrations in each core. The mean pore water velocity and fraction of the soil water that participated in solute transport (the mobile volume) were also calculated. The apparent velocity of Cl movement was always greater than that of 3HOH which suggested that the mobile volume involved in convective and diffusive transport of Cl was less than that for 3HOH. We suggest that Cl and 3HOH diffused at different rates out of flowing water films in a relatively few large conducting channels into essentially immobile water within the surrounding soil matrix. The difference in mobile volume for Cl and 3HOH was used to calculate the perimeter of voids in any horizontal cross-section of the soil through which water flowed, assuming a planar interface between the mobile and immobile water.

The role of pore water circulation during the deformation of foreland fold and thrust belts

Abstract: Volume-loss strain accompanying pressure solution of calcite occurred within both the Umbria-Marches Apennines of Italy and the Appalachian Mountains of western New York. Data from strain markers show that volume-loss strain was greater within the shallow portions of the Apennines than within the Appalachians. Within the deeper portions of both fold and thrust belts, strain was nearly volume-constant. Calcite solubility data suggest that downward circulation of meteoric water is necessary for the 35% volume-loss strain of the limestones within the Apennines. Strain at a depth of about 1 km was volume-constant and is interpretated as indicative of restricted pore fluid circulation. In the Appalachians, calcite comprises less than 1% of the clastic rocks, and a 10% volume-loss of this calcite may occur during circulation of connate or dehydration water derived from dewatering of the shales but in an environment that restricts the circulation of meteoric water. Here, the volume of calcite removed (0.1% of the total rock) is so small that circulation of meteoric water is not necessary for strain by pressure solution.

Rock anelasticity measurements at high pressure, low strain amplitude and seismic frequency

Abstract: A new apparatus has been constructed which will ultimately facilitate the study of rock anelasticity under conditions which closely approach those of teleseismic wave propagation: simultaneous high pressure and temperature, low frequency and strain amplitude, and controlled pore pressure of volatiles. Its performance has been tested in a series of preliminary experiments conducted at room temperature. Measurements on a steel standard demonstrate the sensitivity of the apparatus to very small departures from ideal elasticity (Q>1000). Experimental data for a fine-grained granitic rock show that both the shear modulus G and quality factor Q increase sharply with increasing pressure below ∼100 MPa, beyond which pressure both parameters become markedly less pressure sensitive. These observations are in accord with those of previous studies at higher frequencies and larger strains, and are consistent with the view that the anelasticity of rocks at ambient pressure is dominated by mechanisms operative at open cracks and grain boundaries.

The hydrogeological regime of isolated sediment ponds in mid- oceanic ridges.

Abstract: Temperature logs run on Leg 78B to a sub-bottom depth of 609 m in Hole 395A, Mid-Atlantic Ridge, indicate the presence of 2.5°C bottom water to a depth of about 300 m sub-bottom. Below this, the temperature rises slowly to 15°C near the bottom of the hole and then abruptly to 22°C in a thin layer of presumed sediment fill at the base. This temperature structure, together with the measurement of a subhydrostatic shut-in pressure of 0.9 to 1.5 bars at a sub-bottom depth of 179 m, suggests that water is still flowing down the hole and into the basement five years after the hole was drilled. δ θ 1 8 values in calcite taken from veins in the basement demonstrate that a similar temperature structure existed before drilling. Resistivity logs run in the same hole indicate that the upper 400 m of the crust is characterized by high porosity, but that the porosity drops markedly in the lower part of the section. This is consistent with direct measurements of permeability, obtained with a packer at a depth of 582 m sub-bottom, which range between 2.7 and 9 μdarcies. These constraints, together with measured heat flow values of 40 mW/m (20% of the expected theoretical value) in the overlying sediment pond, suggest that the upper 400 to 500 m of crust around Hole 395A is being cooled by the lateral flow of pore water moving at a rate of about 1 m/yr.

Distributions of Ferrous Iron and Sulfide in an Anoxic Hypolimnion

Abstract: In the anoxic hypolimnion of Lake 227, Experimental Lakes Area, northwestern Ontario, ΣH2S exhibits a mid-depth maximum, while Fe2+ increases with depth. At the mid-depth ΣH2S maximum and below, saturation with respect to amorphous FeS is reached, and the concentration of ΣH2S is limited by the high Fe2+ concentrations, in accord with the FeS solubility product. Values for pKsp for FeS determined from the ΣH2S maximum and below averaged 3.16 in 1979 and agree well with other in situ and laboratory measurements. In the top 10 cm of sediment, pore water ΣH2S and Fe2+ are in equilibrium with amorphous FeS. Analyses of cores confirms the existence of an iron sulfide phase. Fe2+, which is produced in the pore water from the decomposition of organic matter, increases to concentrations at which siderite may form, although the presence of siderite has not been verified. Comparison of calculated pore water fluxes of Fe2+ with the observed increase of Fe2+ in the anoxic hypolimnion reveals that about 90% of the obs...

Laboratory testing and in situ behaviour of peat as embankment foundation: Reply

Abstract: Intact block samples of fibrous peat have been obtained using special equipment from two different sites in the James Bay territory in Quebec. Instrumented test fills were subsequently built at those two sites and the observed behaviour was used to check the laboratory data. The laboratory testing program has included standard 24 h incremental tests and stage creep tests where constant loads were applied for periods of about 30 d. Pore pressure and direct permeability measurements were conducted in most of the tests.The tests have been interpreted using the classical e − log p curves or the tangent modulus approach. The evaluation of compression with time under a constant load has allowed the distinction between primary and secondary consolidation. A certain lag in pore pressure response has been observed and related to incomplete saturation. The coefficient of permeability decreases rapidly with compression as indicated by direct measurements and pore pressure dissipation.The comparison with field observ...

Dewatering of an unvegetated muddy tidal flat during exposure-desiccation or drainage?

Abstract: An unvegetated muddy tidal flat was sampled to determine the changes in surface pore water content and salinity during exposure. Local evaporation accounted for 61% of the upper intertidal surface salinity, with evaporation rates increasing salinity as high as 2.2‰ per hour. In contrast, only 37% of the decrease in pore water content was attributed to evaporative processes. This suggests that drainage (a combination of porosity and permeability) controlled the water content rather than local evaporative conditions.

Phase Relations in Marine Soils

Abstract: Pore water in marine sediments contains dissolved salts. For marine clays and biogenous oozes which have high void ratios, the dissolved salts should be taken into account in determining soil void ratio, water content, dry density, specific gravity, porosity and degree of saturation. This paper presents phase relations for marine sediments and provides correct equations for various index properties. It also provides equations for different relations taking into account the necessary corrections for dissolved salts in marine soils.

Salt-controlled slumping on the Mediterranean slope of central Israel

Abstract: The highly complex morphology of the continental slope of central Israel is the expression of deep-seated rotational block slumping. The overburden of 1.0–1.5 km thick Pliocene-Quaternary sediments that accumulated over 200 m thick evaporites deposited under the deeper portion of the present-day continental slope and in Late Miocene erosion channels, caused the evaporites to flow downslope. This flowage was presumably caused by excessive pore pressures generated by the Pliocene-Quaternary sedimentary overburden in confined layers of clastics embedded within the impervious evaporites.

In Situ Permeability and Pore-Pressure Measurements near the Mid-Atlantic Ridge, Deep Sea Drilling Project Hole 395A

Abstract: The in situ permeability and pore pressure of 7.2-Ma-old ocean crust were measured in a 664-m-deep hole (DSDP Hole 395A) near the Mid-Atlantic Ridge. These measurements indicate that the bulk permeability from 583 to 664 m below the sea floor (490-571 m sub-basement) at this site is very low, about 3 to 9 microdarcies, and that the pore pressure in the interval from 180 to 664 m sub-bottom is at least 1.5 b (bars) below the oceanic hydrostat. The measured permeability is so low that it virtually precludes significant hydrothermal convection between 583 and 664 m sub-bottom at Site 395, although depressed downhole temperatures, anomalously low regional heat-flow values, and the subhydrostatic pore pressure indicate that substantial hydrothermal convection is probably occurring at shallower depths. If this low-permeability zone is pervasive (as more recent measurements of low permeabilities at depth in DSDP Hole 504B, near the Costa Rica Rift, suggest), it places strong constraints on the vertical extent of convective transfer in cooling oceanic lithosphere.

Soil movements on permafrost slopes near Fairbanks, Alaska

Abstract: Many north-facing slopes in the Yukon–Tanana Uplands of Alaska show signs of downhill movements in the form of hummocky surface and leaning trees. Measurements of movements and pore pressures were made at several sites in the Caribou–Poker Creek Research Watershed. It was found that, on slopes with inclinations near 30°, most of the movements occurred immediately after thaw when pore pressures were high.To evaluate the slope stability, the soil strength was measured by direct shear tests. The strength of the moss layer and tree roots was evaluated by performing tension tests on the roots and the moss–root complex. Results of stability analyses show that local failures involving individual wedges are likely. The displacements associated with a wedge slide would result in a hummock or step. The strength of the moss–root complex was found to be a significant factor in the stability of the wedge and of the step. Key words: moss, roots, permafrost, pore pressure, shear strength, slope stability, thaw.

Origins and significance of non-linear temperature profiles in deep-sea sediments

Abstract: Summary. Recent measurements of non-linear temperature profiles have led to the suggestion that vertical pore water advection is present in deep sea sediments in some areas, This paper examines the basis for this hypothesis and considers a number of alternative mechanisms which, even in the absence of fluid advection, may give rise to non-linear temperature profiles. These include changes in bottom water temperature, the effects of rapid deposition or removal of sediment and the release or absorption of heat by mechanical and chemical changes within the sediment column. The downwarping of sediment layers caused by the penetration of a probe is almost certain to cause a small temperature non-linearity which must define a minimum threshold below which natural curvature cannot be con- fidently detected. The influence of irregular topography may also be an important source of error. Stringent criteria must be applied to site selec- tion and data interpretation if sediment temperature profiles are to yield meaningful estimates of pore water advection velocities.

Influence of Drainage Trenches on Slope Stability

Abstract: An unfavorable groundwater condition is a frequent cause of slope sliding. Pore pressure decreases the effective stress level and consequently the shear existance of the soil on the slip surface is decreased. Thus the slope safety against sliding is reduced. A change in the groundwater condition is achieved by drainage systems. The drains introduce a lower potential to the pore water which reduces the pore pressures in the sliding mass. Drainage trenches are suitable for stabilizing shallow translational slides. This paper examines the determination of the necessary spacing of trenches for a required increase in factor of safety. The factor of safety obtained represents the long‐term value for the steady state condition.