Reclamation Fill Research Articles

Experimental study on permeability and strength characteristics of MICP-treated calcareous sand

Calcareous sand is the main fill material for island reclamation projects, but untreated calcareous sand might not be used as a reclamation fill due to its poor mechanical properties. Microbial induced calcite precipitation (MICP) was directly used to consolidate calcareous sands. One-dimensional sand column tests were conducted to identify the optimized solutions and to investigate the effects of cement solution concentration, relative density, and consolidation frequencies on the permeability and mechanical properties of MICP-treated calcareous sands. Finally, three-dimensional model tests were carried out to investigate the effective consolidation range of microbially treated calcareous sands. The results show that the MICP-treated calcareous sand shows a reduction in the permeability of the sample, while the calcium carbonate cementation and its filling effect improves the mechanical properties of the soil. The one-dimentional test results show that the effective values for cement solution concentration, relative density, and consolidation frequencies range from 0.5 mol/L to 1.5 mol/L, 30%–70%, and 5–15 times. The consolidation frequencies have the greatest influence on the permeability and strength properties of the treated calcareous sand. A quadratic polynomial regression model for permeability and strength was established through response surface analysis, and the regression model proved to be highly accurate and reliable through testing. In three-dimentional tests, the consolidation range tends to move downwards in a trapezoidal shape, showing a "big bottom and small top" pattern, with a consolidation range of approximately 34 times the diameter of the pipe. This study serves as a reference for selecting consolidation parameters for subsequent tests and applications of MICP-treated calcareous sands.

Size segregation mechanism of reclamation fill sand due to rainbowing operations in hydraulic dredging activities

Segregation of sand grains is often detected in reclamation lands constructed by hydraulic dredging activities using various placement methods, such as bottom dumping, rainbowing, and discharging via floating pipelines. Heterogeneities in the geotechnical characteristics of the reclamation land, such as the relative density, the hydraulic conductivity, and the shear strength, are caused by the size segregation of the dredged materials. In this paper, a computational fluid dynamic (CFD) method and the discrete element method (DEM) are coupled to investigate the mechanism of size segregation of the dredged sands in the deposit process after the rainbowing operations. A numerical water tank is established with an initially uniform flow field. The sand grains are jetted from the nozzle at a given velocity and are then deposited under their self-weight and the drag force from the atmospheric water. The numerical model is validated through a simulation of the settlement of single grains. The main factors that influence the size segregation phenomenon—the flow velocity, the rainbowing velocity, and the water depth—are investigated based on a parametric study. The complex interlayers observed in real reclamation lands are reproduced by considering the construction sequence of the dredged materials.

Evolution of bubble-bearing areas in shallow fine-grained sediments during land reclamation with prefabricated vertical drain improvement

When land reclamation is conducted in a shallow offshore bubble-bearing sediment, methane bubbles can cause various risks, such as changes in the mechanical properties of soils and explosion during construction. The premise of assessing or addressing these risks is to understand the evolution of the bubble-bearing area during land reclamation. In this study, we selected an island-based land reclamation field in the southeastern part of Hangzhou Bay with a bubble-bearing area in shallow sediments. In this field, soft soil under the land reclamation fill was improved via prefabricated vertical drain (PVD). After the land reclamation, an electrical resistivity tomography (ERT) survey was conducted at the location of the bubble-bearing area and in an adjacent bubble-bearing area. The force and deformation of bubbles in the bubble-bearing area were analyzed at various construction stages of land reclamation. A conceptual model of the bubble-bearing area evolution in each construction stage was proposed based on linear elastic fracture mechanics (LEFM). The results of the ERT survey and theoretical analysis are in good agreement. In the process of land reclamation, although the gas pressure and size of the bubbles in the bubble-bearing area may change with the change in the environmental pressure, the bubble size is always smaller than the critical size for initiating rise; thus, the bubbles cannot rise. After land reclamation, there is no important horizontal/vertical evolution of the gas front in the bubble-bearing area.

Engineered reclamation fill material created from excavated soft material and granulated blast furnace slag

Due to the rapid development and population growth, many coastal areas and countries are in shortage of land space. Large amount soft soil generated from excavation work has been used as an alternative recently, which is also a challenge because of its poor engineering properties. In this study, ground granulated blast furnace slag (GGBFS), a byproduct of steel industry, was employed to stabilize the excavated soft soil (ESS). Toxicity evaluation shows that the prepared engineering reclamation fill (ERF) materials possessed little impacts on marine ecology. Binder is essential to activate GGBFS. Lime was a better binder than cement. After 7 d curing, the unconfined compressive strength (UCS) of the ERF prepared with GGBFS (24%) and lime (1%) increased up to about 1.5 times of ESS, and the friction angle increased from about 15° to 40°. The addition of GGBFS activated by lime inhibits the swelling of ERF samples and increased the coefficient of consolidation tremendously. After soaking in simulated sea water for 113 d, only ERFs with lime/GGBFS/ESS dry ratios of 1/24/75 and 1/16/83 stood still with UCS of 4718 kPa and 3133 kPa, about 7 and 5 times of ESS, respectively. Scanning electron microscopy images further confirmed the formation of hydration products such as calcium aluminates, calcium silicate hydrates and calcium aluminum silicate hydrates responsible to the enhancement of the ERFs mechanical properties. The prepared ERF sample with GGBFS (24%) and lime (1%) has been proved to be a potential economic and environmental friendly alternative for land reclamation.

Slope stability and landform design in a coarse coal refuse reclamation.

Restoration of abandoned mine lands is on-going in Appalachia and across the United States. One potential technique to reclaim these areas is through the use of geomorphic landforming combined with paper mill residuals used as a soil amendment. The geomorphic approach attempts to approximate the long-term, steady state landform condition, leading to reduced erosional adjustment as compared to standard engineered fill designs. This study analyzed regraded mine refuse slopes for safety and erosion sustainability under a 100-year storm event for the Royal Scot refuse pile located near Anjean, WV. The project involved the design and evaluation of a cap and cover system composed of two layers. The first layer is a 60 cm. compacted barrier layer (hydraulic barrier) and second is a 30cm vegetative growth layer. The focus of the analysis was the effectiveness of the hydraulic barrier layer and the final slope stability of the system. A reclamation design was prepared and 3D finite element modeling was performed to assess ground surface terrain profiles considering weather and material strength. Results from the modeling indicate that for a 100-year storm event, the slope remains stable with a factor of safety ranging from 1.3 to 1.9 for different slope profiles. After the 100-year storm event, the hydraulic barrier reached a 20% degree of saturation proving its effectiveness with minimizing infiltration into the reclamation fill.

Influence of sand impurities in cement-treated clays

Cement stabilisation is a well-established technique used in conventional onshore soil improvement works where a large quantity of cement is added to increase the soil's shear strength. However, the application to land reclamation often requires only a low cement dosage and at much higher water content, far exceeding the liquid limit of the soil. In addition, reclamation fill is becoming more heterogeneous with varying amounts of granular material in clayey soils. In this study, a higher unconfined compressive strength of cement-treated clayey soils is attained at a lower water to cement (w/c) ratio. This is persistent for clayey soils with sand impurities. The soil to cement (s/c) ratio produces differing strength development in cement-treated clayey soils, owing to counteracting mechanisms between the degree of bonding and size of pore volume, arising from different amounts of cement and water content, respectively. The normalised strength development of cement-treated clayey soils with curing time follows a logarithmic relationship across different w/c, s/c and sand content. Strong linear correlations between early-age and later-age curing specimens are also observed, which permits early strength prediction of cement-treated clayey soils. In order to account for the variation in microfabric characteristics and pore spaces induced by sand, the free water to cement ratio, (w/wL)/c, is shown to be more appropriate in assessing strength development of cement-treated soils than conventional water to cement ratio (w/c). This is demonstrated with matching strength predictions and measured values for both Singapore marine clay and kaolin clay.

Experimental investigation on effect of curing stress on the strength of cement-stabilized clay at high water content

The application of cement-stabilized marine clay at high water content (HW-CSC) as land reclamation fill has been receiving increasing attention and popularity. In current coastal development, HW-CSC land reclamation projects begin to advance toward deeper water areas, leading to greater filling heights and thus higher curing stresses acting on the lower portions of HW-CSC. In principle the higher curing stress will improve the strength gain of HW-CSC, but this curing stress effect is not taken into account in current design practice. This study aims to bridge the gap. Fifteen different mixes of HW-CSC are first prepared and cured in consolidation tubes for seven days under various curing stresses. Their strengths and some other post-curing parameters are then tested and analyzed to demonstrate the quantitative effect of curing stress on the strength gain of HW-CSC. Results indicate that the strength gain of HW-CSC is significantly influenced by the curing stress; however, the effect of curing stress cannot be simply replaced by reducing the remolding water content during mixing stage and letting the specimens be cured under atmospheric stress condition. Therefore, it is concluded that the effect of curing stress includes two different components—a consolidation effect and another effect termed the “self-stress” effect in this study. Finally, an empirical relationship accounting for the combined effects of mixing proportion and curing stress is developed for characterization of the strengths of HW-CSCs with different mixing proportions and various curing stresses. The proposed relationship is validated by three independent data groups collected from the literature. This empirical relationship can be helpful in optimization of mixing proportions of HW-CSC used for land reclamation purpose.

Piezometer measurements of prefabricated vertical drain improvement of soft soils under land reclamation fill

Piezometers are commonly used for monitoring the dissipation of pore water pressure and therefore the determination of degree of consolidation of soft soils after ground improvement in land reclamation projects. This paper compares the degree of consolidation obtained using pore pressure monitoring data from vibrating wire piezometers and pneumatic piezometers installed in a test site in the Changi East Reclamation project in Singapore. The test site consisted of a vertical drain sub-area at which prefabricated vertical drains were installed at 2.0m×2.0m square spacing as well as an adjacent control sub-area where no prefabricated vertical drains were installed. Pneumatic piezometers were installed at the same elevations as the vibrating-wire piezometers in both sub-areas for comparison purposes. During the surcharge period of 32months, the piezometer monitoring data were analysed at various periods to determine the degree of consolidation of the underlying soft marine clay. The degree of consolidation values interpreted from both types of piezometers were compared with predictions from neighbouring settlement plates to evaluate their performance. The findings of the comparison between pneumatic and electric vibrating-wire piezometers indicate that both types of piezometer are suitable for monitoring the consolidation behaviour of soft soil under land reclamation fills.

Compaction of coal wash to optimise its utilisation as water-front reclamation fill

The coal mining industry in and around Wollongong, New South Wales, Australia, is responsible for producing an ever growing quantity of coal washery reject materials and other mine waste aggregates. Considerable effort is being undertaken to reuse, beneficially, these waste products in the immediate suburbs of Wollongong. In this paper, detailed laboratory investigations carried out on coal wash (i.e. coarse coal washery reject), produced at Dendrobium coal mine near Wollongong, are presented. Geotechnical tests were conducted to determine the particle size distribution, compaction characteristics, shear resistance and collapse potential. Compaction tests were performed under dry and submerged conditions to examine comprehensively the compactability and the strength properties of coal wash. The laboratory tests show that, if properly compacted, Dendrobium coal wash has a good potential as effective low-porosity fill for embankments and port reclamation.

Influence of Settling Behavior of Soil Particles on the Consolidation Properties of Dredged Clay Sediment

Placement of dredged spoils in a slurry form and allowing self-weight consolidation is common in land reclamation works carried out adjacent to water bodies. The influence of the settling behavior of soil particles in clay-water slurry on the homogeneity and consolidation properties of the final sediment were assessed in the current study. Standard one-dimensional and radial consolidation tests were conducted on Port of Brisbane dredged mud, which was remolded at various water contents and salt concentrations and reconstituted. Particle-size distribution curves show the various degrees of segregation that occurred during settlement of soil particles in flocculated and dispersed slurry, prepared and placed in saltwater and freshwater environments, respectively. The vertical and horizontal consolidation coefficients (cv, ch) and the vertical permeability (kv) were found to be fairly uniform throughout the flocculated sediment, whereas for the dispersed sediment significant variation could be observed at various depth levels. The consolidation properties in the horizontal direction (ch, kh) were considerably larger than in the vertical direction (cv, kv) for both the flocculated and dispersed sediment. The degree of anisotropy in the consolidation coefficient, measured by ch/cv, was generally larger for the flocculated sediment than for the dispersed sediment. Proper understanding of the consolidation process and the variability of the consolidation and permeability coefficients with depth is important for the reliable prediction of settlement and the degree of consolidation achieved in the reclamation fill.

Mechanical characteristics of lean-mixed cement-treated granular soil

As an attempt to promote reuse of dredging materials for construction, cement-treated granular soil (CTGS), a coarse-grained granular material, can be produced from dredged marine clays by mixing them with appropriate amounts of cement and polymer. This study focused on lean-mixture designs of cement and polymer for cost reduction. To evaluate the applicability of the lean-mixed CTGS for construction, laboratory tests were conducted to investigate the fundamental mechanical characteristics of CTGS including its durability against degradation, compaction characteristics and strength deformation characteristics. The materials, the production method and the engineering properties of CTGS are also comprehensively presented. The test results showed that CTGS has lightweight, high-porosity properties and indicated beneficial uses for the granular materials. From the engineering properties obtained from the tests, CGTS shows high potential to be useful in reclamation, backfill and embankment fill materials. However, the CTGS particles exhibited a significant degree of crushing and deformation and the particle crushing and deformation properties of CTGS dominated its stress–strain behaviour.

Site-Investigation and Geotechnical Design of D-Runway Construction in Tokyo Haneda Airport

Tokyo International Airport (Haneda Airport) is a domestic hub-airport in Japan; however, the increasing number of passengers has brought it close to its capacity. In addition, there has been strong demand for the development of an international-flight network. Consequently, a new runway, called the “D-runway,” was planned and constructed from March 2007 to October 2010. Because some of the D-runway runs through a river mouth, a hybrid structure consisting of piled pier and reclamation fill was adopted. To overcome the geotechnical difficulties in constructing this hybrid structure on the soft clay deposit, various technologies in design and construction were adopted. This paper provides an outline of the project, the ground investigation, and the design of the D-runway structure from a geotechnical engineering view point. From the results of the site investigation, the stratigraphic model at the site was clarified. For the clay layers, a representative depth-profile for each soil parameter was determined. Some local soil properties which tended to be overlooked when only employing an engineering point of view can be appropriately captured by linking the geological and geotechnical information. In the construction of the D-runway, not only the ground improvement technologies (SD, SCP, and CDM) but also the new developed construction materials (the pneumatic mixing of cement treated soil and air-foam treated lightweight soil) were utilized. In the-D-runway project, various technologies used in previous airport constructions were brought together and applied to the ground investigation, design, construction work, and even maintenance. The construction of the D-runway was completed safely, rapidly, and economically, and it came into use on 21 October 2010, on schedule.

Geotechnical design for the Port Botany expansion project, Sydney

This paper describes ground treatments, foundation preparations and geotechnical design for extensive reclamations and retaining structures forming the Port Botany expansion project in Sydney, Australia. The 63 ha reclamation, up to 30 m deep, is constructed from dredged Botany Bay sands, which are retained by over two hundred 20 m high counterfort and blockwork gravity wall units. These berth structures are founded on vibrocompacted sand backfill placed within a submerged trench excavated into natural fissured clays. The design involved detailed assessment of stability and movement behaviour for the reclamation and berth structures. Geotechnical compliance testing criteria were developed for reclamation fill materials to provide compaction and material quality assurance. In particular, acceptance criteria for berth structure foundations and retained backfill were developed to verify the strength and stiffness of fills so as to meet design requirements at key time milestones following handover of the works. The main fill verification tool utilised cone penetration tests, for which screening rules and site-specific acceptance criteria were developed. Methods of fill compaction included dynamic compaction, vibrocompaction and impact roller compaction. The relative performance of these methods is discussed, along with compliance testing results, effects of vibrocompaction on wall structures, fill variability and actual compared with predicted performance of compacted sand fills.

One-dimensional consolidation of lumpy clay with non-linear properties

When dredged clay lumps are used as reclamation fill, large voids exist between the lumps. Thus consolidation of the lumps themselves, and closing up of inter-lump voids, contribute to the overall settlement, making the fill a complicated non-linear medium. Yang et al. (2002, Géotechnique 52, No. 10, 713–725) have used the double porosity model to describe the consolidation of this type of fill using linear properties to arrive at analytical solutions. In reality, the behaviour is highly non-linear, and the finite element (FE) method is used to solve the non-linear equations. Results from the FE analysis of the consolidation of the lumpy clay fill were shown to be able to match the analytical solutions. Two one-dimensional consolidation tests were conducted for this study. In the first case, water was used to fill the inter-lump voids, whereas in the second case, slurry was used. The procedure for determining the permeability and the e–logσ′ relationships for the inter-lump and intra-lump systems is explained in this paper. All the parameters could be estimated from various tests, except for the fluid transfer parameter, and this was back-analysed, using data from other works as a guide. The results presented show that the model can predict well the measured settlement and pore pressures in inter-lump and intra-lump systems in both experiments, essentially because this is a back-analysis. But the fact that the model can provide consistent prediction of numerous parameters indicates that the model is able to capture the essence of the mechanics of a lumpy clay system.

One-dimensional consolidation of lumpy clay with non-linear properties

When dredged clay lumps are used as reclamation fill, large voids exist between the lumps. Thus consolidation of the lumps themselves, and closing up of inter-lump voids, contribute to the overall settlement, making the fill a complicated non-linear medium. Yang et al. (2002 , Géotechnique 52, No. 10, 713–725) have used the double porosity model to describe the consolidation of this type of fill using linear properties to arrive at analytical solutions. In reality, the behaviour is highly non-linear, and the finite element (FE) method is used to solve the non-linear equations. Results from the FE analysis of the consolidation of the lumpy clay fill were shown to be able to match the analytical solutions. Two one-dimensional consolidation tests were conducted for this study. In the first case, water was used to fill the inter-lump voids, whereas in the second case, slurry was used. The procedure for determining the permeability and the e–logσ′ relationships for the inter-lump and intra-lump systems is explained in this paper. All the parameters could be estimated from various tests, except for the fluid transfer parameter, and this was back-analysed, using data from other works as a guide. The results presented show that the model can predict well the measured settlement and pore pressures in inter-lump and intra-lump systems in both experiments, essentially because this is a back-analysis. But the fact that the model can provide consistent prediction of numerous parameters indicates that the model is able to capture the essence of the mechanics of a lumpy clay system.

Experimental Evaluation of Consolidation Behavior of Stiff Clay Lumps in Reclamation Fill

Abstract When stiff clay dredged from the seabed is deposited by barge as fill material, large voids exist between the stiff clay lumps in the reclamation fill. One major concern is that the voids may close up upon loading, resulting in a large and sudden increase in the fill settlement. In the present study, an experimental investigation is carried out to examine the behavior of fill made up of dredged stiff clay lumps using a large one-dimensional compression apparatus and a centrifuge modeling technique. The results revealed that much of the inter-lump voids close up during the preloading stage. In addition, the rate of consolidation of the lumpy fill is significantly faster than that of homogeneous clay with the difference in consolidation rate diminishing with increase in loading pressure. The effects of lump shape and size, and original shear strength of in situ clay on the performance of lumpy fill, are also investigated.

The status of beaches and shoreline development options on the French Riviera: a perspective and a prognosis

Beach conservation and management on the high-value French Riviera in southeastern France have had mixed fortunes in shoreline economic development strategies over the past half century. Prior to 1965, socio-economic growth related to immigration and tourism resulted in considerable pressure on the coastal zone, leading in particular to beach erosion and degradation of beach environmental quality. Between 1965 and 1980, over 20% of the 132 km-long French Riviera was permanently altered through the implantation of yachting harbours and reclamation fill structures, while beach-based recreation had a rather low ranking as a development choice, except in the two major resorts of Cannes and Nice which exhibit a densely urbanized seafront. On this preponderantly bold rocky coast, the mediocre recreational value inherited by many of the beaches from the regional geologic setting, and from development pressures and earlier errors in coastal management, left them vulnerable to appropriation and so-called ‘valorization’ by yachting harbour and estate developers. Over the last decade, artificial shoreline development has virtually ceased, in response to several more or less interrelated factors. These include relative stagnation of socio-economic growth, increasing development and maintenance costs of yachting harbours, saturation of the yachting harbour market as a result of the burgeoning of new, often cheaper, resorts and of reconversion of commercial and shipbuilding ports to leisure ports in the Mediterranean, more stringent legislation, since 1986, on the implantation of residential and major engineering structures on the coastline, pressure for conservation of the cultural and environmental heritage, and greater demand for beach recreational space. This situation has forced a diversification of shore-based activities, as it has been realised that better managed beaches may balance economic aspirations while contributing to enhanced environmental quality and sensible shoreline conservation. This change in strategy has entailed considerable efforts and money on the improvement of coastal water quality, the provision of amenities for beach-goers, and especially the nourishment of eroding beaches and the creation of several artificial beaches. The extent to which beaches will continue to play a role in the sustainable development of French Riviera resorts will depend largely on the capacity of local authorities to maintain environmental quality in the face of inherited and persistent handicaps such as beach erosion.

Impacts géomorphologiques côtiers des aménagements du Var et de son delta, Côte d'Azur, France

Coastal geomorphic impacts of human-induced transformations of the Var River and Delta, French Riviera The high-relief coast of the Baie des Anges, on the French Riviera, is bounded by two gravel barriers linked to the central Var Delta. This deltaic protuberance forms the terminus of a 135 km-long, steep-gradient river. Over the past century, the lower Var valley and the delta have been massively transformed in various ways. These transformations have included reclamation of large sectors of the river bed for agricultural and transport networks, embankments against flooding of the reclaimed land and construction of several low rock dams across the river to enable gravel accumulation and enhancement of the height of the the alluvial aquifer of the Var, which, by the mid-sixties, had dropped considerably as a result of massive human withdrawal. Two important consequences of these changes were a drastic reduction in the supply of gravel to the coast and destabilization of the morphodynamics of the delta front. The drop in gravel supply to the barrier beaches fringing the Baie des Anges resulted in accelerated barrier breakdown. Following a long period of stability during which permanent loss of gravel to the steep nearshore shelf was compensated in the past by fresh supplies from the Var, the implantation of various installations such as seafront roads, rail links, yachting harbours and reclamation fill structures on the barriers had, by the 1950's, already led to the onset of this phase of erosion. The reclamation of much of the lower valley and the entire delta plain resulted in the irreversible transformation of these morphodynamic units into an artificial system bounded seaward by revetments that have replaced the former gravel beaches. Reclamation of the lower valley and of the subaerial delta plain led to enhanced seaward transport of suspended sediments that could no longer be stored within the fluvial system, and to greater fine-grained sedimentation on the steep and unstable delta front slopes. One of the consequences of these transformations, notably the modification of the upper delta front slope, was a catastrophic submarine landslide, in October, 1979. This landslide caused several casualties, destroyed part of the reclamation works on the upper delta front, including a port under construction at the time, and generated a small tidal wave in the Baie des Anges. Whether gradual, as in the case of beach erosion, or associated with threshold exceedance, as in the case of this landslide, prediction of the effects of human destabilization of coastal systems is hampered by a poor grasp of the functional mechanisms and processes underlying such systems, whose stability depends as much on what goes on upland as in the sea.

Geotechnical properties of pulverized fuel ash as a reclamation fill : Li, K S; Dutton, C Proc Ninth Asian Regional Conference on Soil Mechanics and Foundation Engineering, Bangkok, 9–13 December 1991V1, P405–408. Publ Thailand: Southeast Asian Geotechnical Society, 1991

Geotechnical properties of pulverized fuel ash as a reclamation fill : Li, K S; Dutton, C Proc Ninth Asian Regional Conference on Soil Mechanics and Foundation Engineering, Bangkok, 9–13 December 1991V1, P405–408. Publ Thailand: Southeast Asian Geotechnical Society, 1991

THE EFFECT OF OFFSHORE DREDGING ON COASTLINES

The South of England is well endowed with land deposits of river gravel and sand. Nevertheless, the demand for aggregate and the need to conserve agricultural land have increased to the point where in 1976 sea dredged aggregate was accounting for 11% of the total sand and gravel production. In addition to this annual home consumption of about 12 million tonnes of dredged aggregate a further 3.5 million tonnes was exported to Europe in 1976. Understandably, authorities responsible for coast protection and sea defence view the increase in the removal of marine deposits with concern and a system of licensing by the Crown Estate Commissioners who are responsible for the sea bed from high water to the UK Continental shelf limit has been developed over the years. Dredging by port authorities within their area ofjurisdiction, for navigational purposes, is outside this licensing system. Within the three mile limit local authorities have powers under the Coast Protection Act to regulate dredging. In 1976 a report by an advisory committee to the Department of the Environment1-1^ recommended, among other things, that further studies should be carried out by HRS aimed at reviewing the existing constraints on marine dredging for gravel. In general, the material which is sought for construction purposes is a 60% shingle, 40% sand mixture, but sand is also needed for reclamation fill and for industrial purposes. The areas dredged at present are shown in Figure 1. This paper deals briefly with the licensing procedure and at some length with the involvement of the Hydraulics Research Station in assessing how dredging might affect the coastline. The effect on fisheries, navigation, coastal ecology, and other interests is considered by other organisations. Very little is known of the criteria applied to offshore dredging by other countries, apart from Germany, but with the increased exploitation of the sea bed it is important that information gained by other countries should be used to improve our existing criteria. .