1、Designation: D 4823 95 (Reapproved 2008)Standard Guide forCore Sampling Submerged, Unconsolidated Sediments1This standard is issued under the fixed designation D 4823; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of las
2、t revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers core-sampling terminology, advan-tages and disadvantages of different types of core samplers,core-disto
3、rtions that may occur during sampling, techniquesfor detecting and minimizing core distortions, and methods fordissecting and preserving sediment cores.1.2 In this guide, sampling procedures and equipment aredivided into the following categories based on water depth:sampling in depths shallower than
4、 0.5 m, sampling in depthsbetween 0.5 m and 10 m, and sampling in depths exceeding 10m. Each category is divided into two sections: equipment forcollecting short cores and equipment for collecting long cores.1.3 This guide emphasizes general principles. Only in a fewinstances are step-by-step instru
5、ctions given. Because coresampling is a field-based operation, methods and equipmentmust usually be modified to suit local conditions. This modi-fication process requires two essential ingredients: operatorskill and judgment. Neither can be replaced by written rules.1.4 Drawings of samplers are incl
6、uded to show sizes andproportions. These samplers are offered primarily as examples(or generic representations) of equipment that can be purchasedcommercially or built from plans in technical journals.1.5 This guide is a brief summary of published scientificarticles and engineering reports. These re
7、ferences are listed inthis guide. These documents provide operational details thatare not given in this guide but are nevertheless essential to thesuccessful planning and completion of core sampling projects.1.6 This standard does not purport to address all of thesafety concerns, if any, associated
8、with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. For specificwarning statements, see 6.3 and 11.5.2. Referenced Documents2.1 ASTM Standards:2D 420 Guide to
9、 Site Characterization for Engineering De-sign and Construction PurposesD 1129 Terminology Relating to WaterD 1452 Practice for Soil Investigation and Sampling byAuger BoringsD 1586 Test Method for Penetration Test (SPT) and Split-Barrel Sampling of SoilsD 1587 Practice for Thin-Walled Tube Sampling
10、 of Soilsfor Geotechnical PurposesD 4220 Practices for Preserving and Transporting SoilSamplesD 4410 Terminology for Fluvial Sediment3. Terminology3.1 Definitions: For definitions of terms used in this guide,refer to Terminology D 1129 and Terminology D 4410.3.2 Definitions of Terms Specific to This
11、 Standard:3.2.1 check valvea device (see Fig. 1)3mounted atop anopen-barrel core sampler. As the sampler moves down throughwater and sediment, the valve remains open to allow water toflow up through the barrel. When downward motion stops, thevalve closes. During retrieval, the valve remains closed a
12、ndcreates suction that holds the core inside the barrel.3.2.2 corea vertical column of sediment cut from a parentdeposit.3.2.3 core catchera device (see Fig. 2) that grips andsupports the core while the sampler is being pulled from thesediment and hoisted to the water surface.3.2.4 core conveyora de
13、vice (see Fig. 3) for reducingfriction between a core and the inside surface of a core barrel.3.2.5 core-barrel linera rigid, thin-wall tube mountedinside the barrel of a core sampler. During the core-cuttingprocess, sediment moves up inside the liner.3.2.6 core sampleran instrument for collecting c
14、ores.1This guide is under the jurisdiction of ASTM Committee D19 on Water and isthe direct responsibility of Subcommittee D19.07 on Sediments, Geomorphology,and Open-Channel Flow .Current edition approved Oct. 1, 2008. Published November 2008. Originallyapproved in 1988. Last previous edition approv
15、ed in 2003 as D 4823 95 (2003)1.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The boldface numbers in pare
16、ntheses refer to the list of references at the end ofthis guide.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2.7 extrudeThe act of pushing a core from a core barrelor a core-barrel liner.3.2.8 open-barrel samplerin simplest form
17、, a straighttube open at both ends. More elaborate open-barrel samplershave core catchers and check valves.3.2.9 piston immobilizera special coupling (see Fig. 4)that protects a core from disruptive forces that arise duringsampler pull-out. Piston immobilizers are also called splitpistons or break-a
18、way pistons.3.2.10 piston samplera core sampler (see Fig. 5) with asolid cylinder (piston) that seals against the inside walls of thecore barrel. The piston remains fixed at the bed-surfaceelevation while the core barrel cuts down through the sediment.3.2.11 recovery ratiothe ratio A/B where “A” (se
19、e Fig. 1)is the distance from the top of the sediment core to the bottomof the cutting bit and “B” is the distance from the surface of theparent deposit to the bottom of the cutting bit.3.2.12 repenetrationa mishap that occurs when a coresampler collects two or more cores during one pass.3.2.13 surf
20、ace samplera device for collecting sedimentfrom the surface of a submerged deposit. Surface samplers aresometimes referred to as grab samplers.3.2.14 trip releasea mechanism (see Fig. 5 and Fig. 6(b)that releases a core sampler from its suspension cable andNOTEDark bands represent stiff sediments; l
21、ight bands representplastic sediments. As coring proceeds, sediment below the barrel moveslaterally away from the cutting edge and plastic sediments inside the barrelare compressed. “A” is the cores length and “B” is the barrelspenetration depth.FIG. 1 Deformations Caused by Open-Barrel Core Sampler
22、s (1)1NOTE(a) The leaves separate during penetration and then close duringretrieval. Strips of gauze can be woven around the leaves to provideadditional support. (3) (b) The lever trips down during retrieval to releasethe spring and twist the fabric sleeve shut. (4) (c) The cupped plate dropsduring
23、retrieval to block the entrance and support the core. (4) (d) Thelever releases the spring-loaded blade which pivots downward to hold thecore. (4)FIG. 2 Core CatchersNOTE(a) Strips of metal foil slide up through the core barrel as thecutting edge advances downward. (5) (b) The plastic sleeve unfolds
24、 frompleats stored near the cutting edge. This sleeve surrounds the core as thebarrel moves down. (4)FIG. 3 Core ConveyorsNOTEDuring penetration the shear pins break but the flow-restrictingorifice holds the clevis and piston together. During retrieval, water in thetop chamber flows through the orif
25、ice and allows the piston and clevis toseparate. Cable tension pulls the clevis up against the stop but frictionlocks the piston and core barrel together.FIG. 4 Piston Immobilizer (9)D 4823 95 (2008)2allows the sampler to freely fall a predetermined distancebefore striking the bed.3.2.15 undisturbed
26、 samplesediment particles that havenot been rearranged relative to one another by the process usedto cut and isolate the particles from their parent deposit. Allcore samples are disturbed to some degree because raising thecores to the water surface causes pore water and trapped gasesto expand (10).
27、In common usage, the term “undisturbedsample” describes particles that have been rearranged but onlyto a slight degree.4. Critical Dimensions of Open-Barrel and PistonSamplers4.1 Dimensions of a samplers cutting bit, core tube, andcore-tube liner (see Fig. 7) are critical in applications requiringun
28、disturbed samples. These dimensions control the amount ofdistortion in recovered cores. The recommendations in thissection were developed from tests on open-barrel core sam-plers (11); however, the recommendations are usually extendedto cover piston-type core samplers.4.2 Cutting-Bit AngleThe angle
29、“b” on the cutting bit (seeFig. 7) should be less than about 10; the optimum angle isabout 5. If the angle is smaller than about 2, the bit cutsefficiently but its edge chips and dulls easily.4.3 Core-Liner Diameter, Ds(see Fig. 7)Dsshould belarger than about 5 cm; however, the upper limit for Dsisd
30、ifficult to establish. As Dsincreases, the amount of corecompaction decreases but the sampler becomes heavier andlarger. A survey of existing samplers shows that 10 cm is apractical upper limit. A few samplers have barrels larger than10 cm but these are used only for special applications (12).4.4 In
31、side Friction FactorThe dimensions Dsand De(seeFig. 7) set the inside friction factor defined as Ci=(DsDe)100/De. For a barrel without a core conveyor, the optimumCivalue depends mainly on the barrels length. Cishould besmaller than 0.5 if the barrel is shorter than about 2 m. If thebarrel is longer
32、 than about 2 m, Cishould fall between 0.75 and1.5. For a barrel with a core conveyor, Cishould be smallerthan 0.5 regardless of the barrels length. Notice that in allinstances Dsis lightly greater than De. The small expansionabove the cutting bit minimizes friction where the outside ofthe core cont
33、acts the inside of the barrel or liner. Frictiondistorts the cores strata by bending horizontal layers intocurved, bowl-shaped surfaces shown on the upper part of Fig.8. Friction also causes overall end-to-end compaction of thecore and thereby reduces recovery ratios. If friction becomesvery large,
34、sediment fails to enter the cutting bit. Instead,NOTE(a) The sampler is lowered slowly through the water. (b) Thesampler falls free when the trip weight contacts the bed. (c) The corebarrel cuts downward but the piston remains stationary.FIG. 5 Operation of a Piston-Type Core Sampler (2)NOTE(a) The
35、messenger weight strikes the hook and releases thestring holding the check valve. (6) (b) The trip weight strikes the sedimentand unhooks the sampler. (7) (c) The cable slackens and allows thespring-loaded hook to open. (8)FIG. 6 Release MechanismFIG. 7 Critical Dimensions for Cutting Bits and Core
36、Barrels (11)D 4823 95 (2008)3sediment moves aside as the bit penetrates downward. Thislateral motion, commonly referred to as “staking,” preventsdeep-lying strata from being sampled. It is important toobserve upper limits on Cibecause too large an expansioncauses another form of distortion, the core
37、 slumps against thewalls as the sediment slides up into the barrel.4.5 Outside Friction FactorThe dimensions Dwand Dt(see Fig. 7) set the outside friction factor defined as Co=(Dw Dt)100/Dt. Coshould be zero for barrels used in cohesionlesssediments; but Coshould be between 1.0 and about 3.0 forbarr
38、els used in cohesive sediments. Notice that in all instancesDwis larger than Dt. The small contraction above the bitreduces friction at the outside surface of the barrel and makesit easier to push the core barrel into the bed. On a long barrel,friction can be reduced by installing one or more sleeve
39、s (seeFig. 7). The sleeves not only plough a path for the barrel butthey also serve as clamps to hold barrel sections together.4.6 Area FactorThe dimensions Dwand Deset the areafactor defined as Ca=(Dw2) 100/De2. Cashould be less than10 or possibly 15. Notice that Cais proportional to the area ofsed
40、iment displaced by the bit divided by the area of the bitsentrance; therefore, Cais an index of disturbance at the cuttingedge. A sampler with too large an area factor tends tooversample during early stages of penetration when frictionalong the inner wall of the barrel is low. Oversampling occursbec
41、ause sediment laying below and outside the bit shift inwardas the bit cuts downward.4.7 Core-Barrel LengthA samplers core barrel should beslightly longer than L, the longest core that can be collectedwithout causing significant compaction. L and Ds(see Fig. 7)set the core-length factor defined as Lf
42、= L/Ds. Lfshould be lessthan 5.0 (or possibly 10) for a sampler used in cohesivesediments, but Lfshould be less than 10 (or possibly 20) for asampler used in cohesionless sediments.The constant factors 5,10, and 20 apply to slow-penetrating, open-barrel samplers.Studies suggest that all of these fac
43、tors can be increased byraising the samplers penetration speed or using a pistonsampler instead of an open-barrel sampler.4.8 Barrel SurfacesAll surfaces contacting the coreshould be smooth and free of protruding edges to reduceinternal friction and minimize core distortion. The surfacesshould also
44、be clean and chemically inert if the core is to beanalyzed for contaminants or if the core is to be stored in itsliner for long periods of time.4.9 Chemical Composition of Sampler PartsSamplerparts must not contain substances that interfere with chemicalanalysis of the cores. For example, barrels, p
45、istons, and corecatchers made of plastic should not be used if tests includephthalate concentrations. Misleading data will result fromplasticizer contamination of the sediments.5. Open-Barrel Samplers Versus Piston Samplers5.1 Users sometimes face difficult decisions in choosingbetween an open-barre
46、l sampler and a piston sampler. Thedecision frequently depends not only upon characteristics ofthe two samplers but also upon other factors such as hoisting-equipment capabilities, working platform stability, waterdepth, operator experience, and the purpose for collecting thecores.This section cover
47、s factors to consider before making thefinal choice.5.2 Depth of PenetrationMost open-barrel samplers andmost piston samplers rely on momentum to drive their barrelsinto sediment deposits. Momentum-driven samplers are re-leased at a predetermined point so as to acquire momentumwhile falling toward t
48、he bed. A momentum-driven pistonsampler generally penetrates deeper than a momentum-drivenopen-barrel sampler provided the two samplers have equalweights, equal barrel-diameters, and equal fall-distances (2).5.3 Core CompactionWhen compared under equal testconditions (see 5.2), a piston sampler caus
49、es less core com-paction than an open-barrel sampler. However, the piston mustbe held motionless at the bed-surface elevation while the barrelpenetrates downward. If the piston is allowed to shift downwith the barrel, the core undergoes serious compaction.5.4 Flow-in DistortionFlow-in distortion is caused bysuction at the entrance of a sampler. Sediment is sucked intothe barrel instead of being severed and encircles by the cuttingedge. Flow-in rarely occurs with open-barrel samplers; how-ever, it can be a problem with piston samplers
