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-.i I . Technical Note N-1082 i<l OJETTED-IN MARINE ANCHORS By , JG H. S. Stevenson and W. A. Venezia February 1970 this document has been approved ior public ~release and sale; its distribution is unlimited. NAVAL CIVIL ENGINEERING LABORATORY Port Hueneme, California 93041 M i o h bReproduced by the V' EARINGHOUSE for Federal Scientific & Te ni"c information Springfield Va. 22151 $ I I ~~i1 JTD-IN MARINE ANCHORS I Technical Note N- 082 ~'' fBy LTJG H. S. Stevenson W. A. Venezia Twenty-three lightweight anchors consisting of a 10-foot X 2k-inch pipe with a metal cone welded at the tip end were emplaced and tested in twenty-five feet of water at Lameshur Bay, St. John Island, U. S. Virgin Islands. The anchors were jetted into the coral sand bottom by forcivg water through the pipe and out an aperture in the cone. Two divers guided the anchors into the sediment using the jet of water to exca~ate the soil beneath the cone. Four of the anchors were emplaced with a cement slurry to increase the holding- power. I I It was found that the emplacement procedures were straightforward 11 and posed no problems to the divers. However, the injection of the cement slurry was very time-consuming due to numerous problems. The pullout results and theoretical analysis showed the jetted I anchors to be capable of developing 2,000 to 10,000 lbs holding capaci- ties in the soil at the tes~t site. The holding capacity may be increased by increasing any of the following: anchor cone diameter (area), emplacement depth, compaction of the overburden sediment, or use of a cement slurry. AI Tests indicate that the use of these light weight jetted-in anchors may be of practical use where bottom tie-downs and lght anchor- r ages are required. Further testing is recommended. SThi's document h been approved for public release a saloe; itsdistribution Is e I in ~eny-£ve eetof ate atLamshu Ba, S. Jhn slad, . S VignIlns heacoswr ete notecra adbto y orin aethogthpieadotaapruenthcoe To diesgie h nhr noteseietuigteJto ae o exaaetesolbnahte oeoro teacoswreepae- TABLE OF CONTENTS Page No. ABSTRACT'. i INTRODUCTION .... ...... .. .. .. .... ..... 1 TEST PROGRAM AND PROCEDURES . . . . . . . ....... 3 Emplacement3 Cementlu y Inj to . . . ... . . . . . . . . . . . 3 Cement SlurryIjeto...............3 Extraction ....................... 8 Observations ............. ............ 8 OPERATIONAL ANALYSIS . ..... ............... 8 Emplacement .................... . 8 Ce"Ut Slurry Injection ..................... .... 11 SOIL PROPERTIES . . . . . ................... 11 Summary . . . . . . . . . ............... 11 Hechanical Analysis ...... ................... ... 11 Density Analysis . .............. . . . . . . 11 Direct Shear Tests . . . . . . . . ... . . . 12 Introductfon................14 THOETICAL STUDY OFANCOR PULL-OUT.....................14 D4d ,ihtTheory .............. 14 Cylindrical Failure Surface Theory ........... 14 Torical Slip Surface Theory ....... ........ 18 ANCHOR PULL-OUT REULTS ........................ 20 Summry.. . .. .......... ......... . 20 UnlurriodAnchors ............. 20 Ceenit Slurried Anch r........... . . 25 FIDfIGS s CNCUSONI. ........ .............. 25 RECO4 ATIONS. ........ . . . . . ........... 26 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . 28 ACKNOwr EDG14NTS . . . . . . . . . . . . . . . . . . . . . . . 29 APPENDICES1 A.- Laboratory Soil Tasts A-I through A-17 . -ield Reults B-1 through B-5 C., Theoretical Analya-,.s C-1 pz- -+ ill INTRODUCTION Recent increases in underwater activity and increasing military and industrial requirements for divers to emplace and construct structures on the seafloor have generated new requirements for diver tools and work systems. One &such tool requirement in for an inexpensive, easily em- placed, diver-handled anchor. Such anchors should be capable of sustain- ing vertical loads from 2,000 to 10,000 pounds. Uzes of such anchors include pipe and cable tie-downs, instrument package tie-downs, tie- downs and pulling points for underwater construction and moorings. After completion of Project TEKTITE, one concept for emplaceect of such light-capacity, diver-emplaced anchors was evaluated by the TEKTITE Seabee Construction Divers. The test anchor consisted of a 10-foot pipe" with a sheet metal cone welded at one end, as shown in Figure 1. They were "Jetted" into the seafloor by a stream of water through the pipe (anchor shaft) avd out the nozzle apex of the cone. As the water jet excavated the soil beneath the cone, two divers guided the anchor into. the bottom while holding the shaft in a vertical position. The tests were- conducted at Greater Lameshur Bay, St. John Island[ in the U. S. Virgin Islands in a coral sand bottom at a depth of 25 feet. Twenty-three anchors were jetted in and tested during the program. In an effort to increase the holding power, four of thene were emplaced with a cement slurry injected at the cone. The jet-in anchors were constructed inexpensively by SEABEE steel workers. The objectives of this experiment were as follows: 1. To develop, acquire data for, and validate a standard pro- cedure for diver emplacement of an anchor on the ocean floor by use of a water jet. 2. To invect-itate the use of a cemeat slurry to secure the anchor to the ocean flo>r material in a more peraanent emplecement. 3. To investigate the holding properties of the indigenous coral send. In addition, the parameters of cone diameter (anchor area), cone angle and jetting nozzle size were vart.ed in order to investigate their effect on the ease of emplacement and the holding power. ~ j 4--FIRE HOSE CONNECTION I i 10 ft. X 2h in. Pipe j JSHEET METAL ANCHOR CONE ONE ANGLE 1 6 to iS In. Dia. - 450 JETTING NOZZLE______ I in. Dia. FIGUR 1. ILLSTRATION OF JETTE ANCHOR iI EMPLACEMENT AND TESTING PROCEDURES Emplacement The twenty-three anchors emplaced during the test program were in a plot 40-ft X 40-ft on a flat bottom in 25 feet of water. The plot was divided into 10-ft X 10-ft sections and stakes were driven at the corners. Thus, 25 anchor sites were laid out within the test plot. The equipment used to emplace the anchors consisted of the fully equipped (SCUBA and shallow water gear) diving barge moored at the site, a standard P-250 Gas Driven Fire Pump, 2 -in. fire hose, hose adaptor, safety strap, signal line, and the anchors themselves. The anchors were jetted by two divers to a depth of 6 to 9 feet into the r'and bottom. The divers were equipped with SCUBA or lightweight surface-supplied life support equipmet according to their own prefer- ence. The SCUBA divers were essentially neutrally buoyant and wore fins, whereas the surface-supplied divers used from 10 to 20 lb. weight belts and wore no fins The jetting procedure is shown in Figure 2. Cement Slurry Injection The purpose of these tests was to determine the effect of a cement slurry injected either beneath the anchor cone or just abbve it in order to bind the anchor more permanently into the surrounding sediment. A cement slurry made of one part Portland cement, one part sand, and one part water was injected on four of the anchors emplaced at the test site. These were 9" in diameter. The equipment used, ii addition to that used to jet the anchor, consisted of a slurry pot (see Figure 3), an L. P. air compressor, hose, assorted pipe fittings and valves. The procedure consisted of mixing the slurry, filling the pot with approximately 16 gallons of mix, sealing it, and injectifig the slurry by pressurizing the pot to 35 psi (24 psi over bottom pressure) with air and opening the gate valve at the bottom of the pot. For 'o of the four anchors slurried, the slurry was injected thrbugh te anchor shaft and out the tip of the cone. The other two were slurried through a sepa- rate probe placing the cement just above the anchor cone. Some problems were encountered during the slurrying operation. The fixst anchor that was slurried through the anchor shaft was done with the .ilurry pot attached to the top of the anchor, then jetted as shown in Figure 4. Although this setup was satisfactory for injecting the slurry$, it* was found to be unwieldy due to 'the bulkiness of the pot, and because its buoyancy changed drastically as the slurry was forced out. This procedure also proved to be very costly in terms of use of diver bottom time. Therefore, the other three anchors were slurried by keep- ing the pot on the diving barge and pumping the slurry to the anchor through a fire hose.f 3 IJETTING OPERATION: Figure 2. AllI anchors were jetted by two divers. A water let was used to emplace the anchors six feet to nine feet into the sediment. Equipm. ent used and jetting sequence is depicted below. firehose to surface =, 1arid P-250 portable adptrsafety fire pump (I ) Divers connect hose and diver'ssafety strap, and erect anchor. air Diver signals surface with signal line. "Start pump" dive (2) 'Water jet starts. Divers start pushing. anchor into (3) Anchor jetted about half- way. Water flowing out of 'hole carries soil and builck mound around hole. Same soil stays in 4 JETTING OPERATION Figure 2. Continued. I (4) Anchor 6' to 9' down. J "~~~ S t o p p u m p " i e (5) Divers disconnect hose and refill hole with soil. (6) Diver vibrates-loosi soil with 10 raps from an 8 lb. hammer. ii 5$ I 11 I I ~1. *1I I II L~ I 4 3 V Figure 3. Slurry pot. II (7 6 4 ___________________ __________ _________ ____________I -S. ';- - V '- - - . . . - -- - - -, 'H - - - - - ~-r~~C------t--~<t--- -- - 'I r L 7I - '4 I Figure 4. Diver j~tting anchor with slurry pot attached. - I II I ] AMother problem encountered while injecting the slurry was that of the mix "setting up" in the pot prior to injection. This occurred twice, once necessitating that a hole be cut in the side of the pot, the con- crete chunks removed, and the pot welded together again. This problem was finally traced to the fact that the not had not been completely cleaned after the previous operation. The slurrying effort placed approximately two cubic feet of mix beneath the anchor cone on two of the anchors, approximately two cubic feet above the cone on one anchor, and about one cubic foot of min above the cone on the fourth anchor. Extraction One week after all anchors had been emplaced, the anchors were tested as described in Figure 5. During the pulling operations, it was sometimes necessary to connect two or three chain fall come-alongs in parallel since each com-along was rated at 3,000 lbs and transient loads up to 10,000 lbe were encountered. In addition, in-situ soil samples were obtained and returned to the Naval Civil Engineering Laboratory Iwhere the soil properties were determined. Results of all tests are presented and discussed later in the text. Observations :The data obtained from the experiment were used in determining the easrn of emplacement of the anchors, the holding power of the anchors and the soil characteristics at the site. The basic data recorded at the ": site included: 1. The set-up time, jetting time, brerikdown time* for each i Ianchor, and slurry injection time for the four anchors. 2. The overall time and man-hours required to emplace 15 of the anchors. 1 3. Emoiacement problem. 4. Load vs. displacement for each anchor as it was extracted. OPERATIONAL ANALYSIS Emplacement During the test operations, attention was paid to the ease of emplace- mernt, the jetting procedures and the equipuent used in order to develop ATime to disconnect emplacement. equipment and be ready for next mplacemt. 1It k V F 5. - ~ - EXTRACTION OPERATION: Figure 5. The exiraction and data gathering procedure is described and drpicted below. pulling0- 10 kip (1 ) Divers set up pulling frame tensioneter frame tensiometer, come- a-long and lifting eye over 0 -chain fall anchor as shown. ft.come-a-long Observer marks anchor shaft at sand line. -- ulling eye on , anchor shaft mark on anchor shaft 77 at sand line 6 ft-.- Sft divers anchor- 7- 9 ft. pulling anchor I*8 observer with data o board and measuring stick (2) Divers pull anchor using come-a-long. Observer monitors and records tensiometer and displacement reading approx. once a minute. To take readings, observer stops divers cranking come-a-long, swims to anchor and measures displacement with ex- pandable scale, waits for tensiometer I. V isplacement reading to stabilize (creep down and . measurement stop), andrecords displacement, load A (mark on shaft and time. Observer signals to continue ' to sand line) pulling. This process is repeated until anchor cons breaks out of sediment. *1 9 ....... and validate a standard procedure for the use of jetted anchors. An accounting was made of the time required for each operation of the em- placement sequence. Mean time for each operation and the overall emplacement time for each of the 23 anchors emplaced are given below: Operation Mean Bottom Time (Min) Set-up 3.0 Min Actual Jetting 2.0 Breakdown 1.8 Min Fill-in and compact time 4.3 Min Mean Overall Emplacement Time . .. . 11.1 Min For one series of tests, the diving barge was on station a total of 2 hours and 58 minutes, during which time 15 anchors were emplaced. Thus, the time per anchor was approximately 11,9 minutes. This figure compares reasonably well with a mean overall emplacement time of 11.1 minutes, and indicates that a minimal amount of on-site topside prepara- tion was necessary. No serious difficulties were encountered during the emplacement operations. The actual operation was carried out by eix men. Two divers and one man operating the pump and tending the fire hose were the primary opera- tors; one diving supervisor and a tender for each surface-supplied diver were required backup personnel. Combining this information with the mean time on site per anchor (11.9 minutes) yields 1.2 man hour per anchor using the six-man crew. The economics of emplacing the anchors In the present case (using an experienced crew) included: 1. Fabrication Less than $10.00 2. Transportation Non-fragile, lightweight 3. Emplacement 1.2 m/hr/anchor for an operating crew of six men. To reduce the on-site time requirement per anchor and, in particu-. lar, the diver bottom time requirement, the fill-in and compaction time appears to be the easiest to reduce. Given the proper earth moving hand tools and vibration equipment, this job could probably be done in less thar. 3 minutes. Further time reduction could be obtained by using quick release fittings at the top of the anchor and on the hose supplying the water pressure.. This could reduce the set-up and breakdown times to about one minute each Thus, with minimal procedural changes, the diver bottom tiie per anchor could be reduced to about 7 minutes. This savings would be particularly noticeable when working at depths where the diver 10 'I€ can only stay a short time without making decompression stops during ascent. An example would be an operation in 120 feet of water where the divers have only 15 minutes lo complete their work and still be within the no-decompression limits. Cement Slurry Operations Aa discussed earlier, many problems were encountered during the slurrying operations. As a result of these problems, and due to the fact that during this phase of the experiment many outside interrup- tions of the work were necessary, actual slurry time data were not obtained. However, the effort involved indicates that additional design and testing will be required to develop acceptable, inexpensive slurrying equipment and procedures. Future testing should include a trade-off comparison of the effort involved in placing the slurry to the increase in holding pover attributed to the concrete. SOIL PROPERTIES Several small grab-samples of bottom material were taken frow the test site for laboratory testing. The test site soil consisted of par- tially cemented coarse sand and fine gravel, both derived from coral material. The sand particles were sub-angular to angular in f-rm. Numerous shell fragments were also present. Grain size, density and direct shear analyses were performed on the samples obtained. Mechanical Analysis Following mechanical analysis, the representative sample of the bottom material was classified as a poorly graded sand with Some large I pieces of broken shell and marine rubble. In addition, the mechani- cal analysis showed that there was little or no crushing during the dirict shear tests. This was verified by a standard grain size analy- sis run before and after each shear test. The results of a typical analysis are in Appendix A, Figures A-1 and A-2. Density Analysis To evaluate the range of densities possible in the field, tests were performed to determine maximum and minimum void ratios. The following qualitative definitions are made: Very loose - the minimum density obtainable in the lab using a technique of submerged sedimentation. (See Appendix A.) Loose-- the minimum density ob'tainable in the lab using oven- } ~~ dried soiples. Dense - the sample-underwent three minutes of vibratory packing usirg a load of about four psi. H1 ~ <~.ZC4*orm-- ~ Direct shear tests were run on only the dense and loose samples; the relatively large amount of difficulty involved in preparing samples by submerged sedimentation made it impractical to rut, direct shear tests on vety loose specimens. In each case, three densities were measured: Initial - Density before normal load was applied. Loaded - Density after normal load had been applied. Relaxed - Density at end of direct shear test after the shear load (but not the normal load) had been removed. Quantitative measurements of these procedures and values used are shown in Appendix A, pages A-4 and A-5. Direct Shear Tests The direct shear test procedure was standard2 with the exception that the test samples were oven-dried before the test was performed. A strain-controlled loading unit was chosen so that ultimate resistance and a better measure of the peak resistance could be obtained. Rela- tively large normal loads were used in order to offset the effects of inherent friction within the test apparatus. A shear displacement rate of approximately .015 in/ain was used. Average soil properties for the various test samples are shown below: AVERAGE SOIL PROPERTIES DENSE LOOSE VERY LOOSE Dry Density (lb/Ft ) 93. 82. 76. Porosity 0.46 0.52 0.56 Void Ratio 0.86 1.10 1.28 Buoyant Unit Weight (lb/Ft3 ) 59. 52. 48. Angle of Internal Friction 430 370 350* *This value was assumed since no direct shear tests were run on the very loose samples. Shear load and volume change versus shear displacement for each test were measured and the results are shown in Figures A-6 through A-8, Appendix A. For all of the shear tests performed, the ultimate and peak strengths were noted and a plot of shear stress versus normal stress was obtained, Figure 6. Peak angles of internal friction were determined by usinb -the best straight lit.e fit through the data (Figure 6), and assum- lag that the apparent cohesion depicted in the graphs was caused by 4L~i 12 Mohr Diagram 1600 Cf:1200 p800 0 Dense ult. El Dense pk. 400 Loose ult. 0 Loose pk. 0 0 Normal Stress (I/ft2) Figure 6. 1- 13 frictional resistance in the direct shear device. Peak angles at inter- nal friction were 430 and 370 for the dense and loose sand, respectively. THEORETICAL ANCHORAGE STUDY The resisting force or holding capacity of an anchor is due to the anchor weight and to the resistance to movatzent offered by the confining medium. Vesic 5 suggests that the failure patteras in the overburden soil, which greatly affect the holding capacity, depend on the relative depth of the anchor (depth of burial divided by projected area of anchor), the type of soil and its sensitivity. Several model studies of sand anchors have been corducted but few large-scale field tests have been per- formed. Kalajian4 did conduct a large-scale study on holding capacity of marine anchors in sand but no attempt ia; made to predict the holding capacity of similar anchors based on his experiments. When predicting anchor holding capacity, three failure mechanisms are normally considered. They are: dead weight theory, cylindrical failure surface theory, and the torical theory. Dead Weight Theory This theory is designed to predict the minimum holding capacity developed by a jetted anchor. It applies only when there is no friction developed between the backfilled cylinder above the cone and the undis- turbed soil. The theory assumes that the maximum anchor pullout capa- city (OQ) is numerically equal to the effective weight of the projected cylinder of sand above the anchor, that is; 1 max = Yb A .d Equation (1) where y - buoyant unit weight of soil, A projected anchor area, d a ori;1nal depth of embedment. A Cylindrical Failure Surface Theory ~This theory is particularly applicable for predicting the maximum resistance to breakout of an embedment anchor which is jetted into par- tially cemented or very dense granular materials, then backfilled with little or no densification of the backfill material. In-situ observations indicated that the cylinder of sand jetted out by the anchor upon emplacement, when backfilled, did not reach a density comparable to that of the surrounding sediment, Figure 7. Therefore, it -is reasonable to assume that the resistance to pullout developed by the anchor wa equal to the effective weight of the cylinder of sand above 14 loose backfill partially cemented Figure 7. Anchor after emplacement. 15 the anchor plus whatever friction was developed at the cylinder walls upon pullout. Figure 8 shows a simpler failure surfaca than described by Vosic (see Figure 9) for well-compacted soil but it seems reasonable in the present case. Followin is the development of the cylindrical failure surface I theory. Definition of Terms: J -maximum anchor pullout capacity j A - projected area of anchor[F] K - coefficient of lateral earth pressure at rest 0 d - depth of embedment of anchor Ert a: - vertical stresa [b/Ft2] a - horizontal stress [1b/Ft B - cone diameter [ 'tl 0 - angle of internal friction p - perimeter of cone at base [FtJ dz - differential derth measurement [FtJ Derivation: Assu x - weight term and friction term where; friction term-do () • tan () • p dZ and the weight term- Yb A d Then rd OUSx',bon (s) tans (z) .p dz as first approximatI assume:. (a) 0 function of depth -: ~ ~ ~~~(b ) n " l o • b " S(c) go arching of the soil 1' appled load 1 .. . .A weight of sand column Fiur 8 . irclfiuesfa. '.* I" 2 . *.I" wall shear ". resistance b I ° I* .. .' I .' " I: if. Figure 8. Cylindrical failure surface. (Free body diagram) 2 i!17 Then yI Q a i A d + JK Yb ptan z dz mx b - A * d + K° y p " tan 2 Equation (2) Torical Slip Surface Theory It is suggested by Vesic5 that the assumption of a torical slip sur- face, Figure 9, will yield the maxiim possible effective resistance of the involved soil mass. This assumption is based on observations in small-scale model tests with anchor plates and anchor piles at Duke University. It is noted that this shape occurs only in the fase of relatively shallow anchors in dense sand or stiff silt; clay". It is evident that the difference between the soil weight for an assumiz cylindrical slip surface is small for small diameter objects at shallow depths, but may be very significant for circular objects at greater depth. The predictins for a cohesionless material, based on this theory, follow: Sax "yb A"d*Nq Equation (3) where A, d, and hive previously been defined and N is a theoretical breakout factor tD.' may be obtained from Figure C-i, A4pendix C. The values obtainted from Figure C-1 should be adjusted according to the following pro adure: For very loose sanL d/B 2: Use N at that d/B q d/B > 2: Use N at d/B 2 q *For loose sand dN<4: Use N at that d/B q d/B>4: UseNq at d/B = 4 For dense sand A' 10: Use N at that d/B d/B >1:UseN atd/I 10 *This value was assumed. H I .-'---.- S 4 - -I 5- - I NI - S I I [ I U -0 Ge. 'A, :1 I IL. I- I I - I 19 -- 9 p ~ -~ ~ -- -~------------~---- _________________________________ lI This modification is necessary because available experimental evidence from experiments on 3" 0 plates suggests that the critical relative depth D/B above which embedded objects should behave as rhallow anchors depends upon the relative density of the soil. Thi. limiting depth increases from D/B 2 for a very loose sand to D/b = 10 for a dense sand (Vesic5). ANCHOR PULLOUT RESULTS The field tests indicated that increasing the cone diameter generally increased the anchor holding capacity. Varyfng the cone angle and nozzle size had no apparent effect on the anchor holding capacity. In addition, increased depth of burial increased the holding power, as would be expected. Those anchors that were jetted with a cement slurry generally had a higher holding capacity than those without the cement slurry. Unslurried Anchors Figures B-i to B-5, Appendix B, depict graphically the holding capa- city versus the depth of embedment for the unslurried anchors. An exami- nation of the figures shows that the anchors seemed to fail by two distinct mechanism. 1. The anchor displaced at almost constant load until break- Iout occurred. In this case, the pullout force required was small. 2. The sustained load which the anchor held inc:eased uni- formly to a maximum and then dropped off uniformly with increasing displacement. In this case, the required pullout load was somewhat higher. These two cases re illustrated in Figure 10. A comparison of this figure with Kalajian's data , Figure 11, which shows results of similar pullout tests in loose (relative density, Dr, < 40) and dense sand (Dr < 80), indicates that there is a definite correspondence between in-situ density and force-displacement curve shape. Therefore, it should be possible .o infer the in-situ relative densities from the shape of the field force-displacement curves. This inference has been made, as no measurements of in-situ density were possible. Data from the anchor tests and corresponding theoretical predictions of anchor holding capacity are presented in Table I. The qualitative descriptions of relat-ve density were inferred from the field force-dis- placement relationships, Equations (1), (2), (3) and the average soil properties determined during the direct shear tests were utilized in making the theo retical predictions indicated. E. . KalaJian and S. N. Imben conducted an investigation of the verti- cal pullout capacity of marine anchors embedded in seand by vibration. He also notes that there appears to be two mechanism cf failure within the soil uass. ~ 2D 0A -c c1 CC <1 G %00 0 C4~0 0l t W 0 N G 21 1 TABLE I ACTUAL AND THEORETICAL HOLDING CAPACITIES - - DEPTH STATIC SITE OF CONE Qmax Qmax Qmax qmaI RELATIVE* NUMBER BURIAL DIAMETER MEASURED CYL TORICAL DEW WT. DENSITY FEET INCHES POUNDS POUNDS POUNDS jtOUNDS S4 7.71 6 1100 993 735 79 Loose 5 8.17 6 600 1046 308 77 V, Loose 14 8.02 6 1000 1079[ 772 83 Loose is 8.04 05 2,300 !2979 I 4820 517 Loose 6 7.94 9 1900 1574 1710 184 Loose 7 7.90 9 1900 1553 1700 183 Loose 13 8.67 9 100 1815 740 185 V. uoose 25 8.63 9 800 1744 736 184 V. Loose 9 7.68 9 2800 1760 8200 200 Dense 11 7.28 9 2000 1600 7800 190 Dense 12 8.71 9 3300 2248 9120 228 Dense 16 7.67 9 900 1413 650 163 V. Loose 19** 8.54 9 3700 - - - 20** 8.50 9 7400 .. .. 23*** 8.96 9 2800 .. .. 24*** 8.75 9 4000 - 6.29 12 1800 1489 2410 259 Loose 6.88 12 1200 1743 2630 283 Loose 15 8.15 12 900 2249 I 1236 309 V. Loose 17 7.75 12 3300 2570 110,000 360 Dense 8 8.54 12 1600 2621 3264 351 Loose 10 8.00 12 800 2073 1210 303 V. Loose *Relative density inferrtd from general shape of holding power vs displacement curvr for each anchor I'.] **Cement slurry used under cone ***Cemnt slurry used over cone 22 41' Values of Ko used in the calculations were assumed as follows: Very loose K0 - .56 Loose K - .52 0 Dense K - .43 0 Explanation of Data From Table I, it is readily apparent that the dead weight approach to predict holding capacity is very conservative. This occurs because the soil is assumed to exhibit no shear strength which is in contradic- tion to the results obtained from the direct shear tests previously presented. Results of the Torical and Cylindrical analyses are also presented in Figure 12. Actual holding capacity is plotted against theoretical holding capacity. Ideally, the data should fall on a 450 line through the origin but it is clearly evident that this is not the ca&e. The results of the Torical solution for dense sand were much greater than the actual results and could not be plotted realistically with the reat of the data. A possible explanation is that the bacfil!-Id soil was not actually in a dense state and therefore the coefficient Iq used in hold- ing capacity calculations was much too large and resulted in much greater predicted than actual values of holding capacity. There are fallacies involved in using either the Cylindricse or Torical failure criteria for all values of d/B for this problem. The cylindrical failure theory assumes a general shear type failure at all values of d/B which is simply not the case. A localized or punching type failure occurs in very loose sand at d/B > 2 and in a dense sand at a d/B > 10 (Vesic The Torical failure theory for the particular boundary conditions imposed on the anchor is not applicable. Torical theory assumes that the soil through which the failure q irface will form is uniform. Because the soil is backfilled with very little control over its final density, it is doubtful whether the jetted out soil could be emplaced at its in-situ density. If the backfilled soil is denser than the surrounding soil, then its failure mode should be controlled by the surrounding soil. However, if the backfill is less dense than the surrounding soil, its failure mode should be controlled by the backfill. Since in-situ density was not determined, Torical theory should not be used. 23 3500 ~ 3000 _ __ I-- : 2500 . / I J 6 / ___" 1500 f- w ______ A-l Val zed data fit _00_ _Torical 7 0 Cylindrical 0 1 0 500 1000 1500 2O00 2500 3000 Actual Holding Capacity, Ib Figure I2 Theoretical ve-ss actual holding capoc;ty for wnbedment anchors. 24 24 Cement Slurried Anchors Tabulated also in Table I is the approximate depth of embedment versus the holding capacity for each anchor upon which the cement slurry was injected. The four anchors slurried showed markedly greater holding powers than the non-slurried anchors. The mean maximum sustained load of these 4 was 4,475 lbs whereas the corresponding mean for unslurried anchors of the same size was 1,838 lbs. Two of these anchors were slurried through the apex of the cone. After testing, traces of concrete were found adhering to the surface of the cone and concrete in the shaft was protruding from the tip of the cone which apparently had broken loose from the concrete under the anchor. The other two slurried anchors were slurried by injecting the mix- ture alongside the anchor shaft above the cone as discussed above. The slurry was injected approximately 2 feet above the top of the anchor cone and formed upward displacing the loose sediment above. Evidence of some bonding with the surrounding sediment structure was noted but the primary increase in holding power for this case is attributed to the increased friction surface between the anchor and the side of the anchor hole as the anchor was extracted. For the fourth anchor slurried, only about half the full slurry load was emplaced, as discussed earlier. This anchor exhibited increased holding power, although no cement was found adhering to the anchor after testing. The use of a cement slurry to form additional holding power for the jetted anchor was thus a limited success. The experimental evidence indicates increased holding pow-r due to slurry use; however, the small number of tests performed precludes the possibility of drawing qualita- tive conclusions. A more controlled experiment, using a large number of anchors and an adequately engineered emplacement procedure, is indicated in order to obtain conclusive evidence of the value of the cement slurry for increased holding power. FINDINGS AND CONCLUSIONS From cost and manhour requirement considerations, the jetted-in cone anchors tested here seem to be a feasible means of obtaining easily em- placed, light duty, bottom tie-downs and anchorages . The anchors required approximately 1.2 man hours each to emplace using a six man crew. Several means have been suggested which cculd reduce the em- placement time and, in particular, considerably reduce the diver bottom time required per anchor. In general, the anchors proved easy to handle and emplace. No special skills, other than diving, were required of the Seabee enlisted personnel which emplaced the.. 25 Results of the holding cepacity tests and the resulting analysis indicate that the jetted cone anchors can develop holding powers within the desired range of 2,000 to 10,000 lbs The primary deter- mining parameters of holding power are the anchor size, the depth to which the anchor is jetted and the compaction obtained during backfilling of the hole. The variation of cone angle or nozzle size showed no measurable effect on the holding capacity of the anchors. Correlation of actual with predicted values of holding capacity was not very good. Until better control of the backfill density is realized, it will be very difficult to predict holding capacity with any of the available theories. The attempt to increase the anchor holding power by injection of a cement slurry proved to be time consuming in terms of topside and diver manhours. Encouraging, although non-conclusive, results were obtained indicating the use of such slurry injection may be a most effective means of increasing the holding power of the anchors tested. The addition of the cement slurry may have increased the resistance to pullout by: (1) increasing the dead weight of the anchor; (2) increas- ing the projected area of the anchor; and (3) penetrating into the undisturbed soil. Mechanical and procedural difficulties during the injection of the slurry accounted for the excessive manhour requirement and limited the usefulness of the results. I RECOMMENDATIONS Further tests using the jetted anchor described here are recommended. these tests should be conducted in both sand and clay soils using a larger number of each size anchor tested. Emphasis should be placed on determination of the effects of anchor size, depth of emplacement and soil properties on the holding power. In addition, further design work is recommended to develop the equipment and procedures for injection of a cement slurry on the anchors. This should be followed by further testing of the concept of increasing the holding power with the slurry. Specific suggestions concerning the implementation of these recomenda- tions follow: 1. All experiments should consider diver human factors and the implementation of procedures which will reduce the effort required to emplace the anchors. 2. The backfill soil, if required as in the present case, should be compacted adequately. Either simple rodding or some form of hand vibrator device should be used. This should cause the haldin* capacity to approach the values predicted by the Torical theory explained above. Some attempt at compaction of the surrounding soil should be made in any case. 26 3. Measurements of in-situ soil density should be made before anchor pullout in order to facilitate mathematical analysis of the anchor failure mechanism. It is suggested that a cone penetrometer or similar device be used for this purpose. 4. Larger cone diameters should be tested and more variation in depth of burial should be used. This would bring the ratio of embedment depth/cone diameter down into a range covered more adequately by previous tests. 5. The use of flukes on the anchors which would open after emplace- ment should be invyestigated, 6. When cement slurry is used, procedures for obtaining accurate placemenc of the slurry must be instituted. Excavation around a few slurried anchors may be useful in determining by what mechanism the cement is increasing the holding power. 27 REFERENCES 1. U. S. Navy, NAVSHIPS 250-538, U. S. Navy Diving Manual, July 1963 2. Lambe, T. W., Soil Testing for Engineers, The Massachusetts Institute of Technology, 1951 3. Larbe, T. W., and Whitman, R. V., Soil Mechanics, Wiley, New York, N. Y., 1969 4. Kalajian, E. H., and Bemben, S. M., "The Vertical Pullout Capacity of Marine Anchors in Sand", Project Themis, University of Massachusetts, April 1969 5. Vesic, A. S., "Breakout Resistance of Objects Embedded in the Ocean Bottom", Naval Civil Engineering Laboratory, Report CR .69.031, May 1969 6. Esquirel-Diaz, P. F., Pullout Resistance of Deeply Buried Anchors in Sand, Thesis, Duke University, 1967 28j I! I 28 ACKNOWLEDGMENTS The authors gratefully acknowledge the work of Commander WJ. Eager of the Naval Facilities Engineering Command on this experiment. The jetted-in cone anchor concept, which originated at the Naval Electronics Laboratory, San Diego, California, was developed further by Commander Eager and he ran preliminary on,,site tests of several anchors. Commander Eager originated the concept of using a cement slurry to in- crease holding power and developed the static test apparatus used. He also developed the besic procedure for the experiment and set forth the idea of running a parametric study of the factors affecting the anchor holding power. Without his efforts before and during the testing, this experiment would not have been possible. Senior Chief Utilitiesman R. Miller provided much assistance in organizing and supervising the Seabee Divers who tested the anchors. His efforts aDd those of the Seabees who constructed and tested the anchors are greatly appreciated. The assistance of Messrs. H. G. Herrmann and Homa J. Lee of the Naval Civil Engineering Laboratory is gratefully acknowledged. They provided much needed advice and assistance with the soils testing and the use of the holding power prediction theories. 29 APPENDIX A LABORATORY SOIL TESTS 3D .LHOI3M AS WISMYOD INIDU~dS too0000 10%0 w.o S~ H. 1111I11M I .. E 8 001 a'0 09 *v W6 9. a ~ g ~ ~ cc S IH§W A~ NH IIfit IND13M AG WISWVO3 ±N90kf3d * I* I I N h ojr to M E N w 1L0 IiL T- I Ci! j A a 2 LABORATORY DETERMINATION OF VERY LOOSE DENSITY A. PROCEDURE A cylindrical glass container of known dimensions was partially filled with fresh water. The weight of the water and container was noted. Dry sand was then carefully spooned into the cylinder, and the volume of the sand; and the combined weight of the sand, water, and container were noted. B. DATA Diameter of cylinder .734 inches weight of weight of cylinder, cylinder & water water & sand Height of 3and 66.11 gm 81.62 gm 1 53/64 In 71.04 gm 82.96 gr. 1 34/64 in 70.45 gm 103.19 gm 3 57/64 in C. CALCULATIONS density - weight/volume (wt of sand [gm]) (.002205 l ] (height of sand [in. ) (dia. of cyl. [in.] 2 72 i n3 • " 1728 3i 0. RESULTS Density [ib/Ft3j 75.96 75.54, 75.34 ! ~33 r CALCULATION OF DRY DENSITY, POROSITY, VOID RATIO, AND BUOYANT UNIT WEIGHT A. PROCEDURE During each shear test, three volumes were noted: the initial volume, relaxed volume, and loaded volume.. At the end of each test, the test sample was weighed using only the sand that remained in the test cylinder. B. CALCULATIONS ' wviuht of sample 3] DRY DENSITY d = wolune of sample ~Yd POROSITY (n) I 1 -- Gsyw n VOID RATIO (e) - BUOYANT UNIT WEIGHT s- 1 1 b/Ft WHERE = density of water G 3 specificty gravity of sample (2.76) (2.76) Is 34 K TABLE A-i DIRECT SHEAR AND DENSITY ANALYSIS RESULTS # 2 NORMAL 2 NORMAL 2 NORMAL 244 /Ft LOAD 7710/Ft LOAD 1484#/Ft2 LOAD DENSE LOOSE DENSE LOOSE DENSE LOOSE DENSITY 1 3 I lb/Ft 3 INITIAL 95.44 78.24 90.46 80.24 89.18 80.81 LOADED 95.45 78.39 92.53 81.21 90.14 87.41 RELAXED 89.57 78.49 91.02 80.5 86.57 81.28 POROSITY I INITIAL .4458 .5457 .4748 .5340 .4822 .5308 LOADED .4458 .5448 .4628 .5285 .4766 .4925 RELAXED .4799 .5443 .4715 ,5326 .4973 .5281 VOID RATIO INITIAL .8044 1.2012 .9040 1.1459 .9312 1.1312 LOADED .8044 1.1968 .8615 1.1209 .9106 .97044 RELAXED .9227 1.1944. .8921 1.1394 .9892 1.1191 PK. STRENGTH lb 20.6 10.8 43.2 30.0 60.5 44.1 PK. STRESS lb/Ft2 585.4 306.9 L228. 852.6 1719. 1253. UL. STRENGTH lb 11.6 10.4 25.0 25.0 44.0 43.6 UL. STRESS lb/Ft 2 330. 296. 710. 710. 1250. 1239. 35 600 Direct shear tests 245 lb/ft2 normal fod 0Dense 6~Loose 300 ii 100 8.0 t~4.0 0.0 -2.0 I 0.0 0.1 0.2 Shear Displacement OIn.) 36 1300 4 Direct shear test 771 lb/ft2 normal load 1100 0 Dense 1100 0 I .0 SO J 00 W1 8.0 1 6.04 -24.0 0.0 I0.1 0. Shear Displacement (in.) L 37A Direct shear tests 600 1400 C 6. . . . 400 f 8.0 6.0 -2.0 0 S. 0.1 0. i2 SerDisplacement (in.) 38 APPENDIX B PULL-OUT RESULTS d I Il I 39 $! S 04) '00 40' 1'41 El 0 1 0 *0 CD 0 00 I...D $1 00 (91) Io~ssB nl ii 42 06 -- _ I 044 4 0,I 7 Io _ _ - I 4 1_0 4 44 APPENDIX C PULL-OUT FACTORS CHART 'I *11 %I $** -% IN, 0 . ~~ ~ C ,•" %, AA S*,A % ,,4 -.\ .. * , 4, ,4, 4 , 4. ") % " % % 0ens. Sond' ID I % %| i% 6 - 7 9.0 20 40 O GO 80' 00 1 FACTOR Nq !i Anchor Breakthrough Factors Nq ?46 4.4 . Unclaosified -A St.cunty Classification DOCUMENT CONTROL DATA. R & D S'rurity classhte alto, ol tilll, body .,t aib ltur , ,,,,I.d. . ,fi,, ti,ol , ,n .,t .1 t,, Iti.,e t d h, :thin Ili' c,.,ll rt.p ,t f. , ! s i lI) I ONIGINA TING AC TIVI TV (Corporate A41tghor) 20. HE '-R - SEJRI TY C a 5 1 It . ATI.):I Naval Civil Engineering Laboratory Unclassified Port Hueneme, California 93041 2,. couP LTJG H. S. Stevenson and W. A. Venezia (Authors) 3 REPORT TITLE JETTED-IN MARINE ANCHORS 4 oCSCOIPTIVE NOTES (Type ol report and Inchisive dates) Final S AU THOIQ(SI (First name. middle initial, last name) LTG He S. Stevet.son (Principal) W. A. Venezia 6 REPORT OATE i. TOTAL NO OF PAGES 17b. NO. OF RCFS February 1970 156 6 _ 8. CONTRACT OR GRANT NO Ga. ORIGINATOR'S REPORT NUMBI PIS) , ! b. PROJECT NO YF 38.534.006.01.001 __TN-1082 C. hb. OTHER REPORT NOIS) (Aniy other number, that may be assigned this report) ~d. 10 OlSIRIBUTIOth4 STATEMENT This document has been approved for public release and sale; its distribution is unlimited. I SUPPLEMENTARY NOTES 12. SPONSORING .ILITARY ACTIVITY Naval Facilities Engineering Command I. Z ASTRACT -Twenty-three lightweight anchors consisting of a 10-foot X 2! -inch pipe with a metal cone welded at the tip end were emplaced and tested in twenty-five feet of water at Lameshur Bay, St. John, U. S. Virgin Islands. The anchors were jetted into the coral sand bottom by forcing water through the pipe and out an aperture in the cone. Two divers guided the anchors into the sediment using the jet of water to excavate the soil beneath the cone. Fcur of the anchors were emplaced with a cement slurry to increase the holding power. It was found that the emplacement procedures were straightforward and posed no problems to the divers. However, the injection of the cement slurry was very time- consuming due to numerous problems. The pullout results and theoretical analysis showed the jetted anchors to be capable of developing 2,000 to 10,000 lbs holding capacities in the soil at the test site. The holding capacity may be increased by increasing any of the following: anchor cone diameter (area), empiec-ment depth, compaction of the overburden sediment, or use of a cement slurry. "'es--s indicate that the use of these light weight jetted-in anchors may be of practical use where bottom tie-downs and light anchorages are required. Further testing is recommended. FOR (PAGE 1) Unclassified SA 0 I01. 87l- 680 1 Securttv C|l'ins-fication Unclassified Security Classification -4 KEY WORDS I.NK A LCNK D INK C ROLE W ROLE WT ROLr WT Anchors (structures) Foundations Jetting - Ocean bottom I i ! Placing j 1 Marine sediments I I Slurries Loads (forces) Performance tests JII I-I I ; I I a: I i4 ! DD ,.o,.1473 (BACK) Unclassif ied (PAGE 2) -Security Ciaq-til'ction 52 "I.