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Home My WebLink About; Buena Vista Lagoon Geologic Inv & Soils Analysis; Buena Vista Lagoon Geologic Inv & Soils Analysis; 1982-01-01CALIFORNIA DEPARTMENT OF TRANSPORTATION TRANSPORTATION LABORATORY MATERIALS AND RESEARCH ENGINEERING GEOLOGY BRANCH :i GEOLOGIC INVESTIGATION AND * . SOIL ANALYSIS OF BUENA VISTA LAGOON OCEANSIDE, SAN DIEGO COUNTY CALIFORNIA DEPARTMENT OF FISH AND GAME SACRAMENTO, CALIFORNIA JANUARY, 1982 1 JAN 15 1982 Mr. Charles Tobi Chief Engineer, California Department of Fish and Game Sacramento, California Sir: Submitted for your consideration is a Geologic Report for Buena Vista Lagoon in Oceanside, California. Included in this report is a soil analysis of the recent'lagoon sediments, turbidity and spoil area problems and recommendations for cut slopes. W. C. CAIN Associate Engineering Geologist Registered Geologist 2497 Certified Engineering Geologist 732 WCC: bs cc: GEOLOGIC INVESTIGATION AND SOIL ANALYSIS OF BUENA VISTA LAGOON At the request of the California Department of Fish and Game, a geologic investigation of the recent sediments in the eastern portion of Buena Vista Lagoon was completed in November, 1981. This study was restricted to that portion of the lagoon east of Route 5 and was undertaken to determine the engineering properties of the in situ sediments prior to a proposed dredging of the lagoon. One rotary boring, fourteen 2 inch sampler-probes and seven hand dug test pits were used to obtain the subsurface information depicted on the Log of Test Borings. In addition to the samples taken to determine the engineering properties of the sediments, sixteen samples were taken and tested by a Department of Fish and Game Chemist for pesticide and heavy metal concentrations. SETTING Buena Vista Lagoon prior to 1940 was a tidal-brackish water estuary fed by an intermittent stream called Buena Vista Creek. The sediments being deposited in the estuary at this time, were predominantly fine sands. During the early 1940's a weir was built across the mouth of the estuary. The weir was designed to maintain a constant level of water behind it, and formed the lagoon as it .exists today. The weir installation and subsequent formation of the lagoon created a different depositional environment. As a stream diffuses into a body of standing water, it loses energy and deposits. its load. The loss of energy is gradual"*and the sediments being carried by the stream are deposited in an orderly gradational arrangement. The particles itx the bed load are deposited in order of decreasing size and weight, sand first, then silt and finally clay. A layer of sediments resulting from any one storm is sorted, grading from sand near the stream mouth to finer sediments (silt and clay) as the distance from the source (stream mouth) increases. Geologic Investigation and Soil ^ttalysis of Bueaa Vista Lagoon Page 2 Jtt March 1951, a foundation investigation for the proposed bridge across the lagoon (Route 5), Bridge No. 57-277, was conducted. The Log of Test Borings for 'S&iicture (in the appendix) indicated that since the weir "%on$traet ion, the ts being deposited at the bridge site were predominately organic silts and that the lagoon had filled to elevation +1.5. Recent measMrjHaents near this same location indicate that silty clay has continued to be deposited and the lagoon, is presently filled to an average elevation of + 3.6 feet. In the, last four years (based on aerial photographs provided by -the Department of Fish and Game), severe storms have caused .the normally silt ahd clay laden BuenafVista Creek to deposit layers of sand on top of the silty clay lagoonal deposit. This deposition of sand has been restricted to upper ^eastern) 1/3 of the lagoon and varies in thickness from just a few inches to as much as 2.5 feet. ENGINEERING PROPERTIES Basically, the sediments in the lagoon can be divided into two broad catagories, the silt-sand fraction and the silt-clay fraction. Fraction The silt-sand fraction of the lagoon sediments occur predominantly in the upper 1/3 of the lagoon and are the result of the heavy rains of the last few years. The typical silt-sand sample (average) contains 70% fine sand, 27% silt and 3% clay and can, be classified as a silty fine sand (FAA Textural Classification of Soils^) . A grading analysis of the silt-sand fraction utilizing four separate samples is as follows: Geologic Investigation and Soil Analsis of Buena Vista Lagoon Page 3 Gradation (Unified Soil Classification System) Percent by Weight Soil Samples t • •Sieve Size Coarse sand 4-10 Medium sand 10-40 Fine sand 40 - 200 Silt .074mm to .005mm Clay ,005mm to .00/mm Combined Percent Passing Sieve Size 8 16 30 50 100 200 270 B-l 1% 18% 45% 29% 7% B-19 0 2% 62% 34% 2% Soil B-l 99% 96% 88% 74% 54% 36% 33% B-20 0 10% 75% 14% , 1% Samples B-19 100% 100% 99% 96% 89% 36% 24% B-16 0 5% 61% *32% 2% B-20 100% 99% 97% 84% 40% 15% 12% B-16 100% 100% 99% 90?o 64% 34% 30% Geologic Investigation and Soil Analysis of Buena Vista Lagoon Page 4 In situ soil (silt-sand) samples have measurable moisture contents varying between 10.5% and 19.9%'and unit weights that vary from 110 lbs/ft3 to 123 lbs/ft . This material would make excellent embankmenc material and could be easily compacted. Silt-Clav Fraction The silt-clay fraction -underlies the silt-sand fraction in the upper end of the lagoon and extends westward forming the bottom of the lagoon. This unit has a thickness ranging from about 6.5 feet (near highway bridge) to as much as 2 feet (easterly end of the lagoon), and overlies fine sands containing numerous pelecypods and gastrapods. This silt-clay fraction of the lagoon sediments is highly plastic, has a very low •bearing strength and a shear strength varying from 140 p.s.f. to 1000 p.s.f. The moisture content of it is highly variable and is dependent on where and what depth it is taken. The moisture content varies from 50% to 110%. The samples taken from the western portion of the study area were primarily from the interface zone between the water and the soil and had extremely high water contents (above 90%). Some difficulty was encountered in obtaining samples in this area, because of the liquid nature of the material which caused it to flow out of the sample barrel. Several probes were often necessary to obtain adequate amounts of sample for testing. 3The average unit weight for a sample of this silty clay is 91 lbs/ft . A grading analysis for this silt-clay contains 11% sand, 56% silt and 33% clay. Based on the FAA Textural Classification of Soils, this material would be classified as silty clay., A grading of this silt-clay fraction is as follows: Geologic Investigation and Soil Analysis of Buena Vista Lagoon Page 5 Gradation (Unified Soil Classification System) Percent by Weight Coarse sand Medium sand Fine sand Silt Clay Combined Percent Passing (Sample B-18) Seive Size 4-10 10 - 40 40 - 200 .074mm to .005mm .005mm to ,001mm Seive Size 8 16 30 50 100 200 270 0% 1% 10% 56% 33% 100% 100% 99% 98% 94% 89% AtterburgJLimit s The Atterberg System for classifying silty and clayey soils is the most commont «i\test used on soils that are going to be used for structural purposes. These tests are based on the physical states (liquid, plastic and solid states) that a silt-clay rich soil undergoes with a change in water content. . The liquid limit, that point (moisture content) at which a soil passes from a plastic to a liquid state, for the silt-clay fraction of the lagoon sediments is 66. High liquid limits such as this indicate soils of high clay content and low load-carrying capacity. Geologic Investigation and Soil Analysis of Buena Vista Lagoon Page 6 Th2 plastic limit, that moisture,content at which the soil changes from a semisolid to a plastic state, for the silty clay i,S 32. An important change takes place at the plastic limit of the soil. The load carrying capacity of the soil increases rapidly as the moisture content decreases below the plastic limit. The plastic index is the numerical difference between the liquid limit and the plastic limit and indicates the moisture content range at which a soil is in plastic state. The P. I. (plastic index) for tije silty clay is 34. A high P. I., such as this, indicates the wide range in whicti, this silty clay remains in a plastic state. Shrinkage The silty clay fraction of the lagoon Sfdiments has an average shrinkage factor of 46%. This means that a completely dessieated sample of this material would occupy 54% of its original in situ volume. Thfe resultant net loss in volume upon drying at the spoil area would make a substantial difference in the amount of material that would have to be handled. Turbidity The thorough mixing and agitation that will occur during the hydraulic dredging operation will break down the silty clay into its constituent particles and into a state of suspension. This could present some turbidity problems at the spoil area and in the lagoon in general. To determine the length of time necessary for these very fine particles to settle out of suspension, it was necessary to develop a simple reproducible test. A procedure was developed as follows: (1) A small amount of silty clay (approximately 55 cc) was placed in a clear 150 ml graduated cylinder; (2) Enough water from the lagoon was added to fill the cylinder; (3) This mixture was thoroughly mixed in a mechnical agitator for five minutes; (4),The height of the material in suspension was observed and recorded; ($) The results were graphed (time vs height). Geologic Investigation and Soil Analysis of Buena Vista Lagoon Page 7 Three separate samples of silty clay were tested using this process (samples B-5, 9 & 16). Each sample was run twice and the average results were used to create the graph in figure 1. The graph depicts a major difference in the curves of the three samples. The curve of sample 16, taken nearest the Source, shows a relatively fast precipitation of the suspended material. The curve of sample 9, indicates an intermediate time and the slowest precipitation is exemplified in the curve for sample 5, taken near the Route 5 bridge. This difference is directly related to the size of the constituent particles and the distance from the sediment source area. Material from the upper (eastern) end of the lagoon would settle out of suspension relatively fast, probably within five days. The suspended material from the middle of the lagoon would take approximately two weeks and the toaterial from the Western end of the lagoon would take approximately four weeks to Settle out of suspension. • UJ CL </>r>(/) -J cru .IS- FIGURE TURBIDITY CHART 5- 0- ( ^-—-^ MATERIAL 11 1 " 1 } 1 2 1 ' 1 I 3 4 — — _ SAMPLE is— . — , 4M SUSPENSION ~- : — . _____ — -»,«n ii .. »^- **-*«•*<*• I»MJM»^.»»--^IJH» ; SOLID MATERIAL TIME IN DAYS i i i i i 1 567 8 ^9 10 T 1 1 1 1 11 I l ll 1 12 13 .M 15 -IG 17 18 19 20 21 .. ._ ... ... r i •">'> O"7/-•c /r. j a Vista Lagoon Pago 8 Geologic Investigation and Soil Analysis of Buena Page 9 RECOMMENDATIONS Sediments The Sand-silt fraction of the lagoon sediments is a good structural material and can be easily compacted to 90% or 95% Relative Compaction. However, the location and difficulty in skimming this relatively thin sand layer off the top of the underlying silty clay may prove to be economically unfeasible. The silt-clay fraction of the lagoon sediments is not good embankment laaterial. Its high volume change with the addition of water and elastic nature makes this material extremely difficult to compact. This type of soil is not suitable for use as subgrades under flexible base coarses or bituminous surfaces because of volume changes and loss of bearing capacity when wet. This material could be used to construct the nesting islands and waterfowl loafing bar within the planned lagoon (shown on plans) or for landscaping or agricultural"- use (depending on results of the chemical tests), ChannelSide Slopes Based on the soil parameters obtained in the field, the planned cut slopes within the lagoon, should in theory, stand at a 2:1 angle. This 2:1 figure does not consider a safety factor and perhaps a 2.5:1 or 3:1 figure is more realistic. Sfroiir. Area (not in lagoon) V> If the dredged material is stockpiled away from the lagoon an estimated six months to one year will be required for the dredge spoils to firm up enough to be handled. Upper Lagoon Spoil Area If the dredged material is stockpiled at the upper end of the lagoon it in all probability will never completely dry out (capillary action will pull moisture up from the lagoon). However, the material will firm up and a hard shell like crust will form on the pile. - Geologic Investigation and Soil Analysis of Buena Vista Lagoon Page 10 If the upper lagoon is selected as a spoil area, it may be necessary to build a circular berm of sand to form a basin to pump the dredge slurry into. As the spoil pile increases in height some heaving in the underlying soft silty clay should be anticipated and some repairs to the berm may be necessary during the stockpiling operations. The runoff from the spoil area will undoubtedly carry a considerable amount of suspended clay in the solution and clouding of the upper lagoon should be anticipated. Weir Construction The construction of a sheet pile dam near the Route 5 bridge and the subsequent raising of the water surface to + 8.0 should not cause any serious problems to the existing shore line. The beneficial attributes of the weir construction would be a lessening in the amount of material to be excavated and the containment of material in suspension (caused by dredging) in the upper portion of the lagoon (east of highway bridge). A copy of the Log of Test Borings for the bridge across the lagoon (Route 5) is in the appendix and may prove beneficial during the sheet driving operation. Report by w. c. CAIN . Associate Engineering Geologist Registered Geologist No. 2497 Certified Engineering Geologist 732 WCC: bs cc: APPENDIX — -**- Srf-S,* » 5*«*» 1*™U a *., O PU* ' '^ f ^-«« C <•>< XMUO J^%L_l fc , • tseai* fl J "" ' ^^ *"' *• ' *tw Cut- fcC ' f •11-iV•