AD-A242 676 1 n

i ii | I* .CR 91.012

September 1991

SOIL BIOENGINEERINGCORPORATION

Contract Report MARIETTA, GA

SOIL BIOENGINEERINGMAJOR GULLY WASHOUT REPAIR,

SILVERHILL AIRFIELD,BALDWIN COUNTY, ALABAMA

ABSTRACT This report summarizes research conducted at Navy sites af-fected by gully erosion. Soil bioengineering (or biotechnical) techniques wereused to repair and stabilize a large gully located at Silverhill OLF, Alabama.Biotechnical methods included brush layering, branch packing, live stakes, andlive cribwalls. A portion of the project site used more conventional methods ofslope stablization for a side-by-side comparisons of their effectiveness. Despitedrought conditions subsequent to installation, survival rate was high enough toinitially stablize the site. Subsequent monitoring of the site is recommended todetermine long-term effectiveness. This investigation indicates promising use ofbiotechnical methods for slope stablization and gully repair under various condi-tions.

91-15572

NAVAL CIVIL ENGINEERING LABORATORY PORT HUENEME CAUFORNIA 93043

Approved for publc release; distrbution Is unlimited.

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REPORT DOCUMENTATION PAGE OBAL001Pubbic repoftV is d. w Ift ceitdon of Infowradw isa stiald to averge I ho u spa ense, a 41e .ie1 inw intued".m sawc"n exatum data sorce.gatheirg and nmhntk*ut thes data needad. aNW co~plei eandt neiewmng t"e CONGeo Of tnbmtiOn. Servd co amN ruwdig t buden eatiat a anyotiw a&WeO of tillscoliseeton Ilniaiton, tuding suggesIomu for seiuing Ot buden, to Wahlgow Haaqu.Iw Services. Dkairen for Inbonnelson will Reports. 1215 jierwson Davis Highway,Sudse 1204. Afllngin, VA 22202-4302. and to ONl Ofine of Manaeuneit and Budgat. PapewarI Reckdon Pret (0704-016U). Wasingion. DC 20503.

1. AGENCY USE ONLY (Leave @4Asnki EOTDT 3. REPORY TYPIE AN DOATES COVERED

ISeptember 1991 Finl; October 1985 - July 1988

4. 7TTL ANC SUBTfltE FUDING NUMBERIE

SOIL BIOENGINEERING - MAJOR GULLY WASHOUTREPAIR, SILVERHILL AIRFIELD, BALDWIN COUNTY, PE - 71-061ALABAMA WU - DN660018

.AUTNOR(M C -N62474-8S-C-4963

Soil Bioengineering Corp.

7. PERFORMIING ORGANIZATION RAME1I1 AND ADORESSEdg L PERORING ORGAUnIZTONREOR NUMBER

Soil Bioengineering Corporation CR 91.012Marietta, GA

9. SPONSORINGIMONITORING AGENCY NAME(BI AND AOOAESSE(S1 III. SPONWOUNG00MCNITORING

Naval Facilities Engineering Naval Civil Engineering AGENCY REPORT NMBER

Command (Code 18) Laboratory200 Stovall Street Code L71Alexandria VA 22332-2300 Port Huenemne, CA 93043

11. SUPPLEMENTARY NOTES

i 2&L OESTRISJTION/AVAiLAIUTY STATEMENT 12L DISTRIUTION CODE

Approved for public release-, distribution is unlimited.

13. ABSTRACT (AMaium 200 %inaiij

This report summarizes research conducted at Navy sites affected by gully erosion. Soil bioengineernog orbtotechnical) techniques were used to repair and stabilize a large gully located at Silverhill OLF Alabama.Biotechinical methods included brush layering, branch packing, live stakes, and live cribwalls. A portion of the projec:ite used more conventional methods of slope stablization for a side-by-side comparisons of their effectiveness.Despite drought conditions subsequent to installation, survival rate was high enough to initially stablize the site.Subsequent monitoring of the site is recommended to determine long-term effectiveness. This investigation indicatespromising iise of biotechnical methods for slope stablization and gully repair under various conditions.

11 4 . U B J E T T IE R S I 5 N U M B "R O F P A G E S

Soil bioengineering. erosion, gully repair; brushlayer, cribwall, biotechnical. slope protection 40I&. PRICE CODE

17. SIECURITV CLASSUiCATION I&I SECURITY CLASSIFICATION W S SECUIY CLASSIFCATION 20, LIMITA71ON OF ABSTRACTOF REPOW OF THIS PAWE OF ABTACT

Unclassified I Unclassified Unclassified U.L

4SIN 754041.280-5500 StAn~ani For, 298 IR#.. -89)Pealed b ANSI Sd 2,19. -

TABLE OF CONTENTS

Acknowledgements .. . . ii

List of Figures . . iii

List of Photographs . . .. . iv

Introduction ... . .. 1

Purpose ..... 2

Location Map 3

Soil Bioengineering . 4

Description of OLF Silverhill

Baldwin County, Alabama 6

Climate 6

Physiography and Soils 7

Biotechnical Plant Materials 7

Topographic Plan View 10

Proposed SoilBioengineering Construction Plan View 11

As-built Soil Bioengineering Construction Plan View 12

The Live Systems . 13

Figures 15

Photographs Sequence Location Map 23

Photographs 24

Evaluation of Soil Bioengineering System Success,as it relates to Installation and Project Management 36

Summary . . 39

ACKNOWLEDGEMENTS

We would like to sincerely thank the people who assisted us inproducing this most valuable and worthwhile project. We alsoappreciate the interest which is being shown in soil bioengi-neering technology. Although our list is not complete, we wishto acknowledge the following persons:

Ms. Leslie Karr, Environmental Engineer, Naval Civil Engineer-ing Laboratory, Port Hueneme, California

Lt. Anthony Cox, Resident Officer in Charge of Construction,Pensacola, Florida

Mr. Andy Johnson, Soil Conservationist, Southern Division, Na-val Facilities Engineering Command, Charleston, South Carolina

Soil Bioengineering Corporation wishes to extend its sincereappreciation to Dr. Hugo Schiechtl, Soil Bioengineer/LandscapeArchitect, Innsbruck, Austria, for his personal support and forhis continuing professional dedication to the development ofsoil bioengineering.

ii

LIST OF FIGURES

1. Live Cribwall

2. Live Fascine

3. IBrushlayer - fill

4. Brushlayer - cut

3. Branchpacking

6. Live Siltation

7. Live Soft Gabions (Vegetated Geogrid)

LIST OF PHOTOGRAPHS

PHOTO I. The area of the drop structure prior to construc-tion. This illustrates the undercutting and sideslope erosion headcut problems which threatened thestructure

PHOTO 2. The existing drop structure area during the earlystages of construction

PHOTO 3. The existing drop structure in the final stages ofconstruction activities. Note the soil bioengineeringbrushlayers on the left

PHOTO 4. The existing drop structure three (3) months afterconstruction. Note the soil bioengineering brushlay-ers on the left

PHOTO 5. Left side of the existing drop structure during ini-tial conventional construction

PHOTO 6. Left side of the existing drop structure three (3)months after the soil bioengineering installation

PHOTO 7. The right bank, "downstream" of the first installeddrop stracture, after soil bioengineering brushlayerinstallation

PHOTO 8. The right bank, "downstream" of the first installeddrop structure, three (3) months after soil bioengi-neering brushlayer installation

PHOTO 9. The left bank, "upstream" and alonqside the secondinstalled drop structure, during construction

PHOTO 10. The left bank, "upstredm" and alongside the secondinstalled drop structure, three (3) months after in-stallation of the brushlayers

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PHOTO 11. "Downstream" view of the gully during initial conven-tional construction

PHOTO 12. "Downstream" view of the gully during the soil bioen-gineering brushlayer installation

PHOTO 13. View "downstream" after the soil bioengineeringbrushlayer and live siltation systems had been in-stalled

PHOTO 14. View "downstream" of the reconstructed gully/drainageunit, demonstrating the living soil bioengineeringunits, three (3) months after installation

PHOTO 15. Oblique air photo three (3) months after construc-tion. The photo demonstrates the living soil bioengi-neering systems, the conventional installations, aswell as the airfield drainage source and the pecanorchard

PHOTO 16. Oblique air photo three (3) months after construction

v

Soil Bioengineering Technology Utilizedto Repair Gully Erosion on OLF Silverhill

Baldwin County, Alabama

INTRODUCTION

The Naval Civil Engineering Laboratory (NCEL), Port Hueneme,California, in coordination with Southern Naval Facilities En-gineering Command (SOUTHNAVFACENGCOM), Charleston, South Caro-lina, the U.S. Dept. of Agriculture, Soil Conservation Service(SCS), Auburn, Alabama, and Soil Bioengineering Corporation,Marietta, Georgia, developed soil bioengineering/biotechnicalland stabilization procedures in combination with convention-al engineering design features. These steps were taken to sta-bilize the headcut erosion on the side walls, the head wallsand channel floor of an actively eroding gully. The site was alarge, heavily eroded gully approximately fifteen hundred(1,500) linear feet long, approximately forty (40) feet deep,with twenty (20) to one hundred (100) foot bed widths. The ero-sion was progressing rapidly in the side face areas where sec-ondary gully headcuts were encroaching onto privately ownedproperty. The erosion was jeopardizing a previously construct-ed drop structure and producing heavy sediment loads to be de-posited "downstream".

The site is located at the OLF Silverhill airfield in BaldwinCounty, Alabama.

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PURPOSE

The main purpose of this limited evaluation report is to pro-vide the Department of the Navy with an understanding of thehistory, site conditions and merits of soil bioengineeringtechnology as it applies to this project site. Before, duringand after construction photographs and slides are also includedfor reference.

The broad objective of this work was to test and demonstrate adesigned and constructed soil bioengineering/biotechnical landstabilization project, specifically to repair and prevent prob-lems associated with soil erosion on the Silverhill site.

The site was chosen by the Navy and used as a demonstration insupport of the biotechnical systems, to learn the methods bywhich such a project is developed. Methods discussed and demon-strated were: tender/bidding this technology to outside con-tractors, overseeing correct controlled project management,collecting, handling, and installing procedures, and mainte-nance evaluation performance. The eventual goal, after severalsuch tests/demonstrations, is for the Department of the Navy tobenefit from using rapid repair and stabilization techniquesoffered by soil bioengineering technology on Navy sites.

This report also addresses the system of the tender/bid processas it was handled on this project, and the adequacy of the con-struction techniques used by the construction contractor. Itidentifies and discusses stressed areas of unsatisfactory bio-technical installations and suggests possible repairs. The doc-ument fully discusses the success of each system installed.Finally, it discusses the Navy's support and administration cfthe construction process.

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SILVERILL O 7

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LOCATION MAP

SILVERHILL OLF

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SOIL BIOENGINEERING

A soil bioengineering approach offers many advantages to soilstability: 1) actual field studies have shown that in manyinstances combined structural-vegetative slope protection sys-tems are more cost-effective than the use of structures alone(White, 1979); 2) soil bioengineering slope -rotection sys-tems are more aesthetically pleasing, i.e., they blend into thelandscape; 3) these systems emphasize the use of natural, lo-cally available materials - earth, rock, timber, vegetation;and 4) soil bioengineering systems, by their live nature,offer flexible self-repairing qualities, which contribute tolow long-term maintenance.

The use of vegetation, primarily grasses and forbs, for pre-venting surface erosion on land sites, is fairly common andwell understood. The U.S. Soil Conservation Service and similargovernment agencies around the world have long advocated plant-ings to control both rainfall induced surface erosion and winderosion. However, the protection provided by this herbaceousvegetation is primarily shallow in nature, as the developedroot masses rarely penetrate more than a few inches into thesoil. Ic can indirectly benefit deep-seated, mass stability bydepleting soil moisture, and stabilizing the soil against sur-face erosion, facilitating the establishment of shrubs andtrees (Gray and Leiser, 1982). The "bottom" or "bed" section ofthe gully had (in certain areas) a well developed grass cover.

Woody plants help prevent mass-movement, particularly shallowsliding. Soil bioengineering woody plant installation affectsimmediate sedimentation/soil stabilization. Thesp initial me-chanically functioning systems encourage a high percentage ofseed germination and rapid natural invasion of plants wel!adapted to the site conditions. Woody vegetation affects thebalance of forces in a soil mass through root reinforcement,evapotranspiration, and buttressing and arching, wherebyanchored and embedded stems can act as buttress piles or archabutments in a slope, counteracting shear stresses. Specificinstances where trees were serving as buttress units on thissite were readily available during the initial site visits.

Root reinforcement is the most obvious way in which woody vege-tation stabilizes soils. The intermingled, lateral roots ofplants tend to bind the soil together into a monolithic mass.On slopes, the vertical root system can penetrate tough thesoil mantle into the firmer strata below, thus anchoring thesurface soils to the slope and increasing resistance to sfid-ing.

4

Vegetation, alone, can not control all erosion/sedimentationmovements. Site specific characteristics must be considered inthe decision to use vegetation singularly or in conjunctionwith conventional structures. In the case of OLF Silverhill,the combination of soil bioengineering with conventional engi-neering was expected to produce the best overall structures.The conventional works are intended to give immediate reductionto the velocity of the flood waters, while the soil bioengi-neering systems, as they grow, are expected to consolidate thesoil particles, reduce heavy runoff water velocity by increas-ing the top vegetative growth, and retain and trap sedimentdeposits along the banks.

5

DESCRIPTION OF OLF SILVERHILL

OLF Silverhill:Baldwin County, Alabama

The project site was a large undulating, heavily eroded gully,formed by collected water run-off. The headwall had been par-tially restrained from erosion by a sheetpile gradestructure. The reconstructed gully drainage channel and sideslope area is approximately fifteen hundred (1,500) linear bankfeet. The existing vertical banks, prior to construction,ranged from five (5) feet to approximately forty (40) feet inoverall height at the gully headwall. The floor or bed widthranged from twenty (20) feet wide in the lower "downstream"sections to one hundred (100) feet wide in the upper reaches.

There were several major side slope cut and fill sections, withthe Soil Conservation Service (SCS) channel alignment and dropstructure designs, below the existing grade structure. The areaimmediately adjacent to this was installed with conventionalmethods designed by the SCS.

Soil bioengineering systems "tied into" the area with livesystems to join the required conventional structure to the live"downstream" work, as well as to serve as a back up to theconventional construction. The floor, or bed, of the gully wasfirst mechanically stabilized with two (2) additional dropstructures, which were designed by the SCS. In between the dropstructures, soil bioengineering living systems were put inplace in the channel bed. These are expected to further reducevelocities and cause deposition to occur in the bed.

The sides of the gully have been stabilized with living soilbioengineering units. These are intended to control surfaceerosion, consolidate the soil particles and add fibrous inclu-sions to increase the shear strength of the soils and to resistsliding.

Climate

Information for climate, physiography, and soils was interpret-ed from U.S.D.A. Soil Conservation Service Soil Surveys ofBaldwin County (1960).

Baldwin County has a humid, nearly subtropical climate. Thelong, hot summers are tempered by sea breezes making the aver-age summer temperature 80.2 degrees. The high temperatures

6

prevent a large amount of organic matter from building up inthe soils. Average annual rainfall is normally high, 64.6 inch-es, and well distributed throughout the year. However, overthe last two (2) or three (3) years, springs and summers havebeen much drier than normal. Considering the humid climate andthe normally even distribution of rainfall, the site appearedto be well suited for biotechnical stabilization work.

PhysioQraphy and Soils

Baldwin County is a part of the Gulf Coastal Plain physio-graphic region. The Silverhill site is underlain by Citronellegeologic formation which rests on Hattiesburg clay. The ma-terial in the Cftronelle formation is predominantly sandy withthin layers of clay. The sand is red and crossbedded, the claymottled gray and red. Most of the Citronelle formation in thecounty has been impacted by erosion.

The soil phase at the Silverhill site is Lakeland loamy finesand, 8 to 12 percent slopes. The soils are low in naturalfertility and in organic matter content. The soil texture in-cludes sand, loamy sand and very fine loamy sand. The waterstoring capacity of this soil is low. Water infiltrates rapidlyand the hazard of erosion is moderate to severe.

As waters drain into the Silverhill gully from the watershed,rapid infiltration causes the banks to become saturated duringperiods of high rainfall. As waters quickly drain throulgh thebanks, unconsolidated sands are removed by the waters, causingbank failure.

Biotechnical Plant Materials

Plant species used for the live stabilization systems includedboth woody and herbaceous vegetation. Plant selection was basedon the technical requirements, ecological suitability, and a-vailability of appropriate stock in the local area. A mix-ture of species was used to assure the greatest technical ef-fectiveness by providing a diverse root system as well as en-couraging the ecological health associated with a diverseplant community. Selected woody species were native or natu-ralized to the surrounding area and thus were expected to bewell adapted to the local growing conditions. Herbaceous spe-cies were a mix of grasse- and legumes well adapte to localconditions and commercially available.

7

The Silverhill site showed evidence that vegetation may be ableto establish itself if conditions were modified through grad-ing, proposed by the Soil Conservation Service, combined withsoil bioengineering systems. In areas where rapid soil movementwas not occurring, the invasion of natural species was readilyapparent.

Woody Species

On the Silverhill site, it was expected that several woody spe-cies would provide the major structural support at the time ofinstallation: black willow (Salix niara), coastal plain willow(Salix caroliniana), and Chinese privet (LiQustrum sinense).Experiehce has shown that unrooted cuttings of most willow andprivet species root well when handled and installed correctlyin biotechnical systems.

A major investigation was conducted for plant collection siteswithin a sixty 60 mile radius of the site. Chinese privet wasfound in lowlands or on fence rows in agricultural areas. Ade-quate stands of willow species were found to fulfill the pro-ject requirements.

A relatively high species mix of grasses and legumes was devel-oped by the SCS, for seeding after installation and between thewoody biotechnical systems. This mix may assist in stoppingsurface erosion and improving fertility and organic mattercontent. The species mix plays an important initial role andshould not become either competitive or invasive. The ultimategoal is the natural invasion of native species, which isexpected to occur over a period of years. The system is notinitially strong, but is intended to become more effective andstronger with age.

The biotechnical systems were constructed and seeding performedmainly during the dormant season. The seed mixture consisted ofthe following commercially available species: Pensacola bahia-grass (Paspalum notatum), bermudagrass (Cynodon dactylon),Abbruzzi ryegrass (Lobium multiflorum), and Tibbee crimsonclover (Trifolium incarnatum). The appropriate inoculant wasalso recommended with the clover species. It is expected thateach species will have varying survival rates depending on thelocal micro climatic growing conditions. In particular, clovershave a limited long-term survival rate but help in "fixing"nitrogen and contributing organic matter to _,,e soil.

8

The herbaceous cover holds importance in limiting surface ero-sion during the first months in conjunction with the woody bio-technical systems. The seed mix density needed to be limited toinsure that the surrounding area was not inhibited. Fertiliz-ing, liming, and mulching recommendations were made after eval-uation of the soil analysis. Two (2) types of fertilizer, 13-13-1.3 and 17-7-12 (slow release), were used during the con-struction of the soil bioengineering systems.

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THE LIVE SYSTEMS

The live systems recommended for the OLF Silverhill site weredeveloped to first demonstrate living systems, to repair andprotect the areas against further soil losses in both the cutand fill slope and bed sections. Finally, they were to serve asa learning tool for the Navy as to how such projects should behandled in the future.

Live Cribwall - See Figure 1. This system was built with tim-bers, forming an open, box-like structure. As it was built,live plant materials were incorporated at each layer. The livecribwall began to root internally and is expected to root intothe bank behind, causing the entire area to become stabilized.The cribwall was placed below the first built grade structure.

Live Fascine - See Figure 2. This is a bundle of live plantmaterial, bound together to form sausage-like structures. Theywere installed on contour to create a mini dam across the slopeface. They were used in between brushlayers to secure sectionswhere extra rooting was desirable and to reduce headcutting upthe bank face. These live structures take root along their fulllength and pump water out of the soil through transpiration.

Brushlaver - See Figures 3 and 4. These live systems wereplaced in prepared terraces across the cut or fill slope sec-tions. Branches of live, rootable, biotechnically capableplants were placed at an angle into the slope. The terraceswere placed on contour across the bank face. In the fill sec-tions, the live plant stock was actually placed up to eight (8)feet into the fill site. In the cut sections, the live brancheswere actually placed two (2) feet into the slope. This struc-tural reinforcement is intended to increase as the roots devel-op. The exposed brushy portions of the new brushlayers alsofunction immediately by forming filtered sediment barriers a-cross the slope face. This controls surface runoff, preventsthe formation of gullies and encourages natural invasion to oc-cur.

Branchpacking - See Figure 5. The live branches were placed di-rectly in a critical fill area, at an angle in the bottom ofthe repair fill site. A layer of earth was placed on top ofeach successive layer to give immediate soil reinforcement andto serve as a brush filter/sediment barrier. This constructionis expected to slow the water velocities and capture silts. Thelive branches are expected to root into the placed fill. Thissystem was used to tie together the left side of the upper ex-isting conventional drop structure and live cribwall into thegraded slope.

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Live Siltation - See Figure 6. The live branches were placed atan angle directed "downstream" in the bed of the newly shapedchannel section. The system gives immediate soil reinforcementand serves to reduce flood velocities and to capture sediment.In properly installed systems the roots are expected to consol-idate the soil particles and enhance the stability of the bed.

Live Soft Gabions (Vegetated Geogrids) - See Figure 7. Theselive systems were placed five (5) feet deep in prepared ter-races in the same manner as the previously described brushlay-ers. In addition to the living brush, the soil layers which arein between the live branches, were wrapped with geotextile ma-terial to give added strength in the critical areas.

14

FIGURES

1. Live Cribwall

2. Live Fascine

3. Brushlayer - fill

4. Brushlayer - cut

5. Branchpacking

6. Live Siltation

7. Live Soft Gabions (Vegetated Geogrid)

15

LIVE CRIBWALL

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PHOTOGRAPHS

PHOTO 1. The area of the drop structure prior to construc-tion. This illustrates the undercutting and sideslope erosion headcut problems which threatened thestructure

PHOTO 2. The existing drop structure area during the earlystages of construction

PHOTO 3. The existing drop structure in the final stages ofconstruction activities. Note the soil bioengineeringbrushlayers on the left

PHOTO 4. The existing drop structure three (3) months afterconstruction. Note the soil bioengineering brushlay-ers on the left

PHOTO 5. Left side of the existing drop structure during ini-tial conventional construction

PHOTO 6. Left side of the existing drop structure three (3)months after the soil bioengineering installation

PHOTO 7. The right bank, "downstream" of the first installeddrop structure, after soil bioengineering brushlayerinstallation

PHOTO 8. The right bank, "downstream" of the first installeddrop structure, three (3) months after soil bioengi-neering brushlayer installation

PHOTO 9. The left bank, "upstream" and alongside the secondinstalled drop structure, during construction

PHOTO 10. The left bank, "upstream" and alongside the secondinstalled drop structure, three (3) months Pfter in-stallation of the brushlayers

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PHOTO 11. "Downstream" view of the gully during initial conven-tional construction

PHOTO 12. "Downstream" view of the gully during the soil bioen-gineering brushlayer installation

PHOTO 13. View "downstream" after the soil bioengineeringbrushlayer and live siltation systems had been in-stalled

PHOTO 14. View "downstream" of the reconstructed gully/drainageunit, demonstrating the - living soil bioengineeringunits, three (3) months after installation

PHOTO 15. Oblique air photo three (3) months after construc-tion. The photo demonstrates the living soil bioengi-neering systems, the conventional installations, aswell as the airfield drainage source and the pecanorchard

PHOTO 16. Oblique air photo three (3) months after construction

25

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Photo 2. The existing drop structure area during the earlystages of construction

Photo 3. The existing drop structure in the final stages ofconstruction activities. Note the soil bioengineer-ing brushlayers on the left.

Photo 4. The existing drop structure three (3) months

after construction. Note the soil bioengineer-ing brushlayers on the left.

Photo 5. Left side of the existing drop structure duringinitial conventional construction.

Photo 6. Left side of the existing drop structure three (3)

months after the soil bioengineerinq installation.

I Ii

Photo 7. The right bank, "downstream" of the first installeddrop structure, after soil bioengineering brushlayerinstallation.

Photo 8. The right bank, "downstream" of the first installeddrop structure, three (3) months after soil bioen-gineering brushlayer installation.

I

Photo 9. The left bank, "upstream" and alongside the secondinstalled drop structure, during construction.

Photo 10. The left bank, "upstream" and alongside the secondinstalled drop structure, three (3) months afterinstallation of the brushlayers.

Photo 11. "Downstream" view of the gully during initialconventional construction.

Photo 12. "Downstream" view of the gully during the soilbioengineering brushlayer installation.

Photo 13. View "downstream" after the soil bioengineeringbrushlayer and live siltation systems had beeninstalled.

Photo 14. View "downstream" of the reconstructed gully/drainage unit, demonstrating the living soilbioengineering units three (3) months afterinstallation.

Photo 15. Oblique air photo three (3) monthsafter construction. The photo dem-onstrates the living soil bioengi-neering systems, the conventionalinstallations, as well as the air-field drainage source and the pecanorchard.

I

Photo i6. Oblique air photo three (3) months after

construction.

EVALUATION OF SOIL BIOENGINEERING SYSTEM .JCCESZAS IT RELATES TO INSTALLATION AND PROJECT MANAGEMENT

From a mechanical point of view, i.e., considering the abilityof the installed branch units to halt surface erosion, the soilbioengineering systems are all performing very well at thistime. From a living point of view, those systems which weremore deeply installed, have been most successful.

The deep fill brushlayers, the branchpacking sections, the livecribwall units and the live soft gabions (vegetated geogrids)are all doing very well. The extensive deletions (determined bythe ROICC) of the fill brushlayers, the shallow placement ofthe installed cut brushlayers and the sandy soils, coupled withthe expected and experienced drought conditions, did not givethe cut brushlayer units the success rate that might have beenrealized (see Table 1).

The live fascine units appear to be alive for approximatelytwenty (20) to thirty (30) percent overall. These units, howev-er, are of less importance after the first few months of thebrushlayer establishment. The cut brushlayers which are veryimportant to the success of the project, are doing somewhatbetter. Unfortunately, due to their shallow placement, they areclearly suffering. The ROICC made the decision to reduce thedepth from the recommended five (5) to eight (8) feet to two(2) to three (3) feet. This is a very shallow depth in sandy,droughty soil conditions. Because of this decision, we do notfeel that they will reach the necessary seventy (70) to eighty(80) percent overall survival rate. This should have been ahealthy, self-supporting system, but in reality these areas arein great jeopardy. Since they represent the majority of theinstalled work, the reductions and deletions made by the RC:-Cmay clearly have iamaged the project goal.

The live siltation installations in the center of the channelhave worked as expected in some sections, but due to poor in-stallation, they have not been able to demonstrate the optimumcapabilities of this system.

The brushlayers in fill, the live cribwall units and the livesoft gabions (vegetated geogrids) all appear to have achieved ahealthy seventy (70) to eighty (80) and in some cases up tofinety (90) percent survival rate at this time. These areasshould work well to stabilize the newly repaired draina.= site.

The live stakes and the joint planting systems were totally de-leted (by the ROICC) and thersfore there is no opportunity forevaluation.

36

The systems that have been the most successful appear to bethose that were installed at the greatest depth and those thatwere installed correctly, with freshly cut branches and in thedormant season. Unfortunately, these are the least representa-tive of the soil bioengineering installations.

Several sections of soil bioengineering work were deleted, bythe ROICC, especially on the north bank. Of course, this in it-self may serve as a comparison. The south bank, installed withsoil bioengineering, suffered no bank failures during or afterconstruction. However, several sections were unwisely changedfrom fill to very shallow cut brushlayers, which may carry itsown set of problems with it, from a living point of view.

The bank with mostly conventional grassing suffered severalfailures during construction. Additionally, these sections arestill eroding and are very sparsely vegetated. We expect thatthey may contribute to bank failures and sediment load again inthe future. The area on the right side (facing "upstream") ofthe old existing drop structure has already eroded into a holeat the top. The ROICC simply filled this area and covered itwith "Hold Gro", a loosely woven paper product. The "down-stream" siltation device was heavily damaged at the earlystages of construction. Although this was pointed out to theROICC on-site representative in person, the damage was notrepaired. In June, 1988. it still had not been repaired.

37

TABLE I

Soil Bioengineering System ComparisonsBetween Designed and Installed Units

Proposed &Contracted

to be ActuallySystem Unit constructed Constructed Deletion Addition

Live Stakes each 5,500 0 5,500 -

Joint Planting each 3,800 0 3,800 -

Brushlayer Cut lin.yd. '3,700 3,800 - 100

Brushlayer Fill lin.yd. 2,100 250 1,850 -

Live Fascine lin.yd. 690 65 625 -

Live Cribwall sq.ft. 595 750 - 155

Live Soft Gabion lin.yd. 940 285 655 -

(Veget. Geogrid)

Branchpacking lin.yd. 330 30 300 -

Live Siltation lin.yd. 340 340 - -

38

SUMMARY

The shining light in this entire project from start to finishhas been the Naval Civil Engineering Laboratory (NCEL) group,Port Hueneme, California, that supported the project and thesoil bioengineering technology.

A great deal can be learned from the OLF Silverhill project. Itis obvious that the failures were imposed on the project. Theywere not originally designed into it. The decision makers wereeither unwilling or unable to read and follow the plans andspecifications or to listen to the instructions of the experi-enced on-site Consultants. This inability or unwillingness wasrapidly passed on to the construction Contractors.

The project is now completed or partially completed and whathas been sown is now being reaped.

39

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