aced with rising population anddeclining groundwater levels, in1974 the cities and counties inthe Tampa Bay, FL, area formedthe West Coast Regional Water

Supply Authority (WCRWSA) andbegan providing new water sources on asubscription basis. In 1999 WCRWSAbecame Tampa Bay Water, a wholesalewater provider responsible for develop-ing alternative water sources. Tampa BayWater began working toward develop-ing new sources not only for drinkingwater, but also for the natural environ-ment.

First, Tampa Bay Water developed amaster water plan that meets current

and future needs for the area. The heartof the plan was to reduce dependencyon groundwater by increasing the use ofsurface water and rotation in productionto minimize environmental impacts. Asurface water supply system wasdesigned that consisted of the followingmain components: Tampa Bay Reservoir(TBR), Alafia River Pump Station,Tampa Bypass Canal Pump Station andPipeline, South-Central HillsboroughIntertie, and a 66-mgd surface watertreatment plant.

During high rainy season, the stationsskim water from the Alafia andHillsborough rivers and Tampa Bypasscanal. If not skimmed, this water would

normally be discharged into the gulf.With the new surface water system inplace, a portion of this water is treated atthe surface water treatment plant anddistributed through the main water dis-tribution system. The remaining por-tion of skimmed water is pumped to thenew TBR for storage. During dry seasonwhen river flows are low, water stored inthe reservoir is used to supply the sur-face water treatment plant.

Reservoir Design ConsiderationsTo meet long-range planning require-

ments, the surface water system requireda large storage reservoir with a 15-billiongallons storage capacity. The reservoir

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52 ■ GOVERNMENT ENGINEERING ■ JANUARY–FEBRUARY 2007 www.govengr.com

Soil-cement protects upstream embankment of reservoir against erosive wave action.

Aerial view of Tampa Bay Reservoir during initial filling in August 2005.

By Fares Y. Abdo

Armoring Tampa BayReservoir with Soil-Cement

Armoring Tampa BayReservoir with Soil-Cement

embankment was to withstand wave runup from 110-mph sustained winds and40-in. rainfall. In the late 1990s, 15potential reservoir sites in southeasternHillsborough County were considered.After careful consideration of cost, envi-ronmental effects/benefits, safety, long-range planning, site conditions, archae-ological features, and public multi-pur-pose site uses, a site near the city ofBoyette was selected for the reservoir.The site is located about 20 miles south-east of Tampa.

HDR Engineering, Inc.(www.hdrinc.com) designed the above-ground reservoir using five-mile longearthen embankment with a maximumheight of 65 ft.

To protect the upstream slopes of thereservoir against the erosive wave actiongenerated from hurricane-force winds,the engineer elected to armor the bankwith soil-cement. Soil-cement proved tobe the most cost-effective solution dueto the lack of locally available riprap andthe plentiful supply of sand for the soil-cement within the basin of the reservoir.

Soil-Cement MixDesign and Testing

The project documents specified afour-percent maximum weight loss ofsoil-cement specimens when subjected towet-dry cycles per ASTM D 559 andfreeze-thaw cycles per ASTM D 560.However, during the mix design phase ofthe project, testing for freezing and thaw-ing was omitted since the structure waslocated in southern Florida and wouldnot be subjected to freeze-thaw cycles.

In 2001 Law Engineering andEnvironmental Services, Inc. (nowknown as MACTEC, Inc.,www.mactec.com) performed a series oflaboratory tests on soil samples obtainedfrom the project site. The samples wereobtained at depths ranging from 1 to 15ft below existing ground surface. Theywere described as either tan silty finesand or brown fine sand. The tan sam-ples were combined and identified asSoil Type A and the brown samples werecombined and identified as Soil Type B.

Both soil types were tested in accor-dance with the following test methods:■ ASTM D 698—Standard Test

Methods for LaboratoryCompaction Characteristics of SoilUsing Standard Effort (12,400 ft-lbf/cu ft)

■ ASTM D 422—Standard TestMethod for Particle-Size Analysis ofSoils

■ ASTM D 558—Standard TestMethods for Moisture-DensityRelations of Soil-Cement Mixtures

■ ASTM D 559—Standard TestMethods for Wetting and DryingCompacted Soil-Cement Mixtures

■ ASTM D 1633—Standard TestMethods for Compressive Strengthof Molded Soil-Cement Mixtures

Compressive StrengthTest Results

A first series of tests was performedon Soil Type B using cementitious con-tents ranging from 8 to 15 percent by

dry weight of soil. Ten Type B sampleswere tested per ASTM D 559. Weightloss from wetting and drying testingranged from 10 to 63 percent. This issignificantly higher than the four per-cent maximum allowable weight loss.Two of the ten samples were also testedper ASTM D 1633. The compressivestrength at seven days was below 100psi. Considering only the gradation ofthe soil and the quantity of cementused, the strength and durability valuesshould have been much higher. Thesetest results, however, clearly indicatedthat the Brown Fine Sand at the sitedoes not react normally with cement.

A second series of tests was performedon Soil Type A using 8 percent, 10 per-cent, and 12 percent cement contents bydry weight of soil. Three samples weretested, one for each of the cement con-tents. Weight loss from wetting and dry-ing testing ranged from 1.6 to 2.0 per-cent. Compressive strengths at sevendays were 480, 670, and 830 psi formixtures containing 8 percent, 10 per-cent, and 12 percent cement contents,respectively. These results proved thatSoil Type A at the site reacts normallywith cement and is suitable for soil-cement production. Based on laboratorytest results, a soil-cement mixture withnine percent cement content was select-ed for the project.

Design and ConstructionThe design maximum pool elevation

was set at 136.5 ft. To optimize thequantity and cost of the soil-cementslope protection design, two different

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Figure 1. Typical cross section of reservoir embankment. (Courtesy of Tampa Bay Water)

methods of soil-cement placement wereutilized. (See Figure 1.) For the portionof the embankment below elevation 134ft where the soil-cement would be nor-mally submerged, the soil-cement wasconstructed 16 in. thick, parallel to theslope in a method referred to as “plat-ing.” Above this elevation to the crest ofthe embankment at elevation 145 ft, thesoil-cement was placed in 9-in. thick by8-ft wide horizontal lifts in stairstepfashion. A geotextile filter fabric wasplaced underneath the soil cement toprevent washout of soil particles fromwave action at soil-cement cracks andconstruction joints.

The stairstep method requires moresoil-cement, but provides better protec-tion for the more frequent and higherwave action expected. In addition, thestairstep method dissipates the waveaction and reduces wave run-up. To pro-vide extra protection against possibleuplift from wave action, a water-cementslurry mixture was applied to bond con-secutive lifts. Soil-cement was also usedto construct crest and perimeter roads.To control water seepage, the embank-ment design included a soil-bentonitecut-off wall and a geomembrane.

The contractor reported difficultiesachieving proper density and profile ofsoil-cement when the subgrade was notcompacted well or after rain events. Toprotect the subgrade during rainy sea-sons, the contractor covered the workarea for three days before placing soil-cement.

During construction of soil-cementplate on the embankment slopes, com-paction with vibratory rollers causedsurface cracking. These cracks werespaced at about one-ft intervals. It wasbelieved that the soil was moderatelyplastic and was being pulled up by thesteel drum of the vibratory roller. Thecontractor successfully addressed thesurface cracking issue by switching topneumatic compaction equipmentinstead of steel drum vibratory rollers.

Quality ControlDuring Construction

Before beginning placement, anexploration program determined that anadequate supply of in-situ tan silty finesand (Soil Type A) for the soil-cement

was available at depths of five ft anddeeper along the northern portion of thereservoir. Barnard Construction begansoil-cement placement in December2003 and completed the soil-cementconstruction in November 2004.

The soil-cement mixture with ninepercent cement content (selected basedon the laboratory mix designs discussedearlier) was used during the first twomonths of construction while addition-al field and laboratory tests were beingconducted. Test results obtained duringthe first two months of construction jus-tified reducing the cement content to8.2 percent, which was used for theremaining nine months of construction.

Throughout the project, soil pilesprepared for use in soil-cement weresampled and tested for compressivestrength to confirm that the selectedmaterial would react normally withcement. This was a critical quality con-trol step to ensure that poorly reactingsandy materials did not end up in thesoil-cement structure. Instead, thesematerials were separated and used in theembankment or wasted.

During construction, reference proc-tor tests to determine the maximum den-sity and optimum moisture content wereperformed nearly daily in accordancewith ASTM D 5584. The project specifi-cations required the moving average ofany five consecutive in-place density teststo be at least 98 percent and individualin-place density tests to be at least 95 per-cent of the maximum density. Field den-sity tests using the nuclear gauge wereperformed throughout construction withthe average densities for the plating andstairstep methods being 98.6 percent and98.7 percent, respectively.

To supplement the density tests andto check for consistency of mixture, soil-cement pills were compacted nearlydaily and tested for compressivestrength. The average density of com-pacted pills was 96.4 percent, which isabout 2.2 percentage points less thanthe average density achieved in the field.The average compressive strengths atseven days were 444 and 358 psi formixtures with 9 percent and 8.2 per-cent, respectively. Due to the well-known effect of density on compressivestrength, it is believed that the compres-

sive strength of the compacted fieldmaterial is significantly higher than thestrength of the compacted pills.

In-Place Cost of Soil CementThe project used 260,000 cu yd of

plated soil-cement and 105,000 cu yd ofstairstepped soil-cement. Unit prices forthe in-place material were $20.00 per cuyd for the plating construction and$33.33 per cu yd for the stairstep con-struction. These unit prices includecement cost, handling of soil, and costof soil-cement mixing, transporting,placing, and curing.

Soil-cement proved to be the mosteffective material to construct an erosion-resisting liner and protect the embank-ment slopes at TBR. Design features thatoptimized the use of both plating andstairstep construction methods proved tobe vital to account for wave run-upcaused by sustained high winds. To pro-vide the required freeboard without thesoil-cement stairsteps, the embankmentheight and volume would have been sig-nificantly larger.

Although most sandy soils are suitablefor soil-cement construction, there aresome surface soils in glaciated areas of thenorthern United States and in the easternand southeastern coastal areas thatrequire high cement contents comparedwith average sandy soils. These soils aretypically contaminated by certain organ-ics or other deleterious materials and arereferred to as “poorly reacting” soils.Quality control measures similar to thoseimplemented by the project team at TBRhelp identify these unsuitable materialsprior to their use in soil-cement. On largeprojects similar to TBR, it is recom-mended to use these poorly reacting soilsin the embankment or elsewhere on theproject. Where this is not an option, thesoils should be diluted with normallyreacting soils and the combined materialsshould be tested and approved prior touse in soil-cement.

Mr. Abdo, P.E., is the Program Manger forWater Resources, Portland Cement Association(www.cement.org), and can be reached atfabdo@cement.org. The author would like tothank Amanda E. Rice, P.E., Tampa BayWater; Barry J. Meyer, P.E., HDREngineering, Inc.; and Kevin Ellerton,Barnard Construction, for their valuable con-tributions.

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