Bulletin of the Transilvania University of BraşovCIBv 2015 • Vol. 8 (57) Special Issue No. 1 - 2015

STUDY ON THE SCREEN SEALINGCHARACTERISTICS EXECUTED AT

MILEANCA DAM (CASE STUDY)

A. NICU1 C. NICU2 N. FLOREA3

Abstract: Using the solution of self-hardening mud to complete theMileanca dam screen sealing has stopped the infiltration of water throughthe permeable layers. The screen seal has a width of 60 cm and a variabledepth H = 7 to 11 m. It is embedded in the shale layer, to a depth of 1 m. Thework was done by respecting the technology and the self-hardening mudrecipe was established after laboratory assessment and land verification withthe consent of the engineer and the beneficiary.

Key words: screen sealing, self-hardening mud, guiding beams, bentonitesuspension, impermeable layer (clay marl).

1 Faculty of Civil Engineering and Building Services Centre, “Gheorghe Asachi” Technical University of Iasi2 Cernaconstruct S.R.L. Iasi3 Faculty of Civil Engineering and Building Services Centre, “Gheorghe Asachi” Technical University of Iasi

1. Introduction

The technical conditions for theexecution of the sealing screen from self-hardening bentonite mud, using Kellyinstallation type, are based on the generalrequirements related to the execution ofmolded walls sealing screens.

The technical provisions were requiredfor the smooth execution of the self-hardening bentonite mud sealing screen.

The main operations for the execution ofthe self-hardening mud sealing screenwere:

▪ tracking the works according to theplotting plan coordinates;

▪ earthworks to create the work platform;▪ execution of the facility trench by

excavation with Kelly;▪ filling the trench with self-hardening

mud.

2. Features of the Sealing Screen

2.1. General Features

The main features of the screen are thefollowing:

▪ maximum depth of panels (between thescreen execution upper elevation and therestraint elevation);

▪screen width: 0.60 m;▪ current length of a panel:

- 2.80 m primary panel- 2.10 m secondary panel

The screen has a length of 370 meterswith depth varying from 7 m to 11 m.Bentonite is the key component to achievethe self-hardening mud. The bentonite usedto prepare the slurry of self-hardening mudfulfilled the conditions indicated in STAS9305-81 [1].

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According to STAS 9305-81 [1], theactive bentonite used in the preparation ofself-hardening mud was accomplished by

treating the bentonite with 2% ÷ 4% sodaash in accordance with STAS 2640-82 [2],as shown in Figure 1.

Fig. 1. Aspects from the execution of the sealing screen at Mileanca dam –Excavation of a panel using specialized installation type Kelly

2.2. Physical and Chemical Properties

Bentonite material obtained had thefollowing physical and chemicalproperties:

▪ yield (m3/t), which was determinedaccording to STAS 9484/11-92 [3],representing the quantity of suspension(m3) with the apparent viscosity (36'Marsh') which can be prepared from oneton of bentonite. The minimum value was12.50 m3/t;

▪ free sand content, determined inaccordance with STAS 9484/22-82 [4], themaximum allowable value being 5%;

▪ grinding fineness, determined inaccordance with STAS 9484 / 16-74 [5],the maximum allowable residue 2% on the0.16 sieve and 10% on the 0.063 sieve.

The quality check of active bentonitewas made on lots of max. 10t inaccordance with STAS 2411-75 [6]. Whenthe received material check was notconsidered to comply with the conditionspermissible in STAS 9305-81 [1], thematerial was refused.

Preparation of bentonite slurry was donein IBN mixers with vertical shaft type witha capacity of 3.5 m3.

The order of placing components in themixture will be water - activated bentonite,the mixing time is 5 minutes.

Considering the physical - chemicalproperties of the bentonite, it was indicateda dosage in suspension of approx. 80kg/m3. The dosage was finalized bylaboratory tests in order to obtain asuspension of activated bentonite (hydrated24 hours).

At the placing, the activated bentonitesuspension (hydrated 24 hours) had thefollowing characteristics:

▪ apparent viscosity determined bymeasuring the discharge time by Marshfunnel an amount of 1,000 cm3 ofsuspension, the allowable time being 34 ÷37 sec;

▪ suspension density determined byweighing, the admissibility condition is1.04 ÷ 1.06 kg/dm3;

▪ suspension stability, representing thesediment volume in cm3 to 100 cm3

suspension allowed to stand for 24 hours,the minimum eligibility condition being 99cm3.

Preparation of bentonite suspensions toestablish the physical and chemicalproperties was done in accordance with

A.NICU et al.: Study On The Screen Sealing Characteristics Executed At Mileanca… 135

STAS 9305-81 [1]. The water needed forpreparation of bentonite suspensioncomplied with STAS 790-84 [7].

2.3. The Properties DeterminedAccording to Laboratory Tests

The final recipe for the preparation of1m3 self-hardening mud for the sealingscreen of the upstream toe of Mileancadam was the following: 920 liters of water; 2.5 kg soda ash (required for bentonite

dissolution in water); 100 kg bentonite; 200 kg cement type H II AS, according

to NE012 - 1: 2007 [8].Preparation of the self-hardening mud

was done in the same vertical shaft IBNtype mixer, the entry order of thecomponents being: bentonite suspension,and cement. The mixture stir was done forat least 15 minutes, thus ensuring themixture homogeneity.

The cement met the conditions of SR EN197-1/2011 [9], checking the setting time,the volume constant and the mechanicalresistance.

The transport of bentonite suspensionfrom the preparation place to the placingsite was achieved by pumping using rigidpipes.

2.4. Features of Self-Hardening MudDepending on the Cast Panels Results

Compared to the recommended recipeand given the results obtained on themolded screen panels, the self-hardeningmud characteristics were:

▪ maximum density 1.2 kg/dm3;▪ apparent viscosity determined with

Marsh funnel 39 sec;▪ suspension stability by determining the

percentage of water separated from the freesurface for 100 cm3 of suspension 2.1%.

Some final physical characteristics of the

self-hardening mud screen will be:▪ minimum permeability coefficient of

2x10-6 cm / s;▪ monoaxial compressive strength at least

300 kPaThe definitive recipe of the self-

hardening mud was endorsed by thedesigner and the beneficiary and wasestablished in tests both at the sitelaboratory and on the specialized andcertified units, on the supplied materials.

2.5. Sampling Approach

Taking a sample of bentonite to bechecked was performed at each new batchof supply and during progress in thepreparation and placing.

For new lots of bentonite supplied, thelab supervisor took 2 samples of which onewas used to its controls, and the other wassent to a specialized unit after being placedeither inside or outside the bag two labelswith the following particulars: driveexecution, lab supervisor, bentoniteindication, sampling date. Each samplecontained at least 3 kg of bentonite.

For the tests made on the used bentonite,slurry preparation was done according toSTAS 9484 / 22-82 [4].

2.6. Final Quality Control of Self-Hardening Mud

The quality tests of self-hardening mudscreen were performed on samples takenfrom:

▪ trench immediately after the excavationoperation of the secondary panel wasfinished (phase III of scraping the bottom),the sampling level being at a depth of atleast 4m from the upper panel, the sampleswere kept wet in laboratory (in exicator)and tries were made at 28 days;

▪ nozzles, taken from boreholes executedat least 28 days from the date of the paneltermination, in locations determined by the

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engineer and the beneficiary;▪ minimum three (3) samples for each

molded panel were taken to determine thequality of panel and at 10 panels wasrunned a drill for quality control work.

Attempts were made to determine thefollowing characteristics:

▪ permeability coefficient (K) of thescreen was made depending on the type ofthe embedded screen in a barrier layer, aswell on as the flow of water infiltration.

Determination of permeability was basedon laboratory tests on samples with Ø 7 cmand 2 cm height in parameters according toSTAS 1913 / 6-76 [10].

The permeability coefficient permissiblefor the sealing screen did not have a valuegreater than 2 x 10-6 cm/s.

▪ monoaxial compressive strength, wasmade at 28 days, on prisms samples withdimensions 4x4x8 cm, with the coefficientof slenderness 2.

The screen takes through owndeformations any equal or unequalsubsidence of the surrounding land. Thecheck was made at press according toSTAS 8942/6-75 [11], the minimumpermissible resistance to monoaxialcompression was 3 daN/cm2.

For each panel, from the bottom of thescreen (in the region of the fixing in theclay marl) was yield a probe which waslabeled (date, panel no., the depth of therecessed panel), and was preservedthroughout during execution.

2.7. The Quality Check of ScreenExecution After the Mud Hardening

The checking of the screen after theexecution was done in order to control thework quality and the degree ofwaterproofing. The final verification wasneeded to reveal any deficiencies on worksexecution, but no execution deficiencieswere found.

The screen checking was made by

executing in the body sealing screen Ø80÷100 mm diameter control drilling andextracting cores from different depths,which was examined in the laboratory. Onthese cores were determined samples forpermeability and compression resistance.The panels checked were specified by theengineer and the beneficiary, the checksprogram being required to complete beforethe end. For guidance, a drilling was set ata 100 m distance from the screen.

Supervising the checks and recording theresults was done by the quality controldepartment of the site, in the presence ofthe project supervisor and the assistantgeologist. How to check and all dataobtained from checks was recorded in thefinding minutes attached to the controlledpanel files. In all these cards wasmentioned the verification method adoptedfor each panel and the results obtained.

2.8. Control Boreholes Running in to theSealing Screen Body

The position of control boreholes forscreen tightness could be determined bythe beneficiary who, in advance, hasanalyzed the laboratory results of theimpermeability degree.

The boreholes will be executed only onthe geologist presence. The controlboreholes will have diameters of 80 to 100mm. The optimal diameter can bedetermined experimentally, havingprimarily regarded to ensure verticality.Permeability samples will be carrieddownward.

During drilling execution screen probeswill be sampled and inventoried after depthand stretch and filed in special evidenceboxes. It will be ensured that the drillingsite in panel field to be as close to thescreen spindle. It will be checkedpermanently the horizontality andverticality of the rig. Digging will be madewith diamond cutters, rotary system only

A.NICU et al.: Study On The Screen Sealing Characteristics Executed At Mileanca… 137

and, in no case, by percussion drill. Thedepth of the boreholes will be equal to thatof the panel plus 1.0 m in the bedrock. Insome boreholes, indicated by the engineer,measurements of deflection will be made.

There was no need during the execution,for additional drilling control because theywere not identified areas where the qualitycriteria were not satisfied.

3. Tests, Probes, Determination andResults of the Sealing Screen

3.1. Permeability Probe

The permeability samples werecompleted in top down system, in sections3 ÷ 5 m deep, with simple mechanicalpacker, with open circuit and by raising itfrom the surface.

The permeability samples were carriedout at least 90 days after the execution ofthe screen area. Before starting,permeability tests accurately recorded thewater levels in the accumulation area.

The service pressure at the level of thetest section was calculated and did notexceed 2.0 ÷ 2.5 atm. Samples were madein increments of 0.5 psi./½ hour. Themaximum pressure of (2.0 ÷ 2.5 atm.) wasmaintained constant for 4 hours.

For permeability tests implementationthese steps were followed:

▪ water was put in the mud settling stumpto a level which is determined by a rod or alevel tube. The mud settling stump had acapacity of more than 1 m3, being adaptedto the small water losses, which areestimated. For precision measurementswas used calibrated float rod solidaritywith mud settling stump or a measuringrod with float and micrometer;

▪ water was pumped under the firstpressure stage and, when the measuredgauge pressure was constant, the waterlevel in the mud settling stump wasmeasured at intervals of 5 minutes;

▪ water pumping under a pressure stagelasted until the injected flow remainedconstant at least three consecutivereadings. When this condition was satisfiedit was proceeded to the next pressure stage,the process being similar.

The results of permeability tests wereregistered to a control card, presentinginclusively the specific absorption capacitycalculations with the formula:

qs = Q/L x T x p (l/m × min. × 0.1 at.) (1)

where:-qs - specific absorption capacity;- Q - water flow lost under the regimen

(liters);- L - section length (meters);- T - testing time (minutes);- p - presiunea (pressure 0.1 at.)

The average admissible water loss maybe qs = 0.01 l/m × min. × 0.1 at. (LugeonCriterion).

Samples permeability w ere run on allsections as a whole, without interruption tothe hours. Following measurements are notallowed deviation from the vertical axisdeviation greater than 10 cm.

3.2. Injections

The injections were made only in theboreholes and sections that have exceededthe maximum absorption capacities,respectively qs = 0.01 l / m x min. x 0.1 at.(1.u.L)).

To achieve injections on intermediatesections of the drilling will be used adouble mechanical packer using surfacereinforcement. The injections will be madein the upward system.

The duration of injection, quantity ofcement on ml and the water/cement factor,according to the specific permeability inthe section are shown in the followingtable:

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Duration of injection and dosages depending on the specific permeability Table 1

Specificabsorptioncapacity qs

[l/m x min. x0.1 at.]

FactorA/C

Injectiontime

[min.]

Maximum quantityof cement insuspension

[kg/ml]

Observations onsuspensions change

0.01 10/1 30 20 No change in absorptionbelow 10 l/min persection. Continue to thebrim.

0.01 ÷ 0.03 5/1 30 20

0.03 ÷ 0.05 3/1 30 ÷ 35 50

No change in theabsorption of up to 25l/min per section.Continue to the brim.

0.05 ÷ 0.15 2/1 45 100

No change in absorptionof 30 ÷ 50 l/min persection. Continue toabsorption of 10 l/min persection.

0.15 ÷ 0.40 1/1 45 150

No change in absorptionof 50 ÷ 75 l/m, persection. Continue toabsorption of 10 l/min persection.

> 0.40 1/2 60 200 ÷ 250*

No change in absorptionof 75 ÷ 100 l/min persection. Continue toabsorption of 10 l/minper4 section. It is limitedto a total of 500 kg/m.

* after the consumption of therecommended amount of suspension, theinjection is stopped and allow it to set atleast for 72 hours, after which the injectionis resumed with 5 : 1 suspension.

Refusal could be considered when for ameter of the section, the slurry flowinjected at maximum design pressure doesnot exceed 0.3 ÷ 0.5 l/min. For the specificcase of the screen consolidation in areaswith cracks, are considered sufficient therecipes A/C = 5/1 and 3/1. The other caseswere considered in exceptionalcircumstances. For the suspensionsstabilization will be used bentonite in an

amount of 2 kg per 100 kg of cement (2%).It is recommended to start with unstable

suspensions injections (simple water-cement mixtures) to continue with stablesuspension and eventual ending withunstable suspensions until obtainingrefusal.

The cement used for preparing the A/Cmixture will be of the H II AS (NE012 - 1:2007) [8]. The water was clear and meetthe technical requirements of STAS 790/84[7]. It did not contain solutes above 1.5%,of which sulphates and did not exceed 1%.It did not contain organic substances.

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3.3. Laboratory Methodology

3.3.1. Conventional Viscosity

It is determined using Marsh funnel andrepresents the time in seconds in which1000 cm3 of suspension is flowing fromfunnel, according to STAS 9305-81 [1].

3.3.2. Cone Check

The hopper is filled with fresh water(1000 cm3 water), and the hopper isdepleted. If the emptying time is greater orless than 28 ± 0.5 sec. then it is dirty,deformed or has a threshold between thecylindrical tube and the tapered body. It isforbidden to use deformed funnels, withthresholds in the connecting section ordirty funnels.

3.3.3. Determination of ConventionalViscosity

Soak the funnel with water and close thehole. Fill with 1,000 cm3 suspensionthrough a 1.5 ÷ 2.0 mm sieve. The sievemay be attached to the upper half of thecone section. It is measured the time offilling the 1.000 cm3cup.

3.3.4. Decanting

It is determined in 1.0 l cylindrical glassvessels with a diameter of 60 mm. Thesuspension is left in the cylinder for 2hours. It is read the decanted water columnheight at the top and is related to theoverall height, the ratio being thedecantation.

3.3.5. Density

It is recommended to use the cylindersused to determine the decantation. Weighthe cylinder filled with suspension. Thedifference between the weight of the

cylinder with suspension and the tare is thespecific weight of the suspension. It isrecognized to by determine the densityusing hydrometers after testing the errorsobtained by comparison with data obtainedfrom the weighing method.

3.3.6. Gelation

It is determined using a rod of Ø 3.5 mm,50 mm long, weighing 3.5g. The tip of therod is conical with h = 10 mm. Theopposite end is threaded or grooved for a10mm length. A ring PVC is put on a tileor glass and filled with suspension. Fromtime to time the rod is introduced in thesuspension on a vertical position. Thegelation beginning is recorded when therod does no longer enter till the supportand remain in suspension with the knurledportion of 1 ÷ 2 mm above the suspension.Gelation end is recorded when the conicaltip of the rod enters in the suspension onlyup to half height.

3.3.7. Relative Rigidity

It is determined with a striated bran steelplate. Immerse the plate suspended by adevice with a thin cable on a winding reel.It is hold for 2 seconds in the suspensionand easily manually extracted by rotatingthe drum. The weight of the plate withsuspension is measured by hanging it on aprecision balance pan. The differencebetween the weight obtained and the tare isR, suspension stiffness, which in practiceis reported more convenient in the form:

Rr = R/p (2)

where:- Rr - the thickness of the suspension

layer adhered to each side of the plate;- R - suspension stiffness;- P - specific weight of the suspension.All these determinations are made for

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each batch of material supplied by the siteand each recipe suspension. The number ofmeasurements should be at least one monthseries.

Filling the drilling will be carried outwith a suspension of high consistency (A/C= 1/1 ÷ 1/2). During the drilling filling airholes are not allowed into the drill orincurred due to technological processes.

4. Conclusions

Casting the sealing screen from self-hardening mud, with embedding in thelayer of marl at the upstream toe ofMileanca dam led to stopping the seepageunder the dam, leading to an increase in itsstability and substantial improvement ofthe technical parameters of the reservoir,which holds approx. 14.7 mil. m3 of water.

The adopted recipe following thelaboratory and field studies in agreementwith the engineer and the beneficiary led tothe establishment of parameters andimplementation conditions.

Subsequent determinations and checksshowed that the sealing screen was welldone.

References:

1. STAS 9305-81 – Activated bentonitefor drilling fluids (in romanian);

2. STAS 2640-82 – Raw bentonite(lumps) for drilling fluids protection(in romanian);

3. STAS 9484/11-92 – Silica-aluminamining products. Efficiencydetermination (in romanian);

4. STAS 9484/22-82 – Silica-aluminousmining products. Methods of physicaland mechanical tests. Determination offree sand contents (in romanian);

5. STAS 9484/16-74 – Silica-aluminamining products. Methods of physicaland mechanical tests. Determination of

grinding fineness (in romanian);6. STAS 2411-75 – Silica-alumina

mining products. Sampling and samplepreparation (in romanian);

7. STAS 790-84 – Water for concreteand mortar (in romanian);

8. NE 012-1:2007 – Annex L – Code ofPractice for the production ofconcrete;

9. SR EN 197-1/2011 – Cement – Part 1.Composition, specifications andconformity criteria for commoncements (in romanian);

10. STAS 1913/6-76 – Foundationground. Determination of thepermeability in the laboratory (inromanian);

11. STAS 8942/6-76 – Fondation ground.Method of unconfined compressiontests of soils (in romanian);

12. Mitoiu C. - Technical expertise of theMileanca dam and lake, BotosaniCounty, 2007 (in romanian);

13. Stematiu D., S. Ionescu - Safety andrisk in hydraulic structures, Didacticand Pedagogic Publ. , Bucharest, 1999(in romanian);

14. STAS 1243-88 - Foundation soil.Classification and identification ofland (in romanian);

15. MP 023-2002 - Methodology fordetermining the seepage of earth damsand levees, navigation canals andsewers through thermometrymeasurements in boreholes, BC no.7/2003 (in romanian);

16. NTLH-022 - Methodology regardingthe evaluation of safety in theoperation of dams and reservoirs (inromanian);

17. CERNACONSTRUCT Iaşi (chiefproject engineer. Nicu Adrian) -Secure the Mileanca accumulation onPodriga river, Botosani county(technical design specifications anddetails), Iaşi, 2008-2012 (in romanian).