Capillary Cover Design for Leach Pad Closure, Zhan 2001

8/10/2019 Capillary Cover Design for Leach Pad Closure, Zhan 2001

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Period T

rnillireconds)

TCS

Leach

Pad

1

Lab Regrassion or

TCS

. ab Regress~onor Leach Pad 1

Figure 2 Campbell

6

5 water content reflectometer

and its calibration curves for leach pad and TC S materials:

a) TDR unit and b) calibration curves.

where

T* is the normalized tem perature rise.

Th e temperature unit of Eq. ( 2) is in Celsius.

Five HDS units were calibrated using this method, and the

calibrated results are presented in Fig.

3.

This norm alizing

procedure elimin ates the need for multipoint calibration curves

for each sensor and, thus. greatly simplifies the calibration

process. Through this new calibration procedure, HDS can

provide accurate m atric suction readings in the range of 50 to

10,000 cm (20 to 4.000 in.) .

Test cell instrument installation. Each test cell was instru-

mented with TDR andH DS . The distance between the two cell

locations is approxim ately 2 m 6.6 ft). Figure 4 illustrates the

sensor layout at one test cell and sensor setup at different

depths.

Th e instruments were installed in a trench excavated by a

backhoe a t depths of 15 ,45 ,7 5 and 120 cm

(6,

1 8 , 3 0and 48

in.). Th e top two sensor pairs were located in the cover layer,

while the bottom two senso r pairs were located in the leach

pad. The material excavated from each lift was stockpiled

separately on the groun d surface, so it could be back-filled to

the sam e depth.

Pairs of TDR and HDS were positioned horizontally and

adjacent to each other. The compaction was completed by a

gasoline-pow ered Whacker to repack the trench to approxi-

mately the sam e bulk density as the material prior to

excava-

Dimensionless Temperature Rise T

HDS Probes

abRegression

Figure

3

ampbell

229

heat-dispassion sensor and

calibration curves: a) HDS unit and b) calibration curve.

tion. The in situ field porosities

n )

of the cover and the leach

pad were approximately 0.38 and 0.19, respectively.

In

situ test results. Approximately 120 m3 or 227 cm (4,200

cu ft or 89 in.) of water was ap plied to the test area. Three types

of irrigation ev ents were evaluated:

A large influx fbl lo~+ ~edy long-time drainage:

Ap-

proximately 207 .3 cm (8 1.6 in.)

of

water (equal to about

6.3 years of precipitation) was applied between July 2 4

to August 8. Th e wetting front mov ements in the cover

and leach pad materials and the drying cycle were

monitored in this process.

Rainfall: Rainfall occurred on August 23, August 3

1

Septemb er 1-2 and Septem ber 22. The total rainfall was

about 2 .4 cm (0.95 in.).

Three episodes of adding dlferent volumes of I+,ater:

These occurred on September 12- 13, September 8 and

Septemb er 21. Th e controlled irrigation rate for these

episodes were 1 1.2,2.6 and 3.6 c m 4.4, l O and 1.4 in . ) ,

respective1 y.

Figures 5 and

6

show the volumetric water content

0)

measurements and matric suction

y)

easurements at two

test cells. The observed results of volumetric water content

and matric suction from the two adjacent cells are quite

similar.

During the large influx , it took about three day s for water

to percolate through the 60-cm (24-in.) cover. Th e volumetric

water content for TCS and leach-pad materials reached their

porosity values, which are 0.38 and 0.19, respectively. The

water content measurem ents showed large peaks for both the

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(a> a

Cell

I

(a )

b)

DAY

b

Cell

2

1O.OW

DAY

20

-

15 g

.

-

l o

K

.

d

m

5

DAY DAY

Figure

8 imulated volumetric water content, suction,

Figure

6 Measured m atric suction at each depth interval

and applied irrigation rate: a) simulated volumetric water

and applied irrigation rate.

content and b) simulated matric suction.

rcs

ma)

LEACH PAD GSA)

-

T S FITTED) LEACH PAD FITTED)

4

-

3

25

2 1

5

1 l X1 1

loow

1 1

blittric

Suction

( -cm)

Figure 7 bserved and fitted WRC for the leach pad

and TCS materials.

ing numerical simulations, all GSA points and two high

suction points from DBA are combin ed to form a new repre-

sentative WR C for the TCS and leach pad materials. The new

data sets were fitted to Fredlund's equation using SoilCover

(Fredlund and Xing , 1994; Fredlund et al., 1994: Geo-A naly-

sis 2000 L td.. 1997). The fitted results are also presented in

Fig. 7.

The numerical simulation s were conducted using the one-

dimensional code SoilCover. Detailed daily weather data

were used. The weather station at the mine site recorded

hourly dat a for the test period. T he data includes air tenipera-

ture, wind spe ed, relative hum idity. precipitation and total

radiation. SoilCover computes potential evaporation (PE)

using a modified Pen mam equation that requires net radiation

as input (Wilson, 1990). Net radiation was calibrated to match

the measured total weekly pan evaporation. The simulated

volumetric w ater content and suction values at the four depths,

at which the sensors are installed. are presented in Fig.

8.

The simulated results show reasonably good agreements

with the observed results, as shown in Figs. 5 and

6.

The

wetting fronts are well simulated. The calculated water con -

tent values in the leach pad are similar to the obser ved values,

while the simulated water content valu es in the TC S layer are

slightly lower than the observed values. Becau se the test cells

are on the east-facing slope, less exposure to sun shine could

account forthis difference. Another explanation for the offset

is that the hydraulic properties in situ might be slightly

different than the ones used fo r the calculations. In addition.

only one-dimensional conditions are treated by the code,

which may induce some differences in the results. The

simulated suction values during the large influx period are

much less than the observed values. This may be due to

calibration limitation of HD S that are unable to read suction

values below 50 cm (20 in.) . The simulated suction values at

the shallowest depth shows a significant drop due to the

precipitation events from August

3

to Septem ber 2, but the

observed values do not. It is also possible that the senso rs at

the shallowest depths were installed somew hat deeper than

15

crn

6 in.) or lateral movem ent of water in the cove r, which

a one-dimensional model could not simulate. In spite of the

slight differences, the overall response of the system is well

represented by the numerical calculations.

Long term cover performance predictions

It was demonstrated that the observed rewlts from the pilot

study were simulated reasonably w ell using SoilCover. Dur-

ing long irrigation periods, the volumetric water content of the

unvegetated cover w as as high as 0.26, which is much larger

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Table 1 -Water budget terms of the standard simulation.

Cumulative precipitation, mm

Daily rain period, hr

Cumulative evaporation, mm

Cumulative runoff, mm

Cumulative infiltration, mm

Cumulative transpiration, mm

Cumulative cover bottom flow, mm

Cumulative pad bottom flow, mm

than the residual water content that could be achieved after

extended dry periods or with the addition of vegetation. To

predict long-term performance of the vegetated CCBE, a

quasi-steady-state simulation was conducted for a TC S cover

with thickness of 9 0 cm (36 in.).

Model inputs

In addition to the hydrologic parameters of the

TC S leach pad materials, the following inp uts were also used

in the simulation:

Clirnatic da ta Th e weather station at the mine site has a

complete daily data set for the year 1998. This data set is used

for the weather input with the following modifications:

The 1998 precipitation data were scaled to match the

long-tern1 annual average precipitation from a USGS

weather station. located about km (5 miles) southeast

of the m ine site. This state weather station has 22 years

of precipitation records with an average annu al precipi-

tation of about 33 c m (13 in.) . The shorter record from

the mine-site weather station is consistent with data

from this station.

Net radiation was estimated from incom ing radiation and

pan evaporation. Pan evaporation of about 150 cmlyear

(59 in./year) was measured at the mine site. Soilc over

requires net radiation as input. Net radiation equals

incoming radiation, measured by the sensor, minus re-

flected radiation. Reflected radiation is the product of

incoming radiation and a coefficient of albedo. Coeffi-

cient of albedo was calculated to be 0.25 based on a

potent ia l evapora tion of 150 c d y e a r (59 id ye ar ) .

Boundary conditiorzs

Climatic data were used to define

the suction and temperature boundary con ditions that control

water and heat exchange between the atmosphere and the

upper su rface of the leach pad. Th e lower portion of the leach

pad was represented as fully saturated with a constant tem-

perature equal to the annual average air temperature of 9C

(48F).

Vrgrtat ion Th e method used by the program accou nts for

the effects of canopy cover , root depth an d water stress. Th e

vegetation is characterized as poor, having a leaf area index

(LAI)

from 0 to 1. The growth period is from M arch 16 to

October 15. The root zon e depth is specified as 90 c m (36 in.) .

Th e moisture limiting point of the vegetation is specified as

100 kPa. Th e wilting point of the veg etation is specified as

1.500 kPa.

When suction is smaller than the limiting point (i.e., the soil

is relatively wet), plant transpiration is uninhibited. When

suction is between the limiting point and the wilting point,

plant transpiration is reduced by a factor that is proportional

to the log of suction. Plant transpiration is zero when suction

is greater than o r equal to the wilting point.

Volumetric water content cm3/cm3

Figure 9 -Simula ted water-content profiles of leach pad

with 90 c m

36

in. TC S cover.

Initial con dition dynamic quasi-steady-state condition

was calculated as the initial condition of the sin~ ula tio ns. he

dynam ic quasi-steady-state was reached by inputting the daily

climate information into the model and executing several

iterations until the sim ulated water content profiles were the

same on a particular day for successive annual periods.

Simulated results

The simulated cumulative water budget

terms are summ arized in Table 1. Values presented in the table

show that there is no water seeping to the coarse leach pad

material, and no water flows through the bottom of the pad.

Simulated water content profiles are presented in Fig. 9.

Results from D ays 0 , 16, 21 1 and 365 are p resented fo r

illustrative purposes. Day 16 is a wet winter day during a high

precipitation period, and Day 21 1 is a dry day in the sum mer.

Examination of the profiles indicates that a strong capillary

barrier effect is established. The fine-grain material in the

cover layer is much wetter than the underlying leach pad

material, which h as a very low hydrau lic conductivity, hence,

preventing downward vertical water flow.

The simulated results demonstrate that a 90 cm (36 in.)

TC S cover effectively limits water infiltration to the leach pad

during av erage weather con ditions. It is interesting to note that

the water content in the vegetated cover is approximately

0.17 , which is significantly low er than the w aterco nten t of the

unvegetated cover. Therefore, vegetation will effectively in-

crease the water holding capacity of the cover.

Discussions and conclusions

The observ ed test results demonstrate that the cover perfor-

mance can be monitored using TDR and HDS monitors.

Strong lateral capillary rise was observed during the test. This

phenonlenon can only occur for fine materials with significant

water-retention capacity. With vegetation developed on the

cover, the water content of the cover is sim ulated to be about

0.17 . Wa ter content of the cover at field capacity can reach as

high as 0.30, as demon strated by the test during the irrigation

period of Septem ber 18 and 21, so the additional unit storage

capacity of the cover to 0.13 (0.30 minus 0.17). A cover

thickness of 9 0 to 120 cm (36 to 48 in.) will s tore 12 to 16 cm

(4.7 to 6.3 in.) of w ater, independent of evaporation and lateral

drainage. Based on this analysis, the holding capacity of the

SOCIETY FOR MINING. METALLURGY, AND EXPLORATION, INC.

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TRANSACTIONS2001


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cover has sufficient volume to retain three continuous

100-

year storm events, i .e.. approxim ately 24c m 9.5 in.) of wa ter,

assuming half the precipitation is surface runoff. Therefore,

the cover will operate as designed even under extreme precipi-

tation conditions.

cknowledgements

This paper is aresu lt of the AA leach pad reclamation program

of Barrick Goldstrike Mines Inc. The authors thank Dr. M.

Anken ey, Daniel B. Stephens Asso ciates Inc., for technical

support . Dr. D. Hammermeister and Mr. M. Milczarek, Geo-

~ i s i e m nalysi s Inc . , a re al so acknowledged for the sensor

calibrations and installation.

References

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