Dam Foundation Treatment

7/27/2019 Dam Foundation Treatment

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A CASE HISTORY

OF

EXPLORATION &

REMEDIATION

HIGH PRESSURE

SEEPAGE AT ARAPUNI DAM,

NEW ZEALAND

Presented by:

Saravanan. S

(2010CEU3304)



THE DAM

Arapuni dam is 64 m high curved concrete gravity dam with a

crest length of 94 m.

Arapuni Dam forms Reservoir for 186 MW Power Station

located on Waikato River, 55 km upstream of Hamilton City inNorth Island of New Zealand.

The Dam Construction was started in 1924 and lake filling

was completed in 1928.

The Arapuni Power Station is owned and operated by Mighty

River Power Ltd., a State owned electricity generation

company.



THE CUT OFF PROVISIONS

Original features of the dam include concrete cutoff walls and a

network of porous (no-fines) concrete drains at the

dam/foundation interface (the “under drain”).

The original cutoff walls extend beneath the dam to a depth of

65m below the dam crest and extend 20m and 33m into the left

and right abutments respectively, for the full height of the dam.

There was no grout curtain constructed during Dam

construction.



GROUTING AND DRAINAGE PROVISIONS FOR

FOUNDATIONS

Consolidation Grouting - For filling up the joints, cracks,crevices etc. and there by making the foundation homogeneous.




FOUNDATIONS

Curtain Grouting - For making a curtain to cutoff/ minimize

seepage




FOUNDATIONS

Drainage arrangements - To drain off seepage water and thusreduce uplift. Some rules in this respect are



THE DAM FOUNDATION

The dam site is in an area of multiple ignimbrite flows from

volcanic eruptions over the last 2 million years.

The main dam footprint is founded on a 40-50m thick sheet of

Ongatiti Ignimbrite a point-welded tuff.

The upper part of the unit is very weak, with unconfinedcompressive strength of between 2 and 6 MPa, while below the

original dam cutoff wall the Ongatiti is considerably stronger

(up to 28MPa) and identified as the “hard zone”.

Beneath the Ongatiti Ignimbrite, about 40m below the base of

the concrete dam, are older ignimbrite deposits, identified as

Pre-Ongatiti for this project.



THE DAM FOUNDATION (CONTD.)

At interfaces between ignimbrite sheets there tends to be

unwelded material, either airfall tephras or unwelded

ignimbrite.

The most extensive interface deposit is between the

Ahuroa and Ongatiti ignimbrite units, known as the

Powerhouse Sediments with a thickness of 4 to 8m.



THE DAM FOUNDATION (CONTD.)



THE CUT OFF PROVISIONS (CONTD.)



THE SEEPAGE PROBLEM

Following lake filling and in the first 2 years of operation,

there was considerable leakage from the reservoir both from

the dam drains and from springs in the downstream rock.

Flows typically varied between 2200 liters/min and 4200

liters/min.

In May 1929, a large crack opened in the headrace channel

due to that the Lake was lowered. The lake was not refilled

until April 1932 while the headrace was lined.

During this time a single row cement grout curtain was

constructed along the full length of the dam and both

abutment cutoff walls.



THE GROUT CURTAIN



THE SEEPAGE PROBLEM (CONTD.)

After refilling the reservoir in 1932, leakage flows had been

reduced to 420 liters/min.

In the period 1932 to 1943, the records indicate that there were

several instances of sudden flow increases and a number of

holes on the right abutment were injected with hot bitumen

grout.

From 1943 to 1950 leakage was typically about 750 liters/min

but this declined to about 75 liters/min by 1950.

Leakage flows of about 75 liters/min were typical through the

period 1950 to 1995.



THE SEEPAGE PROBLEM (CONTD.)

In 1995 the dam toe area was cleaned up and seepage

monitoring arrangements rejuvenated.

at this time, eight holes were drilled into the dam foundation

to explore seepage conditions.

Two of these holes, referred to as OP05 and OP06,

encountered high water pressures at discrete depths and

each flowed at several hundred liters per minute after drilling.

It is now evident that the various grouting works only filled

voids where the vertical drill holes connected to open voids

in vertical joints, leaving other leakage paths open.



DETECTION OF HIGH PRESSURE SEEPAGE

Hole OP05 was subsequently used to measure the

pressure in the feature and hole OP06 was used as a

relief well.

With OP06 flow shutoff, the pressure in the feature in

1995 was about RL 97 m, 14 m below reservoir level. With

OP06 flowing at about 380 liters/min, the feature pressure

dropped to RL 87 m.

OP05 and OP06 pressure and flow were included in themonthly dam surveillance monitoring program thereafter.



INVESTIGATION TO LOCATE THE FEATURE



GROUTING THE FEATURE

When sharply rising pressures were identified in OP05 in

September 2000, a possible mitigation measure was to attempt

to grout the feature using OP05 and OP06.

There were two main concerns with this concept. First, little

was known about the nature of the flow paths within thefoundation and so the grouting operation and its effectiveness

would be very uncertain.

The high grouting pressures necessary to inject grout through

OP05 and OP06 could blow out the infill and so significantly

increase flow rates.



GROUTING THE FEATURE (CONTD.)

Grouting took 12.5 hours during which 11.5 cubic meters of

grout was placed.

At the start of grouting, relief well discharge was transferred to

the most upstream well so that grout being injected at the

downstream end was in still water.

After about 2.5 hours, grout was detected at the relief well. The

relief well was closed after 6 hours and a further 4.4 cubic

meters of grout was injected to refusal at 8 bar. Minor

modifications were made to the mix during injection in

response to the field observations.



VERIFICATION OF GROUT EFFECTIVENESS

The immediate response to grouting was that drainage flows

from the dam drains dropped from a total of 600 to 50 liters/min.



UPGRADING THE DAM FOUNDATION

The assessment process following fracture grouting in 2001

identified two key issues relating to the fissure systems:

The presence of highly erodible joint infill in the dam

foundation that is vulnerable to piping erosion.

The presence of near-lake pressure in areas under the dam

due to open fractures hydraulically connected to the reservoir.

Mighty River Power committed to upgrading the dam

foundation seepage control measures so that the risk of further piping incidents would become extremely low and high

pressures under the dam would be controlled.



LONG TERM SOLUTION

A comprehensive investigation took place to determine the

extent of foundation features requiring treatment to prevent

further incidents from developing.

A targeted and cost effective fix involving drilling and

concreting overlapping vertical piles from the dam crest

through the dam and underlying rock formation to a total depth

of 90m was selected to form four separate permanent cutoff

walls at selected locations beneath the dam.



LONG TERM SOLUTION (CONTD.)




An international Alliance between the dam owner

(assisted by their designer) and a contracting consortium

was formed to identify cut-off options, develop them and

implement the selected methodology.

Construction of the cutoff walls commenced in

September 2005 and was completed in mid 2007.

Operation of the reservoir has not been affected and

electricity generation has continued during the project

works.



CONCLUSION Arapuni Dam has had a history of foundation seepage incidents

since first filling in 1927.

The most recent leak was sealed in an emergency groutingoperation in 2001.

The dam’s owner, Mighty River Power Ltd, has undertaken a

dam foundation enhancement project to prevent future leakage

incidents from occurring.

The cutoff walls consist of overlapping 400mm diameter holesdrilled through the dam and underlying ignimbrite sheet with a

full reservoir.

The construction technique required for 90m deep cutoff walls

significantly extends international overlapping / secant pile

technology.

Construction was undertaken with close monitoring of the dam

foundation to ensure that the construction activities did not

generate another leak requiring emergency action and to ensure

that the dam’s safety was not compromised.



THANK YOU

Dam Foundation Treatment

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