Jon Williams dam sftey article townsville.pdf GHD Design Lenthalls Dam Gates

7/27/2019 Jon Williams dam sftey article townsville.pdf GHD Design Lenthalls Dam Gates

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Where is our Weir going an Unusual Upgrade!

Amanda Ament , Jon Wi ll iams, Malcolm BarkerGHD Senior Structural Engineer, Dams

GHD Manager, Dams

GHD Principal Engineer, Dams

Aplins Weir is located on the Ross River in Townsville, downstream from the Ross River Dam. Previouswork had identified Aplins Weir as exhibiting factors of safety below 1.0 under normal operating

conditions, with over 1000 persons at risk today in the event of failure. Originally constructed in the early

1920s, Aplins Weir has been upgraded and repaired following various failures on a number of occasions.

The end result is a complex reinforced concrete and steel sheet pile composite structure reliant for stability

on a number of unreliable components. This paper presents the historical data describing the current

configuration of the weir, and the analyses required to evaluate the extisting structure, leading to the

design of the proposed upgrade works. The final design involves a retrofit of large diameter cast-in-place

lined piles and a heavily reinforced base overlay slab designed to completely bypass all existing vulnerablesubstructure elements.

Keywords: Upgrade, Structural, Piling, Aplins Weir, Ross River.

Introduction

Aplins Weir is located on the Ross River in Townsville,downstream from the Ross River Dam. It is the last weirbefore the mouth of the river, and functioned both aswater supply and tidal barrier for the young Townsville.

Previous preliminary work had identified Aplins Weir asexhibiting Factors Of Safety (FOS) below 1.0 undernormal operating conditions, with over 1000 persons at

risk today in the event of failure. This makes Aplins Weiran extreme hazard dam, which, if it were to becomereferable, would require an upgrade to withstand the

Probable Maximum Flood (PMF) event. Some images of

the existing structure can be seen in Figures 1 and 2.

GHD were engaged by Townsville Water to undertake the

rehabilitation design in 2010.

Figure 1 View of central weir looking towards leftabutment

Data Mining a Timeline History ofAplins Weir

The original Aplins Weir was constructed in the early

1920s, and it has been upgraded and repaired followingvarious failures on a number of occasions. The end result

is a complex reinforced concrete and steel sheet pilecomposite structure. A good deal of historical datamining was performed in order to gain a clear picture ofthe present structure, which is necessary if futureperformance is to be predicted with any reliability.Information was sourced from drawings, historical

photographs, geotechnical investigations, site surveys andinspections.

Figure 2 View of weir and adjacent footbridge,including downstream rock armouring

The Original Apli ns Weir

The original structure constructed in the 1920s consistedof a sheet pile cutoff wall beneath the upstream edge of ahollow reinforced concrete weir and apron. It wasapproximately 2.3m high from top of crest to apron, andextended approximately 110m across the Ross River. Thesheet pile wall appears to consist of 380mm (15in) RSJsections installed flange to flange and held together with

clutches. There is also some indication of the presence ofdownstream cross elements within the sheet piling,however the frequency or presence of these is not clear.

Recent upstream geotechnical investigations in 2004show that there is a zone of very loose sands in the riverbed, with Standard Penetration Test (SPT) N values of


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less than 4. These sands form the top layer over anunknown extent of the river bed, up to a depth of

approximately 6m below current bed level. Consideringthe construction equipment available in the 1920s it canbe expected that ground improvements would have beenrequired in this sandy zone, in the form of additional

dumped rock, to enable the equipment to access the riverbed and operate. This expectation is supported bydrawings of the weir cross section created in 1951, whichshow a significant depth of rockfill downstream of thesheet piles, along with a 2.0m deep concrete cappingbeam on the sheet piles.

1943 Upgrade

In order to increase water storage capacity, the weir wasupgraded in 1943. A new reinforced concrete buttressed

structure was built, on the upstream side of the originalweir. A new upstream sheet pile wall was installed forthe full length of the first weir to form a cutoff. This

sheet pile wall appears to be a flat web type with thumband finger joints at either end. Beyond the extents of theoriginal weir near the right abutment, a reinforced

concrete diaphragm wall was installed in lieu of steelsheet piles.

The 1943 drawings also indicate cast iron pipe piles raked

in the downstream direction, connected to the upstreamend of the base slab. Neither the length nor the rakingangle of these piles is specified, however the drawings

suggest an angle in the order of 45. The same drawingsalso indicate that the base of the piles was belled outusing explosives, but neither the success of this procedure

nor the extent of the belling is known. These piles are infact not shown on some subsequent drawings.

Site investigations were performed with the aim of

verifying the presence or otherwise of these piles usingground penetrating radar, but the results provedinconclusive.

The new buttressed weir appears to have been simply castup to the face of the original weir, with the original weirsubsequently remaining in place for some years. The

nature of the connection between the two structuresduring this time is unclear.

Figure 3 Excerpt from drawing L4152 dated April1943. Note the upstream sheet piles, the raked cast ironpipe piles, the original downstream sheet piles and the

original 1920s cross section

Floods, 1946The first record of flood damage to Aplins Weir occurredduring 1946, when the right abutment and the first two

buttressed bays were lost. A new right abutment wasconstructed, including a 300mm thick concrete apron ofsignificant extent. The crest of the original weir was

demolished at this time, and the new weir was structurallyconnected to the substructure of the original weir. Thedetails of this connection are unknown site

measurements of the area are not consistent with any ofthe drawings. A detail of one version of the drawings canbe seen in Figure 3, which shows the original smaller weirand the buttressed upgrade on the upstream side.

Floods, 1950

During the 1950s floods, large erosion holes formeddownstream of the structure. A major one formedapproximately one third of the way along the structurefrom the left abutment. It was over 2.1m deep, 5m wide

and 7.5m long along the weir. The repair of this hole canbe seen in Figure 4. A second erosion hole of slightlysmaller size formed near the right abutment.

Figure 4 Repaired erosion hole in apron

These holes were filled with rock armour and overlaid

with concrete. Also at this time, large diameter rockarmour was placed approximately 15m downstream of thestructure, tied together with steel cables and bars

embedded into the individual rocks. A capping beamalong the weir crest was also installed for the full lengthof the weir.

And More Floods, 1956

This time the left abutment was lost, along with several

bays, perhaps up to the fifth bay of the central structure.The riverbed surface was scoured significantly.

The left abutment was rebuilt, consisting of a new sheet

pile wall and capping beam constructed upstream of theexisting structure. Here the drawings for this phase differfrom the photographic evidence an upstream clayembankment protected by rock pitching was specified onthe drawings, but this appears not to have beenconstructed. Also, the downstream scour damage appears

to have been rock filled and a concrete apron installed,however no concrete apron was shown on the drawings.

Descript ion of Aplins Weir Today

As the previous Timeline indicates, the reality of thestructure as it exists today is complex. Each component is

discussed in more detail below.


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Left Abutment

The left abutment consists of a sheet pile wall with a 1.2m

wide reinforced concrete capping beam, all of which wasconstructed in 1956. The upstream face of the sheetpiling is largely exposed to the reservoir, indicating that

the fill installed in 1956 has been lost, or was not placedto the full extent shown on the drawing. Significant and

consistent corrosion of the sheet piles can be observed.Downstream of the sheet pile wall there is a concreteapron slab, which may or may not be reinforced.Drainage holes exist at a number of locations on the apronslab. A significant void exists between the fill and the

underside of the apron slab, which was observed usingphotographs taken down the drainage holes. Downstreamof the concrete apron slab a scour hole has developed in

the river bank.

Typical Section

The typical cross section consists of an upstream and adownstream sheet pile wall, founded into clay, and cast

into a 6.9m wide, 457mm thick reinforced concrete baseslab. The upstream sheet pile wall consists of 12mmthick flat web sheet piles, and the downstream sheet pilewall of 380mm RSJs installed flange to flange andcoupled together with clutches. The inclined reinforced

concrete weir face slab varies in thickness from 203mm atthe top to 381mm at the base. The base connectionbetween the weir wall and base slab is a pinned key joint.

The face slab is supported by buttresses at 3.0m centres ina simply supported fashion, and by the continuouscapping beam along the top. The buttresses are

approximately 300mm thick, with a 483mm wide supportbeam immediately behind the face slab.

Drainage holes have been provided in the base slab, and

interconnecting gravel drains beneath the base slab areshown on the drawings. Water levels in the drainageholes were observed to be at, or below, the level of the topof the slab. No pressure build up is therefore presentbeneath the base slab.

Central Section, over Sand Layer

The central section of the weir is very similar to thetypical section, except that the sheet piling extendssignificantly deeper as it passes through the loose sand

layer. Raked pipe piles connected to the upstream end ofthe base slab are shown on the drawings, however the

extents, the raking angle and the founding depths of thesepiles is unknown. They are called up as being 152mmdiameter cast iron pipes which are filled with concrete.The base was belled out with explosives. Based on

geotechnical data and drawings, it is likely that the baseof the piles would have been founded in the stiff claysbeneath the sand layer.

Section Adjacent to Right Abutment

Beyond the extents of the original 1920s weir, the

substructure of the current weir is an upstream reinforcedconcrete diaphragm wall, 2.7m deep and 610mm thick, inlieu of the double sheet pile arrangement.

Right Abutment

The right abutment consists of a vertical reinforcedconcrete wall which appears to be 610mm thick.

Downstream of the wall is fill of an undescribed nature,capped with a reinforced concrete apron slab. Significanterosion and loss of the downstream rock armour hasoccurred adjacent to the vertical apron wall. The very end

of the apron slab has been undermined, and a significantvoid is now present beneath the remaining slab.

Downstream

The condition of the downstream large diameter rockarmouring is variable. Much of the armour itself is still

present, except for the area adjacent to the right abutment.Movement of the individual large rocks has occurred,where the steel cables installed to tie the individual rockstogether have been damaged to the point where many areno longer effective.

The condition of the apron slab is also variable, with a

number of damaged areas, including a zone which

appears to have been caused by movement of theunderlying rockfill during a flood. This suggests the

rockfill beneath the apron slab, or possibly its foundation,is not entirely stable. There are also a number of largeholes which appear to be unrepaired test pits.

Observed Structural Distress

Little structural distress was observed, in the form of

structural cracking or differential deflections. The line ofthe weir face slab is straight with no apparent deviations.

Geotechnical Conditions

The site is generally underlain by stiff clays, with a

variable thickness of loose sands above. The loose sandsexhibit SPT N values of less than 4, and can reachthicknesses of 5.0m in the centre of the river channel, andtowards the left abutment.

Soil testing of the material beneath the base slab showedlow concentrations of sulfates and chlorides, pH values

around 6.5 and resistivities in excess of 5000 ohm-cm.These results suggest that the foundation materials are notaggressive. Groundwater is fresh beneath the structure, inspite of the tidal level reaching up to 2m above the

downstream toe level.

Hydraulic Conditions

Earlier work performed by AECOM in 2009demonstrated that Aplins Weir does not exhibit drownout

even at a flood of at least the 1 in 2000 AnnualExceedence Probability (AEP) event (AECOM, 2009).Indeed, a significant afflux of approximately 2.0mpersists from the 1 in 100 AEP event up to the 1 in 2000AEP event (AECOM, 2009). Discharge for the 1 in 2000AEP event is 1920 cumecs (AECOM, 2009).

GHDs structural stability investigations demonstratedthat the critical hydraulic loading condition occurs duringfloods below the 1 in 50 AEP event. This corresponds toa discharge of 300 cumecs over the weir.


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Analysis Cal ibration

The first task for the desktop analysis was to createstructural models which demonstrated behaviourconsistent with that observed in the real structure. This

was an iterative process performed in conjunction withthe data mining exercise, as the desktop work highlightedcritical structural areas and suggested geometric and

structural requirements which in some cases were hazilydefined by the structural drawings if they were definedat all. The aim of this activity was to gain anunderstanding of the particular features of Aplins Weir

which are vital to its overall stability.

Two series of models were created for this task cross

sectional models, and an overall longitudinal model.

Cross Sectional Models

Three different cross sectional models of the main weirwere created, corresponding with the three differentstructural geometries present along the weir from onebank to the other. These models were three dimensional

plate models of a single bay of the structure. The threelocations modelled were

The cross section adjacent to the right abutment, withthe diaphragm wall in leiu of the two rows of sheet

piling

The typical cross section, founded directly into thestiff clays

The central cross section, across the loose sand profile

Sensitivity analyses were then performed, with the keyoutput parameter being deflection in the downstreamdirection under FSL (Full Supply Level) reservoir

loading, in order to calculate an effective downstreamstiffness value for later input into the longitudinal model.The parameters altered during these analyses were

The presence or absence of the cast iron raked pipepiles

The moduli of subgrade reaction, for both the loosesands and the clays

The section properties of the downstream sheet pilingbetween RSJ sections and flat web type sections

Forcing volumetric conservation of the materialbeneath the base slab

The presence or absence of the downstream rockfillplaced during the original 1920s construction

The structural nature of the connection between theoriginal and upgraded weir, between a pinnedconnection and a rigid connection (as there are no

consistent records of this area)

For the first cross section with the diaphragm wall, themodulus of subgrade reaction was the only variable, as allother elements were not applicable. Deflection ranged

from approximately 20mm to 35mm.

Deflections of the second cross section, the typicalsection, ranged similarly from 15mm to approximately

30mm. Negligible effects were noted from the cast iron

raked pipe piles or volumetric constraint of the foundation the dominant effect was in fact the alteration from a

fixed to a pinned connection between the original weirand the upgrade works.

By far the greatest range of deflections was obtained by

the central cross sectional model, with values anywherebetween 15mm to 260mm depending on parameters. By avery significant margin, the parameter having the mosteffect in reducing downstream deflections was the rockfill

placed downstream of the original sheet pile, underneaththe apron slab.

Longitudinal Model

The final stage of the calibration was to create alongitudinal model of the weir with the three different

section types included where appropriate along its length.This model enabled an understanding to be gained of thestructural implications of having varying downstreamstiffnesses along the length of the weir.

The model consisted of the base slab, at full extent, withtransverse spring stiffnesses applied at 3.05m centres.

The stiffness values corresponded to the appropriate cross

sections, and were calculated using the downstreamdeflection values determined from the cross sectional

models created earlier. A consistent suite of values wasapplied for each run of the longitudinal model.

In this way, the parameters most important in contributingto the overall stability of the weir were determined. Thiswas done by correlating the outputs from the longitudinalmodel most particularly the in plane shear demands in

the base slab with observed structural distress (verylittle) in the actual structure.

Calculations demonstrated that the capacity of the

existing base slab in shear is very low, due to the low

volumes and strength of the reinforcement provided (eventhis has a degree of uncertainty different drawings show

different amounts of reinforcement). This, therefore,places an upper limit on the amount of differentialdeflection the structure can resist along its length before

the base slab exhibits shear distress. The ability for thestructure to span across softer zones in the cross valleydirection is therefore quite limited.

Results

The results of this first calibration phase were very

interesting. In essence, no combination of parameters

except for the inclusion of the passive restraint of the

compacted rockfill downstream of the section over thesand layer accounts for the ongoing structural stabilityof the weir. If that rockfill were to be lost, the analysesindicate that the base slab would fail in diaphragm actionand the weir would be lost.

This event has come close to occurring, with thedevelopment of the large 2.0m deep 7.5m wide erosion

hole which opened up in the apron in the 1950s. If thaterosion hole had extended further upstream andcompromised the degree of compaction of the rockfill atthe toe, a large section of the weir may have been lost at

that time in addition to the right abutment that failed

during that event.Additionally, it was shown that the connection between

the original weir and the upgrade works acts with a fair


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degree of rigidity. Modelling this connection as pinnedalso results in high values of diaphragm shear in the baseslab.

Paths to Failure

The next phase of analysis was an investigation of thepaths to failure of the weir into the future. This

considered issues such as ongoing corrosion of structuralsteel elements, as parts of the structure are 85 years old.

A number of analysis runs were conducted using thecombination of parameters which led to the desktopoutput being consistent with the real structure. In theseruns, the following components were disabled in turn,modelling loss due to deterioration:

Pipe piles Upstream sheet piling Downstream sheet pilingOf the above, only the loss of the downstream sheet pilingwas of any significance from a structural perspective.

Loss of vertical support beneath the toe of the structureresults in bearing failure and rotation of the structure.

Seismic Considerations

The loose sands in the area of the weir have a highpotential for liquefaction under a minor to moderateearthquake. Liquefaction could lead to loss of lateralsupport to the downstream sheet piling, resulting in

buckling and settlement / rotation of the structure.

Therefore, the downstream sheet pile is considered to be

vulnerable under both the deterioration scenario and theearthquake scenario.

Upgrade ConceptTo satisfy current safety criteria, the upgrade concept for

Aplins Weir needed to answer the followingrequirements:

The weir should no longer be reliant upon thedownstream rockfill for sliding stability, which isvulnerable to erosion under flood events and difficult

to quantify from a capacity calculation perspective

The weir should no longer be reliant upon thecompetence of the moment connection between theoriginal and the new weir, which was constructed to

an unknown detail

The weir must have robust vertical support to thedownstream edge of the base slab. Currently the

vertical support provided by the RSJ sheet pile wall iscontingent not only on the condition of the steel, butalso on the ability of the substrates to provide

sufficient lateral restraint to prevent column buckling

failure of the steel members

Any solution should be structurally quantifiableThe chosen upgrade concept answering all the aboverequirements consists of retrofitting a 1200mm diameterlined cast in place pile in each bay, at the downstreamone-third point of the existing base slab, centrally placed

between buttresses. The pile will be structurallyconnected to the existing weir through a 650mm thick

heavily reinforced concrete overlay slab dowelled to theexisting structure. The intent of this concept is tocompletely bypass all existing vulnerable substructureelements, and result in a structure with a quantifiable

capacity.

Design Philosophy

Design scenarios were grouped into base load cases andextreme load cases. The load cases and their individual

design scenarios are described in the followingparagraphs.

Base Load Cases

Base load cases (BLCs) were assigned a required FOS(Factor of Safety) of 1.1. The relevant design scenarios

were:

BLC1: Reservoir at FSL following flood whichcaused loss of downstream rockfill

BLC2: 1 in 10,000 Average Recurrence Interval(ARI) earthquake, with downstream rockfill present

and effective

BLC3: All flood cases, with downstream rockfillpresent and effective

Extreme Load Cases

Extreme load cases (ELCs) were assigned a required FOSof 1.0. The relevant design scenarios were:

ELC1: 1 in 10,000 ARI earthquake, with downstreamrockfill either not present or ineffective

ELC2: All flood cases, coupled with loss ofdownstream rockfill

Detailed Analysis of Upgrade

The aim of the analysis of the upgraded structure was toquantify the design actions within the new weircomponents in order to proceed with the detailed design.In this case, however, this was not a single step process,as is often the case. The main complications were:

The complex nature of the existing foundations andground conditions

The limited ability of the existing structural elementsto plastically redistribute under ultimate conditions

The existing structure will still be under partial loadduring the installation of the strengthening elements,

as a drawdown of the reservoir of only 1.5m isexpected to be possible

Therefore, in order to obtain meaningful results, time stepmodelling which included construction and load

sequencing was performed.

Geometry

A three dimensional finite element model of the entirestructure was created, with emphasis placed on capturingan accurate structural mass and geometry. This was

achieved using plate elements for the existing weircomponents and the new overlay slab, and beam elementsfor the retrofitted piles. The new base overlay slab was

modelled separately from the existing, with the twoconnected using master-slave links. These links were the


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only modelled connection between the existing structureand the retrofitted components, reflecting the reality of

the final structure.

Materials

20MPa concrete was used for the existing weircomponents. The new works have been specified as40MPa concrete.

The final soil parameters used are listed below:

Modulus of subgrade reaction for rockfill atdownstream toe 80,000kN/m

3

Modulus of subgrade reaction for stiff clays80,000kN/m3

Modulus of subgrade reaction for loose saturatedsands 20,000kN/m

3

Spring stiffness applied to retrofitted pile varyingwith depth, and obtained from the SPT values fromthe geotechnical investigations. Values ranged from14,400N/mm at the top of the stiff clay surface, to

44,640N/mm at 7m depthAnalysis Procedure

The nonlinear solver was used to capture the time stepeffects, as follows:

Applying self weight and reservoir load atconstruction drawdown levels, disengage theretrofitted components from the existing structure inthe model. The results of this run represent the pre-existing stress condition in the existing structure at

the time of construction

Re-engage the retrofitted components to the existingstructure, and continue the analysis to apply reservoirload at FSL. This provides results at normal

operating conditions

Continue the analysis applying the 300 cumec floodload case. This provides the results for BLC3,

defined earlier

Continue the analysis, removing the restraining effectof the downstream rockfill. This provides the resultsfor the second of the extreme load cases, described

earlier

Continue the analysis, reducing the water level backto FSL. This provides the results for BLC1. An

example of this output can be seen in Figure 5

Conducting the analysis in this fashion tracks the load

transfer in the structure, as significant events occur.

Figure 5 Preliminary modelling output of nonlinear

time step process, capturing the partially loaded existingstructure and subsequent strain compatibility with the

retrofit components. Smaller preliminary model showninstead of final complete structure for clarity

The seismic load effects were obtained using the spectralresponse solver. The acceleration response spectra fromthe seismicity study for the Ross River Dam were used forAplins Weir, due to the relatively close proximity of thetwo sites.

An initial structural damping factor of 5% was used. This

value is appropriate if the structure is operating entirelywithin the elastic range, and therefore not accessing theenergy dissipation associated with the creation of plastic

hinges. This was later verified following the detailed

design, taking the contribution of the steel liner intoaccount during the calculation of the pile capacity.Concrete cast in place piles with permanent steel linersexhibit capacities greatly in excess of the normalattributed capacity calculated when ignoring thecontribution of the steel liner.

Piping and Seepage Considerations

Seepage is currently controlled by a double row of sheetpiles, which will remain in place following the upgrade.As stated earlier, the soils beneath the structure are not

considered aggressive. As such, a corrosion rate ofapproximately 0.01mm/y can be expected, at which rate

the design life of 100 years will be easily achieved from aseepage perspective.

As the soils under the structure are sandy, piping anddispersion are considered unlikely.

Conclusion

This paper presents the investigation, analysis and design

activities undertaken by GHD during the design of theupgrade of Aplins Weir, on the Ross River in Townsville.

Extensive historical data mining was undertaken due tothe long and interesting history of the structure, in orderto gain an understanding of the existing situation.

Nonlinear time step finite element analysis was requiredto obtain meaningful results for the detailed design phase,as more simple analyses would not have captured thestress history effects. The final upgrade solution is an


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unusual one, requiring an intensively structural retrofit oflarge diameter cast in place piles with a heavily reinforcedconcrete overlay slab.

Acknowledgements

The authors gratefully acknowledge the support of theClient, Townsville Water, in the publication of this paper.

The supply of the photographs and drawings from theirarchives for this project was very much appreciated.

References

AECOM 2009. Aplins, Gleesons and Black Weir RiskAssessment.

Documents

Jon Williams dam sftey article townsville.pdf GHD Design Lenthalls Dam Gates