Recommendations for Slurry Filling

In document Investigation and remedial methods for subsurface erosion control in Banks Peninsula loess (Page 142-175)

No slurry filling should be attemp ted without thorough engineering geological investigation of the site. The sources of water responsible for the erosion and the location of the tunnel inlets and ex its are of particular importance.

- All sources of eroding water should, wherever possible, be eliminated, even if drainage is specifically provided.

- Whenever access allow s, drain coil should � placed along the tunnel floor extending through the area to be filled. This should be place w ith due regard to fall and pinned in position prior to slurry filling.

- The slurry filling of cavities with an average diameter less than 50 0 mm is not recommended. Where hoses can be forced into the area requiring stabilisation and then progressively withdrawn during pumping, infilling may still be possible. The forcing of flex ible hoses down narrow tunnels is seldom easy .

- Prior planning should include provision of more than one slurry entry point.

will seldom travel even under ex treme

Ex perience to date suggests that slurry

more than 4 metres from the entry point, gradients. A series of auger holes spaced at 3 4 metre intervals along the tunnel line i s the ideal arrangement.

- Tunnel ex its should in be blocked off whilst maintaining venting. Box ing must be strong enough to w ithstand the head of high density material (saturated slurry is approx imately twice as dense as water) .

- Slurry mix design should follow the follow ing guidelines: Total binder is ex cess or equal to 8 % by we i g ht of the dry aggregate w ith a minimum hy drated lime content of 5% by weight.

1 38

Additional cement i n c reases the ultimate

sl urry strength at the expe nse of permeability.

Additional lime improves the wet mix workability and ease of pumping .

Aggregate should be graded from very fine sand to

m edium gravel with approximately 40%.

more favourably with

a minimum sand c o ntent of

Crushed hydrated sand lime appears and is to react therefore

preferable in applications where cement content is to be minimised.

The slurry mix water content should be slightly in excess of that required for complete saturation of the aggregate and binder . Final water content can be adjusted on site.

- Placement can be directly from the truck chute or by concrete �· If pumped , careful mix design is vital . Large boom pumps are most suitable in applications requiring high pump pressure . Smaller and cheaper pumps generate lower pressures and can block easily , especially during slurry placement in relatively narrow and restricted tunnel openings.

4.1 1 CONCLU SION

THE SLURRY FIL LING OF SUBSURFACE CAVITIES APPEARS TO BE A FEASIBLE AND EFFECTIVE TECHNIQUE IN THE STABILISATION OF GROUND ABOVE AND AROUND LARGE EROS ION TUNNELS . LIME AGGREGATE S LURRY MIXES OFFER ADVANTAGES IN WORKABILITY AND PERMEABILITY OVER CEMENT EQUIVALENTS , ALTHOUGH A SMALL PROPORTION O F CEMENT IS GENERALLY NECES SARY TO IMPART STRENGTH.

MIXES O F THIS TYPE CAN BE READILY SUPPLIED BY COMMERCIAL CONCRETE FIRMS AND REQUIRE NO ON SITE MIXING. ONCE PLACED THE SLURRY MIXES UNDERGO MINIMAL SHRINKAGE , AND EXPERIMENTS SUGGEST A DIFFUSION STABILISATION REACTION IS INITIATED

BETWEEN THE S LURRY AND THE SURROUNDING LOES S . EROSION

AROUND THE S LURRY INFILL IS ALWAYS POSSIBLE IF SIGNIFICANT WATER STILL ENTERS THE TUNNEL SYSTEM. THE STABILIS ATION REACTION , IN AS S OCIATION WITH A S L URRY P ERMEABILITY

OF AT LEA S T THAT OF THE SURROUND ING LOES S , HELPS

TO REDUCE THIS RISK .

COMPARISON OF THE SLURRY FIL L ING TECHNIQUE W ITH THE

ALTERNATIVE OPTION OF DEEP EXCAVATION AND BACKFIL LING

SUGGESTS SLURRY FILLING COSTS S IGNIFICANTLY LES S .

COST

SAVINGS MAY BE AS MUCH AS HALF WHERE EROS ION HAS OCCURRED

UNDER ROADING .

IN S ITUATIONS WHERE TUNNEL EROS ION AND

SUBS IDENCE HAS OCCURRED UNDER BUILDINGS , SLURRY FILLING MAY

PROVIDE THE ONLY ECONOMIC AND EFFECTIVE SOLUTION .

CHA PTER 5 : C ON CLU SIONS

1 )

Subsurface erosion can d evelop through several mechanisms in a wid e range of fine grained soils und er d iverse climatic cond itions. The hazard s presented by subsurface erosion in Banks Peninsula loess (principally Subsid ence,

significant

Local Flood ing, threat in both

and Siltation) , represent a agricultural and resid ential areas of the Peninsula.

2 ) Geotechnical site investigation prior to d evelopment is recommend ed in loess hill areas. The investigation programme should includ e laboratory tests such as Pinhole Erod ibility, Crumb Dispersion Testing, and U niaxial Expansion which can provid e useful guid es to subsurface erosion risk. The pinhole test is a measure of soil erod ibility and as such is influenced by a number of soil properties (e.g. grain siz e, d i spersion and slaking) •

.!..!..

is not 2..!l effective test £!_ d ispersion and a revised test class criteria and classification is presented which clarifies this.

3) Field method s of subsurface erosion investigation includ e Geomorphic Mapping which provid es the best first approximation to the location, extent and form of subsurface erosion features . Photographic Mapping Method s (Black

&

White, Colour, and Infra Red ) have only limited application as aid s to mapping in d etailed subsurface erosion investigation. Normal colour photographs

cond itions of low angle sunlight, w hich

taken und er emphasise microrelief, appear to be more useful than infra red photographs and are simpler to take.

A lthough Geophysical Method s can be used to define the d epth of soil cover and u nd erly ing bed rock morphology they cannot be used with confid ence to d etect or d elineate subsurface cavities. At present the only method s able to d efine the exact location and extent of su bsurface cavities are augering and cavity entry .

Pumping Dyed Water into cavity entrances and surface collapse holes is a simple and effective technique for d etermining the ex tent of cavity interconnection and the

points of tunnel ex it. By pumping water onto natural slopes ,

culverts, d isguised

road surfaces and

preferred surface

entrances

paths, or into drains and water flowpaths and previously can be located . Smoke Tracing appears to

cavity

be a complimentary investigation tracing tool

with a more restricted application.

4 ) Preventing subsurface erosion from d eveloping is alway s cheaper and preferable to attempting a remed y and this requires early recognition of the erosion hazard. Where erosion has already d eveloped , the potential remedial method s divid e into two general groups, all requiring a complete und erstanding of the site erosion process. The simplest and cheapest methods involve the interception of contributing surface water, subsurface intrasoil seepage, or tunnel through flow before the erosion problem becomes more serious.

S ) Where cavities are already large , subsidence and ground col lapse frequently continue d espite water interception, and more expensive method s of ground support are required. Simply filling the surface collapse holes is seldom effective. If a buiding found ation is threatened , piling through the cavity to bedrock may provid e structural support. In accessible areas, excavation and backfilling of the cavity may be practical, however this requires careful und erdrainage d esign. Well compacted lime stabilised loess can often provid e an' effective and economical non - erodible backfill.

6) The slurry filling of large subsurface cavities throug h

auger holes from the surface appears to be a feasible and effective remed ial technique. Drainage should be provid ed along the cavity floor prior to slurry filling wherever access allows.

L ime/cement/aggregate slurries can be d esigned which pump well and undergo minimal shrinkage on setting yet

provi de suffi ci ent dry strengt h to support loess erosi on cavi ti es.

i ni ti ate

Laboratory studi es suggest slurri es of a di ffusi on stabi l i sati on reacti on

thi s ty pe i n the surroundi ng loess whi ch

around the slurry i nfi ll. to confi rm thi s.

may help prevent renewed erosi on Further fi eld tri als are requi red

Compari son of the slurry fi lli ng techni que w i th alternati ve opti ons such as excavati on and backfi l li ng, suggest that, under opti mum ci rcumstances, slurry fi lli ng can reduce costs by almost a half.

ACKN OWLEDGEMEN TS

The follow ing organisations and ind ivid uals have my sincere thanks for their assistance d uring this thesis stud y: N orth Canterbury Catchment Board for fund ing and guid ance, in particular Mr D. Saund ers.

My supervisors Mr D. H. Bell and Dr J. R . Pettinga of their guid ance and help d uring the project and the ed iting of thesis d rafts.

Christchurch Drainage Board for loan of equipment and help in d ye and smoke tracing, and in particular Tony King and John Hod gson.

L y ttelton Borough Council and Mount Herbert County

Council for personnel and equipment, in particular

County Engineer John Christiansen.

Heathcote County Council for personnel and equipment, in particular Mr Chen.

Messrs D. Chapman and M. Gebbie for the loan of their water pumping equipment.

Mr G. Salt for encouragement and suggestions in research, and the staff at Soi ls & Found ations (197 3 ) Ltd for support and help draughting and typing parts of this thesis.

University of Canterbury Geology Dept. technicians, in particular G . Coates, S. Nichols, A. Downing, D. Jones, A. Nicholas and C. Thornley .

Miles R eay, A. Ad ams, J. Morris, P. Glassy , D.

Johnson, R. Sand ers, J. Hackwell and P. Kelsey for help with field work, d iscussion, sense of humour, and d ata from their own work.

144

Mr N. Crampton for discussion and the use of data from his recently completed thesis.

Lee Leonard for her excellent draug hting and M. Webb for ty ping tables.

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In document Investigation and remedial methods for subsurface erosion control in Banks Peninsula loess (Page 142-175)