CONTROL IN BANKS PENINSULA LOESS
submitted in partial fulfilment
of the requirements of the Degree
Master of Science in Engineering Geology
University of Canterbury
Mark. D. Yetton
University of Canterbury
b;> .NS .Y4&
CHA PTER PAGE
ABSTRACT • • . • • • • • • • • • • • . • • • • • • . • • • • • • • . • • • • • • • • • 1
1.2 Geology of Banks Peninsula
2 3 1.3 The Loess Deposits of Banks Peninsula... 4 1.4 Climate and Vegetation •....•••.••••.•• 7 1.5 Subsurface Erosion
1.5.2 Overseas Examples
1.5.3 New Zealand Examples •.••.•... 1. 5. 4 Banks Peninsula ...•••••••. 1.5.5 Consequences
INVESTIGA TION METHODS FOR SUBSURFACE EROSION
7 10 17 19 22
2. 1 In trod uc t ion
. • . . . • . . . • . . . • . . .24 2.2 Site Investigation Philosophy
2.3 Soil Material Properties
&the Pinhole test 27 2. 3. 2 Other Soil Parameters
A ) Dispersion (Crumb Test) •• B) Slaking (Uniaxial Exp.) •. C) Grainsize
. . .D) A tterberg Limits
. . .E) Dry Density
. . .2.3.3 Erodibility Calssification
2.4 Soil Mass Characteristics 2.4.1 Lay ering
2. 4.2 Defects
2.5 General Investigation Methods 2.5.1 Geomorphic Mapping
2.5.2 Photographic Techniques •••••. 2.5.3 Geophy sical Methods ••.•.••••• 2.5.4 Auger Probing
33 34 37 39 40 41
2.5.6 Dye Tracing 2.5.7 Smoke tracing
66 67 2. 6 Chapter Summary • . • . . . . • . • . • • . . . 71
3 REMEDIAL METHODS FOR SUBSURFACE EROSION CONTROL
3 . 1 In trod uc tion • • • • . • . • • . • . • . . . • . • • 7 3 3.2 Water Interception Methods
3.2.1 Surface Water
3. 2. 2 Seepage
. . . .3.2.3 Tunnel Interception •..•....•.
3.3 Subsidence Prevention
3.3.1 Filling Collapse Holes •..•••• 3. 3. 2 Pi 1 ing • . • • • • • • . • • . • . . . • . • . • • • 3.3.3 Excavation & Backfilling
3.4 Chapter Summary
4 THE SLURRY FILLING OF SUBSURFACE EROSION CAVITIES 4. 1 Introduction
. . . • • . . . • •4.2 Work on Slurry Filling
4.3 Objectives . . . . 74 74 79
83 84 85 97
99 99 100 4.4 Choice of Slurry Binder ••••..••••••..• 101 4.5 Aggregate Properties ••..••...•••• 102 4.6 Properties of Slurry Mixes
4.6.1 Strength •.••.••••.•.•.••... 106 4.6.2 Permeability ...•...•.•••..••• 112 4.6.3 Durability .•..••.••.••.•..••• 113 4.6.4 Swelling ..•..•.•...••••...••. 115 4. 6. 5 Shrinkage . . • . . • . • . . . • . • . . . • • . 11 7 4.6.6 Diffusion Stabilisation •...•• 119 4.6.7 Workability ..•••••..•.•....•• 123
4.6.8 Pumpability 124
4.7 Slurry Placement Techniques
4.10 Recommendations for Slurry Filling .... 137 4.11 Chapter Summary
. . .138
5 CONCLUSIONS • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 1 4 0
6 ACKNOWLEDGEMENTS . . . 143
. . .
Appendix 1: Lyttelton Reserve
. . .Appendix 2 : Andersons Corner
. . .Appendix 3: Parklands Drive
. . .Appendix 4 : Ngaio Lane
. . .Appendix 5: Hender sons Culvert
. . .
Appendix 6: Loess Laboratory Methods
. . .Appendix 7 : Slurry Laboratory Methods
. . .Appendix 8: Slurry Mix Design
. . .Appendix 9 : Geophysical & Tracing Equipment .•
Appendix 10: Sample Storage classification
161 169 177 189 204
Subsurface erosion in the loessial soils of Banks Peninsula has been studied by numerous researchers and the mechanism and form of the erosion process is now well understood. This thesis study is a practical evaluation of potential investigation methods at sites of subsurface erosion, with the additional evaluation of suitable remedial options for erosion stabilisation.
Prefered laboratory procedures include the pinhole test, the crumb test and the uniaxial expansion test. While the pinhole test is the most useful single measure of erodibility, the crumb test is the more effective test of dispersion. When the results are combined, these three tests appear to give a reliable guide to soil material erosion potential. Soil mass layering and defects also provide important controls on subsurface erosion, and auger and testpit logging allows soil mass characterisation.
Geomorphic mapping provides the first approximation to the location, extent and form of subsurface erosion. Photographic methods, as aids to this mapping, appear to have only limited application. Geophysical methods are not able to define cavity location or extent but are still useful as general site investigation tools. Dyed water and smoke can be used to trace cavity connections and establish tunnel outlets.
The interception of surface before it enters the area of principal the cheapest remedial technique.
1. 1 OBJECTIVES
CHA PTER ONE: INTRODUCTION
A ) To assess the usefulness of various laboratory and field methods in the investigation of sites of subsurface erosion in the loess of Banks Peninsula, specifically:
- The pinhole "dispersion" test and other laboratory soil characterisation tests. - Mapping and photographic techniques.
- Geophysical techniques for cavity location. - Dye and smoke tracing methods.
B) To determine the effectiveness of a range of remedial methods for the prevention of subsurface erosion and the stabilisation of erosion affected sites in Banks Peninsula loess.
C) To develop a hydrated lime slurry suitable for pumping into subsurface cavities and to assess the usefulness of this remedial technique.
D) To apply these potential methods of investigation and remedial work to a number of sites of active subsurface erosion and to monitor the result. A total of five sites
were studied in detail (refer A ppendices 1 to 5 for the site investigation reports along with associated maps and '/ crossections).
(I' ·. c. -(I.-, •
These working objectives were developed in view of the theoritical bias of much of the previous work on subsurface erosion in Banks Peninsula loess (see for example
1. 2 THE GEOLOGY OF BA NKS PENINSULA A BRIEF REVIEW
Banks Peninsula is d ominated by the erod ed cald eras of the Ly ttelton and A karoa volcanoes which were active in the Mid to Late Miocene.
mod ified the early view
Recent research has extensively of the nature and origin of the basement rocks and the read er is
1985 for a comprehensive and up to 1.1 summarises the d istribution and the location of the on lapping Plains.
CJBoulder bank < I • SIOddort a Oudl ".
VOlconlcs :l·5 •5·I _: .. � AkaR>a �ccno 8- 9 Ii, Ml Herber!
� �rohon �ca,o 10- II
o 4 8 km I ,__. 1eee,1
referred to Weaver et al, d ate d escription. Figure
of basement lithologies gravels of the Canterbury
1.1 Simplified geological map of Banks Peninsula showing the d istribution of the main volcanic units and the Pliestocene gravels of the Canterbury Plains (after Weaver et al, 1985 ) .
The main volcanic rapid ly d owncutting rad ial
cones d eveloped offshore, d rainage patterns were
--- -- - -
---4established on these by the Pliestocene. The glacial outwash gravels and the interfingering marine sed iments which form the Canterbury Plains, were d eposited d uring the Pliestocene when extensive valley and pied mont glacier systems period ically expand ed
A lps. The coalescing outwash such as the Waimakariri and
and retreated in the Southern fans from the valley systems Rakaia progressively infilled
the area between the alps and the erod ing volcanic centers. During glacial events the sea level was as much as 100m below present and consid erable areas of gravel outwash surface w ere exposed to the East of the Peninsula.
1.3 THE LOESS DEPOSITS OF BA NKS PENINSULA
The w id espread prod uction of quartz and feld spar rich silt sized glacial flour, and later transport and d eposition by the d ominant north-westerly wind , prod uced an extensive mantle of aeolian clayey silt (loess) on Banks Peninsula. The exposed continental shelf to the east of the peninsula also appears
(Raesid e,19 64) . d eposition of
to have contributed The age, origin, the loess has been
to the thickness d iscussed
loess cover and mod e of extensively elsewhere (see Hoskings, 19 62; Raesid e, 19 64; Ives, 19 73; Griffiths, 19 73 & 19 74; Bell, 19 78) .
Paleosols within the loess suggest accumulation was episod ic and there are no marked regional variations in loess thickness or character between the inward and outward facing slopes of the Peninsula. Much of the original airfall loess (in situ loess, after Bell & Trangmar [in prep]) has been subsequently reworked by processes such as slope w ash, solifluction and soil creep with a correspond ing thickening of colluvial loess toward s the lower ground . The majority of serious subsurface erosion within the loess is in this loess colluvium (Trangmar, 19 76; Bell
&Trangmar, in prep) .
variation has resulted from differences in vegetation cover since the Pliestocene, the non calcerous loess having formed in the relatively humid zone below a native forest cover.
The ty pical loess profile shows vertical variations in colour, plasticity , erodibility , density and swelling. This is fully discussed in the next chapter (Section 2.4.1) and Figure 2 .10 summarises these variations. Adopting the terminology first introduced by Hughes (197 0) , the compact lay er immediately below the surface lay er is generally a low plasticity clay ey silt (CL-ML in the Unified Soil Classification system, USBR 1963) which is in contrast to the non plastic silt rich material above and below it (both the lower Surface lay er and the Parent lay er generally plot as ML on the plasticity chart) . This clay ey compact lay er ty pically display s vertical cracking w ith associated gammation and mottling in many areas. While the exact origin of this lay er is in dispute (see discussion in Bell, 197 8 ) it appears to play a critical role in the development of subsurface erosion ( Evans, 197 7 ; Bell & Trangmar, in prep) .
Banks Peninsula loess is dominantly quartzo feldspathic and the few heavy minerals the loess contains confirm a Torlesse Group Grey wacke origin (Raeside, 1964) . Bell (197 8) and Miller (197 1) report a dominance of low to non sw elling clay minerals (illites, hy drous micas, vermiculite [I]) , however montmorillonite and vermiculite
[II], presumably formed by weathering, result in the increase in plasticity and shrink/swell behaviour in the compact lay er. In all the lay ers a proportion of the clay sized loess (i.e. that less than 2 microns) is actually finely ice ground quartzo - feldspathic material with little cohesion, and this further decreases the plasticity while increasing the susceptibility to erosion (Miller, 197 1; Bell, 197 8 ) .
PARAMf;.!J:;B TYPICl\L VALUE OR RANGE OF VALUES SOURCE Rf:FERENCE
POROSITY 30-40\ Birrell, Packard (1958). Miller (1971).
VOID Rl\TIO .6-.7 " " "
ATTERBERG LIMITS LL 18-33 Alley (1966), Hughes (1970). PL NP or 17-22 (in compact layer and c.rampton (1985), This
PI Up to 12
GRl\IN SIZE Adopting sand> , 625mm < Silt > 2 micron < Clay " " Sand around 10\, Silt 65-80\, Clay 11-25\.
Undisturbed Varying with depth, S layer Avera1e c l.54t/m3 Evans (1977). Crampton (1985 Samples (l.39-l.62t/m range) 3 This study,
C layer Avera�e c l.64t/m (l.5 l.88t/m range) P layer Avera�e = l.55t/m3
Recompacted at 1.8 to 1.9 t/m3 Alley (1966). This study, OMC
LINEAR SHRINKAGE From 1-2\ in lower S and the P layer to >5\ in This study. the compact layer.
Undisturbed l. 5 X 10-7 Ill/sec (n. b. for sand rich sample) Birrell, Packard (1953). Samples
In-situ Test Around 10-7 m/sec Sanders (1986). Recompacted
(Standard 10-• to 10-, m/sec Evans, Bell (1983).
Proctor) ANGLE OF FRICTION Residual (ring
shear) 35-37Q Salt (1983) ,
Peak (Triaxial) 30-32 • Hackwell (1986),
COHESION 85 - 112 kPa Hackwell (19B6),
COMPRESSION INDEX Cc• .17' (with a l. 7\ volume change dry to Birrell, Packard (1953). saturated)
Ph Generally acidic but varying from 5 (S layer) Miller (1971). to 7 in deep P Layer
SOWBLE SALT Increasing with depth from l meq/L to 60 meq/L " " CONC. in deep P Layer.
EXCHANGEABLE .9 in S layer to as high as 41 in deep P Layer Hughes (1970), SODIUM PERCENTAGE
SEISMIC VELOCITY 250 - 400 m/sec Crampton (1985) . This
RESISTIVITY Varying with depth from around 90 ohm/metres This study. near surface to less than 10 ohm-metres at
depth in P layer.
CONDUCTIVITY From l. 0 mho/cm x 10-, to 14 mho/cm x 10-, Birrell, Packard (1953).
---with depth. '!'his study.
Peninsula loess with overseas varieties (e. g. Turnball, 19 48 ) indicates the local loess is more dense and less collapsible than most overseas varieties and generally contains more clay sized material. Banks Peninsula loess plots as a clay ey loess adopting the recently introduced loess classification of Browsin, 198 5 .
1.4 BA NKS PENINSULA CLIMA TE A ND VEGETA TION
Figure 1.2 shows how the ty pical distribution of rainfall on Banks Peninsula reflects the general topography . Summers are ty pically hot and dry with a soil moisture deficit most years from December to A pril (see Figure 1.3) .
Both 19 84 and 19 8 5 , the two y ears over which much of the remedial work for this study was carried out, were relatively dry . Table 1.2 gives the rainfall data for these two y ears and shows them both to be considerably below the area average.
The vegetation cover on much of the Peninsula is grassland pasture with
replacing the original
introduced grass species generally native varieties. Native forest cover was not extensive in either the Port Hills area or Lyttelton Harbour, being principally confined to the higher rainfall areas (see Figure 1.4) .
1.5 SUBSURFA CE EROSION
1.5 .1 TERMINOLOGY
Numerous terms have been used for subsurface erosion in its various forms both in New Zealand and overseas i.e. Ground Sinking (Rubey , 19 28) , Subsurface Erosion (Henkel et al, 19 38 ) , Rodentless Rodent erosion (Bond, 19 41) , Subcutaneous Erosion (Cumberland, 19 44) , Tunnel Gully Erosion (Gibbs, 19 45) , Piping (Fletcher & Carroll, 19 48) , Pothole Erosion (Kingsbury , 19 5 2 ) , Psuedokarst (Kunsky, 19 5 7 ) , Sinkhole Erosion (Buckham & Cockfield, 19 5 0) , and Field & Earthwork Tunnelling (Ritchie, 19 61) .
Average rainfall distribution over Danks Peninsula (1940 - 1975). NZ Met. Service,
rfR IO MONTH
J f M A M J
&oll ll\ol1lwte techuge
aon 111ohhua wlilhallon
j A s 0 MOH Hi
H 0 J
CHCH Year Total: 583 Mean ( 1891.rl 980):
CHCH Year Total:2£1
TABLE 1.2: Rainfall data (in millimetres)
for the period of study
(from NZ Met. Service).
usage, while most refer to the process in natural ground. Few d istinguish between the process in soft rocks as opposed to engineering soils, however all the authors make a clear d istinction between subsurface erosion (implying physical or hyd raulic erosion processes) and karst erosion (dominated by solution) in soluble rocks.
The term Subsurface Erosion has been ad opted in this stud y as a general term to includ e soil erosion processes in both fill and _natural ground while excluding karst erosion. The d escriptive term Tunnel Gully as ad opted here has a more restricted usage i.e a tunnel gully is the prod uct of subsurface erosion in natural loess and displays atleast some surface gully expression.
1. 5. 2 OVERSEA S EXAMPLES
Subsurface erosion has been stud ied by five main groups of researchers i. e. Soil Scientists, Geographers, Hyd rologists, Engineers and Geologists. This has led to some confusion in terminology and a d ivision in the literature, with the result that authors are frequently unaware of the full range and scale of the subsurface erosion phenomena.
Table 1. 3 attempts to summarise the international literature available on natural subsurface erosion. The most striking feature of the literature is the d iverse range of climate and vegetation und er which this form of erosion occurs. The d ominant favoured enviroment is und oubted ly arid and semi arid grassland and most of the North American, A ustralian, and African examples are from such areas. The commonly favoured erosion mechanism in these areas is erosion enlargement of soil dessication cracks but a few authors insist that preferential erosion of this type is not a factor (e.g Rubey, 19 28; Massanat, 19 80) . Smith (19 68) -d escribes subsurface erosion in a Wisconsin periglacial
bould er field and considered soil d essication not to be a significant factor.
REFERENCE LOCALITY CLIMATE AND SOIL TYPE EROSION SCALE MECHANIS� PO��IlU.F
VEGETATION � REMEDIAL WORKS
RUBEY, 1928 Nth Texas, Arid Grass- Silty Sand Ground Gullies up Saturated "earth
-U.S.A. land Sinking to 20m wide flowageu under soil
in gully head
Table 1 • 3 Overs eas
--·--FLETCHER & Sth.Arizona, Arid Deserts Clayey sandy Piping
-3 mechanisms - Control of water examples of su bsurfac e
CARROLL, 1948 U.S.A. silts headward erosion �egetation control
from free face !Rodent control erosion
-North America. - Erosion along
- Downward washing
of fines into
BUCKHAM & Kamloops, Arid Grass- Pliestocen_e Lake Sinkhole 16-30m Headward erosion-COCHFIELD British land Silts (Virtually Erosion wide sink- along seepage paths
1950 Columbia no clay) holes. 11:!m back from open face.
HARDY, 1950 Kamloops, Arid Grass- Loess Silt (10% Sinkhole Sinkholes Consolidation of Remove all runoff B.C. and land. Sand, 88% silt, Erosion up to 20m Loess when satur- Deep foundations. Calgary, 2% clay) in diam. ated. Limited Compact natural
sub-Alberta erosion soil to reduce void
FLETCHER et Many sites Arid Grass- Silts, Sands Piping
-. , Need
-source of '
-al, 1954 in South- lands & Desert water
-erodible over-lying layer -hydraulic
gradient -free outlet
BROWN, 1962 Township of Arid Grass and Silty clays and Piping Sinkholes Erosion enlargement - Regrade gully wall
Silt, Bay- Sagebrush clayey silts diam. up to of dessication cracks- Fill and revegetate
field and rich in Mont- 61,m. Tunnels in gully sidewalls. gullies.
Nunn, morillonite up to 2m Dispersion important
PARKER, 1964 Numerous Arid Clays, silts, Piping
-Dessication in rnont.
-sites in sands all con- rich fine grained
Southwest taining dis- soils allows water
U.S.A. persive swell- to disperse and
ing clays erode along cracks.
PARKER et Officers Semi Arid Montrnorillonite Pseudo- 13rn.tunnel Water seepage along
-al, 1964 Cave, rich karst diameter. joints and
dessicat-Dayville, Volcanic ash 250m long. ion cracks.
Oregon Slaking and
rnechan-U.S.A. ical erosion along
LOCALITY Nth.Dakota Badlands, USA Southern Wisconsin USA
CLIMATE AND VEGETATION
Cold, Temper ate. High altitude grassland
BARENDREGHT South East Arid Grass land
& ONGLEY, 1977 Alberta, Canada.
BYRAN et al, 1978
UNITED KINGDOM KNAPP, 1970
Dinosaur Arid Grass-Park Badland land
Benson, Sth. Arid
Grass-Ariz. USA. land
Sth. Semi Arid Saskatchewan grassland Canada Okanagan Valley, British Columbia, Canada Montgomery shire, Wales
Arid Grass land
Silts and silty clays
Silty sandy gravel Piping with large
Sandy and silty bentonitic clays
Sandy and silty bentonitic clays
Silty Sands ( < 3\ clay) Low sodium \
Varved glacio lacastrine silts.
Peat on silty clay
Peat on silty clay Piping Piping Piping Piping Piping Piping Piping SCALE
Tunnels up to 31.,m. diameter
As for Parker, 1964
Stone filled Subsurface erosion con-sinkholes necting previously ice up to Sm. filled voids. Leaves diam. relict boulders.
Tunnel diam. As for Parker
up to 3m. (1964)
Sinkhole i.e. Dessication diam up to crack erosion 9m. enlargement.
Tunnel diam. up to 2m.
Tunnel diam. up to 3m.
High soil perm. allows intrasoil piping if outlet present. Dessication not important
Tunnels up to 200ltUll diam. 5-Gm diam. sink holes
Overland rainfall Control of excess
Tunnels up to lOOrnrn�
runoff enters water. vertical soil
joints until impermeable bed creates subhorizont-al pipes.
Water flows from saturated peat through structural and biological voids with erosion enlarge ment.
AUSTRALIA DOWNES, 1946
NEWMAN & PHILLIPS, 1957
HENKEL et al, 1938
Wales & Derbyshire
Nant Gerig, Wales
Cambria Mts, Wales N. E. Victoria Australia Riverina, N.S.W.
N. S. W.
Urangeline, Riverina, N. S. W. Australia
Natal, Sth. Africa
CLIMATE AND VEGETATION
As above + Woodland
Semi-Arid to Temperate
Semi Arid Pastureland
1.rid and Semi-Arid
Semi Arid Grassland
Peaty Silty Clay
Silty sandy clay
Sandy silty clay with high Na%
Slightly silty sandy clay (CL) High Na%.
Sandy silty clay. High Na%
Clayey gravelly sand
Semi-Arid. Sandy silty clay Winter drought (high swelling) s.ummer rain
Grassland EROSION TERM Piping Piping Piping SCALE
Tunnels up to 150mm diam.
Tunnelling Tunnels up erosion to 500mm
Field tunn elling + Earthwork Tunnelling
Tunnels up to 600m diam.
Tunnels up to 21:im.
Intrasoil piping under head. Dessication not important at his sites
Peat dessicates and erosion enlarges soil joints.
High local infiltrat ion at old stump holes etc. Surface cracking not important.
Grazing control Land Manage ment.
Dessication following Water control
struct-over grazing allows ures. Deep ripping soil joint erosion en- Vegetation control largement through dis- Chemical stabilisa persion above imperme- tion.
As for Newman & Phillips, 1957 And dispersion of clays in earth dams.
Chemical stabil isation. Careful compaction of earthworks.
Tunnels and pot holes
- As for Newman &
Phillips - water control works
Tunnel erosion Field Tunnel erosion 1957
-up to 120mm diam.
- seasonal rainfall - soil subject to
- vegetation change
Subsurface Tunnels Enlargement of Dessic
ation cracks by periodic high
intensity downpour. Erosion up to lm
(Local Term diam.
- deep ripping - planting - Water control
-chemical treat ment
- Improve vege tation
- Deep ripping - Chemical
- control grazing - regrass
- break tunnels to form internal dams at regular intervals.
I • 3 co n tinued Table
Overse as examp les of
subsurface ero s io n
- Un ited Kingdo m,
STOCKING, 1976 BECKEDAHL, 1977 LOCALITY Central Sudan
Umsewe Rv. , RHODESIA
Drakensberg, Natal, Sth. Africa
OTHER ARID AREAS
Hwagn Ho North China
FEININGER, 11969 BANERJEE, 1973 LOFFLER, 1974 BAILLIE, 1975 Central Andes, Columbia Midnapore West Bengal India
New Guinea Lowlands
Sarawak, East Malaysia
CLIMATE AND SOIL TYPE VEGETATION
Semi-Arid. -Summer high
intensity rain. Grassland. EROSION TERM Piping Semi-Arid sparse grassland
Aeolian sedi- Piping
Semi-Arid. Sparse Grassland Semi-Arid Sparse Grassland Arid Grassland
Humid Tropical Pasture on old forest area.
Tropical Wet/Dry climate. Sparse
Tropical. Very wet
(SOOOmm/yr) Savanna and under monsoon forest.
Tropical hardwood forest.
ments on granite.
little shrinkage. Slight clayey and silty sands.
Gypsiferous gravelly silts.
Sandy clay (high ly weathered quartz diorite)
Develop in Ferrallitic soils. No
(up to 60\ clay). No swelling clays.
Leached clay rich soils. No swelling clays. Piping Piping Wells Tubes Pipes Pseudo karst Piping Piping Pseudo dolines Percoline drainage SCALE Tunnels up to 200mm
Tunnels up to 2m. dia.
Eluviation within soil material. No mention of dessication crack enlargement.
;runnels As for Henkel, 1938. up to 1. Sm.
Tunnels up to 2m. diameter.
- form by infiltration through colluvium to intact regolite - under gravel lenses - in tension cracks
parallel to gully sides.
Tunnels up Solution of calcite to lm dia. component in loess then sinkholes up physical erosion. All to 7m. diam. need outlet at gully
Tunnels up to 300mm diameter.
Pipes developed along weathered joints. Did not occur in adjacant metamorphic rocks.
Purely a function of hydraulic conditions nea1 gully sides. Headward erosion comparable to classic dam piping.
Feature of impermeable pan to many tropical soils. Topography concentrates-. seepage to give piping above the layer.
Place silt traps in partially
Table 1 • 3 continu ed
---Overseas examples of subsu rface
to be mainly d essication crack controlled , certainly in the many examples from peat moorland s (peat can shrink up to 50% when d ried ) however atleast one researcher favours eluviation and intrasoil piping und er head to explain examples in his area (Jones, 19 7 8 ) . Tropical forest areas are another favoured enviroment for subsurface erosion, Soil cracking is not generally consid ered an important control und er these cond itions (soil moisture is constantly high and swelling clays and peat are seld om present) .
Although the soils in the examples of subsurface erosion listed in Table 1.3 are all mixtures of sands, silts and clay s1 their properties show consid erable variation. The
presence and ty pe of clay minerals the d egree of natural d ispersion, the sod ium content, the presence or otherwise of calcite or gy psum, and the organic content all vary wid ely . This range of enviroment and soil ty pe suggest natural subsurface erosion can d evelop in a number of way s with the exact mechanism and ty pe of. erosion vary ing from site to site.
One major area of relevant literature has been exclud ed from the table i.e erosion (piping) in earth d ams. While provid ing extremely useful information on the cond itions und er which subsurface erosion can occur, the process of subsurface erosion in
comparable to natural slope erosion.
d ams is not generally Terzarghi (19 31) first pointed out piping in d ams can occur in virtually any material (includ ing gravels) if sufficient head is available. He d istinguished this ty pe of piping by heave from a second ty pe of piping through subsurface erosion (Terzarghi & Peck, 19 67 ) , Once again this latter ty pe of d am piping normally occurs und er saturated cond itions and und er consid erable head in contrast to the natural slope erosion process und er partially or unsaturated cond itions.
16 process (see Perry , 19 65 and particularly Sherard
&Decker, 197 7 ) .
Investigation and Remed ial Method s
Within the overseas literature there is only limited information available on techniques suitable for the investigation of erod ed sites. Gilman (1980) d escribes method s of mapping shallow pipes in the peat upland s of Wales mod ified from the earlier stud ies of Newson ( 197 6) .
Jones (19 7 8) d escribes novel method s of sound tracing whereby
amplify provid es
plastic tubes were inserted d owm augerholes to
the sound of running a d etailed hy d rologic
water. guid e
measuring subsurface water flow volumes use of d y e and rad ioactive tracers.
As is evid ent from Table 1.3
Aitkins on (19 7 8 ) to
techniques d iscusses
possible remed ial
works for sites of subsurface erosion have not been
d iscussed by the majority of authors. Where mention has
been mad e (see for example Fletcher & Carroll, 19 48 ; Brown, 19 6 2 ; Newman & Phillips, 19 57 ) agricultural land is generally consid ered and the range of feasible economic
remed ial options is restricted accord ingly . Noteworthy
exceptions includ e a brief case stud y by Crouch (19 7 7 ) of subsurface erosion in a suburban d evelopment and an excellent stud y by Parker & Jenne (19 67 ) on the structural failure of highway s in the south west USA through subsurface erosion.
@ Oebrls blocks undermined and sapped by pipes 0 culvert
United Sulu Ceotoeiul Survey
(D Shale and sandstone ol Cretaceous Mancos Shale @ Tan 1ill and clay, sandy in placu, ol Quaternary ase.
@ Flood plain ol Aztec Wuh @ Flow ol ephemeral dralnase N
®Ptunse pool @ Block left II natural bridse
.Ei8..:..1 . 5 Ext ensive subsur face er osion below US 1 40 nea r Cor t ez, Cola r a d o (After Pa rker
&Jenne, 1 9 67 ) .
T h e engineer ing lit er a tur e cont ains numer ous exam ples of r em ed ia l m ea sur es suit a ble for d a m s a ffect ed by subsur fa ce er osion includ ing chemica l st abilisa t ion of cor e mater ia ls, inclusion of gr a d ed filt er s, a d just ment s of r eser voir wa t er chem ist r y et c. (see Ingles & Met ca lf, 1 9 7 3 ; Sher a r d & Decker , 1 9 7 7; B ell, 1 982 a ) . Few of t h ese t ech niques ca n be applied t o subsur fa ce er osion pr oblems a t a r ea s ot h er t ha n d a m sites. Ingles (1 9 7 2) ment ions t h e possible inject ion of " t h ic k lime slur r y gr out int o a ny ea r t h work sh owing signs of piping d ist r ess" h owever no publish ed r ecor d exist s of a t r ia l a pplica t ion of t h is t y pe.
1 . 5. 3 SUBSURFACE EROSION IN NEW ZEALAND
---(ot h er t h a n Banks P eninsula)
Gener a l Review
18 d et a il ed s t ud y. Gibbs l ooked a t l oes s ero s ion in t he Wit her Hil l s a nd d evel oped t he now fa mil a r d es s ica t ion cra ck enl a rgement mecha nis m t o expl a in the proces s .
Va n Woul d t (19 54) not ed s ubs urfa ce eros ion in Ta upo rhy ol it e a nd ignimbrit e a s h bed s in t he cent ral Nort h Isl a nd d uring a hy d rol ogical inves t iga t ion of s ubs urfa ce s t ormfl ow. Bl ong (19 65) a l s o d es cribes eros ion in thes e s a me ma t erial s in the vol ca nic pl a t ea u a rea , not ing vert ical coll a ps e hol es up t o five met res in d i a met er. Furt her
nort h, in t he rel a t ivel y humid a rea (19 66) exa mined wha t he ca l l ed
a round Wha nga ra i, Ward pipe/s ha ft phenomena a s s ocia t ed wit h swamp d ra ina ge.
La ffa n (19 73) and La ffa n
&Cul t er (19 7 7 ) l ooked once aga in a t t he Wit her Hills where t hey refined Gibbs (19 45) mod el a nd d es cribe a rel a t ivel y s mall a rea in cons id era bl e d et a il . A more recent s t ud y by Piers on ( 198 3 ) s erves t o furt her empha s is e the wid e ra nge of cl ima t ic a nd s oil cond it ions und er which s ubs urfa ce eros ion occurs , even wit hin New Zea l a nd . Piers on report s exa mpl es from Wes tland pod oca rp fores t and beech fores t nea r Art hurs Pa s s a nd Gl enervie, a s wel l a s the· ' t ra d it ional ' l oes s ma ntl ed a rea s of t he ea s t coa s t of t he Sout h Isl a nd .
The mos t recent publ ica t ion Z ea l a nd is Gold s mit h & S mit h
on s ubs urfa ce (19 8 5) . This
eros ion in d is cus s es fa ct ors invol ved in the forma t ion of eros ion t unnel s in Pl ies t ocene s ed iment s of sout h A uckl a nd .
Remed ial Met hod s
Wit h t he except ion of t he Wit her Hil l s researchers t here ha s been no
for t he cont rol
1 . 5 . 4 SUBSURFAC E EROSION IN BANKS PENINSULA LOESS
C umberl and ( 1944 ) fi r s t de s cribed what he calls subc u t ane o u s ero si on o n Banks Peni n s ul a, de fi ni ng i t as foll ow s :
" the c ul turally i nduced removal by . ru nni ng water o f p ar t s o f the s oil or subs oi l a t varyi ng dep ths be neath the
surface wi tho u t, at fi rs t, di s turbi ng the surf ace sod or soi l . "
Three f orms o f s ubc u t ane o u s ero si on are no ted by C umberl and subc u t ane ous shee t ero si on,
t u n nelli ng . The lat ter form prevai l s o n Ho ski ngs ( 196 2, 196 7 ) considered s ubs urface
di mpli ng and the Peni ns ula . ero si o n on the Ban k s Peni n s u l a i n s o me de t ai l . Table 1 . 4 s u mmari se s the morp ho l ogi cal clas si fi c ati o n and s tage s i n the ev oluti on o f nat ural sl ope t u n ne lli ng a c cordi ng t o H o s ki ngs .
B o th H o ski ngs a�d Cumberl and at tribu te the erosi o n t o p a s ture change ac celerated i n hi s t ori cal ti me s by the
arri v al o f e urope an farmers . The y see the proce s s brie fly as foll ows . Deple ti on or change of vege t ati o n c over, exp o se s the s ur face s oi l t o s u n and wi nd . The bare s oil dries o u t and devel op s deep fi s sures exte ndi ng thro ugh the top soi l and underl yi ng l oe s s hard p an i n t o p are n t materi al . Rai n fall, sl ope wash, and t op s oi l seep age are chan nelled i n t o the dow n sl ope j oi n t s and erode the l oe s s . Loe s s l ade n water reappe ars at t he s urfa ce at some p oi n t downsl ope and all ows the flow p ath to e nlarge . Wi th deepe ni ng and wideni ng the roo f s upp ort i s reduced and sec ti on s o f the hardp an and gras s c over drop i n t o the t u n nel, p o tholi ng the surface . The f orm c hange s through the advanced and fi nal s t age s de s cribed i n Table 1 .4 u n ti l, i n the ideal ul ti mate s t age, an open gull y re sul t s .
STAGE NUMBER 1 . a. b. c . 2. a. b. c. 3 . a. b. c. NAME Youthful stage Early Mid Late Advanced stage Early Mid Late Final stage Early Mid Late
TUNNEL CHARACTERISTI CS
OF TUNNEL OR GULLY
-Begin forming A few ems
Continuous, with Up to
branches, and often 3 00mm a terminal face.
Continuous, but 3 00-600mm uneven profile due
to occasional roof collapse.
Enlarging, transport- 3 00-900mm
ing a considerable
amount of water.
Still enlarging but 300-900mm
being filled by
material dropped from roof.
Tunnel completely �p to 2m collapsed but
largely grass cover-ed preventing rapid scouring .
Initial gullies either Up to enlarge and become 4m. scrubfilled or new
lines of tunnels in interfluves collapse
to form more gullies.
Cracks on surface
Cracks larger, with patches of yellow silt brought to surface.
Still unbroken, but tunnel indicated by grass growth in lines.
Slope dotted by occasional pothole, often hidden by weeds about 1ft. in diameter, but enlarging.
Pot-holes more frequent and increase in size. Tunnel easily apparent, but inter tunnel area still unbroken.
PERCENTAGE OF SURFACE DISRUPTED
Almost continuous gully broken 15-40% only by ' bridges' of turf.
Continuous gullies separated by relatively unbroken inter fluves in which tunnels are appearing.
Large deep gullies separated by relatively flat areas; or a broken slope of small gullies.
Up to 100%
Gullies now cover whole slope, and are subject to normal sheet wash and gully erosion. Slopes and profiles become rounded.
TABLE 1.4: THEORETI CAL STAGES IN TUNNEL GULLY EROSION IN BANKS PENINSULA LOESS ( AFTER HOSKINGS , 1962) .
Table 1. 4 stages in er o s i on
Peninsula Theoretical tunnel in loess gully Ban k s (a fter H oskings , 1 9 62 )
REFERENCE SCOPE OF STUDY SUMMARY AND CQNCLUSIONS
HOSKINGS,1961 One thesis chapter of general Describes morphology. Defines stages in observations erosion progression. Discusses rates of erosion. Outlines mechanism (See text) .
HOSKINGS, 1967 Brief review paper Reviews New Zealand examples of tunnelling erosion and discusses his thesis conclus-ions.
HUGHES, 1970 Thesis on physical setting Examines soil properties of tunnelled
in which subsurface erosion soils. Looked at grainsize, density, occurs, particulary soil chemistry, slaking and dispersion within
conditions. his layer classification. Also considers slope aspect (See Hughes, 1�7?) .
MILLER, 1971 Thesis on soil properties, Looked in detail at dispersion, swelling
affecting tunnel gully erosion. and slaking at one site. Concludes tunnel initiation in cracks most likely
HUGHES, 1972 Brief paper on part of thesis. Presents evidence that slope aspect affects the occurrence of tunnel gully erosion. NNW and W slope most at risk.
TRANGMAR, 1976 Thesis on soil mapping in Looked at distribution of erosion process-Sumner area. es as related to soil type. Found tunnel
gully erosion mainly in shoulder and upper
backslope mantled with loess, loess colluviun and mixed colluviurn.
BELL & TRANGMAR, Paper looking at regolith The subsurface erosion mechanism is dis-In Prep materials and associated cussed and refined. Shallow tunnelling
erosion processes in more above the fragipan in in-situ loess is
detail. contrasted with deep tunnelling in loess colluvium.
t'HLMS, 1979 Thesis on erosion forms in a Concluded soil properties along fissu�es
small valley in Caslunere very important. Notes wide lateral variations in pinhole erodibility.
BATES, 1979 Thesis on hydrological aspects Looked in detail at a single pipe system. of subsurface erosion. Monitored over one winter. Noted high
sediment load in first storms after summer.
LINDLEY, 1985 Thesis on hydrology and sub- Noted abrupt rise and fall in discharge surface flow patterns at site of natural pipes and minimal sustained
in Sumner. seepage.
TABLE 1.5 : SUMMARY AND MAIN CONCLUSI ONS OF THE PRINCIPLE REFERENCES ON SUBSURFACE EROSI ON ON
BANKS PENI NSULA.
In vestigation and Remedial Methods
There is restricted discussion in a few of the referen ces on in vestigation te chniques for eroded sites . Only Bates ( 1979) formalises his in vestigation method and briefly evaluates a few in vestigation te chniques (dye tracing , infra red photography ) .
Similarly remedial methods have re ceived little
attention . C umberland and Hoskings both noted the nuisan ce
subsurface erosion presents to agriculture , emphasising prevention is easier than cure , and proper land management is critic al in preventing initial erosion development . Hoskings makes brief mention of tunnel erosion affe cting new housing de velopment and suggests the use of surface cutoff drains to intercept the slope water responsible . Although
E v ans be c a me in volved in su bsurf ace erosion control , in the literature. Bell stabilisation of soil within
priv ate c onsulting work on none of this work has appeared
( 198 1 ) describes the chemical an active tunnel gully in the most recent published work on the sub je ct .
1 . 5 . 5 C onsequen ces of Subsurf ace Erosion
Su bsurf ace erosion creates
proble m : three main types of
- SUBSIDENC E of ground , either at the water inlet or at some point along the surf ace erosion route .
- Loc al FLOODING or waterlogging at the point of water exit
- SI LTATION at , or near , the point of water exit
In Ban ks Peninsula loess the c onseq uen ces of subsurface erosion have been reported from agric ultural land e.g . Hoskings , 1 967 records the resultant loss of growth on grazing slopes from ground c ollapse and associated dessic ation , the sin khole hazard to sheep , the undermining of orchard trees , and the waterlogging of the trees at tunnel outlets .
The most serious potential roads , structures and other types
d evelopment . Of the five sit es st ud ied , t wo involved a risk t o road ing (Andersons Corner and Hend ersons C ulvert [ see Append ices A2
&AS ] respect ively ) , t wo involved nuisance or risk t o st ruct ures (Ly t t elt on Reserve and Parkland s Drive [ Al
&A3 ]) , and one involved bot h (Ngaio Lane [ A4]) .
Only where relat ively valuable asset s like t hese are t hreat ened can t he consid erable ex pense of t he majorit y of remed ial opt ions out lined in Chapt er Three be warrant ed . At most of t he sit es ex amined, careful sit e invest igat ion prior t o d evelopment would have id ent ified t he risk involved at t he out set and , in many cases, prevent ed serious erosion. It is t o be hoped t hat t he increasing use of geot echnical invest igat ion at t he earliest st ages of d evelopment planning (see for ex ample Bell & Pet t inga, 1984 ) can prevent t he fut ure d evelopment of a high proport ion of subsurface erosion problems,
necessary . The philosophy and
before ex pensive remed ial measures become nex t chapt er consid ers invest igat ion useful met hod s for subsurface erosion
CHA PTER TWO
INVESTIGATION METHODS FOR SUB SURFACE EROSION IN BA NKS PENINSULA LOESS
2 . 1 INT RODUCTION
The primary aim of this chapter is to evaluate
l a b o r a t o r y an d field method s which have poten tial in the characterisation an d in vestig ation of sites of subsurface erosion in B an k s Pen insula loess. The brief in itial section consid ers relevan t site in vestig ation philosophy to provid e a framework for the d iscussion of the specific in vestigation method s.
2 . 2 SITE INVESTIGATION PHILOSOPH Y
T he common ly ad opted en g in eerin g geolog ical approach to site in vestig ation is the ' O bjective Orien ted A pproach ' d eveloped in itially by ISRM (19 7 5 ) an d expan d ed by others sin ce (see for example Stapeld on, 19 8 3 ) . This approach can be summarised as follows:
1) Defin e Objec�ives - Ask Question s.
2 ) Collect and A ssess Existin g Data to prepare a Ten tative Site Mod el.
3 ) Prepare Cost Estimate.
5 ) An alyse t his mod el an d An swer t he q uest ion s asked in 1.
When t his approach is applied t o t he in vest ig at ion of subsurface erosion in Ban ks Pen in sula loess we can d efine t y pic al object ives an d specific met hod s as out lin ed in Table 2 . 1 overleaf. St ep 1 asks - What is bein g erod ed , Where is t his erosion occurrin g, Where is t he wat er comin g from, and Why ? St ep 2 is gen erally rest rict ed t o d et ermin in g t he hist ory of t he erosion problem an d obt ain in g t he limited t opographic an d aerial phot ographic d ata t hat may be available. The in vest ig ation cost est imat e (St ep 3) can only be d on e accurately aft er some familarisat ion wit h t he site problem. When it is at t empt ed at t he commissionin g st age, cost an d t ime overuns are t he t y pical result .
Step 4 is t he most time con sumin g , d urin g which t he
sit e mod el is refin ed an d q uan t ified . Gen eral sit e
charact erist ics, soil mat erial an d mass propert ies, an d t he
active sit e processes are d efined . The subsurface erosion
process can be in vest ig at ed t hroug h bot h surface met hod s (e. g . g eomorphic mappin g and phot ographic t echn iq ues) an d subsurface cavit y in vestigat ion s (e. g . auger probin g, cavit y
en t ry , g eophy sics, d y e. an d smoke t racin g ) . These
in vestig at ions are aimed at d et erminin g t he d imen sion s, d ept h, an d len g t h of t he cavit y , as well as t he locat ion of in let s an d exit s.
The fin al st ages (Steps 4
&5 ) in volve analy sis an d an swerin g of t he q uest ions in St ep 1 and , based on t he establis hed sit e mod el, · t he recommend at ion an d d esig n of suit able remed ial met hod s. To at t empt t his fin al remed ial step, wit hout t he previous 4 st eps, can n ot on ly wast e money in failed repair, but can make t he orig in al erosion problem worse.
TABLE 2.1: .
-- WHAT is being eroded? Consider all possible
WHERE is this erosion occurring? explanations of the problem. - WHERE is the water source?
--is the erosion occurring?
COLLECT AND ASSESS DATA. PRODUCE TENTATIVE SITE MODEL.
Often little geotechnical data available however Question site owner, local bodies can generally obtain History of Problem. Also etc. List Hkely possible
limited topographic information and large scale explanations (Hypothes �) . ' Test air photographs. Plan main investigation these in �ain investigation
(Step 4) . (Step 4) .
PREPARE INVESTIGATION COST ESTIMATE Assess time and material costs
REFINE AND QUANTIFY TENTATIVE SITE MODEL
A) General Site Characteristics (Location, Stadii Survey, Field observation.
Elevation, Aspect, Topography and Slope,
Vegetation, Landuse) .
B) Soil & Rock Material Characteristics (Field
material description, laboratory evaluation In-situ undisturbed and bulk of erodibility, dispersion, and slaking. sampling. Pinhole Test, Crumb
Can also obtain grainsize, atterberg limits Test, Uniaxial Expansion.
and dry density.
C) Soil Mass Features (Layering, Defects, Logging of testpits, exposures
G eometry) and augerholes.
D) Subsurface Erosion Process (define lo cal
areas of subsidence, siltation and flooding. Geomorphic Mapping. Auger Subsurface erosion cavity investigation to probing. Cavity entry. determine dimensions, shape, depth, length, Geophysics. Dye and water
and outlets and inlets.) tracing. E ) Other Active Processes (Groundwater
move-ment, surface water movement, mass move- Piezometers, o�en auger hole� &
ment) . pits. mapping. Dye tracing. Geomorph1c
ANALYSE AND ANSWER WHAT, WHERE & WHY OF STEP 1.
DEFINE SITE EROSION MODEL Combine all available data.
RECOMMEND AND/OR DESIGN SUITABLE REMEDIAL
METHODS BASED ON THIS SITE MODEL.
Summary of investigation steps and method for subsurface erosion investigations in Banks Peninsula Loess.
T a b l e
hlS u m m a r y o f i n v e s t i g a t i o n s t e p s a n d m e t h od s f o r s u b s u r f a c e e r o s i o n i n v e s t i g a t i o n i n B a n k s P e n i n s u l a l o e s s .
2.3 SOIL MATERIAL PROPERTIES AND TEST METHODS
2.3 .1 ERODIBILITY
The most obvious and fund amental control on the susceptibility of loessial soil to subsurface erosion is soil material erod ibility . An erod ible material is d efined here as a rock or soil which und ergoes the particle by particle removal of grains , aggregates or floe s through the action of flowing water. Erosion occurs when the shear stresses ind uced by the fluid flow , either on or in the soil, are great enough to cause the removal of these particles (see Arulanand an & Perry , 19 8 3 ) .
The extent of initial loosening of the rock or soil matrix by slaking , d ispersion and cement d issolution in water, as well as the soil grainsize and magnitud e of the fluid ind uced shear stress, are the fund amental controls on the erod iblity of fine grained soils. Soil erod ibility is thus d epend ant on the interaction of these various soil
properties which Section 2.3 .2
are d iscussed ind ivid ually later
RECOMMENDED ERODIBILITY TEST METHOD: THE PINHOLE TEST
The laboratory method used in most recent research in Banks Peninsula loess as a measure of general erod ibility is the pinhole test, d eveloped by Sherard et al ( 19 7 6a) . The test was originally proposed for recompacted fine grained soils and found particular application in the testing of piping resistance in potential core material for earth d ams. Evans (19 7 7 ) mod ified the method to test und isturbed insitu samples of Banks Peninsula loess and the pinhole test has been used to assess insitu soil erod ibility since that time (see for example Wilms, 19 7 9 ; Saul, 19 7 9 ; Evans & Bell, 1981; Schafer & Trangmar, 19 81; Crampton, 19 8 5 ) .
28 sample a nd the d egree of erosion enla rgement of the pinhole is assessed . The head is increa sed in increments from 50mm to 100 0mm, a nd ea ch head is susta ined for 10 minutes. The total test time is between 10 a nd 40 minutes, d epend ing on the erod ibility of the
cla ss cla ssifica tion of colour of the erod ing
loess tested . The origina l test Sherard et al ( 19 7 6a) wa s ba sed on wa ter a nd ra te of wa ter flow. A revised cla ssifica tin ha s been used in this stud y (see Section 2. 3 . 1 C) .
A d va ntages of realistic mod elling a bility to test both
samples, fa st test
test includ e the compa ra tively subsoil erosion cond itions, the
situ und isturbed and recompa cted a nd reprod ucible results. In
a d d ition, the equipment is not ela bora te a nd takes up little spa ce in the labora tory .
B) Terminology a nd Releva nce of Test
The test wa s initially proposed a nd genera lly a d opted
a s a test for soil d ispersion. The term "d ispersion" wa s
origina lly introd uced to the literature a s a pseudony m for cla y mineral d eflocculation and ma ny a uthors d efine a nd use the term in this manner (Emerson, 19 67; Ry ker, 19 7 7 ;
Holmgren & Fla na ga n, 19 7 7 ; Bell, 197 8 ; Liggins, 197 9 Schafer, 19 8 2) .
Shera rd et a l (19 7 6a) red efine the term d ispersion to mean "colloid a l erod ibility " a nd propose the pinhole test result be used a s a d ispersion ind ex. Thus if the water running through the sa mple und er the lowest test head erod es the sa mple a nd ca rries a cloud y coloured suspension, a nd this suspension d oes not settle out over the 10 to 15 minutes of testing, then the sa mple is cla ssified a s d ispersive (Dl or D2) . Sa mples in which only limited erosion ha s occured , with little or no cloud iness, a re prefixed by ND (non d ispersive) a nd by various number grad es d epend ing on the head required to initia te erosion (ND 1 -4 ) •
wid ely used sense ( i . e . clay mineral d eflocculation) . C onsid ering this evid ence in turn :
1 ) In a comparison between various d ispersion test method s Sherard et al (1 9 7 6b) noted a d iscrepancy between the crumb test for d ispersion and the pinhole test (the crumb test for d ispersion, which consists of immersing a crumb of soil in a beaker of water and observing the extent of the colloid al halo, was first introd uced by Emerson [ 1 967 ] , see full d iscussion in Section 2.3.2 A ) . Sherard et al (1 97 6b) found about 40% of the crumb samples which d isplay ed non d ispersive clay mineral behaviour were classified as "d ispersive" by the Pinhole Test.
2 ) A n examination of existing d ata available for und isturbed samples of Banks Peninsula loess also ind icates a poor correllation between d ispersion measured by the crumb test and pinhole "d ispersion" (see Fig. 2.1 ) .
u 12 ..0
•• • •
e L ____ .._.� __ .... _..._ _ _..,.__ __ __,,--_..._..,.---1E > 1000
E IOOO E 380 E 180
N02 N03 N04
E 180 · :l0
02 E:10 (RavlMd Cla"ificarion) DI (Sherord •' al, 1976)
If d ispersion was the only variable being measured by erosion in the pinhole test a straight line relationship could be reasonably expected . In Figure 2 .1 no such relationship is apparent.
3) To further highlight this discrepancy a sample of loess
was cleaned of the clay sized fraction, of the resulting sand y silt showed (colloid al cloud iness) in the crumb test. material was pinhole tested it proved
so that a crumb no d ispersion When this same to be the most experience, and erod ible material tested in the writers
thus the correspond ing Sherard et
classification class was Dispersive 1 (Dl ) .
al (19 7 6a)
4) O ther tests for d ispersion fail to correlate with the pinhole test results. Schafer
&Trangmar (198 1) present a comparison of d ata for recompacted loess from Banks Peninsula and ad jacent areas. In their stud y they compared pinhole test results with the porewater analy sis and the SCS d ispersi on tests (refer Sherard et al, 19 7 6b for d etails of these test method s) . Schafer
&Trangmar conclud e that although there was reasonable correlation, in many cases, neither the SCS test, nor the porewa ter analy sis method , correlate completely with the Pinhole Test.
5 ) More recently Craft d iscrepancies between the pore water analy sis
Acciard i pinhole test
(19 8 4) noted
results and for d ispersion (see Figure 2.2) . O nce again materials classified as non d ispersive by the Pore Water test were "d ispersive" by the pinhole test.
� 40 (.J
KEY Zone B
Zone C • D ispersive (DI ,02) e N03 and 4 Non -0 NO I and 2 Dispersive
O·I 0·2 0·5 l·O 5 10 50 100
Sum of Cations in Mllllequivalents/ Litre
(After Goldsmith and Smith , 198 5 )
i40 a. 20
O·I 0·2 0·5 l·O 5 10 50 100
Sum of Cations in Milliequivalents/Litre ( After Craft and Acciard i , 1984)
Porewater Method Zone A - Dispersive Zone B - Transition
Zone C - Non Dispersive
Soils : All Clayey Silts and Silty Clays
( MH - CH )
FIGURE 2,2: DISCREPANCI ES BETWEEN THE POR EWATER M ETHO D OF DISPERSION TESTI NG A N D TH E PINHOLE
' DISPERSION ' TEST.
In conclu sion the re su lts out line d above c o nfirm that although t he pinhole te st measure s ero d ibility , it is not alway s an accurate in dicator of d i s pe r s i o n. Wh i le i n some
s oil s , high erodibility may i n d i cate the pre sence of dis per sive c l a y mine r al s , in Banks Pen i n s u l a loe s s h igh ero dibility is the resu lt of the in teract i o n of a number of
C ) RECOMMENDED CLASSIFICATION
The conclusions above do
test as an effective measure of suggest a reclassification of
TO THE PINHOLE TEST
no invalidate the pinhole erodibility but they do the pinhole test result classes is required to remove the dispersive non dispersive (D
ND) implication inherent in
modifications of the
classification have been suggested by Evans (19 7 7 ) and Bell (19 81) . These changes both emphasis the extent of erosion under the various heads as the principal criteria separating the classes. The degr ee of water cloudiness is less important. The D - ND terminology of Sherard et al (19 7 6a) is retained. Schafer & Trangmar (19 8 1 ) define a quantitative '' erosion index " based on the volume increase in the pinhole size as calculated from the volume of eroded soil in the test water. Scaling equations adjust the index for different pinhole diameters and flow rates. This is an improvement on the original classification as it removes the
dispersive non dispersive distinction, however the
collection , evaporation and weighing of sediment in the test water, along with the calculations required, are all time consuming.
The relatively quick and simple classification procedure briefly outlined below was adopted throughout this study and is proposed for general use (see Appendix A6.3 for a detailed account of the method and classification) . During the test the usual record is kept of water discharge per unit time and this is plotted up as a graph at the test completion. From this graph the head at which sustained
erosion first occurs is established and this is defined as follows i.e. the head which produces a progressive increase in discharge over three or more minutes while itself remaining constant (see examples in Figure 2 .3 overleaf) . An erosion category is then assigned according to this head
i . e E S O • E 180 • E 380 • E 10 0 0 · If significant erosion