csiro ^ivision of iilding research
TRANSCRIPT
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CSIRO ^ivision of
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THE CLASSIFICATION OF THE EXPANSIVE
BEHAVIOUR OF MELBOURNE SOILS
FOR DOMESTIC CONSTRUCTION
by
P.F. WALSH * J.E. HOLLAND ** T. KOUZMIN ***
CSIRO DIVISION OF
BUILDING RESEARCH
AUSTRALIAN ENGINEERING AND BUILDING INDUSTRY
RESEARCH ASSOCIATION (AEBIRA)
Published by Cement and Concrete Association 1976
*Principal Research Scientist, CSIRO Division of Building Research. "Principal Lecturer, Swinburne College of Technology.
" Experimental Officer, CSIRO Division of Building Research; now Lecturer, Caulfield Institute of Technology.
© CSIRO
CSIRO
Division of Building Research,
Graham Road,
Highett, Vic. 3190
ABSTRACT
This paper presents a guide to the expansive behaviour of the
soils of Melbourne and outer suburbs. These soils have been
broadly divided into major types based on their geological origin,
and each type has been classified into one or more of three
potential swell categories with respect to their stability for
domestic construction. The classification may be directly applied
in the design of footings for domestic structures by simple
correlation to standard designs having the stiffness and depth
required to combat each swell category soil.
Aids to the recognition and identification of the soil types
covered in the classification system are presented. It is expected
that, in general, professional advice should not be required to
classify sites. No attempt, however, has been made in this report
to provide data on the behaviour of soft alluvium or fill.
In as much as the major division is basically one of geologic
origin, the classification system may be applied with confidence
to other areas of Victoria which are geologically and climatically
similar.
(i)
1.
INTRODUCTION
Swelling and shrinking, or more simply, reactive or expansive clays have been responsible for
many house foundation problems in Melbourne. Recently, extensive research at CSIRO Division
of Building Research (1, 2, 3) and at Swinburne College of Technology (4, 5) has led to
improvements in the design of foundations to resist expansive clay movements. These studies
have led to changes in the building regulations where the following three classes of sites are
defined: stable, intermediate and unstable. At present, however, no simple accurate method
exists for evaluating the reactivity of a given site. It is the aim of this paper to present a set of
workable standards for the classification of soil reactivity with direct applicability to the standard
raft and footing designs in the Uniform Building Regulations.
The authors would like to point out that this paper is not intended as a theoretical treatise on the
behaviour of expansive soils, but as a purely practical approach to the problem of predicting soil
behaviour. The classifications contained herein can only be described as broad, and any soils
encountered which do not correspond with the types described should be dealt with in the usual
manner of specific investigation prior to design.
EXPANSIVE BEHAVIOUR OF SOILS
A soil is said to be expansive or reactive when it undergoes appreciable volume change as a
result of changes in moisture content. This volume change occurs as shrinkage upon drying, and
swelling upon wetting. As a rule, heavy plastic clay soils will always display a higher degree of
reactivity, whereas more sandy lighter clay soils will be less reactive. The movement of expansive
clays can be inhibited by pressure. Thus, only lightly loaded structures usually are damaged from
expansive soil movements. Also by their nature, clays and clayey soils are fairly impervious to
water, and any significant moisture change takes a long time to develop; thus the volume change,
accompanying moisture change, will also be slow.
Structures built on an expansive clay will cause long term changes in the moisture environment
of the clay. Also, changes in moisture content, particularly near the edge of the building, will
occur from seasonal changes. Under lightly loaded buildings such moisture changes will tend to
cause uneven movements of the clay and unless the structure has been properly designed, brick-
work cracking and jamming of doors and windows may result.
As a result of these short and long term movements, the shape of the foundation tends to adopt
one of the forms shown in Fig. 1 for strip footings and Fig. 2 for slabs. For strip footings, the
overall soil-structure shape depends on the underfloor ventilation and drainage. If this area is
dry, shrinkage of the clay occurs and dishing of the foundation results. If moisture can enter the
underfloor area through permeable soil layers or poor site drainage, and is restricted from escaping
due to poor ventilation, the foundation may tend to dome. For slabs, dishing usually only occurs
as a transient stage and in the long term the foundation adopts the domed state.
The design of the footing or slab to resist this movement depends upon the amount of potential
movement and its shape, the structural type, and to a lesser extent the effective stiffness of the
soil. Perhaps the parameter most characteristic of these factors is the differential movement that
would occur between the centre of the structure and the outside edge at the level of a light strip
footing or slab edge beam. This expected differential movement should theoretically be determined
by ignoring the weight and stiffness of the slab or footings and considering only the effect of
moisture changes. This quantity is generally less than the seasonal surface movement of the soil.
2.
0 DOMING
DISHING
Fig. 1. Typical foundation movement for strip footings.
SEASONAL
DRY
DISHING
Fig. 2. Typical foundation movement for slabs.
Distress and cracking in houses can also be caused by localized variations in either moisture or
soil type. Large, abrupt variations in moisture causing localized heave or shrinkage over short
distances are frequently a cause of severe stress in the structure. Common causes of local soil
movement due to large moisture change are:
(a) high moisture ingress and heave due to
— cracked, broken or poorly detailed water, sewerage or stormwater drainage pipes
— grossly excessive watering of flower-beds and shrubs planted close to the structure
— removal of large trees prior to construction;
3.
(b) drying out and shrinkage due to
— sorption of moisture by roots of trees
— porous drainage paths close to footings
— change from septic to sewered system and subsequent drying out of previously
constantly wet effluent area;
(c) variation in soil type and thus reactivity to moisture due to
— variation in depth to bedrock
— Gilgai formation in more expansive soils, e.g. northern and western suburbs
— substratum of steeply dipping bedrock with outcrops of varying residual soils occurring
in narrow bands, e.g. eastern suburbs.
Localized movements are indirectly allowed for in the design of slabs and strip footings by the
provision of adequate strength, stiffness and depth.
CLASSIFICATION OF SOIL BEHAVIOUR
The soil classification system detailed below has been derived from a wide study of the behaviour
of Melbourne soils both in the laboratory and the field, and from a major investigation into the
performance of actual and experimental footings and slabs. Details of these studies are not in
context for this report but will be or are published elsewhere.
The classification system relates the expected expansive behaviour of the foundation to the
performance of the minimum standard footing design recommendations. Three categories of
movement are used, viz, stable, intermediate and unstable. The stable classification does include
clay soils that move moderately, but this movement is expected to be within the capacity of the
corresponding footing or slab design.
The soils of Melbourne are subdivided primarily in terms of their geological origin. A simplified
map of the major soil types referred to in this paper is given in Fig. 3. Soils of the one geological
origin may then be further subdivided on the basis of their typical soil profile. An assessment of
the expansive behaviour of the major soil types results in the classification system given in Table 1.
The steps involved in using this classification system are as follows:
1. The builder, building designer or his consultant should check the site for obvious defects
and should also inquire from the building authority, selling agents and developers about the
presence of fill. For recently developed estates the plan of subdivision may indicate the
presence of fill. (If the site consists of filled soil or soft alluvium, it is outside the scope of
this report; and unless adequate local knowledge exists a professional consultant should be
engaged.)
2. A geological map should then be consulted to determine the relevant geological origin of
the soil. If the site is close to a boundary, note should also be made of the neighbouring soil. Suitable detailed maps of the greater Melbourne area (the 1/63 360 series is particularly
recommended) are available from the Department of Mines from the first floor of their
offices in 107 Russell Street, Melbourne. Terrain studies of the Melbourne area by Grant (6,7) may also be used to supplement the geological maps. The notation of the 1/63 360
geological maps, Grant's provinces and the terminology used in this paper are cross-
referenced in Table 2.
3. Table 1 should then be consulted and, if the classification is clear cut, i.e. only one class for
all soil profiles, or where the site is near a geological boundary and the neighbouring classif-
ication is the same, then no site testing to determine expansive behaviour is needed.
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4. If several options are given in Table 1 the builder may simply choose to adopt the least stable
category, and again no testing is needed. If doubt exists, it should be resolved by determining
the actual profile by test boring (On whole subdivisions only a few test borings may be
necessary.) No laboratory testing is required; i.e. no linear shrinkages. The site classification
should now be established and this can be submitted to the building authority for approval.
5. At the time of excavation of the footings the soil profile encountered should be checked
against that on which the design was based. Note that the depths given in Table 1 are very
approximate. Particular attention should be given to checking that'the site does not consist
of loose fill or other unexpected soil conditions.
Not all soil profiles are included in the classification scheme given above. If, at any stage in the
above sequence, it becomes apparent that the profile is not one of those listed, the site must
receive individual assessment. If local knowledge is inadequate, a professional consultant should
be used. Although the classification may still be used in areas outside Melbourne, some attention
should be given to the climate. For more severe climates, i.e. more arid, a more stringent class may
be appropriate than for the same profile in Melbourne.
FOOTING AND SLAB DESIGN
The design of the strip and stump footings, slab or footing slab shall be in accordance with the
Uniform Building Regulations of Victoria and reference should be made to the "Standard
Foundation Design Sheets" (8) for suitable design details arid specifications.
WARNING
This report is concerned solely with the problem of expansive movement, and is not intended to
comment on the suitability of the site in its load carrying capacity. Soft soils or filled sites should
receive individual special attention.
ACKNOWLEDGMENTS
This work is the result of research at the CSIRO Division of Building Research and Swinburne
College of Technology. The study at Swinburne was sponsored by the Australian Engineering and
Building Industry Research Association and their generous assistance is acknowledged. The.work
at Swinburne involved the following higher degree students: A. Crichton, J. Washusen, D. Cameron
J. Jackson. The work at CSIRO was assisted by B. Budgen and S. Towstoless.
The assistance of the CSIRO Division of Applied Geomechanics, in particular K. Grant and B.G. Richards, is also acknowledged. Additional data used in the preparation of this class- ification were provided by Soilmech P/L., Universal Soils Laboratory, and the Soils Conservation Authority.
REFERENCES
Copies of the reports by CSIRO are available from the Divisions concerned and the report from
Swinburne College of Technology from that organization.
1. Walsh, P.F. (1974) — The design of residential slabs-on-ground. CSIRO Aust. Div. Bldg.
Res. Tech. Pap. (Second Series), No. 5 (2nd edition).
2. Walsh, P.F., Lewis, R.K. and Beresford, F.D. (1974) — 3 Recent concrete research projects
of the Division of Building Research, Chartered Builder, 10 June/July:45-54.
7.
3. Walsh, P.F. (1975) — Residential floors. Concrete slab-on-ground construction for
Victoria. CSIRO Aust. Div. Bldg. Res. Special Report.
4. Holland, J.E., Washusen, J. and Cameron, D. (1975) — Seminar on Residential Raft Slabs.
Dept. of Civil Engineering, Swinburne College of Technology. (Due for revision 1977).
5. Holland, J.E., Washusen, J. and Cameron, D. (1975) — Ground movement information
for Melbourne soils as required for residential raft slab design. Inst. of Eng. Aust.
Symposium on In Situ Testing for Design Parameters'
6. Grant, K. (1972) — Terrain classification for engineering purposes of the Melbourne area,
Victoria. CSIRO Aust. Div. Appl. Geom. Tech. Pap. No.11.
7. Grant, K. (1972) — Terrain classification for engineering purposes of the Queenscliff area,
Victoria. CSIRO Aust. Div. Appl. Geom. Tech. Pap. No. 12.
8. Walsh, P.F. and Holland, J.E. (1977) — Standard foundation design sheets, Nos. S1, S2,
FS1 and F. Cement and Concrete Association, Australia.
8.
TABLE 1.
SOILS CLASSIFICATION FROM GEOLOGICAL ORIGIN AND
TYPICAL SOIL PROFILE FOR MASONRY VENEER OR
TIMBER CONSTRUCTION
Geological Typical soil profile Classification
description
of soil Approximate Description Strip & Stump Slabs or
depth Footings Footing Slabs
(mm)
Quaternary
Alluvium
Werribee Delta
— sand, silts
and clays Geol. Map 7822, Ref. Qpw Grant() Ref. 52010-01/2
0-100 Red brown clayey silt Stable Stable
100+ Red brown silty clay to clay
Carrum Swamp
Geol. Map 849 7922, Ref. Qvm,
859 Ref Q5. Grant Ref. 52010-00/3
0-300 Dark grey sandy topsoil Stable Stable
300-2000+ Grey and brown sandy clay with
occasional gravel layers
0s-600 Black clay topsoil Stable Stable
600-2000+ Grey and brown sand to clayey sand
with very occasional sandy clay layers
0-300 Black silty sand or clay topsoil Stable Stable
300-2000+ Brown grey yellow sand, silty sand
or clayey sand with occasional
sandy clay layers.
The following profile is uncommon but
may occur near creeks and rivers.
0-200 Black clay topsoil Intermediate Intermediate
200-2000+ Grey and brown clay
Port Melbourne — South Melbourne Area
Geol. Map Qrs
0-600 Black stratified silty clay Stable Stable
600+ Sand silt or silty clay
Geol. Map Qrp
0-400 Dark grey sandy topsoil Stable Stable
400-2000 Grey sand
9.
TABLE 1 (contd)
Geological Typical soil profile Classification
description
of soil Approximate Description Strip & Stump Slabs or
depth Footings Footing Slabs
(mm)
Quaternary 0-150 Dark grey brown silty sand topsoil Stable Stable
Aeolian 150-300+ Light yellow grey sand
— sands 300+ Various clays
0-2000+ Uniform grey to dark grey sand Stable Stable
Quaternary 0-100 Brown to black clay topsoil
Basalts 100—rock Brown to black highly plastic clay may
— clays contain floaters
(a) Deeper clay soils (>1 m) or soils Unstable Intermediate
for which local knowledge indicates
past problems.
(b) Shallower clay soils or soils for Intermediate Intermediate
which local knowledge indicates
satisfactory past performance.
(c) Very shallow soils ( <200 mm clay) Stable Stable
where edge beams of slabs or
footings may be founded on rock.
0-150 Brown to light brown silty clay topsoil Intermediate Intermediate
150—rock Red to red brown clay
Tertiary Sediments
— sands and
clays
0-1000+
Deep uniform grey sand
Stable Stable
0-200 Grey silty topsoil Stable Stable
200-500 Grey to yellow sand or clayey sand
500—rock Light grey to yellow sandy or silty clay
0-300 Dark brown or grey sandy topsoil Stable Stable
300-800 Loose brown or grey sand
800+ Brown clay to sandy clay generally
becoming more sandy with depth
0-300 Brown sandy topsoil Stable Stable
300-1000 Loose brown sand
1000+ Medium dense red brown, or brown sand
with occasional gravel layers
0-600 Black sandy topsoil Stable Stable
600+ Yellow brown and grey sandy clay.
becoming more sandy with depth.
10.
TABLE 1 (contd)
Geological Typical soil profile Classification
description
of soil
Tertiary 0-400 Dark grey to reddish clayey topsoil Intermediate Intermediate Basalts
400—rock Dark grey or reddish brown to brown — clays
highly plastic clay
Devonian
Granodiorite
and Granite
— clays
0-200 Grey or brown sandy to silty topsoil Stable Stable
200-600 Grey and brown silty sand to clayey silt
600—rock Mottled red, brown and grey clay
Devonian
Ahyodacite
— clays
0-200 Grey to brown silty topsoil
200-700 Grey brown, yellow brown or red
brown silty or sandy clay, mottled
700—rock Mottled red brown and grey yellow
brown or orange clay generally stiff to
very stiff, may contain silt, sand or
gravel
Stable Stable
Ordovician, 0-100
Silurian and
Devonian 100-400
Sediments
— clays
400—rock
Grey or grey brown silty topsoil
Grey, grey brown or yellowish silt
to silty clay. Soft when wet, hard
if dry.
Mottled yellow grey or reddish brown
clay
Stable Stable
* This table may be applied to solid masonry construction if the internal walls are articulated (for example, by
the use of full height door openings) otherwise the standard designs may be inadequate.
Approximate Description Strip & Stump Slabs or depth Footings Footing Slabs (mm)
11. TABLE 2
CROSS REFERENCE BETWEEN GEOLOGICAL DESCRIPTION
AND GRANT'S PROVINCE NOTATION AND NOTATION FOR THE 1/63 360 SERIES GEOLOGICAL SURVEY MAPS .
Geological description
Grant
Reference in geological survey in
province
Victoria
notation
52010
52002
52009
51009
51007
34001
Quaternary alluvium
— sands, silts and clays
Quaternary aeolian
— sands
Quaternary basalt
— clays
Tertiary sediment
— sands and clay
Tertiary basalt
— clays
Devonian granodiorite &
granite
— clays
Devonian rhyodacite
— clays
Qri, Qpw, Qpa, Qrc, Qra, Qrm, Qrt,
Qrc, Qrt, Q5, Q3, Q1.
Qpd, Q4, Q2, Qpe, Qrp
Qvn, Qpl
Qvn
Tpr, Tpa Tewr, Tpb,
Tb, Tp, Te
Tvo, Tob
Dgl, Dg, Ddt, Dgd, Dgb, Dgr, Ddp
34002 Dvf, Dvy, Dvk, Dum, Dvc
Ordovician, Silurian & 33001 Dlu, Dlh
Devonian sediment 32001 Sud, Sla, m, S, Slk, Sum
— clays Ou, Om, Omd, Oly, Olc, Olb
