Western Cape Unit P.O. Box 572 Bellville 7535 SOUTH AFRICA c/o Oos and Reed Streets Bellville Cape Town Reception: +27 (0) 21 946 6700 Fax: +27 (0) 21 946 4190

A review on Problem Soils in South Africa

S. Diop, F. Stapelberg, K. Tegegn, S. Ngubelanga, L. Heath

Council for Geoscience Report number: 2011-0062

© Copyright 2011. Council for Geoscience

1

Contents Contents.................................................................................................................................................. 2 Figures..................................................................................................................................................... 4 Tables ...................................................................................................................................................... 4 1

Introduction .................................................................................................................................... 5

2

General background and overview of problem soils in South Africa.............................................. 5

3

4

5

2.1

Expansive soil......................................................................................................................... 5

2.2

Collapsible soil ....................................................................................................................... 6

2.3

Soft clays................................................................................................................................ 7

2.4

Dispersive soils ...................................................................................................................... 7

Mechanism and conditions leading to hazard events .................................................................... 8 3.1

Expansive soil......................................................................................................................... 8

3.2

Collapsible soil .....................................................................................................................10

3.3

Erodible soils........................................................................................................................11

3.4

Soft clays..............................................................................................................................13

3.5

Dispersive soils ....................................................................................................................14

Distribution ...................................................................................................................................15 4.1

Expansive soil.......................................................................................................................15

4.2

Collapsible soil .....................................................................................................................17

4.3

Soft clays..............................................................................................................................17

4.4

Dispersive soils ....................................................................................................................18

Effects and cost implications ........................................................................................................19 5.1

General problem statement ................................................................................................19

5.2

Problem soil effects .............................................................................................................21

5.2.1

Erodible soil effects .........................................................................................................21

5.2.2

Active soil effects ............................................................................................................23

5.2.3

Collapsible soil effects.....................................................................................................25

5.2.4

Compressible soil effects.................................................................................................25

5.2.5

Acidic soil effects.............................................................................................................26

5.3

Structural manifestation of problem soil damage ..............................................................27 2

5.3.1

Erodible soils ...................................................................................................................27

5.3.2

Active soils.......................................................................................................................27

5.3.3

Collapsible soils ...............................................................................................................28

5.3.4

Compressible soils...........................................................................................................29

5.3.5

Acidic soils .......................................................................................................................29

5.4 5.4.1

Erodible soils ...................................................................................................................30

5.4.2

Active soils.......................................................................................................................30

5.4.3

Collapsible soils ...............................................................................................................34

5.4.4

Compressible soils...........................................................................................................35

5.4.5

Acidic soils .......................................................................................................................36

5.5

Remediation costs associated with problem soil damage: .................................................37

5.5.1

Erodible soils ...................................................................................................................37

5.5.2

Active soils.......................................................................................................................37

5.5.3

Collapsible soils ...............................................................................................................38

5.5.4

Compressible soils...........................................................................................................38

5.5.5

Acidic soils .......................................................................................................................38

5.6

International experience and response to problem soils....................................................38

5.6.1

Erodible soils ...................................................................................................................38

5.6.2

Active soils.......................................................................................................................39

5.6.3

Collapsible soils ...............................................................................................................42

5.6.4

Compressible soils...........................................................................................................42

5.6.5

Acidic soils .......................................................................................................................42

5.7

6

Mitigation options and costs ...............................................................................................30

Conclusions & recommendations........................................................................................43

5.7.1

Erodible soils ...................................................................................................................43

5.7.2

Active soils.......................................................................................................................43

5.7.3

Collapsible soils ...............................................................................................................43

5.7.4

Compressible soils...........................................................................................................44

5.7.5

Acidic soils .......................................................................................................................44

References ....................................................................................................................................44

3

Figures Figure 1: Schematic image of (a) Swelling clay, (b) Non-swelling clay.................................................... 8 Figure 2: Expansive clays (a) Polygonal pattern of shrinkage cracks observed on the surface of bare soils; (b) saturated wet expansive clay .................................................................................................10 Figure 3: Schematic image of a collapsing sand structure....................................................................11 Figure 4: Examples of erodible soils: (a) Embankment erosion; (b) Donga erosion.............................12 Figure 5: Damage to structures placed on Problem soils, Moretele and Bela Bela, Nothh-West Province. ...............................................................................................................................................13 Figure 6: Schematic image of (a) Dispersed clay (b) Deflocculated clay...............................................14 Figure 7: Regional distribution map of swell clay occurrence in South Africa (after http://www// publicworks.gov.za/PDFs~) ...................................................................................................................16 Figure 8: Regional distribution map of potentially collapsing sand in South Africa (after http://www// publicworks.gov.za/PDFs~) ...................................................................................................................17 Figure 9: Dispersive Clay occurrences (modified from Elges, 1985).....................................................18 Figure 10: Examples of sheetwash and gully erosion ...........................................................................22 Figure 11: Map of soil susceptibility to water erosion in the RSA (From: AGIS Comprehensive Maps: Online) ..................................................................................................................................23 Figure 12: Typical crack configurations associated with active soils. ...................................................28 Figure 13: Typical effects of collapse phenomena................................................................................29 Figure 14: Swelling clay map of South Africa (from: AGIS Comprehensive Maps: Online) .................. 33 Figure 15: Swelling clay map of the USA (Olive et al, 1989) .................................................................40 Figure 16: Swell-shrink map of the UK..................................................................................................41 Figure 17: Compressible & collapsible soils of the United Kingdom.................................................... 42

Tables Table 1: Problem soil ranking................................................................................................................21 Table 2: Expansive soil swelling pressures............................................................................................24 Table 3: Corrosion rates for buried metal elements............................................................................26 Table 4: Heave and construction type costs .........................................................................................31 Table 5: Expansive soils: House foundation mitigation measures and state subsidy variation ...........32 Table 6: Collapsible soils: House foundation mitigation measures and state subsidy variation.......... 34 Table 7: Compressible soils: House foundation mitigation measures and state subsidy variation .....36

4

1 Introduction Problematic soils can be naturally occurring or man-made soils. This includes natural soils that have been displaced naturally or by man. Problem soils can give rise to many geotechnical difficulties including inadequate bearing capacity, the potential for unacceptable settlements and slope instability (Slocombe, 2001). Damage to structures in South Africa is commonly related to soil characteristics, with expansive and collapsing soils causing the most problems. There are, however, many types of problem soils, some of the most noteworthy being expansive soils, collapsible soils, soft clays and dispersive soils. It is noteworthy that significant developments occurred in the methods for civil engineering development on problem soils in South Africa from the mid 1960s until the 1980s (Conference on Problem soils in South Africa, Geotechnical Division of SAICE, July 1985). However, such developments generally overshadow more recent advances, so that the State of the Art in dealing with South African problematic soils did not change significantly over the past two decades (Jacobsz, 2009). In addition to the well documented historic concerns dealing with specific problem soils, recent encounters with significant problem situations have highlighted the need for a comprehensive documentation on the role of remote sensing and GIS technologies for mapping, characterizing and monitoring problem soils in South Africa.

2 General background and overview of problem soils in South Africa 2.1 Expansive soil Expansive soil is a term generally applied to any soil or rock material that has a potential for shrinking or swelling under changing moisture conditions. In many parts of South Africa, expansive soils pose a significant hazard to foundations for light buildings. They owe their characteristics to the presence of swelling clay minerals. As they get wet, the clay minerals absorb water molecules and expand; conversely, as they dry they shrink, leaving large voids in the soil. Expansive soils are also known under the terms swell clay, active clay, shrinkable clay or heaving clay. In South Africa with its predominantly dry climate, this soil type is more often referred to as swelling clay, while in countries such as Great Brittan with its wetter climate; it is more often referred to as shrinkable clay. Expansive clays are thus those clays for which variations in the moisture content results in a volumetric change of the soil skeleton. This is problematic since inter alia seasonal moisture changes in the foundation and sub-foundation horizons of especially lightly loaded fixed structures gives rise to volumetric changes. Volumetric change in the soil skeleton in turn induces stresses in the footings and super-structure, leading to super-structure strain and cracking. Due to the repetitive nature of the stress variation, conventional crack repair measures generally are unsuccessful.

At the root of the problem of expansive clays lies the fact that the magnitude of the soil movement is often not recognized timely (structural damage is in fact possible when as little as 2 to 3% of soil volume expansion-contraction occurs). Furthermore, there is often a lack of knowledge of the benefits to be obtained by applying proper investigation and design techniques to counteract the potential soil movement. Extensive studies have been undertaken on the origin and formation mechanism as well as the soil & geology types of expansive clay. Their identification, effects on structures as well as countermeasures and additional construction costs to prevent structural damage are now well understood. It is thus relatively simple to allow for extra design and construction pre-emptive measures once the potential problem has been identified and the end user convinced of the cost-savings in adopting a pro-active approach. The key to a pro-active approach is identification of the possibility of a swell clay condition in an area targeted for construction. The array of identification tools which can be utilized for identification include existing geological and topographical maps and remote sensing imagery, field investigation and identification, and laboratory testing of soil samples.

2.2 Collapsible soil Collapsible soils are typically poorly graded with respect to particle size with a porous texture and generally exhibit low in-situ density. Collapsible soil may undergo a sudden large reduction in volume when a sufficient triggering mechanism or event occurs. They undergo slight compression due to imposed stresses at low in-situ moisture content, but exhibit a significant decrease in volume (large settlements) under the same stress when wetting of the soil occurs. The mechanism and conditions are further explained in section 3.2. The occurrence of a collapsible soil fabric is limited to very specific geological conditions which generally lead to the formation of silty sand or sandy silt with a low percentage of clay sized particles. A collapsible soil fabric generally occurs in wind deposited sands (loess); old, highly weathered and leached granite soils; or residually weathered so-called “dirty” sandstones, but in some instances also in soils which have been deposited by sheetwash, gulley wash, wave action, or termite activity. Collapse often do not occur below the entire extent of a structure, but only in localities where collapse conditions are “favorable”, thus leading to a pattern of localized damage to structures. If a collapsing soil structure is identified prior to construction, preventative construction measures are fairly straight foreword, centering on proper densification or compaction of the founding horizons.

2.3 Soft clays By definition, soft clays are of low shear strength, high compressibility and give rise to time related settlement problems. Generally, they are sensitive, have undrained shear strengths of less than 40 kPa, and consolidate for long time spans (years in many instances) after a structure has been placed on them. They generally have high moisture contents – at or near saturation point. In addition, these materials often lead to slumping, slope instability and structural damage or failure when underlying road embankments, dams or other fixed structures.

2.4 Dispersive soils Dispersive soils are more common in certain type areas and geological settings than others. Elges (1985) indicated that dispersion can occur in any soil with high exchangeable sodium percentage (ESP) values and they generally exhibits pale colour. These soils were once believed to be confined to arid and semi-arid climates, but erosion problems associated with the presence of dispersive soils have been found in areas with humid climate in recent years. Where dispersive clays are used in earth dams and embankments, desiccation cracks may be deep and large, and also settlement cracks might provide water flow paths from where piping can start, eventually leading to failure. Failure caused by usage of dispersive soil construction material is normally associated with rapid filling after construction, or rapid filling after prolonged drought. In South Africa, dispersive clays are mainly derived from argillaceous sedimentary rocks of Karoo Supergroup, Cape Supergroup, Uitenhage group and residual soils of all granites (e.g. Swaziland Basement Complex). However, the majority of these occur in areas of relative water scarcity. Typically, their presence is indicated by gully erosion in the field and milky runoff water and they are widely associated with damage to natural slopes and/ or failed embankments such as earth dams. Field identification of these soils is generally easy since they are often associated with areas of erosion. However, an eroded area does not necessarily indicate the location of a dispersive soil since this condition can also be induced by factors such as loss of vegetative cover, steep slopes and high intensity precipitation. Laboratory testing of soils to determine their dispersivity is complex and a number of tests are needed to be performed to enable their identification. Particular tests may give varying results, causing misleading and possibly erroneous deductions. The properties which need to be tested are the pH, electrical conductivity, cation exchange capacity and exchangeable sodium potential (ESP) of the soil as well as total dissolved salts and sodium absorption ratio (SAR) of the pore water. Apart form the chemical testing, physical tests performed to determine soil erosive potential are the crumb test, pinhole test and double hydrometer test.

3 Mechanism and conditions leading to hazard events 3.1 Expansive soil Clay minerals are complex aluminum silicates (less than 2 microns) composed of two basic units: Silica tetrahedron and aluminum octahedron (Das, 2002). Clays belong to a family of minerals called silicates. Because of electron sharing, the silicon tetrahedrons link together with one another to form thin tetrahedral sheets. The aluminum octahedrons also link together to form octahedral sheets. The actual clay crystals are a composite of aluminum and silicon sheets which are held together by intra-molecular forces (Figure 1). 2:1 clay 1:1 clay Tetraedra Octaedra Tetraedra

Tetraedra Octaedra Tetraedra

Weak cation / water interlayer bond

Octaedra Tetraedra Octaedra

Tetraedra Octaedra

Tetraedra (a) clay

(b) non-swelling clay

Figure 1: Schematic image of (a) Swelling clay, (b) Non-swelling clay

In the weathering process, hydrolysis, or decomposition by the union with water, plays a dominant role. Hence the physical drainage conditions together with climate and length of time during which soil-forming processes act are also important factors. In regions of high temperature and high rain fall, the bases are removed as soluble compounds and are carried downward or out of the soil, leaving an insoluble weathered residue of silicates in which kaolinite is the dominant, non-swelling 1:1 lattice type clay mineral. With decreasing rain fall or impeded drainage, chemical weathering becomes less intense and soluble bases released by weathering are not leached from the soil. This leads to the formation of the 2:1 lattice clay minerals between whose successive sheets in the crystal structure varying amounts oriented water molecules occur. It is the change in the amount of this water which causes swelling or shrinking of the sheet structure and hence of the soil mass as a whole.

For a group of prominent and highly expansive clay minerals called smectites, one octahedral sheet is sandwiched between two tetrahedral sheets to create the mineral structure. In expansive clay, groupings of the constituent clay crystals will attract and hold water molecules between their crystalline sheets in a sort of “molecular sandwich”. The electrical structure of water molecules enable them to interact with other charged particles. The mechanism by which water molecules become attached to the microscopic clay crystals is called “adsorption”. Because of their shape, composition and resulting electrical charge, the thin clay crystals or “sheets” have an electro-chemical attraction for the water dipoles. The clay mineral “montmorillonite”, which is the most notorious in the smectite family, can adsorb very large amounts of water molecules between its crystalline sheets and therefore has a large shrink-swell potential (Arora, 2000). When potentially expansive soil becomes saturated, more and more water dipoles are gathered between the crystalline clay sheets, causing the bulk volume of the soil to increase or swell. The incorporation of the water into the chemical structure of the clay will also cause a reduction in the capacity or strength of the soil.

Figure 2: Expansive clays (a) Polygonal pattern of shrinkage cracks observed on the surface of bare soils; (b) saturated wet expansive clay During periods when the moisture in the expansive soil is being removed, either by gravitational forces or by evaporation, the water between the clay sheets is released, causing the overall volume of the soil to decrease or shrink. As the moisture is removed from the soil, the shrinking soil can develop gross features such as voids or desiccation crack. These shrinkage cracks can be readily observed on the surface of bare soils (Figure 2) and provide an important indication of expansive soil activity at the property.

3.2 Collapsible soil Collapsible soils are typically poorly graded with respect to particle size, have a porous texture, are derived from residual, aeolian or hillwash soil forming processes and comprise of slightly clayey fine sand and silt mixtures. The soil texture is open textured (voided), and these particles are interconnected by clay “bridges” – refer to Figure 3 Upon being subjected to stresses larger than those imposed by the soil density, and receiving a triggering mechanism such as wetting

under load, stress changes or even dynamic loading due to earth tremors, the “bridges” are unable to withstand the increased stress, and collapse, leading to a sudden decrease in the soil volume. The collapse is sudden and non-reversible. In severe instances the percentage collapse may comprise more than twenty percent of the original soil volume.

Sand grains

Clay “bridges” Voids

Figure 3: Schematic image of a collapsing sand structure

3.3 Erodible soils Erodible soils are soils affected by flowing water to physically remove particles from the exposed surface (Figure 4). Soil erodability is an estimate of the ability of soils to resist erosion, based on the physical characteristics of each soil. Generally, soils with faster infiltration rates, higher levels of organic matter and improved soil structure have a greater resistance to erosion. Sand, sandy loam and loam textured soils tend to be less erodible than silt, very fine sand, and certain clay textured soils.

Figure 4: Examples of erodible soils: (a) Embankment erosion; (b) Donga erosion

The impact of raindrops on the soil surface can break down soil aggregates and disperse the aggregate material. Lighter aggregate materials such as very fine sand, silt, clay can be easily removed by the raindrop splash and runoff water; greater raindrop energy or runoff amounts might be required to move the larger sand and gravel particles. Surface runoff, causing gully formation or the enlarging of existing gullies, is usually the result of improper outlet design for local surface and subsurface drainage systems. Gully formations can be difficult to control if remedial measures are not designed and properly constructed. Control measures have to consider the cause of the increased flow of water across the landscape.

3.4 Soft clays Clayey soil deposits, especially recent deposits, which have not been buried during their history by overlying soil or water, are referred to as “normally consolidated”. When an additional load is placed on these soils, they are temporarily “over-pressurized” and need to re-adjust their soil skeleton in order to reflect their altered state of loading. This is achieved by expulsion of soil moisture. However, due to the low permeability of clay materials, many years may pass before the soil moisture can be sufficiently expelled to complete the consolidation process - particularly so when the clay horizon is thick. Since clay horizons often contain lenses of sand or gravel, the rate and magnitude of consolidation may additionally vary somewhat, giving rise to differential consolidation. The magnitude of damage to structures constructed on the clay gradually worsens as consolidation progresses.

Figure 5: Damage to structures placed on Problem soils, Moretele and Bela Bela, Nothh-West Province.

Apart from the consolidation process, the clay may also fail upon load application. This is the case when the shear strength of the clay is too low to support the applied load, with the result that the bearing capacity of the soil is superseded and sudden collapse or partial collapse of the structure takes place.

3.5 Dispersive soils Clays in a dispersed state are clays of which the platelets are largely orientated parallel while for deflocculated clays the orientation is largely random – refer to Figure 6. Dispersive erosion depends on variables such as the clay mineralogy and chemistry as well as the dissolved salts in the soil water and eroding water.

Smectitic clays (e.g. montmorillonite) and illite potentially have high (ESP) values. The repulsive electrical surface forces between individual clay particles exceed the attractive (Van der Waal’s) forces, causing the progressive detachment of particles from the surface when in contact with water.

(a)

(b)

Figure 6: Schematic image of (a) Dispersed clay (b) Deflocculated clay

The main clay property causing repulsion is the percentage absorbed sodium cations on clay surfaces relative to other exchangeable cations (calcium, magnesium and potassium). Due to the high repulsive forces between particles, once in suspension, they take do not settle out over the normal time span to be expected for that particular particle size of material and thus lend a high total dissolved solid content to the erosive water. The second most important factor governing dispersion is the total content of dissolved salts in the water - the lower the content the greater the susceptibility of the clay to dispersion.

Since montmorillonitic clays absorb water and thus swell at a slower rate than kaolinitic clay, they are less capable of ‘plugging holes’ increasing their susceptibility to dispersion.

4 Distribution 4.1 Expansive soil Expansive clay is widely distributed throughout South Africa, its distribution largely being dictated by geology, soil type and by local climatic conditions and land form. The importance of their origin cannot be over-emphasized as it provides an informed observer with essential information regarding their engineering/ geotechnical characteristics. Generally, these soils originate from either basic igneous rocks or argillaceous sedimentary rocks. Basic igneous rock units associated with expansive clays in South Africa include the norite from the Bushveld Igneous Complex, dolerite of the Karoo Supergroup, diabase and andesite from the Pretoria Group and also andesite from the Ventersdorp Supergroup. Furthermore, argillaceous rock units particularly those of Karoo Supergroup, are the most important source of expansive soils in Southern Africa. The shales and mudrocks of Dwyka, Ecca and Beaufort Group weathers to heaving clays which are characterized by their slope stability problems in some parts of South Africa, i.e. KwaZulu-Natal Province. The following is a summary of known occurrences of expansive soils in South Africa: - Soils derived from lava occurring in a number of areas of the Limpopo Province [area west of Lepalale (Ellisras); area between Makhado (Louis Trichardt), Alldays and Musina (Messina); a strip all along the eastern borders of the Limpopo and Mpumalanga Provinces; the Mookhophong (Naboomspruit) area and the Bela Bela (Warmbaths) area], the northern part of the Eastern Cape Province (north of Dortrecht). - Black “turf” in the Onderstepoort to Rustenburg area and northwards towards Thabazimbi (residual norite soils) - Andesite and diabase soils in the Pretoria and Lydenburg areas - Soils derived from lava occurring in the south eastern parts of the Northwest Province and in some areas south of Johannesburg - Soils derived from mudstone/shale covering the western parts of the Northern Cape, northern (largest) parts of the Free State, eastern parts of Mpumalanga, western (largest) parts of Kwa-zulu Natal, northern (largest) parts of the Eastern Cape and northeastern pars of the Western Cape provinces (so-called Karoo mudrock and tillite)

- Soil derived from mudstone/shale in the Port Elizabeth and Uitenhage area (Eastern Cape Province) - Soil derived from clayey sandstones and shale in the Malmesbury area (Western Cape Province) - Soil derived from dolerite rock (intruded). These occur as small irregular bodies all over the interior of the county in particularly Karoo sedimentary rock. Figure 7 is also of interest in that it shows a comprehensive regional distribution map of swelling clays in South Africa.

Figure 7: Regional distribution map of swell clay occurrence in South Africa (after http://www// publicworks.gov.za/PDFs~)

Figure 8: Regional distribution map of potentially collapsing sand in South Africa (after http://www// publicworks.gov.za/PDFs~)

4.2 Collapsible soil Due to the fact that quite a number of deposition mediums can lead to the formation of a collapsible soil structure, the phenomenon is very widespread, however generally on a localized scale. Areas of more continuous distribution however include the granite soils in the area between Pretoria and Johannesburg, in the Mpumalanga Lowveld and Limpopo; windblown sand covering the northern parts of the Northern Cape, and large parts of Northwest Province, Limpopo, the Free State, Gauteng, the interiors of Kwa-Zulu Natal and the Eastern Cape. Refer to Figure 8 for a distribution map.

4.3 Soft clays In South Africa soft clay occurrences are generally restricted to the eastern and southern coastal areas and occur particularly in the immediate vicinity of existing or old river channels, but can also occur locally in the interior in poorly drained areas. These clays occur primarily as transported soils and are predominantly found in a number of depositional environments along the coastal areas like Durban, Richards Bay, Knysna and Langebaan. However, they also occur in some areas further inland as fairly shallow deposits:

-

Durban, Richards Bay, KwaZulu-Natal North and South Coasts The Eastern Cape Coast at a number of estuaries – notably in the Knysna area The Western Cape south coast at a number of estuaries and at localised estuaries in the Cape Town area Localized pans in the interior

Figure 9: Dispersive Clay occurrences (modified from Elges, 1985)

4.4 Dispersive soils In South Africa most dispersive clays have in the past largely been encountered in soils derived from the following geological formations and groups – the distribution of which is indicated in Figure 9: Karoo Supergroup:

Beaufort Group, Ecca Group, Molteno Formation and the Dwyka Formation.

Cape Supergroup:

Witteberg Group, Bokkeveld Group, Table Mountain Group and Malmesbury Group

Uitenhage Group:

Cretaceous Enon Formation, Kirkwood Formation and Sundays River Formation.

Swaziland Complex:

Basement All granites and granodiorites.

Dispersive clays can also develop under the following circumstances: -

Low-lying areas where the rainfall is such that seepage water has a high SAR value, especially in dry regions (so-called ‘Weinert N lines’1 displaying high values). Soils developed on granite are especially prone to the development of high ESP values in low-lying areas.

-

Areas where the original sediments contain large quantities of illite smectitic clays (montmorillonite, vermiculite) with high ESP values. This is particularly the case with the mudstones and siltstones of the Beaufort Group and the Molteno Formation in regions where the Weinert N-value is higher than 2. Soils in low-lying areas of these formations are dispersive.

-

In the more arid areas, where the Weinert N-value exceeds 10, the development of dispersive soil is generally inhibited by the presence of free salts despite high SAR values. Highly dispersive soil can develop if the free salts with high SAR values are leached out.

5 Effects and cost implications 5.1 General problem statement The SAICE State of the Art Conference on Problem Soils held in Stellenbosch, South Africa, from 11th to 12th September 1985 considered: -swelling clays -collapsible soils -dispersive soils -soft clays -dolomite (incl. wad & ferroan soils) These same topics were addressed again in 2008, 23 years later, at the Problems Soils Conference held at the University of Pretoria. However the NHBRC via its Home Builder Manual site geotechnical classification system (incorporated into GFSH2-2002), only consider three (3) types of problem soils, namely: expansive 1

: (H, H1, H2, H3 classes) – 35% of South Africa

The Weinert’s N-value is calculated from climatic data as follows:

N=

12 ⋅ E j Pa

, where Ej = evaporation during January; Pa = annual precipitation

residual mafics/ultra mafics, alluvial clays collapsible

: (C, C1, C2) - % of South Africa residual granite, colluvium

compressible : (S, S1, S2) – no data estuarine clays, inland peats, residual & colluvial clays, loose sands

The Council for Geoscience (CGS), by contrast, via its 1:50 000 scale regional geotechnical mapping program incorporates the identification of 13 geotech parameters including five (5) problem soil types, namely: Active (Act), Collapsing/settling (Col), Dispersive (Dis), Acidic (Aci) and Erodible (Ero) soils. For example the Vereeniging 2627DB sheet (boundary between Gauteng & Free State provinces), lies within a region broadly defined as being underlain by both moderately expansive and potentially collapsible soils (Anon, 1991). Mapping showed the following actual spatial distribution with respect to the total sheet area: -Active soils (expansive/shrinking) = 87% -Collapsing or settling soils = 18% -Erodible soils = 6% -Acidic soils = 1% Other geotechnical conditions (slope instability, karst, outcrop, inundation areas etc), occupy remaining areas. It must be borne in mind that both heave and collapse condition can occur at one locality (viz: collapsible aeolian soils over active residuum). Thematic maps are thus prepared for each of the 13 geotechnical parameters, when present and spatial extent over the approximately 620km2 sheet area, calculated in GIS. Examination of a second 1:50k sheet example, the White River 2531ac sheet (Mpumalanga province), which falls in a region dominated by potentially collapsible soils, indicates problem soil distribution to be: -Active soils: (expansive/shrinking) = zero % -Collapsing or settling soils = 53% -Erodible soils = 64% -Acidic soils = no data.

As engineering geological maps for: (i) South Africa, (ii) each province, (iii) each district municipal area or (iv) each local municipal area; have yet to be prepared and only 22 of the two thousand (2000) 1:50 0000 sheet areas, or conversely only three of the five metro regions, have been

geotechnically mapped; it is difficult to accurately rank the overall effect and cost implications of problem soils, in South Africa. However, a provisional ranking is shown in Table 1.

Table 1: Problem soil ranking EFFECTS:

[ie: noticeable to society]

COST IMPLICATIONS

1

erodible soils (incl dispersive)

1

expansive soils

2

expansive soils

2

erodible soils (incl dispersive)

3

collapsible soils

3

collapsible soils

4

compressible soils

4

compressible soils

5

acidic soils

5

acidic soils

Subsequent paragraphs follow the above ‘effects’ ranking scheme for the five noted problem soil types, described in this report. The effects, manifestation, mitigation options & costs, remediation activities & costs, plus some insights into international experiences; are briefly examined for each of these problem soils.

5.2 Problem soil effects 5.2.1 Erodible soil effects It was stressed at the Annual Land Degradation Conference of the International Erosion Control Association (IECA) South Africa Branch, in Port Elizabeth (Lindique, 2010), that South Africa is heading for a national disaster, through the loss of productive farmland due to poor farming practices, low levels of government funding and soil erosion control. One measure to determine soil erosion is the Universal Soil Loss Equation (USLE), which is commonly expressed as:

A = RKLSCP A = The computed soil loss in tons/acre/year R = Rainfall factor K = Soil erodibility factor for a specific soil type L = Slope length S = Slope steepness C = Crop-management factor P = Conservation practices that reduce soil loss

In areas of moderate to highly dispersive soils, where poor land management and cropping practices occur, sheet and rill erosion soon develops into badlands type terrain and the development of dongas (Figure 10).

Sheetwash erosion at a cemetery site Note the high colloid suspension load. [CGS photo archives]

Donga erosion in sodic colluvial soils in the former Transkei [J Stapley, web site]

Typical wide donga erosion in Kwa Zulu Natal, Ozisweni area, KZN province, in 2005. [CGS photo archives]

Deep donga erosion in the Sinathingi area, PMBurg, KZN. [CGS photo archives, 2005]

Figure 10: Examples of sheetwash and gully erosion

The AGIS online Comprehensive Maps (Anon, 2011), include a national erodibility map (Figure 11), which highlights the widespread extent of areas susceptible to water erosion.

Figure 11: Map of soil susceptibility to water erosion in the RSA

(From: AGIS

Comprehensive Maps: Online)

The impacts and effects of erodible soils on land degradation in South Africa, are more fully described by Le Roux (2007), Meadows & Hofmann (2002) and Garland et al (2000). Maharaj and Gilli (2008) refer to three (3) men as having fallen to their deaths in the 10-30m deep Dumbuza erosion gulley near Pietermaritzburg, KZN, in 2005. The clayey fine grained colluvial soils derived from the Karoo Super Group in particular, normally have a high exchangeable sodium content (>10%), making them dispersive and highly erodible (Brink, 1985). Deep and pervasive donga development is thus typical across Kwa Zulu Natal and the former Transkei, along the footslopes of the Drakensberg Escarpment.

5.2.2 Active soil effects The effect of vertical heave on structures is evaluated via a consideration of differential heave. Structures are subjected to varying insitu heave conditions as soil moisture content varies spatially i.e the wet outer edges versus dry inner areas under a building. The deflection

resistance of walls and foundations to these movements is directly dependent upon their design and any additional reinforcement present.

Swell pressures exerted can be determined directly via a one dimensional oedometer test, under constant volume conditions (Jennings & Knight, 1957, Burland, 1975); or through empirical relationships with other easily measured soil parameters (Vijayvergiya & Ghassahy, 1973; Chen , 1988). It is important to note that about six metres of clay soil overburden stress (kPa), balances out vertical heave pressures, while the depth limit for housing site geotechnical investigation heave calculations, is invariably set by the excavator (TLB) maximum reach of 3,50m. For the ubiquitous single storey housing developments now being seen in many parts of South Africa; the mandatory NHBRC Home Builders Manual site geotechnical classification system applied, is based on a maximum foundation loading of 50kPa (i.e.: single storey masonry buildings). Furthermore the class ranges of expected vertical soil movements on expansive soils (viz: H, H1, H2, H3 as per NHBRC standard) and their corresponding appropriate foundation designs to mitigate these deflections; correlate to a differential heave movement of just 50% of total heave. Clay soils can exhibit swell pressures well in excess of 1000kPa (Table 2), easily lifting and damaging both single and double storey housing. The value at risk varies from low cost state subsidized housing that used to be constructed for 35

>30

60-95

Liquid (%)

Limit

Degree of expansion

Swelling pressure (kPa)

>60

very high

>1000

20-30

40-60

high

250-1000

30-60

10-20

30-40

medium

150-250