Geotechnical characterisation of the upper quaternary sands of the Cape Flats

Fouche, Nanine (2021-03)

Thesis (PhD)--Stellenbosch University, 2021.


ENGLISH ABSTRACT: The sand-covered coastal plain connecting the Cape Peninsulamountain chaintothe South-Western Cape mainland,is known as the Cape Flats (Hill andTheron, 1981). The area is home to many residents of the greater Cape Town area and is characterised by densely populated townships comprising an amalgamation of formal and informal housing. Rapid on-going civil development is taking place in theCape Flats to improve existing infrastructureand todevelop new facilitiesfor the local communities. The construction of educational, community and public transport facilities and housing developments,are some of the recent and future envisaged improvement projects.The whitish windblown sands covering this area of approximately 460 km2, referred to as the upper Quaternary sands of the Cape Flats, will provide founding material for the proposed infrastructure. A knowledge of the geotechnical properties and engineering behaviour of these sandsis essential for design of suitable foundations for proposed structuresand earthworks associated with such developments.Duringthe initialplanning stages of a project,whenassumptions must be made regarding investigation and construction methods best suited to the project, a knowledge of general characterisation can also be beneficial. This research isthe first major contribution towards comprehensively characterising the Quaternary-aged sands of the Witzand, Springfontyn and Langebaan Formations fromtheCape Flats in terms of their physicalproperties and engineering behaviour. By combining, analysing and interpreting the wealth of existing and available geotechnical information from previous and new soil investigations undertaken in the area, this aim was achieved.In-situand laboratory datafrom 155site investigationsundertaken in the study area were collected and documented.In addition, new or different methods of investigation and testing not commonly used in the area have been researched, including, but not limited to piezocone penetrometer tests (CPTu), continuous surface wave (CSW) tests, and monotonic and repeated load triaxial tests. The sands from the study area were classifiedbased on theirgrading properties, Atterberg limits, maximum dry density and optimum moisture content, minimum dry density, California Bearing Ratio (CBR), erodibility and corrosivity, and characterised in terms of its compressibility, shear strength, permeability, volumetric behaviour during shear including liquefaction potential, in-situ density and moisture content and specific gravity. The geotechnical properties that characterise the distinctive sand formations wereexplored to produce statistical results,revealing underlying patterns, distinctive trends, distributions and correlations, and the resultant practical importance and probable implications explored. Focus wasplaced upon determining the nature of the relationships between the soil parameters, specifically for the sands of the Cape Flats, and any inter-formation variation in such relationships. The typically steep particle size distribution (PSD) curveassociated with the shallowCape Flats sands reflects the predominance of particles with sizes between 0.075mm and 0.6mm (fine and medium sand). The aeolian sands are typically non-plastic or slightly plastic with a typical fines content (<0.075mm) of around 5%. It is the predominance of fine, and to a lesser degree, mediumsand size particles, and the shape of these quartzose particles, that typically govern the engineering behaviour of the windblown deposits. In this regard, typical descriptions for some of the soil properties are given below: Mainly G7, G8 and G9 quality “gravel-soil” (TRH14 system), Class A-3 materials (AASHTO system), and SP materials (USCS). •Rarely compacted to densities exceeding 1850kg/m3using modified AASHTO compaction effort (owed to the uniform gradation).•Minimum index density ranging from approximately 1420kg/m3to 1590kg/m3.•Specific gravity (Gs) of soils with predominantly sand-size grains ranging from 2.62 to 2.69. •Buried structures susceptible to deterioration via electrochemical processes in certain areas of the Cape Flats.•Typically, low susceptibility of soil grains to detach and be transported by rainfall and runoff.•Mostly semi-pervious (k = 5.9 x 10-4to 5.7 x 10-6m/s).•Typically, moderate to high surface infiltration rates.•Highly compressible, and either non-collapsible or possessing a low collapse potential. •Peak friction angles varying from about 30° to 40°, with cohesion values up to about 13kPa.•Predominantly dilative and locally prone to cyclic liquefaction during dynamic loading (e.g. an earthquake). Thesedescriptions, which provide abroad and simplified indication of the engineering properties of the material underlying the Cape Flats, could create the impression of a uniform deposit of mainly sandy soils. However, the soils werefound to be highly variable(with both inter-and intra-formation variation), with a wide range of values assigned to many material properties; principally a function of soil texture, gradation and degree of cementation. For instance, the calcretised sands of the Witzand and Langebaan Formations will be associated with significantly decreased permeabilities and infiltration rates (thereby increasing run-off and erosion potential of the overlying sands), a potentially collapsible soil fabric (calcium carbonate as bonding agent), increased bearing strength, and shallow perched water tables. The higher fines content associated with the Springfontyn Formation sands resulted in higher compacted densities, whereas peaty layers from the same formationare associated with lower specific gravities (ranging between 2.4 and 2.64), and acidic and potentially corrosive soils. The consistency of the Cape Flats sands variessubstantially, both vertically and horizontally (over short distances), and so also itssusceptibility to settlement. The variability also showed in the proposed predictive models.Grading parameters were found to be relatively poor predictors of maximum dry density and optimum moisture content, and therelationship between individual California Bearing Ratio and dry densityvalues was weak and only marginally improved by including the grading modulus.In addition, SPT blow count and overburden pressure are not very efficient predictors of shear wave velocity (Vs) in the non-uniform sands. Settlementpredictions based on small-strain stiffness (obtained from Vs) have the advantage of considering non-linear stress-strain behaviour of soil and the degradation of stiffness with increasing strain. From the above it is evident that the physical properties of the Cape Flats sandsare far from simpleand cannot be narrowed to ageneral definition. The findings of this research may be used to form initial appreciation of the likely properties of the material and potential problem areas. This will guide the planning and execution of appropriate site-specific investigationsand aid the interpretation of results.This research has shown that there is merit in broadening the current approach to site investigations of the Cape Flats,which traditionally rely heavily on shallow test pits and penetrometer testing. In particular, increased use of CPTu testing and the determination of shear wave velocity by means of CSW or MASW testing should be considered.

AFRIKAANSE OPSOMMING: Die sandbedekte gelyktes van die kusgebiedwat die Kaap Peninsula bergreeks verbind met die Suidwes-Kaapse vasteland is bekend as die Kaapste Vlakte (Hill en Theron, 1981). Hierdie streek is die tuiste van baie inwoners van die groter Kaapstad area en wordgekenmerk deur digbevolkte dorpsgebiede, bestaande uit ‘n samestellingvan formele en informele behuising. Siviele ontwikkeling op die Kaapse Vlakte vind vining en deurlopend plaas om die bestaande infrastruktuur te verbeter en om nuwe fasiliteite vir die plaaslike bevolking daar te stel. Die konstruksie van opvoedkundige, gemeenskap en openbare vervoer fasiliteite en behuisingsontwikkeling is sommige van die huidige en toekomstige verbeteringsprojekte wat beplan word. Die witterige, eoliesesand wat hierdie area van ongeveer 460km2bedek, is bekend as die boonste Kwartêrne sand van die Kaapse Vlakte en sal fondamentmateriaalvir die voorgestelde infraskruktuur voorsien. ‘n Kennis van die geotegniese eienskappe en ingenieursgedrag van hierdie sand is noodsaaklikvir die ontwerp van geskikte fondamente vir voorgestelde strukture en grondwerke wat met die sodanige ontwikkelings gepaardgaan. Gedurende die aanvanklikebeplanningsfases van ‘n projek, wanneer aannames gemaak moet word rakende ondersoek-en konstruksiemetodes mees geskik vir die projek, kan ‘n kennis van die algemene karakterisering ook voordelig wees. Hierdie navorsing is die eerste grootskaalse bydrae tot omvangryke karakterisering van die Kwartêrne sand van die Witzand, Springfontyn en Langebaan Formasies van die Kaapse Vlakte, interme van hul fisiese eienskappe en ingenieursgedrag. Deur die kombinering, analiseen interpretasie van die groot hoeveelheidbestaande beskikbare geotegniese data van vorige en nuwe grondondersoeke in die area, is hierdie doelwit bereik. In-situ en laboratorium data van 155 terreinondersoeke in die studie-area is bymekaar gemaak en gedokumenteer. Voeg hierby dat nuwe of verskillende ondersoek-en toetsmetodes, wat nie algemeen gebruik is in die area nie, nagevors is, insluitend maar nie beperk tot piësokegel penetrasie toetse (CPTu), deurlopende oppervlakgolf (CSW) toetse en monotoniese en herhaalde las drieassige toetse nie. Die sand van die studie-area is geklassifiseer, gebaseer ophul graderingseienskappe, Atterberg grense, maksimum droë digtheid en optimum voginhoud, minimum droë digtheid, Kaliforniese drakragverhouding (KDV), erodeerbaarheiden korrosiwiteit. Die sandis gekarakteriseer in terme van saamdrukbaarheid, skuifsterkte, deurlatendheid, volumetriese gedrag tydens skuifbeweging insluitend vervloeiings potentiaal, in-situ digtheid en voginhoud en spesifieke digtheid.Die geotegniese eienskappe wat die spesifieke sandformasies kenmerk, is ondersoek om statistieseuitslae te produseer, en dui op onderliggende patrone, spesifieke tendense, verspreiding en korrelasies. Die gevolglike praktiese belangrikheid, asook moontlike implikasies, is ondersoek. Fokus is geplaas op die bepaling van die verhouding tussen grondparameters, spesifiek vir die sand van die Kaapse Vlakte en enige inter-formasie variasies in sodanige verhoudings. Die tipiese styl partikelgrootteverdeling-kurwe wat met die Kaapse Vlakte sand geassosieer word, toondie oorheersingvan deeltjies in grootte tussen 0.075mm en 0.6mm (fyn en medium sand). Die Aeoliese sand is tipies nie-plasties of effens plasties met ‘n tipiese fynstof inhoud (<0.075mm) van ongeveer 5%. Dit is die oorheersing van fyn, en in ‘n mindere mate, medium sandgrootte deeltjies wat tipies die ingenieursgedrag van die windverwaaide afsettings beheer. In die opsig word tipiese beskrywings van sommige grondeienskappe hieronder aangedui. •Hoofsaaklik G7, G8 en G9 kwaliteit gruisgrond (TRH14 sisteem), klas A-3 materiaal (AASHTO sisteem), en SP materiale (USCS).•Selde gekompakteer tot digthede wat 1850kg/m3oorskry, deur gebruik te maak van gewysigde AASHTO kompaksie energie (as gevolg van die eenvormige gradering).•Minimum droë digtheid wat varieer van ongeveer 1420kg/m3tot 1590kg/m3. •Spesifieke digtheid (Gs) van grond wat grootliks sandgrootte korrels bevat varieer van 2.62 tot2.69. •Ondergrondse strukture vatbaar vir verswakkingvia elektro-chemiese prosesse in sekere areas van die Kaapse Vlakte.•Tipiese lae vatbaarheid van grondkorrels om los te raak en weggevoer te word deur reënval en afloop water. •Hoofsaaklik semi-deurlatend (k = 5.9 x 10-4tot5.7x 10-6m/s).•Meestal ‘n matige tothoë infiltrasietempo.•Hoogs saamdrukbaar en nie-swigbaarof ‘n lae potensiaal om swigbaar te wees.•Maksimum wrywingshoeke wat varieer tussen ongeveer 30° en40° met kohesiewaardes tot en met13kPa.•Hoofsaaklike uitdyend gedurende skuif en lokaal geneig tot sikliese vervloeiing gedurende dinamiese belasting (bv. ‘n aardbewing). Bogenoemde beskrywings, wat ‘n breë en vereenvoudige aanduiding van die ingenieurseienskappe van die materiaal watdie Kaapse Vlakte onderlê verskaf, kan die indruk skep van ‘n eenvormige afsetting van hoofsaaklik sandgrond. Dit is egter gevind dat die grond hoogs veranderlik is (met beide inter-en intra-formasie variasie) met ‘n wye verskeidenheid van waardes geassosieer met heelwat materiaal eienskappe, hoofsaaklik ‘n funksie van gradering en graad van sementasie. Byvoorbeeld, die gesementeerde(kalkhoudende) sand van die Witzand en Langebaan Formasies sal geassosieer wordmet ‘n laer deurlatendheiden infiltrasietempo (daarby toenemende afloop en erosievan die oorliggende sand), ‘n potensiële swigbare grondstruktuur (kalsiumkarbonaat as bindingsstof), verhoogde dravermoë en vlak tydenlike grondwatervlakke. Die hoër fynstof inhoud wat geassosieer word met die Springfontyn Formasie sand het gelei tot hoër kompaksie digteid, terwyl grond met ‘n hoë inhoud organise materiaal in dieselfde formasie met laer spesifieke digtheid (tussen 2.4 en 2.64) geassosieer word, en suur en potensieel korrosief is. Die digtheid van die Kaapse Vlakte sand wissel beduidend, beide vertikaal en horisontaal (oor kort afstande), asook die vatbaarheid vir versakking. Die wisselbaarheid is ook getoon in die voorgestelde transfomasie modelle, met die graderingsparameters wat maksimum droë digtheid en optimum voginhoud swak voorspel, asook‘n swak verhouding tussen individuele Kaliforniese drakragverhouding en droë digtheid waardes (ietwat verbeter deur die graderingsmodulus in te sluit). Bykomend, is SPT houtelling en bo-gronddruk nie goeie aanduiders van skuifgolfsnelheid (Vs) in die nie-eenvormigesand nie. Sullke aanduiders het die voordeel dat dit die nie-lineêre spanning-vervormings gedrag in ag neem wanneer elastiese versakking bereken word. Uitdie bogenoemde is dit duidelik dat die fisiese eienskappe van die Kaapse Vlakte sand vêr van eenvoudig is en kan nie vereenvoudig word tot ‘n enkele definisie nie. Die bevindinge van hierdie navorsing kangebruik word om ‘n aanvanklike idee van die waarskynlike eienskappe van die materiaal en potensiële probleemareas te vorm. Hierdie sal die beplanning en uitvoering van terrein-spesifieke ondersoeke lei en die interpretasie van die resultate aanhelp. Hierdie navorsing het aangedui dat daar meriete is in die verbreding van die huidige benadering tot terreinondersoeke indie Kaapse Vlakte, wat tradisioneel meestal staatmaak op toetsgate en penetrasie toetse. Die vermeerde gebruik van veral CPTu toetse en die bepaling van skuifgolf snelheid (Vs) by wyse van CSW of MASW toetse moet oorweeg word.

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