SAGIT WIND FARM Wolseley, Western Cape - GIBBprojects.gibb.co.za/Portals/3/App H_SAGIT Wind Farm...

SAGIT WIND FARM Wolseley, Western Cape

PRELIMINARY

GEOTECHNICAL REPORT

16 November 2012

Prepared for: Aecom South Africa (Pty) Ltd

REFERENCE NUMBER: 1-137212

R.A. Bradshaw & Associates cc 17 Midwood Avenue

Newlands 7708 South Africa

Contact person:

R. Bradshaw +27 21 6711995

[email protected]

SAGIT Wind Farm, Wolseley Preliminary Geotechnical Investigation

R.A. Bradshaw & Associates cc page i

Contents SUMMARY ____________________________________________________________ii

1. INTRODUCTION ______________________________________________________ 1

2. DESCRIPTION OF THE SITE AND THE DEVELOPMENT ________________ 1 2.1 The Site __________________________________________________________________ 1 2.2 The Development __________________________________________________________ 1

3. OUTLINE OF THE INVESTIGATIONS __________________________________ 3 3.1 Desk Study _______________________________________________________________ 3 3.2 Field Investigation and Inspection ______________________________________________ 3

4. REGIONAL GEOLOGY ________________________________________________ 3

5. SEISMICITY OF THE WOLSELEY AREA ________________________________ 5

6. GEOLOGY OF THE TURBINE SITES ___________________________________ 6

7. GROUNDWATER CONDITIONS ________________________________________ 8

8. POSSIBLE FOUNDATION LAYOUTS ____________________________________ 8

9. ROADS AND LAY-DOWN AREAS _______________________________________ 9

10. USE OF ON-SITE MATERIALS FOR CONSTRUCTION PURPOSES _______ 10

11. CONCLUSIONS AND RECOMMENDATIONS ___________________________ 10

Figures FIGURE 1 LAYOUT OF TURBINES __________________________________________________________ 2 FIGURE 2 REGIONAL GEOLOGY ___________________________________________________________4

Tables TABLE 1 POSSIBLE SOIL PROFILES, FOUNDATION LAYOUTS AND SUBGRADE CONDITIONS FOR ROADS AND LAY-DOWN AREAS ________________________________________________ 7

Annexure Photographs of Soil Exposures in Trial Pits and Other Excavations


R.A. Bradshaw & Associates cc page ii

Summary This report presents the results of the preliminary geotechnical investigation for the SAGIT Wind Farm which is located between 5km and 10km south of Wolseley. The investigations comprised a desk study and filed investigation and inspection of the thirty turbine positions. Engineering properties in this report are based on visual assessments. The objectives of the investigations were to obtain preliminary data to assist with planning design and costing for the project. The investigations revealed that the turbines will generally be located in areas underlain by variable thicknesses of fine and mainly coarse alluvium overlying residual Malmesbury soils and weathered Malmesbury bedrock. Variations to this general profile do occur, notably at Turbine 10, or where rocky outcrop occurs. Shallow seasonal groundwater is expected, particularly in the southern half of the site. The water will have adverse effects on the trafficability around the site and construction of turbine bases and clayey soils would also be softened. The site is located in an area of seismic activity. Published data for Tulbagh suggests that the maximum credible earthquake (MCE) might be 6.6 on the Richter Scale, and the operating basis earthquake (OBE) with a 63% probability of exceeding in 100 years is 5.7. The peak ground acceleration (PGA) for the MCE could be 0.3g, but, for example, a PGA of 0.05g has a return period of 27 years and a probability of exceedance in 100 years of approximately 98%. Although the data on seismicity quoted above is for Tulbagh, the data for the Wolseley site is expected to be similar. Seismicity and differential settlement will therefore be major factors affecting the design of the foundations. Piled foundations are currently anticipated for seventeen of the thirty turbines with another twelve possibly requiring piling unless it is proven that weathered rock suitable for founding occurs at shallow depth.

Other foundation options and the design of the piles should be investigated during the detailed phase of the geotechnical investigations. Subgrade conditions for roads will be fair in the northern parts of the site, but the shallow groundwater will significantly adversely affect subgrade conditions in the southern parts. Large quantities of materials for gravel surfacing for roads and lay-down areas are unlikely to be sourced on the site. Because of the poor subgrade conditions in the southern parts of the site, construction of roads and turbines should be programmed for the dry window in the year between, say, November and April. Aecom South Africa have scheduled ten exploratory boreholes for the detailed phase of investigations, but it is probable that between twelve and eighteen boreholes might be required in conjunction with trial pitting to determine ground conditions and to optimise foundation layouts.


R.A. Bradshaw & Associates cc page 1

1. INTRODUCTION Planning and design for the proposed SAGIT Wind Farm, which will be located to the south of the town of Wolseley in the Western Cape, are currently in progress. Geotechnical conditions and specifically the seismicity in the area and the foundation layout for the turbines will influence the design of the turbine structures and the efficacy and cost of the project. Mr R Valentyn of Aecom South Africa (formerly BKS (Pty) Ltd), the Consulting Civil and Structural Engineers for the project, therefore requested R.A. Bradshaw & Associates cc to undertake a geotechnical investigation at the site. Both a preliminary geotechnical evaluation and a detailed geotechnical investigation were required. A proposal for both the preliminary and detailed investigations was e-mailed to Mr Valentyn on 23 August 2012. Authorisation to proceed with only the preliminary investigation was received from him on 30 August. This report presents the results of the preliminary geotechnical investigation. The method of investigation, regional and local geology, seismicity and an initial evaluation of founding conditions and foundation layouts for the turbines and subgrade conditions for access roads and lay-down areas are provided.

2. DESCRIPTION OF THE SITE AND THE DEVELOPMENT

2.1 The Site The site is located between approximately 5km and 10km south-southwest of the town of Wolseley in the Tulbagh Valley and effectively immediately to the east of the R43 Regional Road to Worcester. It extends up to approximately 2.4km to the east of the road and the area in which the wind farm will be constructed is some 1200Ha. The site is located within farmland, which includes orchards, fallow ground and pasturage, and seven farms fall within the general site. The Breede River lies to the west of the R43 and hence the ground generally slopes westwards towards the river, with the steepest gradients occurring, in places, in the eastern parts. Access to the different parts of the site is via a combination of farms road and tracks local tarred and mainly gravel roads. The main railway line between Cape Town and the interior runs north-south through the southwestern parts of the site and it effectively sub divides that area into two parts.

2.2 The Development The development will include thirty turbines with associated access roads, lay-down areas and presumably offices and associated buildings for permanent maintenance and control staff. The locations of the turbines are illustrated on Figure 1.



FIGURE 1: LAYOUT OF TURBINES



No details of the turbine structures and their foundations have been provided. However, based on other wind farms investigated by R.A. Bradshaw & Associates, turbine masts up to 80m high, base diameters up to 20m and base thicknesses between 2m and 3m might be relevant. For the configuration described above, peak bearing pressures on the outer edges of the footings might be in the order of 300kPa.

3. OUTLINE OF THE INVESTIGATIONS

3.1 Desk Study The desk study comprised sourcing published data on the regional geology and seismicity of the area. In this respect, the following references were used:

• Geological Survey of Southern Africa, 1992. 1:250000 Geological Series Map, Sheet 3319, Worcester.

• Gresse, P.G. and Theron, J.N., 1992. The Geology of the Worcester Area. Department of Mineral

Affairs, Geological Survey.

• Kijko, A., Retief, S.J.P. and Graham, G., 2002. Seismic Hazard and Risk Assessment for Tulbagh, South Africa: Part 1 - Assessment of the Seismic Hazard. Natural Hazards, Vol. 26, pages 175-201. Kluwer Academic Publishers.

• Wooldrige, J. 2004. Geology of the Breede River Valley: Worcester to Robertson. ARC Inftuitec,

Nietvoorbij, Stellenbosch.

3.2 Field Investigation and Inspection The locations of all thirty turbines sites were visited on 6 November and ten pits and other exposures were inspected. Photographs of the soil profiles in the pits and exposures are presented in the Annexure to this report.

4. REGIONAL GEOLOGY The regional geology for the area to the north of Tulbagh and almost to Worcester in the south and extending eastwards into the Little Karoo is depicted on Figure 2 which is taken from the 1: 250000 map referenced above. The site is located in an area known as the Cape Syntaxis where the north-south and the east-west orientated branches of the Cape Fold Belt meet and curve towards the south-southwest. Wolseley and Tulbagh are located in a valley underlain by ancient Malmesbury Group rocks of the Late Proterozoic, whereas the mountains flanking the western and eastern sides of the valley comprise the mainly arenaceous rocks of the Paleozoic Cape Supergroup.



Qg Quaternary gravel SS Skurweberg Group Sg Goudini Group Table O-Sc Cederberg Group Mountain Opa Pakhuis Group Group Ope Peninsula Group Npo Porterville Formation Malmesbury Nn Norree Formation Group

FIGURE 2: REGIONAL GEOLOGY



The metasediments of the pre-Cambrian Malmesbury Group in the Tulbagh Valley are assigned to the Porterville Formation which comprises predominantly phyllitic shale and fine to medium grained greywacke. The Malmesbury Group rocks have been subject to several phases of tectonism with associated folding and faulting.

The Cape Supergroup rocks in the mountains to the west of the valley occur in a synclinal setting with the rocks of the Peninsula, Pakhuis, Cederberg, Goudini and Skurweberg Groups, which comprise sub groups within the Supergroup, represented. The same groups of rocks also occur in the mountains to the east where they dip predominantly eastwards into the Karoo Basin, but synclinal structures are also present. The Cape Supergroup rocks on the eastern side of the valley lie unconformably on the Malmesbury Group metasediments. In contrast, the Cape Supergroup rocks have been down-faulted against the Malmesbury Group on the western side of the valley. Displacement has occurred on the Worcester Fault which extends from the Nuwekloof Pass near Tulbagh to Robertson and eastwards from Robertson. The throw across the fault is variable with a maximum throw of up to 6000m quoted by Gresse and Theron. The faulting was a response to tension in the earth's crust associated with the rifting that culminated in the separation of Africa from South America and the linked Falklands Plateau during fragmentation of the southern supercontinent of Gondwana. The mountains of the Cape Fold Belt were created in the Triassic Period during tectonic episodes between 278 and 230 million years ago (mya). Rifting of Gondwana began in the late Jurassic (about 154mya), but tensional faulting probably did not peak until the early Cretaceous (between 135 and 130mya). Erosion of the continental land mass began with the initial uplift and rifting of Gondwana and peaked after the breakup. Erosion was rapid at first, slowing with time. Erosion carved the geological structures and rocks in ways that reflected their differing abilities to resist the erosive process. The sandstone-capped mountains of the area remained as prominent features in the landscape whereas structures composed of softer formation (e.g. Malmesbury Group) eroded to form valleys including the current Tulbagh-Wolseley Valley. The detritus from the erosion of particularly the sandstone was carried over large distances and broad areas by the Breede River. More recent detritus comprises the talus and scree deposits evident at the base of the mountains to the west and east of the site.

5. SEISMICITY OF THE WOLSELEY AREA The Southwestern Cape can be considered as one of the regions with the highest level of seismic activity of tectonic origin in South Africa. Earth tremors have been recorded in the Southwestern Cape from as early as 1620. Numerous earthquakes have record been recorded since that date with at least seven of them considered to have had local magnitudes between 5.1 and 6.5 on the Richter Scale. Many of the events are linked to seismicity in the Tulbagh-Wolseley area and include the well documented event on 29 September 1969 which had a local magnitude of 6.3.



Considerable uncertainty exists as to the location of the 1969 event, but probably the most accurate determination places it at 33.28°S 19.14°E which is approximately 65km northeast of the site. Kikjo et al have developed a probabilistic seismic hazard analysis (PSHA) which they have applied to the Tulbagh area in general and specifically to Tulbagh itself. This information provides a good approximation of the seismic hazard and associated parameters at the Wolseley site which is located some 20km to 25km south-southeast of Tulbagh. Their method gives a maximum credible earthquake (MCE) of 6.6. Return periods vary according to the magnitude of an earthquake and details are provided in their publication. As an example, a magnitude of 4.5 earthquake is expected to occur once every 7 to 19 years and, on average, once every 10 years. The operation basis earthquake (OBE) is defined as the earthquake for which an engineering structure is designed to remain operational. The recurrence interval of this earthquake is frequently estimated as 100 years. For Tulbagh, the 63% probability of exceedance curves indicate an event with a return period of 100 years would have a local magnitude of approximately 5.7. Based on various procedures including assuming a hypocentral distance of approximately 27km and an epicentral depth of 10km for a seismic event, the maximum peak ground acceleration (PGA) expected in the Tulbagh area is 0.30g with an 84% confidence upper level of 0.74g. These values are from a deterministic analysis and represent the worst-case scenario. The mean return periods associated with selected accelerations are also presented in their publication. As an example, an acceleration of 0.1g is expected to occur once every 92 to 111 years, but on average once every 101 years. A PGA of 0.05g is usually considered to be of engineering interest. For Tulbagh, a PGA of 0.05g has a return period of 27 years and has a probability of exceedance in 100 years of approximately 98%. In terms of site characteristic response spectra, probabilistic analyses indicate that for an event with a mean return period of 10 000 years, a peak spectral acceleration of approximately 0.63g at 20 Hz with a PGA of just less than 0.3g could be expected. It is reiterated that the site-specific data in the publication by Kijko et al is for Tulbagh, but the data for the Wolseley site is likely to be similar.

6. GEOLOGY OF THE TURBINE SITES Based on exposures in trial pits, eroded areas and excavations in platforms and borrow pits, soil profiles have been evaluated in the areas of all thirty turbines. The summarised descriptions of soils profiles at the turbine sites are presented in Table 1, and photographs of typical profiles in some areas are presented in the Annexure. From Table 1, it is apparent that:

• Gravelly and cobbly alluvium is present in many areas. The thickness is variable, but in places it ranges up to several metres. Boulders also occur within the coarse alluvium.



TABLE 1: SUMMARY OF POSSIBLE SOIL PROFILES, FOUNDATION LAYOUT AND SUBGRADE CONDITIONS

Turbines Soil Profile Foundation Layout Subgrade Conditions 1 and 2 Sandy colluvium over gravelly and cobbly alluvium over

residual Malmesbury soil Piled foundations

Fair subgrade with possibly G6 layer riding course and local filling to raise road level

3 and 4 Gravelly and cobbly alluvium over residual Malmesbury soil

Piled foundations but possibility of a conventional founding if weathered bedrock occurs within 2.5m of ground surface

5 Thin colluvium and alluvial gravel over residual Malmesbury soil and highly weathered Malmesbury bedrock

6, 7, 11 to 13 Gravelly and cobbly alluvium over residual Malmesbury soil

8 Thin colluvium and alluvial gravel over residual Malmesbury soil and highly weathered Malmesbury bedrock

9 Clay sandy colluvium over residual Malmesbury soil and weathered bedrock

Piled foundation

10 Rock outcrop and sloping ground

Conventional founding on rock Rock in places. Local blasting might be required.

15 to 18 Sandy colluvium and alluvium over thick alluvial gravels and cobbles. Wet areas in and after winter.

Piled foundations

Potentially poor subgrade mainly due to seasonal shallow water tables. Selected layer and/or thin fill to raise road level. Pioneer layer and surface and sub surface drainage required.

14 and 19 to 28 Thin clayey silty sandy colluvium overlying, in places, gravelly alluvium over residual Malmesbury soil. Very wet in and after winter

Piled foundations

29 and 30 Thin gravelly alluvium over residual Malmesbury soil over weathered Malmesbury bedrock

Piled foundations but possibility of a conventional founding if weathered bedrock occurs within 2.5m of ground surface



• Turbines 1 and 2 are possibly located on a younger alluvial wash fan.

• Turbines 3, 4, 6, 7 and 11 to 13 are located in areas covered by gravelly and cobbly alluvium

over residual Malmesbury soils with highly weathered Malmesbury rock at unknown depth.

• Turbine 8 has thin colluvial and gravelly soils (less than 0.75m) overlying Malmesbury soil and highly weathered Malmesbury rock. Wet seasonal conditions are reported in this area.

• Turbine 10 is located in rocky sloping ground.

• Turbines 15 to 18 are located in relatively low-lying areas with thin sandy colluvium and alluvium

over thicker alluvial gravel and cobbles. These areas become seasonally wet.

• Turbines 14 and 19 to 28 have a similar soil profile to that at Turbines 15 to 18, but the alluvium is thinner and the residual soil probably extends to greater depth.

• Turbines 2, 9 and 30 have a similar soil profile to that described for Turbine 5 and 8.

7. GROUNDWATER CONDITIONS Groundwater conditions are currently not fully understood and they must be evaluated in the detailed phase of investigations. Shallow, seasonal perched water tables are anticipated in many areas and specifically the areas in the vicinity of Turbines 14 to 28. The anticipated impact of groundwater would be to:

• create poor subgrade conditions for roads such that vehicles could not cross the natural terrain for possibly six months a year without formal layer works being placed.

• interfere with construction of turbine bases

• soften clayey soils

• cause overbreak and consequent over-excavation in excavations for turbine bases and sub

surface services particularly where fine and/or coarse alluvium is present. The permanent groundwater table is probably a significant depth.

8. POSSIBLE FOUNDATION LAYOUTS Assuming, for purposes of the current evaluation, that the configuration and bearing pressure described in Section 2.2 are relevant to the SAGIT Wind Farm and in the knowledge that seismicity and differential settlement will be major factors affecting the design of foundations and possible foundation layouts are also indicated in Table 1 overleaf and discussed below.



From Table 1, it is apparent that:

• Piled foundations are anticipated for seventeen of the turbines.

• Piled foundations will probably be required in twelve other turbine locations unless it is proven that weathered rock suitable for founding occurs at shallow depth.

• Only Turbine 10 is located in an area of relatively high strength rock and it can be founded

conventionally on bedrock possibly using passive, grouted tendons to resist uplift forces and reduce the diameter of the base.

• The depth of piling and type of pile can only best be determined during the detailed phase of

investigations. Factors which will influence piling include the presence of cobbles and, in places, boulders, the fact that weathered bedrock, which would be difficult to penetrate, occurs at shallow depth and the uplift forces, which will necessitate designing the piles for uplift, will be present.

Other foundation options, such as stabilised soil rafts, could be considered when the actual ground conditions are revealed during the detailed investigations.

9. ROADS AND LAY-DOWN AREAS Access roads to each turbine position will be required during both construction and the operating life of the turbines. Large lay-down areas are generally required next to each turbine to facilitate assemblage of the turbines and for future major maintenance. No road layout has been provided and it is not clear how the large vehicles would manoeuvre through the orchards. It is also uncertain whether or how the lay-down areas will be constructed in the orchards. Nevertheless, the following factors and conditions are relevant to the design and construction of gravel-surfaced roads and lay-down areas:

• The subgrade for roads to the turbine sites in the northern part of the site (Turbines 1 to 9 and 11 to 13) will probably comprise coarse alluvium over residual soils with local surficial sandy colluvium or alluvium. Fair subgrade conditions are anticipated with probably only a G6 type riding course required in many areas after normal subgrade preparation.

• Sandy subgrade with coarse alluvium possibly within the material depth is expected in the areas

of Turbines 14 to 28. The sands are possibly of G8 or worse quality and many of the areas are probably subject to seasonal wetting or saturation. Road design in these areas would therefore possibly include a selected layer and/or thin fill to raise the road above general ground levels.

A pioneer layer of gravelly soil might be required so that construction vehicles can enter these areas if the ground is saturated.

Provision of both surface and possibly local subsurface drainage will be required in the area of these turbines.



• The roads near Turbines 8, 28 and 30 would have cohesive soils in the subgrade. These soils would have low wet strength and the road and layer works design must ensure that water does not penetrate the subgrade.

• Part of the final section of the road to Turbines 10 would be over rock. Local blasting might be

required. The subgrade for the lay down areas would naturally be the same as that described above for the roads in the various areas. Because of the sloping ground and the anticipated large size of the laydown areas, cut and fill platforms would ideally be constructed. It should be noted that the subgrade for the laydown areas would be loosened by the presence of seasonal groundwater. The subgrade and the material excavated from the upslope parts of the platform would be impossible to compact if the subgrade and/or the material is too wet.

10. USE OF ON-SITE MATERIALS FOR CONSTRUCTION PURPOSES

Assuming that the conditions are ideal, excavated material from the cut could be used as engineered fill provided that the coarse material is removed. Material for filling would generally include the fine and coarse alluvium and, locally the residual Malmesbury soil and possibly even weathered rock. The coarse alluvium (mixed with the finer cohesionless or near cohesionless alluvium) could be crushed to provide a G6 or possibly G5 quality material. However, the quantity of suitable material from the excavations from bases will be small and only use of a small mobile crusher might be viable to produce some surfacing material. A small borrow pit occurs to the northwest of Turbine 12. This pit contains gravel and it might be possible, with the appropriate authorisations, to mine small quantities of gravel for crushing from this area. Materials for concrete, subsurface drains and even road layer works and surfacing of lay-down areas would be required.

11. CONCLUSIONS AND RECOMMENDATIONS

a) The turbines will generally be located in areas underlain by variable thicknesses of fine and mainly coarse alluvium overlying residual Malmesbury soils and weathered Malmesbury bedrock. Variations to this general profile do occur, notably at Turbine 10, or where rocky outcrop occurs.

b) Shallow seasonal groundwater is expected, particularly in the southern half of the site. The water will have adverse effects on the trafficability around the site and construction of turbine bases and clayey soils would also be softened.

c) The site is located in an area of seismic activity. Published data for Tulbagh suggests that the maximum credible earthquake (MCE) might be 6.6 on the Richter Scale, and the operating basis earthquake (OBE) with a 63% probability of exceeding in 100 years is 5.7.



d) The peak ground acceleration (PGA) for the MCE could be 0.3g, but, for example, a PGA of 0.05g has a return period of 27 years and a probability of exceedance in 100 years of approximately 98%.

e) Although the data on seismicity quoted above is for Tulbagh, the data for the Wolseley site is expected to be similar.

f) Seismicity and differential settlement will therefore be major factors affecting the design of the foundations.

g) Piled foundations are currently anticipated for seventeen of the thirty turbines with another twelve possibly requiring piling unless it is proven that weathered rock suitable for founding occurs at shallow depth.

h) Other foundation options and the design of the piles should be investigated during the detailed phase of the geotechnical investigations.

i) Subgrade conditions for roads will be fair in the northern parts of the site, but the shallow groundwater will significantly adversely affect subgrade conditions in the southern parts.

j) Large quantities of materials for gravel surfacing for roads and lay-down areas are unlikely to be sourced on the site.

k) Because of the poor subgrade conditions in the southern parts of the site, construction of roads and turbines should be programmed for the dry window in the year between, say, November and April.

l) Aecom South Africa have scheduled ten exploratory boreholes for the detailed phase of investigations, but it is probable that between twelve and eighteen boreholes might be required in conjunction with trial pitting to determine ground conditions and to optimise foundation layouts.

R.A. Bradshaw Pr.Sci.Nat. R.A. BRADSHAW & ASSOCIATES cc


R.A. Bradshaw & Associates cc

ANNEXURE

PHOTOGRAPHS OF SOIL EXPOSURES IN TRIAL PITS AND OTHER EXCAVATIONS



PIT TO SOUTH OF TURBINE 2 SHOWING COARSE ALLUVIUM

PIT TO SOUTHEAST OF TURBINE 3 SHOWING THIN RESIDUAL MALMESBURY SOIL OVER HIGHLY WEATHERED MALMESBURY BEDROCK



OPEN EXCAVATION TO THE SOUTHEAST OF TURBINE 5 SHOWING THIN ALLUVIAL SOILS OVER RESIDUAL MALMESBURY SOIL

PIT NEAR TURBINE 8 SHOWING HIGHLY WEATHERED MALMESBURY BEDROCK EXCAVATED FROM SHALLOW DEPTH



EXCAVATION FOR NEW PLATFORM EAST OF TURBINE 6 SHOWING COARSE ALLUVIUM AT TOP OF CUT WITH LIGHTER COLOURED RESIDUAL MALMESBURY SOIL AND WEATHERED MALMESBURY BEDROCK

EXISTING BORROW PIT NORTHWEST OF TURBINE 12 SHOWING COARSE ALLUVIUM OVER MALMESBURY SOILS



COARSE ALLUVIUM IN PIT EAST OF TURBINE 16

THIN ALLUVIUM OVER RESIDUAL MALMESBURY SOIL IN PIT NEAR TURBINE 30

SAGIT WIND FARM Wolseley, Western Cape - GIBBprojects.gibb.co.za/Portals/3/App H_SAGIT Wind Farm...

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