Communs geol. Surv. S.W Africa/Namibia, 2, (1986), 7-12 A. REPORTS BY THE GEOLOGICAL SURVEY

THE ENGINEERING GEOLOGY OF SWAKOPMUND

by

B.G. Bulley

ABSTRACT of the area has been shaped largely by luvial, marine and arid pedogenic processes. Old drainage lines of the An engineering geological survey of Swakopmund, Swakop River, up to 5 km north of it’s present course, one of South West Africa’s major developing centres, are indicated by alluvial terraces and silty deposits. The has been undertaken in order to provide an informa- Swakop River loods through to the Atlantic Ocean dur- tion base for further expansion. The town is located on ing periods of heavy rainfall in the catchment. Severe Cenozoic luvio-marine and alluvial deposits that over- loods occurred in 1931 and 1933, when the railway lie Precambrian Damara Sequence rocks and Karoo age bridge, houses and gardens were washed away (Sten- dolerites. The coastal environment is highly corrosive, gel, 1964). Northerly longshore drift has resulted in due to abundant airborne salts, the frequent fogs and a marked degradation of Swakopmund’s beaches and salty, moist soils. Protective anti-corrosion measures rockill breakwaters have been constructed to prevent should therefore be provided and construction mate- further erosion (Swart, 1982). rials must be chosen with care. Although foundation An engineering geological survey of Swakopmund conditions are generally good, differential settlement was undertaken during May and June 1983, in order can occur on some of the unconsolidated supericial de- to provide an information base for future development. posits. Construction materials, in the form of ine and The investigation commenced with a literature study, coarse aggregate for concrete, are readily available in followed by detailed aerial photographic interpretation the area. and ground truth checking. Relevant literature includes, inter alia, Brandt (1981); Brink (1981); Logan (1960); 1. INTRODUCTION Marker (1981); Martin (1963); Porada and Hill (1974); Rust (1980); Schulze-Hulbe (1979) and Smith (1965). Swakopmund was founded in 1892 to provide the The scope of the survey did not allow for detailed sub- German territory of South West Africa with an alter- surface investigations, such as drilling or excavation of native port to the British-held harbour of Walvis Bay, inspection pits. Existing development also imposed lim- 30 km to the south. Although the town no longer func- itations on the investigation, in that the natural ground tions as a port it thrives as South West Africa/Namibia’s is largely obscured and access restricted. major holiday resort and as the nearest town serving This paper summarises salient features of the engi- the massive Rössing Uranium Limited open-cast mine neering geology of Swakopmund, additional details of some 55 km to the east. which may be obtained from the investigation report Swakopmund falls within the extreme arid sector of (Bulley, 1983). the Namib Desert, but it’s coastal location ensures mod- erate to cool temperatures and a high frequency of fogs 2. GEOLOGY (95 - 215 days/yr) generated by the cold Atlantic Ocean (Met. Services, R.N. and S.A.A.F., 1943). Fog precipi- The Swakopmund area is underlain by rocks of the tation yields some 34mm/yr, whereas rainfall is erratic Damara Sequence, intruded by dolerite dykes of Karoo and averages 15mm/yr (op. cit.). The dominant wind di- age. The complex stratigraphic relationships within the rections are SW (summer) and NE (winter). The combi- Damara Sequence have not as yet been clearly deined nation of high relative humidity and abundance of salts and formation names attributed to the different rock in the atmosphere and soil make the SW.A. coast one types must be regarded as provisional. Cenozoic super- of the most highly corrosive environments in southern icial deposits, comprising thin colluvial soils, alluvium Africa (Callaghan, 1983). and luvio-marine deposits overlie the bedrock to vary- The broad sandy bed of the ephemeral Swakop River, ing depths. occurring along Swakopmund’s southern municipal A summarised stratigraphic column for the area is boundary (Fig. 1), forms a natural barrier between the given in Table 1 and a description of each unit, with an coastal dunes of the Namib Sand Sea to the south and the engineering assessment, is provided in the legend of the sand and gravel plains to the north. The geomorphology geological map (Fig. 1). TABLE 1: Stratigraphic column for the Swakopmund area. about 0,5 m and greater have a suficiently high bearing capacity to support most light structures. GEOLOGICAL UNIT THICKNESS Examination of soil proiles indicates that the sands Colluvial soils < O,5m generally have a low collapse potential, but the upper Swakop River Generally < 10m, but up to 30m in loose surface layers are compressible and subject to alluvium deep channels substantial settlement under load. Fluvio-marine deposits 1 - 5m Structural cracking, mostly of a minor nature, has oc- ROCK TYPE curred in some dwellings in Mondesa and Tamariskia. Karibib Calc-silicate rock This is attributed largely to founding at too shallow a Formation depth on loose compressible sands with resultant dif- ferential settlement. In some instances the situation has Karibib Marble Damara Formation been aggravated by wetting the subsoils, induced by broken water pipes and watering of gardens adjacent to Sequence Rössing Dolomitic marble the external walls. Formation Differential settlement may also be caused by vari- - Gneissic-granite able founding conditions across a site, e.g. when a dwelling is founded partly on bedrock and partly on 3. ENGINEERING GEOLOGICAL ASSESS- ill or loose colluvial sand. Settlement of foundations MENT on medium dense to dense sands can be expected to be well within tolerable limits for most normal buildings 3.1 Founding Conditions with low to moderate bearing pressures. However, for larger buildings with a high bearing pressure or low set- 3.1.1 Bedrock tlement tolerance (e.g., water towers or buildings con- taining sensitive machinery or panel walls), it is recom- Bedrock occurs on or near-surface in much of Swa- mended that laboratory and ield testing be carried out kopmund’s eastern and central suburbs. The slightly to predict the likely settlement of footings on the sand. to moderately weathered bedrock generally has a high Precautionary measures, in the form of increasing the bearing capacity and provides a good base for foun- depth of founding onto denser sands or widening the dations. Problems are, however, encountered during footings, will generally prove to be suficient. Prewet- excavations for foundation and services trenches, par- ting and compaction of the sand at founding level may ticularly in areas of prominent surface outcrop, where also be considered. it is necessary to resort to the use of jack hammers and In the case of high end-bearing loads, like water tow- blasting. Low ridges of marble, up to several metres ers, it will be necessary to key the foundations into the high, occur in parts of Industrial Extensions 5 and 10, unweathered bedrock underlying the sands. near the railway line. Future development should pref- Extensions 8 and 9 of Vineta, zoned for future resi- erably avoid these ridges, as site levelling and founding dential development, are located on the luvio-marine excavations would require blasting, thereby substantial- deposits. The medium dense sands at a depth of about ly increasing the cost of development. Localised dykes 0,5 m are expected to provide suitable founding con- of dolerite (e.g., in Mondesa Ext. 2) will present similar ditions for most normal residential dwellings. Deeper problems. founding levels or widened strip footings, as outlined Foundation excavations in rock should be checked, as above, should, however, be considered for shopping a matter of routine, for isolated pockets or zones of deep centres, schools and blocks of lats. weathering or fracturing. Such zones, which may occur Adverse founding conditions on gypsiferous materi- adjacent to dolerite dykes, should be removed and ex- als have been reported from various countries, particu- cavations taken through the highly weathered material larly Turkey (Yuzer, 1982). Problems are related largely into competent rock. For larger developments, it may be to removal of soluble gypsum by groundwater, with the necessary to key the foundations into bedrock, but for resultant development of cavities or voids, decrease smaller structures with a low bearing pressure, it should of shear strength or weakening of sands cemented by be suficient to cast the footings into the clean bedrock gypsum. Groundwater activity in Swakopmund is rela- surface using shutters. tively restricted, therefore it is expected that dissolution of gypsum will be minimal and more likely to be caused 3.1.2 Supericial Deposits artiically, e.g., associated with broken water pipes. In areas with a shallow water table, excavations should be Many buildings in Swakopmund are founded on the checked carefully for the presence of solution cavities deep luvio-marine deposits. These deposits consist or a porous, honeycombed texture in the subsoils. largely of non-cohesive, granular, gravelly, medium- Swakopmund also has the advantage of a relatively grained sands. The upper surface layer is generally deep and static water table and allowances do not nor- loose, but the medium dense to dense sands at depths of mally have to be made for saturated soils at founding

8 level. Rare zones of seepage or shallow water table do, both atmospherically and by sulphate salts in the soil however, occur, e.g., on Strand Street, and this should and groundwater. Spalling, pitting and general deterio- be taken into account prior to construction. Groundwa- ration of the concrete can result, thereby considerably ter conditions are relatively stable due to the low rain- reducing it’s durability and life-span. Equally important fall and the detrimental effects due to periodic satura- is the disintegration of reinforcing steel within the con- tion and desiccation of soils are unlikely. crete, a process which is accelerated by the presence of Zones of micaceous sandy silt have been identiied, chlorides. particularly in parts of Vineta and Tamariskia. The silts A dense concrete of low permeability will be consid- are cohesive but sometimes loose to very loose due to erably more resistant to deterioration than a porous or desiccation. Therefore, these silts are likely to be highly low quality concrete. A well proportioned mix, with a compressible, particularly when saturated, and are con- low water-cement ratio, is essential and attention should sequently not suitable for foundations. As the silt often be paid to thoroughly vibrating the mix and correct cur- forms layers and lenses, it will often be possible to in- ing. Care should be taken to clean steel reinforcing be- crease the depth of founding into more dense sands, or fore use, particularly if it has been stockpiled in Swako- to remove the silty material. pmund’s corrosive climate. Good cover should always The shallow colluvium overlying the bedrock in be provided for steel reinforcement. A good example of some parts is usually loose to very loose. It is prefera- the effects of corrosion is provided by the water pipe- ble to found through the colluvium onto the underlying line to Rössing, portions of which disintegrated within bedrock surface. Precautions should be taken to ensure two years after installation. The subsurface pipes, con- that founding conditions are consistent throughout the sisting of concrete-coated steel casings reinforced with development and that any pockets of colluvium or weak prestressed wire, were subjected to penetration by salty material are removed. moisture from the surrounding soil. Rapid corrosion and failure of the reinforcing wires took place, followed 3.2 Corrosion by disintegration of the pipe itself. These subsurface pipes have since been replaced by coated steel pipes The highly corrosive environment can be attributed to on surface. a combination of fog moisture, high relative humidity and an abundance of airborne salts, notably chlorides 3.3 Excavations and sulphates. In addition, there is an abundance of corrosive salts in the soils, the surface layers of which Excavations for services, pipelines and foundations are frequently wet due to fog moisture condensation. frequently require blasting in areas of shallow bedrock. Therefore, construction materials have to be chosen The variable, heterogeneous nature of the red gneissic- with care, particularly as regards steel work. granite presents dificulties in obtaining a smooth, even Long-term investigations on atmospheric corrosion proile during blasting. This was exempliied during carried out by the Council for Scientiic and Industrial blasting for the secondary settlement tank at the sew- Research (CSIR) indicate that the Walvis Bay environ- erage works, where the brittle pegmatite veins tended ment is particularly corrosive as regards zinc and zinc to shatter whereas the xenoliths proved to be relatively coatings, and recommends against their use in this area resistant. This resulted in an uneven excavation proile, (Callaghan, 1983). Similar conditions may be assumed with large areas of overbreak in the pegmatite veins. to apply in Swakopmund. Aluminium and stainless steel Consequently, portions of the excavation had to be alloys generally proved to have relatively low corrosion backilled with a weak concrete. rates (Callaghan, 1983, p. 28-29). Layers of gypsum or gypsum-cemented sands and Sand-cement bricks, commonly used in construction, gravels frequently have to be blasted during the course are prone to attack by salts. The corrosive solutions of excavations for sewerage and water pipelines. The containing most signiicantly, sulphate (SO4), are drawn relatively hard, dense layers tend to absorb the impact into the absorbent brick by capillary action and concen- of the blast without fragmenting into blocks of work- tration of salts occurs by evaporation. Chemical reaction able size. with the cement and subsequent crystallisation of the products in pores induces expansive forces which result 3.4 Further Investigations in cracking and disintegration of the brick. The normal procedure to prevent this process is to use bricks with It is recommended that further site-speciic investiga- a very low absorption below the damp proof course, or tions should be carried out for any proposed major de- for one course below ground surface and three courses velopments in the area. The scope, nature and intensity above. Naturally occurring rock material, such as mar- of the investigations should be geared to suit the indi- ble, dolomite or even the red gneissic-granite is suitable vidual requirements of the proposed development. The for this purpose. Certain types of dense face-brick are main objective of most investigations will be to provide sometimes used, and surface-coatings can also provide geotechnical information on the supericial deposits an alternative. Concrete work is also subject to attack, and the underlying bedrock. The depth to competent

9 bedrock will be a primary consideration in determining piers for the bridge should be founded on bedrock, us- founding levels. In the event of relatively deep depos- ing either large cylindrical caissons or large diameter its overlying bedrock, it will be necessary to establish bored piles. strength and settlement characteristics of the suricial materials, with identiication of possible low strength, 4. CONSTRUCTION MATERIALS compressible silt layers. Investigations for most struc- tures can, however, be limited to the supericial deposits 4.1 Sand and the upper bedrock surface except for larger under- ground works such as shafts and tunnels. For most de- Abundant, good quality sand for construction is velopments in areas of shallow soils, judiciously spaced available in the Swakop River bed. A large borrow pit inspection pits will probably be suficient. The pits will area is currently being worked immediately to the south allow a detailed examination of soil proiles, an assess- of Swakopmund which provides all of the town’s sand ment of their engineering properties and provide an requirements. The sand is used in concrete and also for indication of depth to and nature of the bedrock. Sam- the manufacture of sand-cement bricks. It is recom- pling for laboratory testing can also be carried out in the mended, however, that before use in high grade con- inspection pits. crete, the mica and sulphate content of the sand should Standard Penetration Tests (SPT) and Dutch Probe be checked. soundings can provide valuable information on subsoil Expansion of the borrow pit area is largely in a west- strength, bearing characteristics and expected settle- erly direction, and depth of excavation is between 2 and ments, particularly in areas of deep supericial deposits. 3 m below surface, to the water table. Expansion in a The effectiveness of these methods in the Swakopmund southerly direction is inadvisable due to the presence area, may however, be limited in places by the pres- of yellowish brown, micaceous clayey silt layers up to ence of gravel, boulders and gypsum-cemented layers. about 1 m thick. Good quality, clean sands occur along Auger drilling will be likewise affected. Plate loading the northern margin, but expansion in this direction is tests provide an alternative method for determining set- limited by the presence of a large marble ridge. In gen- tlement and allowable bearing pressures of the uncon- eral, the deposits in the central portion of the river bed solidated sediments. tend to contain a number of clayey silt and gravel lay- Depending on the nature of the development, consid- ers, whilst the sands along the margins are relatively eration may also be given to rotary core drilling or per- clean. cussion drilling. These methods are, however, relatively expensive and are normally only required for major en- 4.2 Rock Aggregate gineering works. Good quality rock aggregate for concrete is obtained 3.5 Seismicity from the Rössing Stone Crushers (Limited) quarry at Rössing Mountain, some 40 km to the east. The rock, Data on seismic events in S.W.A. are sparse and de- which is also used for railroad ballast and road construc- termination of earthquake hazard is therefore based on tion, consists of massive feldspar-pyroxene-hornblende relatively little information. From available records, quartzite of the Khan Formation, Damara Sequence. however, the coastal regions of S.W.A. have a low seis- The physical properties of this material (data supplied mic risk (Korn and Martin, 1952; Fernandez and Guz- by SA Transport Services) are provided below:- man, 1979). Relative density 2,71 Aggregate crushing value % 21-23 3.6 General Flakiness % on 19mm sieve ±20 L.A. abrasion value % 26,2 The irst rail-bridge crossing the Swakop River on the Weathering % 3,10 main Walvis Bay line collapsed during a lood in Janu- Shrinkage % 107,01 ary 1931 (Stengel, 1964). Remnants of the piers are still Absorption % 0,75 visible near the river mouth. The piers, supported on A disused open-pit quarry occurs 6,5 km to the south large concrete pads founded at depths of up to 5,5 m of Swakopmund near the Walvis Bay road. The quarry, below river bed level, failed after extensive scouring some 200 m from the sea, consists of a massive green- of the subsoils below founding level. Some piers disap- ish-grey meta-quartzite containing occasional large peared completely. pegmatite veins. This rock type, also belonging to the Seismic survey and drilling results obtained during Khan Formation, is suitable for use as concrete aggre- preliminary investigations for the existing road bridge gate. over the Swakop River (Ninham Shand and Partners, 1965), indicate average depths of between 3 and 9 m 4.3 Road Building Materials and rarely up to 30 m, of alluvial deposits overlying the bedrock. The report strongly recommended that all Apart from the major tarred roads and streets, most

10 roads in Swakopmund are constructed by compacting have a reduced bearing capacity. If necessary, steps layers of gypsiferous sand which have been prewetted should be taken to found through such layers into with a concentrated brine solution obtained from the underlying denser sands at greater depth. Excava- salt works. This method of construction, ideally suited tions for services are generally simple, but blasting to the coastal climate, provides a relatively cheap and may be required in zones of gypsum-cemented sand satisfactory road surface. or gypcrete. Detailed site investigations should be Gypsiferous sands are ubiquitous in the area and the undertaken for major developments located on the road construction materials are obtained from a number luvio-marine deposits. of shallow borrow areas to the north of Swakopmund, 5.4 Coarse and ine aggregate for concrete is readily adjacent to the Henties Bay road. Sands containing a available in the area. ine gypcrete gravel or powder are the most suitable. 6. ACKNOWLEDGEMENTS 4.4 General The assistance provided by the Swakopmund Town Sand-cement bricks are the most important source Engineer’s Department, in the form of discussion and of building bricks in Swakopmund. Clay or face bricks access to maps and records, is gratefully acknowledged. have to be imported from South Africa and are conse- John Ward is thanked for his comments on an earlier quently far more expensive. draft. Local stone, generally marble, is also used, particu- larly in the plinth brickwork of many houses. The red 7. REFERENCES gneissic-granite has also been used in the past and was obtained during the German colonial period from a Brandt, R. 1981. Unpublished geological map, 2214 quarry (since inilled) near Strand and Dolphin Streets. Walvis Bay. Geol. Surv. S.W.A./Namibia. This material was also used in the construction of the Brink, A.B.A. 1981. Engineering Geology of Southern yacht mole. Large blocks of red gneissic-granite, ob- Africa, Vol. 2. Building Publications, Pretoria, 255 tained from excavations for trenches and pits in the area, pp. have been used to construct the breakwaters at Vineta. Bulley, B.G. 1983. The engineering geology of Swa- A number of disused, open-pit quarries occur in white kopmund. Geol. Surv. S.W.A./Namibia, Open File Karibib Formation marbles about 3 km east of the Swa- report 13/4/2/90, 26 pp. kopmund salt works. The stone was used in the past for Callaghan, B.G. 1983. Long-term atmospheric corro- ornamental gravel on driveways and gardens in Swa- sion programme - results after 10 years exposure. kopmund. The material may also be suitable for use as Corrosion and Coatings South Africa, June 1983. concrete aggregate. It is also worth mentioning here that Fernandez, L.M. and Guzman, J.A. 1979. Earthquake many of the occurrences of Karibib Formation marble hazard in Southern Africa. Geol. Surv. S. Afr., Seis- in the area consist of relatively pure calcitic marble (> mol., 10, 22 pp.

95% CaCo3) which is suitable for the manufacture of Korn, H. and Martin, H. 1952. The seismicity of South cement. As South West Africa’s cement requirements West Africa. Trans. geol. Soc. S. Afr., 54, 85-88. are currently imported from South Africa, it is possible Logan, R.F. 1960. The Central Namib Desert, S.W. that the Swakopmund area, among others, may be con- Afr., Pub. 758, National Assembly of Science, sidered as a site for a cement factory. Washington. Marker, M.E. 1981. The geomorphological signiicance 5. CONCLUSIONS AND of some central Namib materials: J.S.W.A. sci. Soc., RECOMMENDATIONS 34/35, 49-55. Martin, H. 1963. A suggested theory for the origin and a 5.1 Engineering geological conditions in Swakopmund brief description of some gypsum deposits of South are generally favourable for development. Precau- West Africa. Trans. geol. Soc. S. Afr., 66, 345-350. tions do, however, have to be taken to prevent de- Meteorological Services of the Royal Navy and the terioration of construction materials in the highly South African Air Force, 1943. Weather on the corrosive environment. Coasts of Southern Africa Vol. II (1). The West 5.2 Areas underlain at shallow depth by bedrock pro- Coast of Africa from River Congo to Olifants River, vide good founding conditions although dificul- 3-61. ties may be experienced with excavations in rock Ninham Shand and Partners. 1965. Proposed road for services or site levelling. bridge across the Swakop River at Swakopmund. 5.3 The medium dense to dense gravelly sands of the Conid. Rep., S.W. Afr. (Roads Branch). luvio-marine deposits provide adequate founding Porada, H.R. and Hill, R.S. 1974. The marble deposits conditions for most types of structure. However, of South-West Africa. Mem. geol. Surv. S. Afr. (SWA. foundation excavations should be inspected for Series), 5, 57 p.p. the presence of lower strength layers of silt, which Rust, U. 1980. Models in geomorphology - quaternary

11 evolution of the actual relief pattern of coastal cen- ter Affairs in S.W.A. Afrika-Verlag Der Kreis, Wind- tral northern Namib Desert. Palaeont. afr., 23, 173- hoek, 467 pp. 184. Stengel, H.W. 1964. Die Riviere der Namib und ihr Schulze-Hulbe, A. 1979. First and second geologi- Zulauf zum Atlantik. Scientiic Papers of the Namib cal feasibility reports, desalination plant, Mile Desert Research Station, 22. 4, Swakopmund. Geol. Surv. S.W. Afr., Ref. GO Swart, D.H. 1982. Stranderosie te Swakopmund tydens 16/1/1/11/74. 1980 en 1981. WNNR verslag C/SEA 8216. Smith, D.A.M. 1965. The geology of the area around Yuzer, E. 1982. Engineering properties of evaporites the Khan and Swakop Rivers in South West Africa. and evaporitic formations of Turkey. Bull. Int. As- Mem. geol. Surv. S. Afr. (SWA. Series) 3, 113 pp. soc. Eng. Geol., 25, 107-110. Stengel, H.W. 1963. Wasserwirtschatt-Waterwese-Wa-

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