Geotechnical challenges underlying and circumscribing subsurface city development in Nairobi

J.N. Mukabi, Y. Kimura, S. Kotheki, & A. Ngigi Kensetsu Kaihatsu Limited

P. Murunga, & V. Sidai, Kenya Rural Roads Authority, & Bamburi Cement Ltd

K. Onacha, & J. Wambugu Office of the DPM and MoLG, Nairobi City, & B.P. Contractors Ltd.

ABSTRACT: Population explosion from 350,000 people in 1963 to 5Million currently has transformed Nairobi into a candidate mega city. Against this background and Vision 2030 therefore, the Research & Development (R&D) for underground city development that will include modern subsurface mass transit systems, parking bays, shopping malls, amenity parks, etc has been initiated. In this paper, some of the major challenges facing such an undertaking are introduced. A preamble is also given on some technologies and concepts that may be further developed to solve some of the geotechnical engineering problems. The importance of advancing com-prehensive investigations, testing and analysis in relation to enhanced R&D is also briefly discussed.

1. INTRODUCTION

1.1. Background

Figure 1. Physical development changes of Nairobi

Nairobi, the capital city of Kenya, was founded in 1899 as a supply depot for the Kenya-Uganda railway constructed under the British Empire. The city is located in the south central part of Kenya within Nairobi Province. Figure 1 is a depiction of the physical development of the city from the early 1900s to present. As the hub of the East and Central Africa Region, the increasing population and economic growth in some of the neighbouring countries and the

instability in others, has impacted on its dynam-ics from various perspectives.

Consequently, in order to solve some of the immediate problems, the Office of the Deputy Prime Minister and Ministry of Local Govern-ment is reviewing the necessary policies, whilst the Government of the Republic of Kenya has established the Ministry of Nairobi Metropoli-tan Development mainly to ensure the realisa-tion of the Nairobi Metro 2030 Vision. Within this vision, Nairobi is to be transformed into a world class metropolis with a robust, interna-tionally competitive, and dynamic economy.

1.2. Underground city development attraction

Increasing urbanisation and the desire to main-tain environmental quality in the face of in-creased demands for above surface space has transformed focus to the possibility of utilising the underground space beneath cities. Further-more, underground structures are intrinsically more robust than above surface structures in the face of such disasters as earthquakes.

The fact that subsurface transit systems can serve as an invaluable emergency solution in a post earthquake situation during which surface transportation systems are disrupted and/or rendered obsolete, is indeed important. Recent advances in tunnelling technologies have also made underground space development attractive due to the reduction in construction costs. In developing subsurface civil engineering

structures in Nairobi, it is also considered that immense progress will be made in terms of technological advancement and capacity, which in turn will contribute to more effective under-ground resource recovery in the entire region.

1.3. Main projects considered for advanced case studies

Due to the complexity of tunnelling through soft clays and problematic geomaterials, some of the attractive subsurface models in megacities that are being considered for intensive study include the Alice City proposed for Tokyo (Figure 2), structures within the Minato Mirai City in Yokohama, and the Tokyo Bay Aqualine Tunnel (Figures 3 to 8).

Figure 2. Model of Alice City terminal office (Source: Internet, Taisei Corporation)

Figure 3. Trans Tokyo Bay Highway 15.1km route connecting Kawasaki & Kisarazu Under Sea Tunnels (9.4km) and two Man-made Islands

Fugure 4. Trans Tokyo Bay Highway (Tokyo Wan Aqualine) traversing mainly Above Sea vide Bridges (4.4km) (after Trans-Tokyo Bay Highway Authority)

Figure 5. Plan and cross-sectional profile of Trans-Tokyo Bay Highway (after Trans-Tokyo Bay High-way Authority) Some of the results adopted for design, model-ling, prediction and construction control pur-poses for the Trans-Tokyo Bay Highway are depicted in Figures 6, 7, and 8 (Mukabi et al., 1989 and Mukabi, 1991k, 1995a). These partic-ular results were also used to simulate and monitor the effect of disturbance during tunnel-ling and construction.

Figure 6. Soil profiles, Gf and Gmax values of Tokyo Bay Clay (after Mukabi et al. 1989, Mukabi, 1991)

Tokyo Bay Clay

εa=0.01%/min U: "Undisturbed"FD: Fairly disturbedST: Short term consolidationLT: Long term consolidation

Figure 7. Small stress-strain relations up to εa=0.01% (after Mukabi, 1995a)

Figure 8. Strain level dependency of norm. secant Young’s modulus (E/qmax) (after Mukabi, 1995a) The underground parking lots over subway stations and tunnels constructed by the cut and cover method in Moscow (Figure 9) are also considered interesting.

1562

0

9970

5650

27002700 12900

1300

3600

2700

2700

2100

2100

350

350

450

100

300

300

100

900

400

200

18300

20100

650400

200

2100

2100

Figure 9. In-situ reinforced concrete lining for a single-vault subway station, integrated with under-ground car parking bays (© Yurkevich Engineering Bureau Society, Ltd)

2. SOME GEOTECHNICAL RELATED CHALLENGES

2.1. Problematic soils in existence

The overburden soils of Nairobi city and the environs are predominantly Black Cotton Soils, which constitutively are residual expansive soils formed from in-situ weathering of basic igne-ous, metamorphic and pyroclastic rocks with salient properties that are usually dark-coloured heavy clays. The classification of these soils is presented in Tables 1 while the typical nature of problematic Black Cotton Soils is shown in Figure 10. These soils typically occur in poorly drained areas and are usually underlain by lava and tuffs. They are also known to be highly susceptible to moisture-suction matrices charac-terised mainly by swelling due to wetting and shrinkage due to drying that prompts accelerat-ed reduction or gain in strength respectively. This characteristic is quantitatively depicted in Figure 11. Table 1. Classification of Black Cotton Soil

Soil Parameter Classification

Moderate Swellability

High Swellability

Very high Swellability

Dry Density dγ <15kN/m3 15< dγ < 16 >16

Clay content C<0.002 <40% 40< C < 55 >55 Liquid Limit (LL) <48% 40<LL < 55 >65

Plasticity Index (PI) <30% 30<PL < 40 >40 Shrinkage Index (IS) <30% 30~60 >60

Swell Pressure <120kN/m2 120~600 >600

Figure 10. Nature of Typical problematic soils in East Africa and the Region

Figure 11. Plasticity characterisation of Black Cotton Soil subjected to moisture~suction variations In their study for mapping swelling soils in Nairobi and the surrounding centres through remote sensing and spatially varying micro-topography consisting of gilgai topography and evident in the ETM+panchromatic band 8 to identify areas with swelling soils, Kariuki et al., 2004 concluded that most areas are predomi-nantly overburdened by swelling soils.

Although these layers are hardly thick, usu-ally ranging between 1~3m deep on the aver-age, they overlay highly weathered rock which may prove problematic for the departure and arrival shaft construction.

2.2. Topography

The city of Nairobi is characterised by undulat-ing hilly topography with elevations ranging from 1460m to 1960m. Figure 12 shows the general topography superimposed on the soils map and the topographic data depicting the altitudes at landmark locations and main urban centres.

2.3. Geology and soils

The geology of the project area comprises the Nairobi Trachytes extending from Dagoretti – Karen up to the east of Nairobi, and towards north of Kiambu and Githunguri; Nairobi Phonolites exist between the Nairobi National Park and Kiambu and resting directly on the Athi Series; and the Kapiti Phonolites which are also overlain by the Athi Series. The Upper Athi Series formations mainly consist of sandy sediments, gravel, or pebble beds, tuffs and pyroclastic sediments. Welded tuffs are con-spicuous in and along the streams around Thika, and give rise to a number of falls along the rivers. Soils between Thika, Ruiru and Kahawa

are shallow, yellow-brown to yellow-red friable clays overlying a laterite horizon, while in Nairobi, black cotton soils with calcareous and non-calcareous variants are dominant.

The geological history of Nairobi Metropoli-tan Area is dominated by volcanic activity whereby a thick succession of alkaline lavas and associated tuffs began accumulating in mid-Miocene to upper Pleistocene eras.

The geological cross-sections of the Nairobi area are shown in Figure 13

Figure 12. Nairobi area soils along topographic altitude from Ngong Hills in SW to Thika in NE

Figure 13. Geological Cross section of Nairobi area

2.4. Hydrogeology

The rift region within the area is characterised by subsidiary internal drainage; outgoing drainage being virtually absent. Surface water runoff either collects in numerous depressions or disappears in fissures (Mulwa, 2001). Only storm water in the eastern border zone leaves the area through a few gaps in faults occurring in the area (Gevaerts, 1964). Two major peren-nial rivers draining through the area are Kiseri-an and Ngong rivers. Seasonal streams include Kandizi and Nol Chora.

Aquifers in the area are replenished by part of the precipitation that infiltrates underground. The aquifers are tapped through boreholes and constitute an important source of water supply in the area. Lithological variations coupled with varied degree of weathering and fracturing attribute a high degree of in-homogeneity in the hydro-geological characteristics of different aquifers. This inhomogeneous character causes aquifer yields to vary over a continuous area.

This inhomogeneous character causes aqui-fer yields to vary over a continuous area.

2.5. Environmental considerations

There are a number of major environmental constraints that have to be considered and the impact comprehensively assessed prior to the underground development and construction of major structures in Nairobi. The Nairobi city area in particular and most areas in the metropo-lis have already been developed. The study therefore has to adopt designs and methods of construction which take this fact into account noting that the population is continuously increasing astronomically.

The existence of the 117km2 Nairobi Na-tional park also requires careful considerations for the ecology. On the other hand, spontaneous unplanned settlements have resulted into the degradation of the Ngong River Basin as a water catchment area affecting availability, quality, and quantity of freshwater resources. Being porous and permeable, it is expected that the Kerichwa Valley Tuffs aquifers are polluted with confluents discharged from domestic and industrial wastes. Fluorides content in most of the volcanic rocks is above the recommended 1.5ppm3, while shallow groundwater exhibits low mineralisation frequently dominated by calcium and bicarbonates.

Figure 14. Nairobi Dam infested by hyacinth, reeds, and bulrushes (after IUCN and UNEP, 2002)

Most of the river beds consist of thick clays or lateritic soils. On the other hand, the Nairobi Dam is heavily silted by sediments from erosion and solid waste dumped at various locations to reclaim land for agriculture (IUCN, UNEP, 2002). As depicted in Figure 14, water hyacinth as well as various other aquatic macrophytes such as common reeds and bulrushes have infested the water body practically transforming it into a geotechnical engineering hazard.

2.6. Potential disaster zones

Nairobi is located within the vicinity of the eastern flanks of the Great Rift Valley. Recent studies indicate that this valley is expanding at a rate of 2~3mm per year due to tensional forces in the crust (Waithaka, 2001). The geothermal activities at OlKaria, Bogoria, and Kapedo in Turkana district is a manifestation of the high heat flow beneath the crust (Von Seht et al, 2000) while the high seismicity around Lake Magadi is evidence of continental plate breakup (Prodehl et al, 1991).

Seismic data for the period 1994 to 1999, obtained from the University of Nairobi, De-partment of Geology indicate that, whereas the western part of Nairobi is susceptible to seismic activity, the eastern side has virtually no fault lines. The data further indicates minimal earth-quake activity in the central area of Nairobi but appreciable in the western and south-south-west of the metropolis. By correlating the data of the various studies by layering onto digital geologi-cal maps and using the MapInfo GIS software, Wayumba (2001) modelled the potential disas-ter sites in the city of Nairobi and confirmed these findings. 3. PREAMBLE OF SOME ONGOING

PRELIMINARY R&D

3.1. Advances in Geophysics and Geotechnical Engineering

Apart from the advantages of enhancing under-ground construction of civil infrastructure, expanded underground utility and secure struc-tures, subsurface engineering is vital in resource recovery related to petroleum and natural gas, in-situ mining, hydrothermal and engineering geothermal systems, potable water supply and mining hydrology. It also facilitates for the monitoring and control of contaminants such as industrial and nuclear waste.

Model experiments for advanced Geo-science and Geo-engineering Research as proposed by Deep Underground Science and Engineering Laboratory (DUSEL), 2006, are of interest in this respect. The scientific stabilization of a theoretical foundation that relates particle acceleration or velocity in the inelastically deforming zone to stresses and/or energy as well as material properties will be comprehen-sively investigated. Complementary studies on the research on fault motion associated with these properties conducted by NELSAM (Natu-ral Earthquake Laboratory in South African Mines) is certainly an essential initiative to-wards achieving this end.

Nevertheless, fundamental approaches from “the known” are indeed vital in enhancing a clear understanding of the earth sciences, mainly comprising of geophysics and geotech-nical engineering. In this regard, based on preliminary in-situ experimental testing and research findings, Mukabi et al., 2009a devel-oped empirical relations correlating the ge-otechnical engineering parameters determined from mechanical tests to those based on geo-physical methods. Some of the proposed rela-tions are given in the generalized Eqs. (1) to (3).

( ) SPTSPTSPT BnAN −= ρ (1)

( )uu qqu BnAq −= ρ (2)

( )ii EE BnAE −= ρmax (3)

where, NSPT is the number of blows from a Standard Penetration Test, qu is the bearing strength, Emax is the elastic (Young’s) modulus, ρ is the geo-electromagnetic resistivity, and ASPT, BSPT; Aqu, Bqu; and AEi, BEi are material and ground related constants for the geo-parameters.

3.2. Applicable technologies

Various State of the Art tunnelling technologies have been developed for varying geomaterials and soil strata. Based on enhanced research in the engineering geology of Nairobi Metropolis, it is considered that determination of the most appropriate technologies will be made.

Advanced research related to the OBRM (Mukabi and Shimizu, 2001b) and OPMC (Mukabi, 2004a) concepts will be incorporated in the advanced technologies based on innova-tive approach for the sake of realizing more cost-effective and VE subsurface geo-structures.

3.3. Analytical techniques

Recently developed analytical techniques will be further advanced in order to characterize the behaviour of the geomaterials and grond as precisely as possible. Specifically, functions related to Consolidation and Shear Stress Ratio (CSSR), Modified Critical State theory (MCST), basic physical and environmental factors, strength, deformation resistance and Quality Control (QC) will be further researched.

3.4. Prediction and modelling

Further research to enhance the precession of prediction and modelling will be undertaken rigorously.

The proposed Dynamic Loading/Time-dependent SCDR (Structural Capacity Deterio-ration) Model briefly introduced in 3.4.1 and which considers the influence of environmental factors (3.4.2), will be modified to account for seismic effects in relation to strength and deformation resistance (stiffness and Elastic Limit Strain).

3.4.1. Deterioration of Structural Capacity with Time Progression

The deterioration with time of the structural capacity factor fI

SC after Nt = 2.2 years can be defined by Eq. (4) below,

( ) 15.1log −= t

eSC

tSC Nxff (4)

Based on the concepts and equations intro-duced by Mukabi, 2004a, the following equa-tion is introduced for soft clayey soils.

SCtSCtSCSCt CNBNAf +−= (5)

where, ft

SC=Time dependent Structural Capacity Factor, ASC=0.001, BSC=0.0507 and CSC=1.13 are Structural Capacity~Time related constants, Nt=Time Progression in Years.

On the hand, for OPMC, mechanically and/or chemically (treated) stabililized geo-material, stiff soils and relatively hard rock, the following equation is proposed.

(6) where fRSC=Reference Structural Capacity

when Nt=2.2Years

3.4.2. Analysis of Influence of Envi-ronmental Factors

Environmental factors such as moisture-suction variation due to seasonal cycles, inferior materi-al intrusion as a result of the combined effects of dynamic loading and water infiltration (pumping) and land use affecting the effective structural ground layer and/or foundation thickness are known to affect the performance of structures.

In order to determine in a quantitative man-ner, the magnitude of the influence of these factors in relation to the depreciation (deteriora-tion) of the structural capacity of geo-structures, the following equations are adopted.

The environmental factors time dependent generalized equation is factored as ft

df and expressed as,

(7)

The environmental factors time dependent depreciating variation factor, fd

vf is defined as,

d

thdd

msvd

vf ffff ××= .int (8)

Where, fd

msv=Moisture~Suction Depreciating Factor, fd

int=Inferior Material Intrusion Depreciating Factor, fd

th=Pavement Layer Thickness Depre-ciating Factor,

The time dependant Structural Capacity de-preciating factor is therefore computed as,

(9)

fdSC=Structural Capacity Depreciation Factor,

fiSC=Initial Structural Capacity (pre-

consolidation) Nt= Time Progression in Years NR

t = 2.2years (Reference Time Period) fR

SC= 0.824 (Reference Structural Capacity Factor).

The application of the SCDR Model is demonstrated in Figs. 15 and 16 for varying design criteria of the Songwe International Airport Runway Pavement in Mbeya, Tanzania

3.5. Design and performance

Further research on the Comprehensive Method of Design (CMD) Mukabi et al., 2007a, Quality Control and Maintenance Techniques will be undertaken with the aim of enhancing precision and innovatively advanced construction meth-

ods that can realize immense cost-savings and sustainable underground geo-structures.

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 2 4 6 8 10 12 14 16 18 20 22

Stru

ctur

al Ca

pacit

y Dep

recia

tion F

acto

r, f sc

Time Progression, Nt (Years)

Structural Depreciation Factor Vs. Time Progression "WITH Maintenance"

ORIGINALUSAFAA/ICAO BasedOPTION1 Proposed Type IOPTION3 VE BasedOPTION2 Proposed Type II

Critical Zone

Terminal Line

Figure 15. Structural depreciation factor vs. time progression “with maintenance”

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

0 2 4 6 8 10 12 14 16 18 20 22

Depr

ecia

ted S

truc

tura

l Cap

acity

Fact

or

Time Progression, Nt (years)

Depreciated Structural Capacity Factor vs. Time "WithOUT Maintenance"

ORIGINALUSFAA/ ICAOOPTION1 Proposed Type IOPTION2 : Proposed Type IIOPTION3: VE Based

Critical Zone

Terminal Line

Figure 16. Depreciated structural capacity factor vs. time progression “without maintenance” 4. PROPOSED RESEARCH PROGRAMME

4.1. Geotechnical investigation and experi-mental testing

The proposed programme is intended to employ such methods as airborne sensing, electrical and electromagnetic methods, ground penetrating radar, magnetic, microgravity, seismic, thermal, advanced laboratory techniques, and other new methods as are advanced.

4.2. Evaluation of subsurface coupled processes

Application of advanced CSSR concepts, MCS theories, advanced numerical analysis and modelling will be adopted in evaluating subsur-face coupled processes under long term stress changes, moisture variations, chemical/mineral redistribution, thermal transfer and other preva-lent active processes.

4.3. Technology development

OBRM and OPMC technologies are to be advanced to scientifically determine optimum batching ratios of multi-materials for most geomaterials as well as for concrete. Scientific research on the soil~geogrid interaction is also ongoing.

Figure 17 shows a postulation, as an exam-ple, of an OPMC-Ribbed-Shield element.

Figure 17. Schematic impression of OPMC ribbed shield element. 5. CONCLUSIONS

The main conclusions drawn from this study are set out as follows: 1) Based on the population explosion and

other related factors introduced herein, it is imperative to consider underground space development for Nairobi.

2) Problematic soils are the predominant overburden soils in Nairobi. Consequently, it is necessary to enhance the research on these soils as well as other geotechnical engineering challenges facing the city.

3) In order to realize the Nairobi Metro 2030 Vision, scientific and technological ad-vancement are inevitable. Advances in ge-ophysics, geosciences, and geo-engineering as proposed in this Study are an absolute prerequisite.

6. REFERENCES

Mukabi, J.N, Ampadu, S.K, Tatsuoka, F. 1989: Small strain behaviour of undisturbed Tokyo Bay Clay in triaxial compression, Proc. of the 25th Annual Symposium of the Japanese Society of SMFE, Okayama, Vol. 1, pp.589-592. Okayama, Japan.

Mukabi, J.N 1991k. Behavior of clays for a wide range of strain in triaxial compression. MSc. The-sis. University of Tokyo. Japan.

Mukabi, J.N. 1995a. Deformation characteristics at small strains of clays in Triaxial tests. Geotech-nical Engineering Laboratory Report, Ph D The-sis. Institute of Industrial Science, University of Tokyo. Tokyo, Japan..

P.C. Kariuki1, T. Woldai, and F. van der Meer. 2004. The Role of Remote Sensing in Mapping Swell-ing Soils. Asian Journal of Geoinformatics, Vol. 5, No. 1. Amsterdam.

Mulwa, J.K., 2001. Geological and structural set-up of Kiserian-Matathia area and its influence on groundwater distribution and flow. M.Sc. Thesis, University of Nairobi, Unpubl.

Gevaerts, E.A.L., 1964. Hydrogeology of the Nairobi area. Techical Report No.1, Water Dev. Dept.

IUCN, UNEP, 2002. Resource Booklet on Pollution Monitoring Activities. NETWAS Nairobi River Basin Programme Phase II. Nairobi, Kenya.

Waithaka, H., 2001. Determination of Crustal Movements Across the East African Rift Valley using GPS, Internal Publication, Kenya Institute of Surveying and Mapping, Nairobi, Kenya.

Seht, M.I. S. Blumenstein, R. Wagner, D. Hollnack, and J. Wohlenberg, 2001. Seismicity, Seismotec-tonics and crustal structure of the southern Kenya Rift – new data from the Lake Magadi area. Geo-physical Journal Int. (146) pp. 1 – 20.

Prodehl, C., G. R. Keller, M. A. Khan, 1991: Crustal and Upper Mantle structure of the Kenya Rift, Proceedings,. Kenya Rift International Seismic Project (KRISP) Workshop, Karlsruhe, Germany, 4 – 12, August, 1991. Tectonophysics, Vol. 236 Nos 1 – 4. Publication No. 26 of the International Lithospheric Program.

Gordon O. Wayumba, 2001. Modelling Potential Disaster Sites for City of Nairobi, Kenya. Inter-national Conference on Spatial Information for Sustainable Development, TS17.3 Nairobi.

DUSEL, 2006. Geo-Science and Geo-Engineering Research at DUSEL. Technical Report: National Science Foundation. Washington, D.C. USA

Mukabi J.N, Njoroge B.N, Kotheki S, 2009a. De-rived empirical relations of vital geotechnical engineering parameters. Submitted for publica-tion for the 2010 IEEE International Geoscience and Remote Sensing Symposium, Hawaii 2010 IGARSS, Honolulu, Hawaii, USA

Mukabi, J.N., Shimizu, N. 2001b. Strength and deformation characteristics of mechanically stabi-lized road construction materials based on a new batching ratio method. Proc. of the 14th IRF Road World Congress. Paris France.

Mukabi, J. N. 2004a. A Comprehensive Engineering Report on Proposed Countermeasures to Slope, Embankment and Pavement Structure Failure at Sta. 9+310 ~ 9+346km. Engineering Report Vol. I. (for Ethiopian Roads Authority and JICA.

Mukabi J.N 2007a. Unique Methods of Enhancing Engineering Properties of Geomaterials for Slopes, Embankments and Pavement Structures. Proc. 23rd World Road Congress. Paris, France.