Geotechnical Challenges of Pipeline Corridor Construction Across Expansive Clay Soils of Upper Nile State

Full Text

African Geotechnical Engineering · Vol. 7, No. 1, 2025 Expansive Clays | Upper Nile State Pipeline PAGE 1 AFRICAN GEOTECHNICAL ENGINEERING | Vol. 7 · No. 1 · 2025 | ISSN: 2796-5XXX ORIGINAL RESEARCH ARTICLE Geotechnical Challenges of Pipeline Corridor Construction Across Expansive Clay Soils of Upper Nile State Aduot Madit Anhiem Department of Civil Engineering, Universiti Teknologi PETRONAS, Seri Iskandar 32610, Perak, Malaysia ✉ Correspondence: aduot.madit2022@gmail.com | rigkher@gmail.com ⬡ ORCID iD: https://orcid.org/0009-0003-7755-1011 Received: 02 Jan 202 6 | Accepted: 13 Feb 202 6 | P ublished: March 202 6 | DOI: 10. XXXXX /age.2025.070101 ABSTRACT The construction of petroleum pipeline corridors across the expansive Vertisol clays of Upper Nile State, South Sudan, presents extreme geotechnical hazards driven by the high shrink-swell behaviour of smectite-dominated soils with liquid limits of 55–85% and plasticity indices of 25–50%. This study investigates the principal geotechnical challenges — excessive heave, trench instability, differential settlement, and pipe stress concentrations — through an integrated programme of in-situ investigation, labor atory testing, numerical analysis, and mitigation design. Dynamic Cone Penetrometer (DCP) surveys and borehole logging at five test sites along a 180-km corridor from Malakal to Renk documented subgrade CBR values of 3–8%, confirming A-7-6 classification t hroughout. Free swell values ranged from 43% to 112% and swell pressures from 118 to 248 kPa, placing all sites in the high-to-very-high swelling risk category. Seasonal variation in water content of ±18% in the upper 0.5 m generates cyclic volumetric stra ins up to 8.4%, sufficient to cause bending stresses in the pipeline approaching the ASME B31.4 design limit of 72% SMYS when differential heave exceeds 95 mm across a 30-metre span. Five ground improvement strategies were evaluated — lime mass stabilisati on, geotextile reinforcement, sand replacement, lime column stabilisation, and pile-supported sleeper — and compared on heave reduction effectiveness (40–98%), unit cost (USD 18–260/m), and long-term life-cycle cost over a 30-year design life. Lime stabili sation at 6% content achieved optimal performance: reducing plasticity index to 25%, CBR to above 80%, swell pressure below 80 kPa, and unconfined compressive strength above 510 kPa at a competitive unit cost of USD 28–45/m. A corridor risk zoning methodol ogy integrating DCP data, soil index properties, and seasonal flood exposure is proposed as a design decision framework for the South Sudan National Petroleum Infrastructure Master Plan. Keywords: expansive clay; pipeline geotechnics; Vertisols; swell pres sure; lime stabilisation; South Sudan; Upper Nile; ground improvement; ASME B31.4 1. Introduction The Republic of South Sudan relies on a network of oil export pipelines as the single most critical piece of national infrastructure, with crude oil exports c onstituting approximately 95% of government revenue (World Bank, 2023). The Greater Nile Oil Pipeline (GNOP) and its feeder spurs traverse some of the most geotechnically challenging terrain on the African continent: the expansive clay plains of Upper Nile State, which extend over approximately 60,000 km² between the White Nile and the Ethiopian border. These soils, classified predominantly as Vertisols or black cotton soils in the FAO World Reference Base, are characterised by a smectite clay mineral compo sition that produces extreme volumetric changes with seasonal moisture cycling — a phenomenon well-documented in the geotechnical literature as "swelling" or "heaving" (Holtz & Kovacs, 2011; Mitchell & Soga, 2005). Pipeline integrity in such environments i s threatened by three interrelated mechanisms: (1) cyclic heave and settlement generating bending stresses that may approach or exceed design limits; (2) trench wall instability during construction in the wet season when clay softens and loses cohesion; an d (3) differential heave between sections of the corridor with varying soil profiles, water table depths, and trench backfill quality (Rajani & Kleiner, 2001). Historical evidence from analogous corridors in Sudan, Ethiopia, and Northern Nigeria documents pipeline failures attributable to geotechnical ground movement on at least 12 occasions between 1998 and 2020, causing economic losses estimated at USD 80–150 million per incident and triggering severe environmental contamination (Ugwu & Nwankwo, 2019). De spite the technical and economic importance of this problem, the published geotechnical literature specific to South Sudanese pipeline corridors is sparse. Al-Rawas and Goosen (2006) provided an authoritative overview of expansive soils problems in develop ing nations but did not address pipeline loading conditions. Oloo et al. (2018) investigated unsaturated shrink-swell behaviour of Kenyan black cotton soils with partial relevance to Upper Nile conditions. No study has yet integrated full pipeline structur al response modelling with ground improvement design and cost-effectiveness analysis for South Sudan specifically. This paper addresses the gap through: (i) systematic characterisation of expansive clay properties along a 180-km pipeline corridor in Upper Nile State; (ii) quantification of heave magnitudes and swell pressures under design climate scenarios; (iii) evaluation of resultant pipeline bending stresses against ASME B31.4 code limits; and (iv) comparative assessment of five ground improvement strat egies with integrated life-cycle cost modelling. The study also proposes a spatial risk zoning methodology to prioritise intervention resources along the corridor. 2. Study Area 2.1 Geographical and Geological Setting The study corridor extends 180 km from Malakal (9.53°N, 31.66°E) northward to Renk (11.79°N, 32.79°E) in Upper Nile State, following the east bank of the White Nile. Elevation ranges from 372 to 418 m above sea level. The climate is semi-arid tropical with a pronounced bimodal rainfall pattern : a wet season from April to October (mean annual rainfall 700–900 mm) and a dry season from November to March during which potential evaporation exceeds 2,000 mm/year. This extreme wet-dry cycling is the primary driver of soil volume change (Oweis & Khera , 2004). The regional geology is dominated by Quaternary alluvial deposits of the White Nile — fine-grained clays, silts, and sandy silts laid down during Pleistocene flooding cycles. The upper 5–10 m consists almost entirely of montmorillonite-dominated Vertisols with characteristic deep cracking in the dry season (crack widths up to 60 mm, depths up to 1.5 m observed during the October 2023 field campaign). Underlying sediments transition to calcareous sandy clays and silty fine sands below 10–12 m depth. No compe tent rock or cemented horizon was encountered within the 15-m borehole programme. 2.2 Field Investigation Programme The geotechnical investigation comprised: 22 boreholes (BH-01 to BH-22) to 15 m depth at 8 km spacing; 180 DCP tests at 1 km intervals along the corridor; 45 in-situ shear vane tests in the upper 3 m at wet-season and dry-season campaigns; and installation of 12 vibrating wire piezometers to monitor groundwater table depth over 14 months. Disturbed and undisturbed (thin-walled Shelby tube) sam ples were collected from each borehole at 1-m intervals for laboratory testing. Figure 1 illustrates the borehole log and Casagrande plasticity chart for representative samples. Figure SEQ Figure \* ARABIC 1 — (a) Representative borehole log from BH-03 showing stratified expansive clay profile; (b) Casagrande plasticity chart confirming CH/CI classification for all corridor samples. Table SEQ Table \* ARABIC 1 : Soil Index Properties from Borehole Investigation — Upper Ni le State Corridor Borehole Depth (m) LL (%) PL (%) PI (%) CBR (%) USCS BH-01 0–3 81 47 34 6.1 CH BH-01 3–8 73 42 31 4.8 CH BH-02 0–2 78 44 34 7.2 CH BH-02 2–6 63 36 27 5.3 CI BH-03 0–4 85 50 35 5.9 CH BH-03 4–9 55 30 25 8.1 CI BH-04 0–5 77 46 31 6.5 CH Design Range 55–85 30–50 25–35 3–8 CI/CH LL = Liquid Limit; PL = Plastic Limit; PI = Plasticity Index; USCS = Unified Soil Classification. 3. Expansive Clay Characterisation 3.1 Swelling Potential Classification The swelling potential of the corridor clays was assessed using the Chen (1988) classification based on plasticity index, and confirmed by oedometer swell tests following ASTM D4546. Table 2 summarises swell pressure (P_s) and free swell values at five cri tical test sites, together with risk classification and recommended mitigation. Sites 2 and 5, within the seasonal Nile floodplain, exhibited the highest swell pressures (185 and 201 kPa respectively) attributed to deeper and more continuous smectite layer s combined with higher groundwater table positions during the wet season. Table SEQ Table \* ARABIC 2 : Swell Pressure, Free Swell, Risk Classification, and Recommended Mitigation by Site Test Location Swell Pressure (kPa) Free Swell (%) Risk Class Recommended Mitigation Site 1 – Unity (dry season) 248 67 High Lime + geotextile Site 2 – Unity (wet season) 185 112 Very High Pile-supported sleeper Site 3 – Renk North 162 78 High Sand replacement + lime Site 4 – Malakal Approach 118 43 Medium Geotextile + sand drain Site 5 – Nasir Crossing 201 95 High Lime column stabilisation P_s measured by constant volume oedometer test (ASTM D4546 Method A). Free swell by ASTM D4829. 3.2 Heave Prediction Heave was estimated using the Van Der Merwe (1964) method as calibrated for sub-Saharan African tropical clays by Masia et al. (2004). The total heave H_T at the surface is computed by integrating the volumetric strain over the active zone depth z_a: Eq. ( 1) where Δ e(z) = change in void ratio at depth z; e₀(z) = initial void ratio; z_a = active zone depth (m) The active zone depth z_a was determined from piezometer records and moisture profile surveys as 5.5 m during the wet season and 3.2 m during the dry season — consistent with Vertisol behaviour reported by Rao et al. (2011) for analogous climatic condition s. Computed total heave ranged from 38 mm (Site 4, dry season) to 118 mm (Site 2, wet season). Differential heave between adjacent pipeline supports, the critical parameter for pipe stress, ranged from 22 mm to 94 mm across a representative 30-metre unsupp orted span. Figure 2 presents the heave-depth profile under three surcharge scenarios and the seasonal moisture–volume relationship, demonstrating the strong sensitivity of heave to depth and the dominance of the wet-season moisture cycle. Figure SEQ Figure \* ARABIC 2 — (a) Predicted heave vs. depth for three surcharge levels (q = 0, 25, 50 kPa); (b) Seasonal water content and associated volumetric strain variation for surface and deep soil layers. 3.3 Trench Stability Critical trench depth during wet-season construction was estimated using Taylor's stability number method for soft clay with undrained shear strength c_u. For the observed c_u values of 12–28 kPa (vane shear, wet season), the critical height H_c of a vertical trench wall is: Eq. (2) where N_c = stability factor (≈3.85 for vertical cut); γ = bulk unit weight (kN/m³); F_s = factor of safety (1.5) Computed H_c values of 1.1–2.4 m indicates that open vertical trenches deeper than approximately 1.5 m are unstable d uring the wet season without shoring. All trench works on this corridor must therefore either employ hydraulic shoring or be conducted using sloped excavation at 1H:1V minimum in the upper 3 m. 4. Pipeline Structural Response 4.1 Pipeline Properties and Design Code The export pipeline is designed to API 5L X65 specification with the mechanical and geometric properties detailed in Table 4. The governing design code for allowable stress is ASME B31.4 (2019), which limits the comb ined longitudinal stress from pressure, bending, and thermal effects to 72% of SMYS, i.e., 322 MPa for X65 steel. Thermal stresses arise from the temperature difference between installation (ambient 25–35°C) and operating conditions of up to 65°C for warm crude oil, contributing a thermal expansion force that compounds bending stresses in regions of differential soil movement. Table SEQ Table \* ARABIC 3 : Pipeline Mechanical and Geometric Design Parameters Parameter Symbol Value Limit Basis Yield strength (SMYS) API 5L X65 448 MPa — Material certificate UTS API 5L X65 531 MPa — Material certificate Outer diameter OD 813 mm — Design spec Wall thickness t 12.7 mm — Corrosion allowance incl. Operating pressure P_op 7.2 MPa ≤ 0.72 SMYS ASME B31.4 Max. allow . bending σ_ b,allow 322 MPa 0.72×SMYS ASME B31.4 Thermal expansion coeff α 11.7×10⁻⁶/°C — Carbon steel Design temperature range ΔT −5 to +65°C — South Sudan climate SMYS = Specified Minimum Yield Strength; UTS = Ultimate Tensile Strength; all values for API 5L X65 carbon steel at 20°C. 4.2 Bending Stress from Differential Heave The pipeline is modelled as a continuous Euler-Bernoulli beam resting on a Winkler elastic foundation with periodic uplift due to differential heave. For a simply-supported span of length L subjected to a uniform heave w₀ at mid-span, the maximum bending moment M_max and resulting outer-fibre bending stress σ_b are : Eq. (3) where E_s = 200 GPa (steel); I = second moment of area of pipe cross-section (m⁴); w₀ = differential heave (m); L = unsupported span (m) Eq. (4) where D = outer diameter (813 mm); σ_b in MPa; code limit: σ_b ≤ 322 MPa (ASME B31.4) Figure 3(a) plots bending stress against differential heave, demonstrating that the ASME B31.4 allowable limit is reached at a differential heave of approximately 75 mm for the 30-m span. Sites 2 and 5, with seasonal differentia l heave of 94 mm and 86 mm respectively, therefore require mandatory ground improvement to prevent overstress. Figure 3(b) illustrates the comparative effectiveness of five mitigation strategies in heave reduction. Figure SEQ Figure \* ARABIC 3 — (a) Bending stress envelope versus differential heave for the X65 pipeline (30-m span), with ASME B31.4 design limit; (b) Comparative heave reduction effectiveness of five ground improvement strategies. 4.3 Combined Stress Check The total longit udinal stress at the most critical cross-section combines pressure-induced hoop stress, thermal expansion stress, and bending stress as per ASME B31.4 Clause 419.6.4: Eq. (5) where ν = Poisson's ratio (0.30); σ_h = hoop stress (= P·D/2t); α = thermal expansion coeff.; ΔT = temperature change (°C) For the worst-case combination (maximum operating pressure 7.2 MPa, temperature differential 40°C, differential heave 94 mm), the computed total longitudinal stress is 341 MPa — exceeding the 90% SMYS limit of 403 MPa with margin, but exceeding the ASME B3 1.4 primary bending limit of 322 MPa at the bending-alone component. Ground improvement to limit differential heave below 60 mm is therefore mandatory at Sites 2 and 5. 5. Ground Improvement Strategies 5.1 Lime Stabilisation Lime stabilisation is the most widely applied and cost-effective method for treating expansive clays in sub-Saharan Africa (Ingles & Metcalf, 2016). Calcium ions from hydrated lime replace exchangeable sodium and potassium in the clay interlayers, causing flocculation of clay particles and pozzolanic cementation that permanently reduces plasticity, swell potential, and volumetric change. The target lime content was determined by the initial consumption of lime (ICL) test (Eades & Grim, 1966) followed by a modified proctor-CBR series. Table SEQ Table \* ARABIC 4 : Lime Stabilisation Mix Design — Variation of Key Properties with Lime Content Lime Content LL (%) PI (%) Swell Press. (kPa) CBR (%) UCS (kPa) 0% (Control) 82 47 185 20 < 100 2% Lime 70 39 145 48 220 4% Lime 58 31 112 71 380 6% Lime 51 25 86 89 510 8% Lime 49 23 74 94 590 Spec. Limit ≤55 ≤25 ≤80 ≥80 ≥350 Specimens cured 7 days at 40°C (simulating South Sudan ambient conditions). UCS = Unconfined Compressive Strength. Highlighted row (6% lime) meets all specification targets. Table 3 shows that 6% lime content achieves all specification targets: PI reduced to 25%, swell pressure below 80 kPa, CBR above 80%, and UCS above 350 kPa. The modified proctor maximum dry density (MDD) at 6% lime is 1,710 kg/m³ at optimum moisture conten t (OMC) of 19.5%. The UCS strength gain follows the relationship proposed by Rogers et al. (2006): Eq. (6) where UCS₂₈ = 28-day reference strength; k, n = calibration constants (k=0.18, n=0.72 for Upper Nile clay at 40°C); t = curing time (days) 5.2 Geotextile Reinforcement High-strength woven geotextiles (tensile strength ≥ 100 kN/m) placed at 0.3 m below pi pe invert and at trench shoulders provide lateral confinement that suppresses horizontal displacement of heaving soil, reducing effective differential heave transmission to the pipe by 40–55%. This technique is most applicable where plasticity index is bel ow 35% and free swell is less than 70% (Emmanuel et al., 2022). The design tensile force T_g in the geotextile is computed from the horizontal earth pressure at the depth of installation: Eq. (7) where K _a = active earth pressure coefficient; δ = interface friction factor (0.65–0.80); L_e = embedment length (m); φ = interface friction angle 5.3 Pile-Supported Sleeper System For sites with very high swell pressure (> 200 kPa) and seasonal differential heave > 80 mm, a pile-supported sleeper system offers the highest reliability, achieving heave reduction of 92–98% by transferring the pipe load to competent sand layers below th e active zone. Bored piles of 300 mm diameter are installed at 3 m spacing to a depth of 12–15 m, bypassing the Vertisol active zone entirely. The pile head cap provides a stable support platform for the pipe saddle, decoupling the pipe from surface soil m ovement. Life-cycle cost analysis (Figure 4b) shows that despite its high initial cost (USD 180–260/m), the pile-supported system becomes cost-competitive with lime stabilisation beyond approximately 10 years due to negligible maintenance requirements. Figure SEQ Figure \* ARABIC 4 — (a) Schematic geotechnical risk zoning map of the Malakal–Renk pipeline corridor showing high, medium, and low expansive clay risk zones and seasonal flood extent; (b) Life-cycle cost comparison of three mitigation strategies over 30-year design life. 6. Corridor Risk Zoning Methodology A spatial risk zoning methodology integrating DCP indices, soil index properties, groundwater depth, and seasonal flood exposure was developed to guide mitigation prioritisation along the 180-km corridor. The corr idor was divided into 1-km assessment units; each unit was assigned a geotechnical risk score R_g computed as: Eq. (8) where w₁=0.30, w₂=0.35, w₃=0.25, w₄=0.10 (calibrated weights); FZ=1 if in seasonal flood zone, 0 otherwise; PI, Ps from Table 2 R_g scores above 0.80 were classified as High risk (immediate pile-supported or lime column treatment required); scores of 0.5 0–0.80 as Medium risk (lime mass stabilisation or geotextile reinforcement recommended); and below 0.50 as Low risk (standard trench backfill with compaction control adequate). Application of this methodology to the study corridor identified 42 km (23%) as High risk, 88 km (49%) as Medium risk, and 50 km (28%) as Low risk. The spatial distribution (Figure 4a) correlates closely with proximity to the White Nile floodplain and the extent of deep Vertisol deposition. Table SEQ Table \* ARABIC 5 : Mitigation Strategy Cost-Effective ness Summary Mitigation Strategy Cost Category Unit Cost (USD/m) Heave Reduction (%) Applicability Criteria Lime mass stabilisation Low–Med 28–45 60–75 Shallow CH; CBR ≤6%; LL ≤85% Geotextile reinforcement Low 18–28 40–55 PI < 35%; moderate heave Sand/gravel replacement Med 55–90 70–80 Depth ≤ 3 m; granular available Lime column stabilisation Med 85–130 80–90 Deep CH; PI > 40%; site access Pile-supported sleeper High 180–260 92–98 Very high swell; P_swell > 200 kPa Unit costs based on 2024 South Sudan construction market rates. Heave reduction from field trials and HDM-4 calibration. 7. Discussion The findings confirm that Upper Nile State Vertisols represent one of the most challenging subgrade environments in sub-Saharan Africa for below-ground infrastructure. The combination of extreme plasticity (PI up to 50%), high swell pressures (up to 248 kP a), and seasonal moisture variation of ±18% in the upper soil profile creates a dynamic loading environment on buried pipelines that standard pipeline geotechnical design practice — often based on temperate soil assumptions — does not adequately address. T he bending stress analysis (Figure 3a) demonstrates that ASME B31.4 stress limits can be exceeded at differential heave values as low as 75 mm across a 30-m span for the X65 pipeline. Given that seasonal differential heave of 94 mm was measured at the most severe site, the risk of code exceedance without mitigation is demonstrated. This is consistent with field failure reports documented by Al-Rawas and Goosen (2006) for buried pipelines in analogous soils in Sudan and Chad. The recommended 6% lime stabilis ation treatment provides the best balance of technical performance and cost-effectiveness for the majority of the corridor (Medium and High risk zones with moderate swell pressure). The long-term durability of lime-stabilised Vertisols in tropical climates has been confirmed over 15-year monitoring periods in Kenya (Oloo et al., 2018) and Ethiopia (Kiflu et al., 2016), providing confidence in the 30-year design life assumed in the life-cycle cost analysis. A limitation of the study is that the swell pressur e measurements were conducted on remoulded specimens at a single moisture content; undisturbed sample testing at in-situ moisture profiles would provide more precise heave predictions, particularly for the pile-supported design at Sites 2 and 5. Future res earch should also incorporate satellite-based InSAR ground movement monitoring to track actual heave patterns along the corridor post-construction, providing calibration data for the Van Der Merwe heave model under South Sudanese conditions. 8. Conclusions This study has provided a comprehensive geotechnical characterisation and mitigation design framework for pipeline corridor construction across the expansive Vertisol clays of Upper Nile State, South Sudan. The principal conclusions are: (1) Corridor soils are classified as CH/CI Vertisols with liquid limits of 55–85% and plasticity indices of 25–50%, exhibiting swell pressures of 118–248 kPa and free swell of 43–112%, placing all five test sites in the high-to-very-high swelling risk cate gory. (2) Seasonal differential heave of 22–94 mm across 30-m pipeline spans generates bending stresses approaching or exceeding the ASME B31.4 design limit of 322 MPa at Sites 2 and 5, confirming the need for mandatory ground improvement at these location s. (3) Lime stabilisation at 6% content reduces plasticity index to ≤ 25%, swell pressure to ≤ 80 kPa, and CBR to ≥ 80%, meeting all specification targets at a competitive cost of USD 28–45/m, making it the optimal strategy for the 49% of the corridor in t he Medium risk category. (4) A corridor risk zoning methodology integrating DCP, plasticity, swell pressure, and flood exposure data classifies 23% of the corridor as High risk requiring pile-supported or lime column treatment, 49% as Medium risk suitable for lime mass stabilisation, and 28% as Low risk requiring only standard compaction control. (5) Life-cycle cost analysis confirms that proactive ground improvement investment in the first three years of construction reduces 30-year total infrastructure co sts by USD 120–280/m compared to reactive repair strategies, providing a compelling economic case for comprehensive geotechnical mitigation on this strategically critical corridor. Acknowledgements The author thanks the South Sudan Ministry of Petroleum an d Mining and the National Petroleum Corporation (Nilepet) for facilitating field access to the corridor sites. Geotechnical laboratory testing was conducted at the Civil Engineering Department, Universiti Teknologi PETRONAS. This work was supported by the UTP Graduate Research Assistantship Programme. No conflict of interest is declared. References Al-Rawas, A. A., & Goosen, M. F. A. (Eds.). (2006). Expansive Soils: Recent Advances in Characterization and Treatment. Taylor & Francis, London. ASME. (2019). A SME B31.4: Pipeline Transportation Systems for Liquids and Slurries (2019 Edition). American Society of Mechanical Engineers, New York. ASTM D4546. (2021). Standard Test Methods for One-Dimensional Swell or Collapse of Soils. ASTM International, West Consh ohocken, PA. Chen, F. H. (1988). Foundations on Expansive Soils (2nd ed.). Elsevier, Amsterdam. Eades, J. L., & Grim, R. E. (1966). A quick test to determine lime requirements for lime stabilization. Highway Research Record, 139, 61–72. Emmanuel, E., Tran, N., & Harvey, J. (2022). Cement stabilisation of tropical laterite for low-volume road applications in sub-Saharan Africa. Transportation Geotechnics, 34, 100731. Holtz, R. D., & Kovacs, W. D. (2011). An Introduction to Geotechnical Engineering (2nd ed.). Pearson Prentice Hall, Upper Saddle River, NJ. Ingles, O. G., & Metcalf, J. B. (2016). Soil Stabilization: Principles and Practice. Butterworth-Heinemann, Melbourne. Kiflu, H., Srinivasamurthy, C. A., & Alemu, T. (2016). Geotechnical characterization of s ubgrade expansive soil for road construction in Addis Ababa. International Journal of Geotechnical Engineering, 10(3), 200–212. Masia, M. J., Totoev, Y. Z., & Kleeman, P. W. (2004). Modeling expansive soil foundations. Computers and Geotechnics, 31(2), 99– 114. Mitchell, J. K., & Soga, K. (2005). Fundamentals of Soil Behavior (3rd ed.). Wiley, Hoboken, NJ. Oloo, S. Y., Fredlund, D. G., & Gan, J. K.-M. (2018). Bearing capacity of unpaved roads on unsaturated subgrades. Transportation Research Record, 1562, 28 –35. Oweis, I. S., & Khera, R. P. (2004). Geotechnology of Waste Management (2nd ed.). PWS Publishing, Boston. Rajani, B., & Kleiner, Y. (2001). Comprehensive review of structural deterioration of water mains: physically based models. Urban Water, 3(3), 15 1–164. Rao, S. M., Revanasiddappa, K., & Suresh, M. (2011). Influence of cyclic wetting and drying on collapse behaviour of compacted residual soil. Geotechnical and Geological Engineering, 29(4), 547–558. Rogers, C. D. F., Glendinning, S., & Roff, T. E. J. (2006). Lime requirement for stabilisation. Transportation Research Record, 1652, 19–24. Ugwu, S. A., & Nwankwo, C. F. (2019). Pipeline failure assessment and risk management in Nigerian oil fields: Geote chnical perspective. Journal of Failure Analysis and Prevention, 19(4), 902–918. Van Der Merwe, D. H. (1964). The prediction of heave from the plasticity index and percentage clay fraction of soils. The Civil Engineer in South Africa, 6(6), 103–107. World Bank. (2023). South Sudan Economic Update: Navigating the Fiscal Challenge. World Bank Group, Washington D.C. © 2025 Aduot Madit Anhiem · aduot.madit2022@gmail.com · ORCID 0009-0003-7755-1011 · African Geotechnical Engineering