Slurry Microtunnelling Challenges by Soil Type and a UAE Soil Map
Ground behaviour, slurry challenges and a regional overview of UAE ground conditions.
Download the complete article as PDF
Summary and method
Under water, a slurry microtunnel is in its natural working environment; in dry ground, the machine works in an environment it was not designed for. Under water, the main enemy is face stability and slurry loss. In dry ground it is slurry escaping into voids, the filter cake failing to form, and frac-out. In clay the problem in both cases is clogging, in rock it is cutter wear, and in karstic and evaporitic ground it is voids and dissolution.
Classification method
In this document I group the world’s soils into four main groups:
- Fine-grained soils: clay, silt, organic soil and peat. These correspond to CL/CH/ML/MH/OL/OH/Pt in USCS and Cl/Si/Or in ISO 14688.
- Coarse-grained soils: sand, gravel, cobbles and boulders (USCS SW/SP/SM/SC, GW/GP/GM/GC; ISO 14688 Sa/Gr/Co/Bo).
- Rock and mixed face: by ISRM UCS classes, soft rock, hard rock, karstic limestone and the soil–rock transition.
- Special and problem soils: sabkha and evaporites, swelling clay, collapsible soil (loess), fill and obstructions, contaminated ground, frozen ground.
Two extreme conditions
Fully submerged: The tunnel axis and the whole face are below the groundwater table, and the ground is fully saturated. Here the slurry pressure works against the water pressure; the filter cake forms from this pressure difference.
Fully dry: The tunnel is entirely above the groundwater table, and the ground is unsaturated. There is no counter water pressure to hold the slurry; the fluid seeps freely into empty pores.
On real sites, conditions are usually somewhere in between (partial saturation, the water table inside the tunnel cross-section). These in-between cases are generally a combination of the challenges at the two extremes, and they also bring a mixed-face risk.
Note: The points below are supported by published case histories and research. General engineering inferences that are not directly supported by a source are marked “(engineering judgement)”.
Classification of the world’s soil types
| Group | Soil type | Grain size / strength | Typical permeability k (m/s, approximate) | Typical examples around the world |
|---|---|---|---|---|
| Fine-grained | Soft–firm clay (low plasticity) | Mostly < 2 µm | 1e-9 – 1e-11 | Delta and estuarine clays (Shanghai, Bangkok) |
| Fine-grained | Stiff, overconsolidated, high-plasticity clay | < 2 µm, Su > 100 kPa | 1e-10 – 1e-12 | London Clay, Frankfurt Clay |
| Fine-grained | Silt and silty sand | 2–63 µm | 1e-6 – 1e-8 | River floodplains, loess derivatives |
| Fine-grained | Organic soil and peat | Fibrous/amorphous organic | Highly variable | Bogs of Northern Europe and Canada, Malaysia |
| Coarse-grained | Fine–medium sand | 0.063–0.6 mm | 1e-4 – 1e-5 | Coastal and windblown sands (Gulf, Netherlands) |
| Coarse-grained | Coarse sand and gravel | 0.6–63 mm | 1e-2 – 1e-4 | Alluvial fans, river terraces |
| Coarse-grained | Cobbles and boulders (GCB), glacial till | > 63 mm | > 1e-2 (matrix-free) | Alpine foreland, Canada, Boston, Lyon alluvium |
| Rock | Soft rock (mudstone, calcarenite, weak sandstone) | UCS 1–25 MPa | 1e-5 – 1e-9 | Barzaman/Fars (UAE), marl |
| Rock | Hard rock (granite, basalt, quartzite, ophiolite) | UCS 50–250+ MPa | Depends on fractures | Shields, mountain belts |
| Rock | Karstic limestone/dolomite | Variable, with voids | Conduit flow | Mediterranean, Virginia (USA), China |
| Rock | Mixed face (soil + rock) | Variable | Variable | Rockhead transitions, weathering zones |
| Special | Sabkha and evaporites (gypsum, anhydrite, salt) | Cemented crust + loose sand | Variable | Gulf coasts, Abu Dhabi |
| Special | Swelling clay (montmorillonite) | High activity | Very low | Texas, Sudan, black cotton soils of India |
| Special | Collapsible soil (loess, dry sand) | Loose, cemented skeleton | 1e-5 – 1e-7 | Chinese Loess Plateau, Central Asia, US Midwest |
| Special | Fill, old foundations and obstructions | Heterogeneous | Variable | Urban areas, old ports |
| Special | Contaminated ground | Any | Any | Industrial and fuel station sites |
| Special | Frozen ground (permafrost) | Ice-bonded | Very low (while frozen) | Siberia, Alaska, northern Canada |
The permeability values are general textbook ranges (engineering judgement). Values above 1e-2 m/s for matrix-free gravel–cobbles were measured on the Lyon metro (Tunnels & Tunnelling – Lyon).
1. Fine-grained soils
1.1 Clay (soft, stiff and high-plasticity)
Fully submerged
Cutterhead and line clogging: The stickiness of clay makes it adhere to the cutters, the cutterhead openings and the slurry line. Adhesion, bridging across cutterhead openings and failure to disperse in water are the main mechanisms of clogging (Oxford ORA – clogging in pipejacking). As the plasticity index of the clay increases, the need for conditioning rises markedly.
Slurry line blockage and pressure fluctuation: If the line blocks, the slurry pressure fluctuates sharply and the face becomes unstable. In the extreme case the line bursts, the pressure drops suddenly and the face collapses towards the cutterhead. Clay sticking to the cutterhead also reduces excavation efficiency and the advance rate (Slurry shield in clay – summary).
Persistent adhesion and increased torque/thrust: In CFD-DEM modelling, fewer than 70% of the particles could be washed away in time by the circulating slurry, and about 9% stuck to surfaces, increasing torque and thrust (Underground Space 2024).
Rapid rise in slurry density: Fine clay particles cannot be removed by desanders/desilters; a centrifuge or filter press is needed. Otherwise viscosity rises and advance drops (Herrenknecht separation technology).
The adverse effect of high slurry strength: As the shear strength of the slurry increases, the stickiness of the excavated clay can also increase. A strong slurry that helps in coarse ground raises the clogging risk in a mixed face (Applied Sciences – clogging criteria). Polymer additives, on the other hand, can protect the clay surface and give high slurry resistance with low clogging (Oxford ORA).
High friction in overconsolidated clay: High jacking forces and deviations occur. Continuous bentonite/polymer lubrication and a conservative jacking force calculation are needed (Eurohinca – risks by ground type).
Heating at the cutterhead–soil interface: In cohesive ground, the continuous thrust of the cutterhead raises the interface temperature and makes adhesion worse (Arabian J. Sci. Eng. 2023).
The upside: The low permeability of clay makes it easy for a filter cake to form at the face. The long stand-up time also reduces the risk of collapse (Slurry shield in clay).
Mitigation: Adding a dispersant to the slurry reduces the stickiness of the excavated clay and the viscosity of the suspension (Adv. Mater. Sci. Eng. 2021). Cutterheads with a high opening ratio and flushing nozzles are also used.
Fully dry
Hard, dry clay: Instead of plastic behaviour, excavation is brittle and lumpy. The lumps travel through the line without dispersing in the slurry and overload the crusher and pumps (engineering judgement).
Swelling and softening on contact with slurry: Dry clay absorbs water and forms a sticky crust around the cutterhead and shield. This brings the clogging potential close to the submerged case (engineering judgement; for the clogging mechanism see Oxford ORA).
Slurry loss through shrinkage cracks: Desiccation cracks in dry clay open paths for slurry to escape. On shallow drives there is a risk of frac-out to the surface (engineering judgement; for frac-out see Maryland MDE tunnelling memo).
High pipe friction: In dry clay the lubricant in the annulus is absorbed by the clay and buoyancy is lost. The jacking force increases (engineering judgement).
1.2 Silt and silty sand
Fully submerged
Flowing and piping: Low-cohesion silt flows into the face under high water pressure and causes settlement through overexcavation (Eurohinca – high water table).
Difficult separation: Silt-sized particles (2–63 µm) are below the cut point of a desander. Desilters and centrifuges are essential; otherwise the slurry density rises quickly (Herrenknecht).
Low friction with good lubrication: On two DN 1200 drives in silty sand (> 550 m), the average friction stayed very low and the pipeline was largely buoyant. The increase in friction after a stoppage depends on the stoppage duration and the bentonite volume (Univ. Galway – Underground Space 2020).
Fully dry
Loose, powdery structure and sudden collapse: Dry silt can lose its structure when it gains moisture (see collapsible soils, 4.3) (engineering judgement).
Rapid absorption of slurry: Dry silt draws water out of the slurry and a thick, soft mud zone forms at the face. Face pressure control becomes difficult (engineering judgement).
1.3 Organic soil and peat
Fully submerged
Very low bearing capacity and machine diving: Peat is the most extreme soft soil, with extremely high water and organic content, very low undrained strength and high compressibility (TCD – study on peat). Line and grade control under the weight of the MTBM is difficult.
Buoyancy: Saturated peat generates large uplift forces (TCD). Light pipes float upwards; in Edmonton, a heavier concrete pipe was used in the worst ground sections to counter buoyancy (Stantec – Big Lake).
Fibrous material clogging the cutterhead and screens: Plant fibres build up on the separation screens (engineering judgement).
Ground loss during shaft construction: When excavating a shaft in peat and wet sand, ground loss was prevented by advancing the casing ahead of the excavation bucket (Horan, GeoQuébec 2015).
Fully dry
Flammability and shrinkage of desiccated peat: Dry peat shrinks and cracks; slurry escapes into these cracks (engineering judgement).
Gas (methane/H2S) build-up: Organic decomposition creates a gas risk in the shaft and the machine. Gas monitoring is needed (engineering judgement).
2. Coarse-grained soils
2.1 Fine–medium sand
Fully submerged
Short stand-up time, ground loss and shaft collapse: On a microtunnel in fine sand on the Caspian coast, there was ground loss, a possibility of shaft collapse and a massive inrush of water from the sea (Jebelli et al., TUST 2010).
Slurry pressure must exceed water pressure: To prevent seepage towards the face, the slurry pressure must be higher than the groundwater pressure. Bentonite slurry forms a filter cake at the face, and the less the slurry spreads, the greater the stabilising force (Jebelli et al.).
Piping and settlement in loose sand: Fine control of chamber pressure, suitable rheology, continuous lubrication and sufficient cover are needed (Eurohinca).
Slurry loss in permeable sand: Slurry loss increases cutterhead torque and reduces penetration. At Al Jadaf in Dubai, friction was reduced with overcut and continuous polymer lubrication (MATEC 2017).
Cut point problem at the separation plant: Very fine sand of ~60–100 µm is close to the cut point of a desander (e.g. BE 300 d50 = 0.060 mm: Bauer); a desilter is needed.
Bentonite flocculation in saline groundwater: Salt lowers the zeta potential and makes the slurry clump and separate; the effect of Ca²⁺ and Mg²⁺ is greater than that of Na⁺ (CJGE 2020). Xanthan gum is among the polymers least affected in saline conditions (Gels 2022).
Fully dry
No counter water pressure, no filter cake: The slurry–water pressure difference that forms the filter cake under water does not exist in dry ground. The slurry seeps quickly into the empty pores; instead of support, a wet, weak zone forms at the face (engineering judgement; for the filter cake mechanism see Jebelli et al.).
Running of cohesionless dry sand: Dry, uniform sand (e.g. windblown sand) runs freely. If the face pressure is kept low, a void and a sinkhole form above (engineering judgement).
Frac-out and slurry reaching the surface: With shallow cover, the slurry pressure can fracture the ground and reach the surface. Sufficient cover and, where needed, ground improvement (jet grouting, deep soil mixing) are recommended (Maryland MDE).
Loss of lubricant in the annulus: The lubricant is absorbed by the dry sand, the pipe does not float and friction rises markedly (engineering judgement; for the importance of buoyancy see Univ. Galway).
Water consumption: In dry ground, large volumes of water and bentonite are needed to make up for losses. On desert sites this is a logistical problem (engineering judgement).
2.2 Coarse sand and gravel
Fully submerged
Severe slurry loss due to high permeability: In Wellington (New Zealand), an unexpected stretch of open-graded coarse gravel caused very large slurry losses; the problem was overcome with additives, constant topping up and a booster pump in the feed line (Trenchless Technology – New Zealand).
Failure to maintain continuous face pressure: In matrix-free gravel–cobbles, a slurry TBM can suffer fluid loss, poor pressure control, excessive surface deformation and sinkholes. In Lyon, in ground with k > 1e-2 m/s, a pressure of 2.5 bar was maintained with a “technical slurry” containing clogging additives, and a volume loss of ~0.15% was observed (Lyon Metro B).
Wear: Quartz-rich gravel causes rapid wear of the cutterhead, crusher, intake ports and slurry line (Hunt – GCB).
Fully dry
Very high loss volumes: With no water pressure, the slurry flows down through the gravel pores under gravity. Pressure cannot be established (engineering judgement; for losses in permeable ground see Lyon).
Ravelling at the face and crown: As dry gravel ravels onto the cutterhead, overmining occurs and sinkholes form at the surface (Hunt – GCB).
Alternative: In dry gravel, auger or EPB-type methods, or ground improvement by pre-grouting, are sometimes considered (engineering judgement).
2.3 Cobbles and bouldery ground (GCB), glacial till
Fully submerged
The hardest condition: Wet, cohesionless, highly permeable gravel–cobbles–boulders is probably the most difficult environment for microtunnelling. The risks are: a jammed excavation chamber, high torque and machine stalling, settlement or sinkholes from overmining, a severe drop in advance rate, excessive wear of the cutterhead/crusher/slurry system, and impact damage to gears/bearings (Hunt – GCB).
Machine breakdowns in boulders: On the Boston Chelsea River crossing, the till contained far more boulders than expected, causing breakdowns and slow progress. Because of the high permeability of the alluvium, bentonite escaped, and it was reported that the river bed may have been breached (Underground Infrastructure 2023).
Mismatch between high-strength, abrasive boulders and machine capacity: On an 8.3 km heterogeneous alluvium project, this mismatch led to rescue and repair operations. The lesson: design for the worst case and have robust contingency measures (Marulanda & Gutiérrez, T&F).
Mitigation: Reinforced picks and discs, high torque, stabilisation grouting, entry/exit buffers (Eurohinca). Sometimes a boulder is pushed aside or broken up by drilling from the surface (Tunneling Online – DSC).
Fully dry
Boulders move and rotate in front of the cutterhead: Without a support fluid, boulders can rotate with the cutterhead instead of breaking. Crusher efficiency drops (engineering judgement).
Slurry loss in an open skeleton: The loss is even greater than under water (engineering judgement).
3. Rock and mixed face
3.1 Soft rock (mudstone, calcarenite, weak sandstone, marl)
Fully submerged
Clogging in clay-rich rock: Slurry TBMs also suffer clogging in sedimentary rock containing clay minerals. A case in clay-rich siltstone under the Ganjiang River in China and the mitigation measures applied have been documented (Frontiers of Structural and Civil Engineering 2023).
Water and slurry escaping through fractured rock: In Dubai, the high groundwater, fractures in the rock and the proximity of the Creek made it difficult to lower the water level in the pits, and breaking the hard ground inside the pits was difficult (MATEC 2017).
Slaking and disintegration: Mudstone breaks down on contact with slurry and the fines load increases. In this case a centrifuge/filter press is needed (Herrenknecht).
Hard concretions in cemented sand: Caliche-like cemented zones form obstructions at the face (Tunneling Online – DSC).
Fully dry
Excavation as dust and fragments: Dry, brittle soft rock breaks into large pieces. This loads the crusher (engineering judgement).
Loss of strength when wetted by slurry: Dry mudstone and marl soften quickly on contact with slurry and can ravel from the crown (engineering judgement).
Slurry loss through rock fractures: With no counter water pressure, the fractures draw slurry freely (engineering judgement).
3.2 Hard rock (granite, basalt, quartzite, ophiolite)
Fully submerged
Disc cutter wear and change time: Quartz content, UCS and jointing determine the cutter choice. In abrasive rock wear dominates, while in very hard rock bearing and metallurgical failures dominate. In hard rock tunnels, cutter changes usually take more than 10% of the time (Tunnels & Tunnelling – disc cutters).
Wear rate multiplies as UCS increases: In small-scale disc tests, the wear rate gradually doubled as UCS increased; in the hardest rocks, deformation of the disc tip was observed (Rock Mechanics Bulletin 2024).
Constraints on small-diameter machines: At microtunnel diameters, the number and size of discs are limited. In non-man- entry diameters (≲ DN 1200), a recovery shaft may be needed to change cutters (engineering judgement).
Water pressure in fracture zones: High-pressure water inflow can occur. On one drive in Canada, an airlock was designed for an average water pressure of 1.4 bar (World Construction Network).
Fully dry
Heat and dust: Even with slurry cooling, cutter heating increases in dry, hard rock (engineering judgement).
Easier stability: In hard rock the face supports itself. Here the slurry’s role is mostly to carry the spoil. Losses are limited to fractures (engineering judgement).
3.3 Karstic limestone and dolomite
Fully submerged
Unpredictable voids and conduits: Karst can contain random voids, caverns or underground streams, and these may not be detectable by common site investigation. Large voids may not support the MTBM; a continuous void or stream leads to unmanageable slurry loss. Grouting voids during the drive can create additional risk (Trenchless Technology – karst, February 2025).
Uncontrolled water inflow and unstable voids: Limestone, dolomite, gypsum and anhydrite are the most unpredictable rocks in tunnel excavation (PIK Potsdam – karst modelling).
Face access for cutter changes: Although limestone is not very abrasive, face access for cutter changes must be planned (Trenchless Technology – karst).
Fully dry
Sudden slurry drainage into dry voids: When a void is entered, the slurry is lost entirely and the machine can “drop” into the void (engineering judgement; for the void risk see Trenchless Technology – karst).
Clogging in clay-filled voids: Karst voids are often filled with terra rossa clay, which brings a clogging risk (engineering judgement).
3.4 Mixed face (soil–rock transition)
In both cases
Sudden changes in lithology: Cohesive–granular or soil–rock transitions lead to pressure imbalance, loss of lift and accelerated wear of the cutterhead (Eurohinca – common risks).
Steering: The machine deviates towards the softer side (engineering judgement).
Impact damage to discs: A disc rotating in soft ground strikes hard rock and can break (engineering judgement).
Conflict in slurry rheology: Coarse ground needs a strong slurry, but the same slurry increases clogging in the clay section (Applied Sciences – clogging criteria).
Additional risk under water: Collapse in the sand section, slow advance in the rock section; long stoppages increase friction.
Additional risk in dry ground: Slurry loss in the sand section, wear in the rock section (engineering judgement).
4. Special and problem soils
4.1 Sabkha and evaporites (gypsum, anhydrite, salt)
Fully submerged
Hypersaline water and aggressive chemistry: On the Dubai Metro, the groundwater was described as hypersaline, and the ground and water were classified as “extremely aggressive”. Loose sand and sabkha with hard concretions and voids were encountered (Tunnels & Tunnelling – Dubai Metro).
Bentonite degraded by salt: Saline water rich in Ca²⁺/Mg²⁺ flocculates the slurry (CJGE 2020). Salt-tolerant polymers (xanthan, PAC) are needed (Gels 2022).
Gypsum dissolution and karst voids: Gypsum exposed to flowing water dissolves about 100 times faster than limestone and can form voids at any depth (Taylor & Francis – karst). In Abu Dhabi, gypsum dissolution and karst formation are a common ground hazard; they have been studied using data from the STEP tunnels (Al Kaabi & Sousa, WTC 2019).
Hydration of anhydrite (swelling): Anhydrite increases in volume as it turns into gypsum with water. This can create pressure and damage in the tunnel lining (PIK Potsdam; Gysel 2002).
Sudden changes in hardness: Rapid transitions between the cemented sabkha crust and loose sand lead to mixed-face problems (T&T – Dubai Metro).
Fully dry
The salt crust dissolving in slurry: A dry sabkha crust dissolves in the slurry water and loses its cementation; ravelling from the crown and settlement can follow (engineering judgement).
Slurry contaminated by salt: Dissolved salt upsets the slurry chemistry; in this case the flocculation risk comes from within (engineering judgement; for the mechanism see CJGE 2020).
Slurry loss into dry voids: When gypsum karst voids are dry, the loss is sudden and total (engineering judgement).
4.2 Swelling clay (montmorillonite/smectite)
Fully submerged
High activity and clogging: The type of clay mineral (especially smectite) is one of the main factors governing adhesion and clogging potential (Open Civil Eng. J. – clogging; T&T – clay mineralogy).
Shield squeeze: Swelling clay closes the annulus around the shield and pipe and increases the jacking force (engineering judgement).
Fully dry
Sudden swelling as it absorbs water: Dry swelling clay swells rapidly on contact with slurry. This is the most severe case: the annulus closes and the pipe can become stuck (engineering judgement).
Loss through shrinkage cracks: Desiccation cracks can carry slurry to the surface (engineering judgement).
4.3 Collapsible soil (loess, cemented dry sand)
Fully submerged
Saturated loess has already collapsed and softened; it behaves like silt (see 1.2) (engineering judgement).
Fully dry
Sudden volume loss on wetting (hydrocollapse): The cemented skeleton of dry loess breaks down in the slurry water. Sudden settlement occurs around and above the tunnel. This is the most dangerous outcome of a slurry system in dry ground (engineering judgement).
Mitigation: Dry methods instead of slurry, a low-water polymer foam, or prior ground improvement (engineering judgement).
4.4 Fill, old foundations and obstructions
In both cases
Buried obstructions: On one documented project, variable fill, timber piles and obstructions left from historic land uses made the drives difficult, and cracks formed at the reinforced concrete pipe joints (SME – Adventures in Microtunneling).
Unexpected coarse material in old fill: In Wellington, unexpected coarse gravel turned up within fill placed between 1880 and 1964, and slurry was lost (Trenchless Technology – New Zealand).
Under water: Loose fill can flow and pipe.
In dry ground: Open fill draws in slurry (engineering judgement).
4.5 Contaminated ground and groundwater
Under water: Contaminated groundwater mixes into the slurry circuit. The separated water and spoil are disposed of as hazardous waste; treatment of the contaminated water is needed (SME; GZA – East Boston).
In dry ground: Hydrocarbon vapour carries a fire/explosion risk in the shaft and the machine; gas monitoring is needed (engineering judgement).
4.6 Frozen ground (permafrost)
While frozen (behaves as “dry”): Ice-bonded ground is excavated like hard rock. The slurry temperature melts the ice and causes thaw settlement around the tunnel (engineering judgement).
Thawed and submerged: Very soft ground with a high water content forms; the risk of flowing and settlement is high (engineering judgement).
Freezing of the slurry: In cold climates, the lines and tanks need heating (engineering judgement).
5. Summary risk matrix
The worst combinations for a slurry system are: submerged cobbles–boulders, dry collapsible soil (loess), dry swelling clay and karst/gypsum voids. The most suitable environment is submerged fine–medium sand and silty sand; the slurry machine was designed for exactly this.
The risk levels are a qualitative assessment based on a synthesis of the referenced points above.
| Soil | Fully submerged: risk | Dominant problem under water | Fully dry: risk | Dominant problem in dry ground |
|---|---|---|---|---|
| Soft–stiff clay | Medium–High | Clogging, line blockage, separation | Medium–High | Swelling and adhesion, loss through cracks |
| Silt / silty sand | Medium | Flowing, fine separation | Medium | Slurry absorption, tendency to hydrocollapse |
| Organic soil / peat | High | Buoyancy, steering, fibres | Medium | Shrinkage cracks, gas |
| Fine–medium sand | Low–Medium | Face stability, salt, cut point | High | No filter cake, running, frac-out, lubricant loss |
| Coarse sand / gravel | High | Slurry loss, continuous pressure, wear | Very high | Pressure cannot be established, ravelling |
| Cobbles–boulders / till | Very high | Jammed chamber, impact, overmining | Very high | Rotating boulders, large losses |
| Soft rock | Medium | Clogging in clay-rich rock, fracture water | Medium | Softening when wetted, loss through fractures |
| Hard rock | Medium | Cutter wear, changes, high water pressure | Low–Medium | Heat, wear |
| Karstic limestone | High | Voids, uncontrolled water, slurry loss | Very high | Sudden total loss into voids |
| Mixed face | High | Pressure imbalance, deviation, disc damage | High | Loss in the sand section, wear in the rock section |
| Sabkha / evaporite | High | Saline water, gypsum dissolution, anhydrite swelling | High | Crust dissolution, slurry contamination |
| Swelling clay | High | Clogging, shield squeeze | Very high | Sudden swelling, stuck pipe |
| Collapsible soil (loess) | Medium | Silt behaviour | Very high | Sudden settlement on wetting |
| Fill and obstructions | High | Obstructions, flowing | High | Loss into voids, obstructions |
| Contaminated ground | Medium–High | Waste and treatment | Medium–High | Gas and fire |
| Frozen ground | High (thawed) | Flowing, settlement | Medium (frozen) | Thaw settlement, slurry freezing |
6. UAE soil map
The map is schematic and the boundaries are approximate. From a slurry microtunnelling point of view, the ground changes as you go from west to east: submerged saline sand and sabkha on the Abu Dhabi–Dubai coast, dry dune sand inland, coarse gravel along the Al Ain–Dhaid line, and hard rock in the Hajar Mountains.
6A. UAE soil map by 5 m depth slices
Each small map shows one depth slice, and each region is coloured by its average soil type in that slice. On the coast (A, B, C), soft rock is reached at 10–20 m; in inland Abu Dhabi (D) this transition is at about 25 m. At Liwa (E) sand continues down to 60 m, while in the east (F, H) thick gravel sits on ophiolite or Barzaman. The slice values are given in the table below.
6B. Region × 5 m depth slice: average soil type
This table gives a representative average profile for each region. There is no publicly available country-wide soil model of the UAE at 5 m resolution. I compiled the profiles from the BGS 1:100,000 map descriptions, the published ground descriptions of Dubai/Abu Dhabi tunnel projects and general geology. The region boundaries and transition depths are approximate; they can change within a few hundred metres even inside the same region.
| Depth | A West Abu Dhabi coast | B Abu Dhabi island | C Dubai– Sharjah coast | D Inland Abu Dhabi (Sweihan) | E Liwa / southern sand sea | F Al Ain–Dhaid gravel plain | G Hajar Mountains | H East coast (Fujairah) | I RAK mountains |
|---|---|---|---|---|---|---|---|---|---|
| 0–5 m | Sabkha | Fill / sabkha | Sand (fill, windblown sand), sabkha in places | Dune sand | Dune sand | Gravel–boulders | Wadi gravel | Gravel beach / fan | Scree, gravel |
| 5–10 m | Calcarenite / aeolianite | Calcarenite, sand | Sand + calcarenite | Dune sand | Dune sand | Gravel–boulders | Ophiolite | Gravel–boulders | Limestone/dolomite |
| 10–15 m | Miocene sandstone/mudstone + gypsum | Calcarenite / Hili sandstone | Sand + calcarenite | Dune sand | Dune sand | Gravel–boulders | Ophiolite | Gravel–boulders | Limestone/dolomite |
| 15–20 m | Miocene + gypsum | Miocene mudstone + gypsum | Barzaman | Fossil dune / aeolianite | Dune sand | Gravel–boulders | Ophiolite | Gravel–boulders | Limestone/dolomite |
| 20–25 m | Miocene + gypsum | Miocene + gypsum | Barzaman | Fossil dune / aeolianite | Dune sand | Cemented gravel | Ophiolite | Gravel–boulders | Limestone/dolomite |
| 25–30 m | Miocene + gypsum | Miocene + gypsum | Barzaman | Barzaman | Dune sand | Cemented gravel | Ophiolite | Gravel–boulders | Limestone/dolomite |
| 30–35 m | Miocene + gypsum | Miocene + gypsum | Barzaman | Barzaman | Dune sand | Cemented gravel | Ophiolite | Ophiolite | Limestone/dolomite |
| 35–40 m | Miocene + gypsum | Miocene + gypsum | Barzaman | Barzaman | Dune sand | Cemented gravel | Ophiolite | Ophiolite | Limestone/dolomite |
| 40–45 m | Miocene + gypsum | Miocene + gypsum | Barzaman | Barzaman | Dune sand | Barzaman / limestone | Ophiolite | Ophiolite | Limestone/dolomite |
| 45–50 m | Miocene + gypsum | Miocene + gypsum | Barzaman | Barzaman | Dune sand | Barzaman / limestone | Ophiolite | Ophiolite | Limestone/dolomite |
| 50–55 m | Miocene + gypsum | Miocene + gypsum | Barzaman (Fars at Jebel Ali) | Barzaman | Dune sand | Barzaman / limestone | Ophiolite | Ophiolite | Limestone/dolomite |
| 55–60 m | Miocene + gypsum | Miocene + gypsum | Barzaman / Fars | Barzaman | Dune sand | Barzaman / limestone | Ophiolite | Ophiolite | Limestone/dolomite |
| Groundwater (average) | 0–2 m | 1–3 m | 1–3 m | Shallow between dunes, deep under dunes | Very deep if measured from the dune crest | Deep, variable | Shallow in wadis | Shallow at the coast | Shallow at the coast |
Basis and notes
A, B: Coastal sabkha and the Abu Dhabi Formation (BGS – Gheweifat); fill, Ghayathi aeolianite and the Hili Formation (BGS – Al Wathba); mudstone and gypsum at depth (MAPEI – STEP). Gypsum karst voids can occur in any slice (Al Kaabi & Sousa).
C: Quaternary deposits → Pleistocene calcarenite → Barzaman (Macklin et al.); the deepest Bur Dubai sections enter the upper Fars near Jebel Ali (T&F – DSST summary). The depth of the transition to Barzaman varies between 10 and 20 m along the alignment.
D: Rub al Khali dune sand, fossil dune cores; the Barzaman conglomerate may underlie the region (BGS – Sweihan).
E: Megabarchan dunes rise up to 130 m (BGS – Umm Azzimul); if depth is measured from the dune crest, sand is expected down to 60 m. Between the dunes, rock is much shallower.
F: Alluvial fan gravels derived from the Hajar (BGS – Al Qua’a); the gravel thickness and the depth to the underlying rock are approximate.
G, H, I: The Hajar Mountains consist mainly of ophiolite (Springer – UAE landforms). The gravel thickness on the east coast and the RAK limestone profile are based on general knowledge (approximate).
7. Soil types and depths by UAE region
The surface of the UAE is largely sand desert. The sand stretches from the Gulf coast south to the Empty Quarter (Rub al Khali), and east to the gravel plains around the Hajar Mountains (Gulflink – UAE geology). The coastal plains consist of beaches and sabkhas; the coastal sabkha west of Abu Dhabi is considered one of the finest examples in the world. The Hajar Mountains consist mainly of ophiolite (former ocean crust and upper mantle) (Springer – UAE landforms).
| Region | Surface (0–5 m) | Intermediate depth (5–30 m) | Deep (30 m+) | Groundwater | Main problem for slurry microtunnelling |
|---|---|---|---|---|---|
| West Abu Dhabi coast (Sabkha Mati, Ruwais, Ghayathi) | Coastal sabkha, Abu Dhabi Formation (beach ridges, intertidal deposits, algal mats) (BGS – Gheweifat) | Shuwaihat Formation sandstone/mudstone (BGS) | Miocene sedimentary rocks (approximate) | Very close to the saline water table in the sabkha (BGS) | Saline water, sabkha crust, gypsum |
| Abu Dhabi island and suburbs (Musaffah, MBZ, Al Wathba) | Fill (dredged carbonate sand), sabkha, Ghayathi aeolianite (BGS – Al Wathba) | Hili Formation sandstone and channel gravels (BGS) | Miocene rocks; mudstone and gypsum (on the STEP tunnel) (MAPEI – STEP) | Shallow at the coast, hypersaline (approximately 1–3 m) | Gypsum karst voids (Al Kaabi & Sousa), fill, salt |
| Dubai, Sharjah, Ajman coast | Fill, windblown sand, sabkha | Marine sand + calcarenite interbeds | Barzaman (≈10–60 m+), Fars below (Macklin et al.) | 1–3 m (Springer 2018) | Mixed face, slurry loss, salt (MATEC 2017) |
| Inland Abu Dhabi (Sweihan, Remah, east of Al Wathba) | Rub al Khali dune sand, interdune sabkha (BGS – Sweihan) | Fossil dune cores, fluvial sands | Barzaman conglomerate may underlie the whole area (BGS – Sweihan) | Shallow between dunes; deep under dunes (approximate) | Dry sand: slurry loss, frac-out |
| Liwa and the southern sand sea | Megabarchan dunes (up to 130 m), flat interdune sabkhas (BGS – Umm Azzimul) | Dune sand | Miocene rocks (approximate) | Shallow in interdune sabkhas, can exceed 100 m at dune crests (approximate) | Fully dry sand; tunnels usually run between dunes |
| Al Ain, Dhaid and the gravel plains | Alluvial fan gravels (of ophiolite and Hamrat Duru origin) (BGS – Al Qua’a) | Gravel, sand, distal fluvial deposits; continental sabkhas (BGS) | Limestone (around Jebel Hafeet) and Barzaman (approximate) | Variable, generally deep (approximate) | Cobbles–boulders, wear, losses in dry gravel |
| Hajar Mountains (Masafi, Hatta, inland Fujairah) | Wadi gravels, weathered ophiolite | Ophiolite: peridotite, gabbro (hard rock) (Springer) | Ophiolite | Shallow and fresh in wadis; fracture water in the rock | Hard rock wear, gravel–boulders |
| East coast (Fujairah, Kalba, Khor Fakkan, Dibba) | Gravel beach, alluvial fan | Gravel–sand, large boulders (approximate) | Ophiolite (approximate) | Shallow at the coast (approximate) | Submerged bouldery gravel: the hardest combination |
| Northern Ras Al Khaimah | Coastal plain, sabkha | Alluvial gravel | Limestone/dolomite mountains (karst) (approximate) | Shallow at the coast | Karst voids, mixed face |
The depth and groundwater values marked “(approximate)” are based on general knowledge and have not been verified directly against a source. Every project must confirm them with boreholes.
References
- Clay and clogging
- Oxford ORA – clogging in slurry-supported pipejacking
- Slurry shield tunnelling in clayey soils
- Underground Space 2024 – clogging with CFD-DEM
- Adv. Mater. Sci. Eng. 2021 – dispersant
- Applied Sciences – cutting head clogging criteria
- Frontiers of Struct. Civ. Eng. 2023 – clogging in sedimentary rock
- Arabian J. Sci. Eng. 2023 – interface temperature
- Open Civil Eng. J. – clogging potential
- Tunnels & Tunnelling – clay mineralogy
- Sand, gravel, boulders
- Jebelli, Meguid & Sedghinejad, TUST 2010
- Hunt – Gravel, cobbles and boulders (GCB)
- Underground Infrastructure 2023 – Chelsea River
- Trenchless Technology – New Zealand
- Marulanda & Gutiérrez – heterogeneous alluvium
- Lyon Metro B – technical slurry
- Eurohinca – risks by ground type
- Eurohinca – common risks
- Eurohinca – high water table
- Univ. Galway – lubrication in silty sand
- Tunneling Online – differing site conditions
- Rock, karst, evaporites
- Tunnels & Tunnelling – disc cutters
- Rock Mechanics Bulletin 2024 – abrasivity index
- Trenchless Technology – microtunnelling in karstic rock
- PIK Potsdam – karst modelling
- Taylor & Francis – karst and gypsum
- Gysel 2002 – anhydrite dissolution
- Al Kaabi & Sousa, WTC 2019 – gypsum cavities in Abu Dhabi
- Slurry and salt, separation
- CJGE 2020 – effect of cations on slurry stability
- Gels 2022 – bentonite in saline water
- Herrenknecht – separation technology
- Bauer BE 300 data sheet
- Maryland MDE – frac-out memo
- Special soils
- TCD – peat and buoyancy
- Stantec – Big Lake, Edmonton
- Horan, GeoQuébec 2015
- SME – Adventures in Microtunneling
- GZA – East Boston
- UAE geology
- BGS – Gheweifat and As Sila; BGS – Al Wathba; BGS – Ghantoot; BGS – Sweihan and Remah; BGS – Umm Azzimul; BGS – Al Qua’a
- Springer – UAE landforms
- Gulflink – UAE geology
- Macklin et al. – Barzaman Formation
- Tunnels & Tunnelling – Dubai Metro
- MATEC 2017 – Al Jadaf, Dubai
- Springer 2018 – foundation design in Dubai
- MAPEI – STEP Abu Dhabi