Domain

Explanation

What is groundwater?

• Some may say "blessings of essence from Mother Earth"
• To engineers, groundwater is subsurface water occurring below water table in soils & geologic formations, but the treatment (engineering) of groundwater is more often a headache than a blessing

Why does the ground have water?

• Groundwater is part of a global phenomenon - the hydrologic cycle
• Due to this cycle of hydrological movement, the groundwater is seldom, if ever, stationary, thus it is constantly changing in states, composition & motion
• Let's treat the ground like a reservoir. This reservoir has incoming water from sources like infiltration, precipitation and drainage, moving reservoir water due to physical flows, seepage, potential differences and exfiltration as well as outgoing water through evapotranspiration, drainage and outflows
• Hence, groundwater can be treated like the above reservoir water, seemingly stationary, but is actually dynamic

Why is groundwater a headache to engineers?

• Whether the groundwater is liquid or solid (frost), it causes the following problems to engineers & construction:

1. Inflows into underground structures like tunnels
2. Excessive dewatering causing land subsidence
3. Dam failures due to piping (seepage digs a hole in the dam)
4. Landslides of loose, poorly-drained, exposed slopes
5. Movements of retaining structures due to seepage, hydraulic pressures & inadequate drainage
6. Failures of pavements & roads due to trapped water

How do engineers deal with groundwater?

• The way of engineering groundwater depends on (geologic conditions, structures - both surrounding & to-be-built, weather conditions, expertise & construction resources available, design needs, funds allocated)
• A recent famous example of groundwater engineering (2001) is the reduction of the angle of inclination (not straightening) of the Leaning Tower of Piza
• Basically, groundwater engineering can be split into:

1. Darcy's Law
2. Flow in unsaturated flow
3. Seepage types & flow nets
4. Foundation dewatering & drainage
5. Filters & drains
6. Applications

Darcy's Law?

• Henry Darcy lived in the 19^th century & his Darcy's Law (1856) is as fundamental to groundwater engineering as Newton's Laws are to physics
• Darcy's Law: v = -k.i
• The velocity of low through a porous medium (like soil), v, is proportional to (surprisingly) only two parameters - k, a permeability coefficient & i (=dh/dl), the hydraulic gradient
• Darcy is saying that in porous soils with high permeability (lots of holes for water to run), when there is a potential gradient (like water ready to run from high towards low), the speed of water flow is higher, thus faster
• Darcy's Law can be used to model, predict & design for subsurface flows; the basic equation can be expanded in various forms for multi-dimensional, temporal analysis
• Limits: being linear, valid for 1<Re<10 (soils & aggregates)
• There also exists a Coupled Flow concept that is less often used: v=-L₁.dh/dl - L₂.dT/dl - L₃.dc/dl - L₄.dV/dl
• h: hydraulic head, T: temperature, c: chemical concentration, V: electrical voltage, L_i: conductivity constant

What is seepage and what are the types of seepage?

• Seepage, as the name implies, is relatively slow leakage of water through the porous ground (you won't get a river - that's a flow)
• There are two types of seepage in both saturated (filled with water) & unsaturated (water-air-soil) conditions:

1. Steady state seepage: the flow characteristics are constant over time & space; applied to confined (impervious dams) & unconfined (earth dams)
2. Transient seepage: variable flow characteristics over time & space; water levels filling, drawdown & mounding

How do engineers deal with seepage?

• The fundamental steps include:

1. Geotechnical investigation: to find out all required ground characteristics like permeability, groundwater table, flow conditions, boundary conditions, construction, etc.
2. Model: as engineering is as much about modeling as construction, predictive models are constructed & analyzed
3. Flow nets: originating from Darcy, flow nets model the seepage flow paths of water through the ground; flow nets provide insights into flow path, volume, velocity, induced pressures as well as associated design issues; drawing of flow nets used to done manually, but has now been efficiently computerised depending on the model used
4. Design: with the known flow characteristics & engineering forces induced, engineers apply principles & judgement (since analysis is still a developing science) to mould their structures to withstand the seepage forces within constraints

• All construction starts at the foundation

What are the problems & solutions of foundation dewatering & drainage?

• Groundwater dewatering under foundations causes:

1. Quick conditions in sand (fines)
2. Base heave in clays (ground bulges up)
3. Settlement in clays (the ground is sinking)

• Dewatering principles:

1. Acquifers
2. Wells
3. Flow from circular or line sources

• The right dewatering for the right groundwater: NAVFAC DM-7.2 & BS8004 pp.62-76

What are filters & drains?

• Filters: prevent erosion & reduce/cutoff inflows by seepage reduction methods
• Drains: removal of water & relief pressure build-up for safety of structures
• Commonly used filters are geotextiles and geonets for separation of soils & filtration, sometimes for reinforcement
• Design of filters:

1. Prevent piping: D₁₅ (filter)/D₈₅(soil) < 4,5
2. For permeability: D₁₅ (filter)/D₁₅(soil) > 4,5
3. Prevent soil movement: D₅₀ (filter)/D₅₀(soil) < 25
4. For slots: D₈₅ (filter)/slot width > 1.2
5. For circular holes: D₈₅ (filter)/hole diameter > 1.0

What insights into groundwater engineering?

• From engineering experience, the following are inferred:

1. Dam with deeper, wider cutoff wall has lower upward thrust due to hydraulic pressures
2. Dam with a longer, wider impervious apron lower upward thrust due to hydraulic pressures
3. Dams strengthened with pervious rocks along edges, impermeable core (concrete) & cutoff walls
4. Dewatering at a point (e.g. 10m) with surrounding wells with deeper drawdown (e.g. 12m)