Karst, subsidence, permafrost and other hazards

Landslides and earthquakes are the most dramatic geologic hazards, but many others quietly damage infrastructure and cost far more in total. Ground can collapse into hidden cavities, sink as fluids are pumped out or old mines cave in, heave as frost or swelling clays push upward, or soften as frozen ground thaws. These hazards share a pattern: they often give little visible warning on the surface, they depend on geological conditions that can be mapped and investigated, and they are far cheaper to deal with by recognition and avoidance than by repair. This lesson surveys the most important ones for North American practice.

Karst

Karst is the landscape and subsurface drainage system that develops where soluble rocks, mainly limestone, dolostone, gypsum and salt, are dissolved by groundwater. Rainwater absorbs carbon dioxide from the air and soil to form weak carbonic acid, which slowly dissolves calcite along joints and bedding planes. Over thousands of years the openings enlarge into fissures, conduits and caves. At the surface, karst produces sinkholes, disappearing streams, large springs and an irregular, pinnacled rock surface buried beneath soil.

Sinkholes are the main engineering hazard. Dissolution sinkholes form slowly as the rock surface is lowered. Cover-collapse sinkholes form when soil above a cavity or widened fissure ravels down into it, creating a void in the soil that grows upward until the roof collapses suddenly, sometimes overnight. Cover-subsidence sinkholes form more gradually where sandy cover slowly trickles into openings below. Human activities often trigger sinkholes: concentrated drainage from roofs, roads and leaking pipes washes soil into openings, and pumping groundwater lowers the water table and removes buoyant support from soil arches.

Karst complicates foundations because the rock surface can vary by many metres between closely spaced points, and borings that hit sound rock may sit on a pinnacle beside a soil-filled slot. Investigation uses closely spaced probes and borings, geophysics such as electrical resistivity tomography and ground-penetrating radar to find voids, and careful attention to drainage. Mitigation includes avoiding sinkholes, controlling surface water, grouting voids, bridging over them with reinforced mats or beams, and founding on piles or drilled shafts socketed into sound rock after probing beneath each one.

Karst occurs in several parts of Canada, including the limestones and dolostones of southern Ontario and the Niagara Escarpment, parts of Manitoba, the Rocky Mountains and Nova Scotia, where gypsum karst is common. Gypsum dissolves much faster than limestone, and sinkholes in gypsum near Wood Buffalo National Park and in Nova Scotia can form within human time scales.

Subsidence

Subsidence is the gradual or sudden sinking of the ground surface. Apart from karst, the main causes are:

• Fluid withdrawal: pumping groundwater, oil or gas from compressible sediments reduces pore pressure and increases effective stress, so clay layers consolidate. Regional subsidence from groundwater pumping has exceeded several metres in California's San Joaquin Valley, around Houston and Galveston, and in Mexico City. It damages wells, canals, pipelines and drainage, increases flooding in low-lying areas and is largely irreversible.
• Underground mining: collapse of old coal, salt and metal mines produces sudden pit-type sinkholes over shallow workings and broad troughs over longwall panels. Abandoned mines are common in coal regions of Nova Scotia, Alberta, Pennsylvania and the Appalachians, and in old metal-mining towns. Mine records are often incomplete, so investigation may need drilling and geophysics.
• Dissolution of salt, either naturally or through solution mining.
• Drainage of organic soils: peat that is drained shrinks and oxidizes, lowering the land surface by centimetres per year.
• Hydrocompaction: dry, loose soils such as loess and some alluvial fan deposits collapse when wetted for the first time, as discussed in lesson 6.

Subsidence from groundwater withdrawal can be estimated with one-dimensional consolidation theory. When the head in an aquifer falls by \(\Delta h\), the effective stress in the confining clay layers rises by about \(\gamma_w \Delta h\) once the clay has drained, and the clay compresses accordingly. For a normally consolidated clay, the settlement is approximately:

where \(C_c\) is the compression index, \(H\) the clay thickness, \(e_0\) the initial void ratio and \(\sigma'_0\) the initial vertical effective stress at the middle of the layer. Overconsolidated clays compress much less until their preconsolidation pressure is exceeded.

Frost action

In cold climates, seasonal freezing of the ground causes frost heave. Heave is not simply the 9 per cent expansion of pore water; it is mainly caused by ice lenses that grow as water is drawn upward by suction to the freezing front, especially in silts and fine sands that combine high capillarity with moderate permeability. Heave lifts shallow footings, pavements, retaining walls and utilities, and the following thaw leaves the soil weakened and saturated, causing pavement breakup in spring. Frost susceptibility is assessed mainly by grain size, particularly the percentage finer than 0.02 mm, and by soil type.

Design responses include placing footings below the design frost depth, using non-frost-susceptible granular materials beneath pavements and slabs, providing drainage to keep water away from the freezing zone, and using rigid insulation to reduce frost penetration. Design frost depths vary from less than a metre in mild coastal areas to more than two metres in the northern interior, and are given in local codes and practice.

Permafrost

Permafrost is ground that remains at or below 0 °C for at least two consecutive years. It underlies large areas of northern Canada, Alaska and other polar and mountain regions. Above the permafrost is the active layer, which thaws each summer and refreezes each winter. Permafrost that contains little ice, such as dense gravel or sound rock, is thaw-stable: it loses little strength or volume on thawing. Permafrost in fine soils often contains large volumes of excess ice in lenses and wedges, and is thaw-unstable: when it thaws, the ice melts, the ground settles and the soil may turn to mud.

Construction disturbs the thermal balance. Removing the insulating vegetation and organic layer, placing dark pavements, and building heated structures directly on the ground all cause thawing and settlement. Northern engineering therefore follows two strategies: keeping the ground frozen, by building on piles with ventilated air spaces, using insulated and ventilated pads, and installing thermosyphons that remove heat from the ground in winter; or, where the ground is thaw-stable or thin, removing or pre-thawing the permafrost before building. Climate warming is degrading permafrost across the north, threatening existing roads, airstrips, pipelines and buildings, and making thaw settlement and thaw-related landslides more common.

The settlement caused by thawing ice-rich permafrost can be estimated from the thaw strain, the fractional reduction in thickness when a layer thaws and drains. If a layer of thickness \(H\) has thaw strain \(\varepsilon_t\), the thaw settlement is roughly \(s = \varepsilon_t H\), plus any further consolidation under load. Thaw strains of 10 to 30 per cent are common in ice-rich silts, so even a few metres of thaw can cause large settlement.

# Subsidence estimates: aquifer drawdown (consolidation) and permafrost thaw
import math

# Consolidation of a clay layer from aquifer drawdown
Cc, e0, H, sig0, gw = 0.35, 1.10, 6.0, 85.0, 9.81
for dh in (2.0, 4.0, 8.0):
    dsig = 0.5 * gw * dh
    s = Cc * H / (1 + e0) * math.log10((sig0 + dsig) / sig0)
    print(f"head drop {dh:3.0f} m -> about {s*1000:4.0f} mm settlement")

# Thaw settlement of ice-rich permafrost layers: (thickness m, thaw strain)
profile = [(1.0, 0.05), (2.0, 0.25), (1.5, 0.15), (3.0, 0.03)]
depth = 0.0
for thaw_depth in (1.0, 3.0, 4.5, 7.5):
    s = 0.0
    top = 0.0
    for h, eps in profile:
        thawed = max(0.0, min(h, thaw_depth - top))
        s += eps * thawed
        top += h
    print(f"thaw to {thaw_depth:3.1f} m below the active layer -> about {s:.2f} m settlement")

head drop   2 m -> about   47 mm settlement
head drop   4 m -> about   90 mm settlement
head drop   8 m -> about  165 mm settlement
thaw to 1.0 m below the active layer -> about 0.05 m settlement
thaw to 3.0 m below the active layer -> about 0.55 m settlement
thaw to 4.5 m below the active layer -> about 0.78 m settlement
thaw to 7.5 m below the active layer -> about 0.86 m settlement

Expansive and collapsible soils

Expansive soils rich in smectite, introduced in lesson 2, swell when they absorb water and shrink when they dry. They damage light structures such as houses, slabs and pavements through differential heave and through seasonal cycles, especially where trees, leaking pipes or poor drainage cause uneven moisture changes. Their potential is assessed from plasticity, clay content, swell tests and suction measurements. Mitigation includes deep foundations or stiffened slabs, moisture control around buildings, removal and replacement of the active zone, and chemical stabilization with lime. Collapsible soils, such as loess and some fan deposits, have the opposite problem, losing volume suddenly when wetted under load.

Other hazards worth recognizing

• Flooding and erosion: floodplains, alluvial fans and coastlines are dynamic. Rivers migrate laterally, erode banks and scour around bridge piers; coastal bluffs retreat; and floods deposit sediment and debris. Geological mapping of floodplain and channel history complements hydrological flood mapping.
• Volcanic hazards: in the Cascades and Alaska, lahars (volcanic mudflows) travelling down river valleys are the greatest threat to communities far from the volcanoes themselves, as shown by the 1980 Mount St. Helens eruption. Volcanic ash also affects infrastructure over large areas.
• Radon and naturally occurring hazardous materials: some granites, shales and uranium-bearing rocks release radon gas into buildings; serpentinites can contain asbestos; and some rocks release metals or acid drainage when excavated.
• Methane and other gases from buried organic deposits, old landfills or coal can accumulate in excavations, tunnels and basements.