Foundation Engineering — Geotechnical Engineering

Foundation Engineering applies the soil mechanics principles of the previous module to the design of the structural elements that transfer building loads to the ground — from selecting a foundation type and computing bearing capacity and settlement, through shallow and deep foundation design and field testing, to slope stability, embankments, dams, earth-retaining structures and ground improvement. Every formula, IS code reference, diagram, solved example and exam-pattern table is included.

Learning Objectives

After studying this chapter you will be able to:

• Select an appropriate foundation type (shallow vs. deep) based on soil conditions, loads and site constraints
• Compute the ultimate and safe bearing capacity of shallow foundations using Terzaghi's and Meyerhof's theories, including water table and shape/depth/inclination corrections
• Compute immediate, primary consolidation and secondary compression settlement, and check against IS 1904 allowable limits
• Design isolated footings and rafts for shear and bending, and interpret plate load, SPT and CPT field tests
• Compute the static and dynamic capacity of pile foundations, pile group efficiency, and design well (caisson) foundations
• Analyse slope stability using infinite-slope, Swedish circle (Fellenius) and Bishop's methods, and Taylor's stability chart
• Design earthen embankments and gravity dams, checking seepage, filter criteria, and overturning/sliding/bearing stability
• Design earth-retaining structures (Rankine/Coulomb pressure, cantilever walls, sheet piles, reinforced earth) and select ground-improvement techniques

Exam Relevance

Concept Map

Where This Fits

Prerequisite: Soil Mechanics (bearing capacity, settlement and earth pressure formulas here build directly on the shear strength, consolidation and classification concepts covered there). Leads to: Hydraulics, which covers flow and hydraulic structures relevant to dam spillways and seepage analysis introduced here.

1.1 Classification of Foundations

1.2 Selection Criteria for Foundation Type

• Soil bearing capacity: Low capacity → raft or pile; high capacity → isolated footing
• Depth to firm strata: If firm strata is deep → piles or caissons
• Water table: High water table → raft or buoyancy raft; avoid open excavation
• Type and magnitude of load: Heavy or eccentric loads → combined or raft
• Adjacent structures: Nearby buildings → avoid excavation-induced settlement; use piles
• Settlement sensitivity: Differential settlement sensitive structures → raft or pile cap
• Scour depth: Bridge foundations in rivers → well foundation below scour depth
• Economy: Shallow footings are cheapest; piles are expensive but unavoidable in soft soils

1.3 Key Definitions

Gross pressure intensity \(q\) is the total load divided by the base area of the footing. The net pressure intensity subtracts the overburden pressure that already existed before excavation. Safe Bearing Capacity (SBC) divides the ultimate bearing capacity by a factor of safety, and the final allowable bearing capacity is the lesser of the SBC and the bearing capacity based on the settlement criterion — settlement, not shear failure, is often the governing check for wide footings on sand.

1.4 IS Codes for Foundation Engineering

2.1 Terzaghi's Bearing Capacity Theory (1943)

Terzaghi's theory applies to shallow foundations (\(\dfrac{D_f}{B} \le 1\)) undergoing general shear failure. It assumes a rigid, perfectly rough base and neglects shear resistance of the soil above the base level (treating it purely as a surcharge).

2.2 Types of Shear Failure

2.3 Meyerhof's General Bearing Capacity Equation (1963)

Meyerhof extended Terzaghi's equation into a general form (adopted by IS 6403) that adds shape, depth and load-inclination correction factors to each of the three bearing-capacity terms, making it applicable to rectangular footings, footings embedded deeper than \(B\), and footings under inclined loads — none of which Terzaghi's original equation handles well.

2.4 Bearing Capacity of Footings on Saturated Clays (φ = 0 Analysis)

For saturated clays under the undrained (short-term) condition immediately after construction, the friction angle \(\phi_u = 0\), which collapses the general equation dramatically: \(N_q = 1\), \(N_\gamma = 0\), and \(N_c = 5.14\) (Prandtl). This φ=0 analysis governs the critical short-term stability of foundations, embankments and excavations in clay.

2.5 Bearing Capacity from Field Tests

2.6 Effect of Water Table on Bearing Capacity

A high water table reduces bearing capacity significantly, especially in sands, because the effective (submerged) unit weight \(\gamma'\) is roughly half the bulk unit weight — and bearing capacity is directly proportional to unit weight through the surcharge (\(N_q\)) and self-weight (\(N_\gamma\)) terms. The correction to use depends on whether the water table lies above, at, or below the footing base.

3.1 Components of Settlement

Total settlement of a foundation comprises three components: immediate (elastic) settlement \(S_i\), primary consolidation settlement \(S_c\), and secondary compression (creep) settlement \(S_s\). In sands, consolidation is essentially instantaneous, so total settlement is approximately just \(S_i\). In clays, \(S_c\) dominates, \(S_i\) is also significant, and \(S_s\) becomes important for organic soils.

3.2 Immediate (Elastic) Settlement

Immediate settlement is computed by treating the soil as an elastic half-space and the footing as a flexible or rigid loaded area. The influence factor \(I_s\) depends on the footing's length-to-width ratio and increases without bound for an infinitely long strip footing, so a practical finite value is used instead. A rigid footing settles somewhat less at its centre than an equivalent flexible footing, since rigidity forces the settlement to be more uniform across the footing.

3.3 Primary Consolidation Settlement (Terzaghi, 1925)

For a normally consolidated clay layer (OCR = 1), settlement follows the standard \(C_c\)-based logarithmic formula. For over-consolidated clay (OCR > 1), the calculation depends on whether the final stress stays below the preconsolidation pressure \(\sigma_p'\) (in which case the flatter swelling index \(C_s\) governs throughout) or crosses it (in which case \(C_s\) governs up to \(\sigma_p'\) and \(C_c\) governs beyond it).

3.4 Rate of Consolidation — Terzaghi's 1D Theory

The dimensionless time factor \(T_v\) links elapsed time to the degree of consolidation \(U\%\) through Terzaghi's parabolic approximation for \(U\le60\%\) and a logarithmic approximation for \(U>60\%\). The coefficient of consolidation \(c_v\) is back-calculated from laboratory oedometer data using either Casagrande's log-time method (using \(t_{50}\)) or Taylor's square-root-of-time method (using \(t_{90}\)).

3.5 Secondary Compression (Creep)

After primary consolidation is essentially complete, void ratio continues to decrease slowly under constant effective stress due to plastic re-adjustment of the soil skeleton — this secondary compression is governed by the secondary compression index \(C_\alpha\), important for highly organic soils (peat, muskeg) and soft marine clays.

3.6 Stress Increment in Soil (Boussinesq Method)

Boussinesq's solution gives the increase in vertical stress at depth due to a point load on an elastic half-space; it is integrated to give an influence factor for a uniformly loaded rectangular area (via Newmark's chart or tabulated formulas). The simplified 2:1 approximation assumes the load spreads at a slope of 2 (vertical) to 1 (horizontal) and is convenient for quick hand estimates. Westergaard's equation instead assumes alternating rigid horizontal layers (appropriate for stratified/anisotropic soils) and gives lower stresses than Boussinesq for the same load.

3.7 Allowable Settlement Criteria (IS 1904)

4.1 Design of Isolated (Spread) Footing

The design proceeds in a fixed sequence: (1) size the base area from the allowable bearing capacity, (2) fix the footing dimensions \(B\times L\), (3) compute the net upward pressure that governs the structural design, (4) check one-way (beam) shear at distance \(d\) from the column face, (5) check two-way (punching) shear at \(\dfrac{d}{2}\) from the column face — this often governs the required depth, (6) compute the bending moment at the column face for flexural reinforcement, and (7) check the development length of that reinforcement.

4.2 Raft Foundation Design — Conventional Method

• Conventional rigid method: assume the raft is rigid so the soil pressure varies linearly; compute contact pressure from biaxial bending and design each strip as a continuous beam
• Flexible plate method: model the raft as a flexible plate on Winkler springs (\(k_s\) = modulus of subgrade reaction); solved by FEM or grid-beam analogy for large or irregularly-loaded rafts
• Modulus of subgrade reaction \(k_s\): relates to the soil's elastic modulus \(E_s\); for sand, \(k_s\) reduces with increasing footing width (\(k_s(B)=\dfrac{k_s(0.3)}{B}\)); for clay it is approximately independent of \(B\)
• Compensated (buoyancy) raft: the depth of excavation is chosen so that the weight of soil removed approximately equals the structural load, giving a net bearing pressure near zero — the standard solution for tall buildings on soft clay

4.3 Plate Load Test (IS 1888)

A 300 mm or 450 mm square steel plate (minimum 25 mm thick) is loaded in increments and settlement measured, until failure or twice the design load or 25 mm settlement is reached. For clay, the ultimate bearing capacity is essentially independent of the plate size, so \(q_{ult}(\text{footing}) = q_{ult}(\text{plate})\) directly. For sand, however, bearing capacity increases with size, so settlement must be scaled using an empirical relation. The test has significant limitations: it is affected by local heterogeneity and scale effects, cannot capture long-term consolidation settlement, and is unreliable for deposits thicker than about twice the plate width.

4.4 Standard Penetration Test (SPT) — IS 2131

A split-spoon sampler (50.8 mm OD, 38.1 mm ID) is driven by a 63.5 kg hammer falling 750 mm; a 150 mm seating drive is discarded, then N is recorded as the blow count for the next 300 mm. Three corrections are applied before using N in design: an overburden correction (accounting for the fact that the same soil resists penetration less at shallow depth), an energy correction to a standard 60% energy ratio, and a dilatancy correction for fine saturated sand when \(N>15\) (dense saturated fine sand dilates during rapid shearing, temporarily raising apparent resistance, and the correction removes this artifact).

4.5 Static Cone Penetration Test — CPT (IS 4968 Part 3)

• Cone tip resistance \(q_c\) and sleeve friction \(f_s\) are measured continuously as the cone is pushed, without any borehole
• Friction ratio \(F_r=\dfrac{f_s}{q_c}\) distinguishes soil type: clay gives \(F_r>4\%\), sand gives \(F_r<1\%\)< /li>
• Undrained shear strength is estimated from \(q_c\) using a site-specific cone factor \(N_{kt}\) (typically 10–20)
• A piezocone (CPTu) adds pore-pressure measurement, useful for detecting layer boundaries and estimating permeability
• Advantages over SPT: continuous profile, no sample disturbance, repeatable, and better performance in soft clays where a split-spoon sample is easily disturbed

5.1 Types of Piles

5.2 Static Analysis of Pile Capacity

The ultimate capacity of a single pile combines end-bearing resistance at the tip and skin friction along the shaft. In clay, skin friction is usually computed by the α-method (total stress, using an empirical adhesion factor that decreases as undrained strength increases). In both sand and clay, the more general β-method (effective stress) can be used, relating skin friction to the effective overburden stress through a lateral earth pressure coefficient and the pile-soil interface friction angle.

5.3 Pile Group Efficiency

Piles in a closely-spaced group interact through overlapping stress zones, so the group capacity is generally less than the sum of individual pile capacities — captured by the group efficiency \(\eta\), most commonly estimated by the Converse-Labarre formula. For pile groups in clay, a block failure check (treating the entire group plus enclosed soil as a single large pier) must also be performed, and the design capacity is the lesser of the two. Settlement of a pile group is typically estimated using Meyerhof's equivalent-raft concept, placing a fictitious raft at some depth within the pile length and analysing settlement of that raft.

5.3a Negative Skin Friction (Downdrag)

Negative skin friction develops when the soil surrounding a pile settles more than the pile itself — typically where a pile passes through a recently placed fill, a consolidating soft clay, or a layer subject to groundwater lowering. Instead of resisting the load, the downward-moving soil drags the pile down, so the skin friction over that zone acts as an additional downward load rather than a supporting resistance.

• Effect on capacity: the negative-friction force \(F_{nf}\) is subtracted from the available capacity — allowable load \(= \dfrac{Q_{ult} - F_{nf}}{\text{FoS}}\), and \(F_{nf}\) is added to the working load the tip must carry.
• Single pile (clay): \(F_{nf} = \alpha\,\bar{c}_u\,(\pi D)\,L_f\); (sand/fill, effective stress): \(F_{nf} = K\tan\delta\cdot\bar{\sigma'_v}\,(\pi D)\,L_f\), where \(L_f\) = thickness of the settling layer.
• For a pile group, negative friction may act on the perimeter of the whole block; the larger of the individual-sum and block values governs.
• Mitigation: bitumen (slip) coating on the pile shaft within the fill zone, or isolating sleeves, to reduce the mobilised drag.

5.4 Dynamic Formulae for Pile Capacity

Dynamic formulae relate pile capacity to the observed "set" (penetration per hammer blow) during driving. The Engineering News Record (ENR) formula is simple but crude (implicit FOS ≈ 6); the Hiley formula is more refined, accounting for hammer efficiency and elastic compression losses. Modern practice (and IS 2911) prefers static analysis or an actual pile load test over dynamic formulae, and for large projects, wave equation analysis (based on stress-wave propagation theory) gives the most accurate prediction.

5.5 Pile Load Test (IS 2911 Part 4)

• Maintained load test: load applied in increments up to twice the safe load; maintained for 24 hours at the design load and at maximum load
• Cyclic load test: used to separate the skin friction and end-bearing contributions
• Failure criterion: total settlement reaching 10% of the pile diameter, or the load at which the settlement rate exceeds 2 mm per 1 kN load increase
• Uplift test: tension applied to measure uplift capacity; failure defined at 12 mm net upward movement
• Lateral load test: load applied at the pile head to measure deflection and back-calculate the soil's sub-grade reaction

5.6 Well (Caisson) Foundation

A well foundation consists of a cutting edge, well curb, steining (the main hollow shaft wall), bottom plug, top plug and well cap, sunk under its own weight (reduced by buoyancy) with skin friction resisting sinking. The grip length below the scour level provides lateral stability and increases with the proportion of live/dynamic load carried. Tilt and shift during sinking are critical construction risks, tightly limited by IRC 78.

6.1 Types of Slope Failures

6.2 Factor of Safety Definitions

Slope stability FOS is defined as the ratio of resisting moment to driving (overturning) moment, or equivalently the ratio of the available shear strength to the shear stress actually mobilised on the trial slip surface. IS 7894 specifies different minimum FOS values depending on the loading condition — the required FOS is lowest for the transient rapid-drawdown condition (1.2), since it involves the least certainty about pore pressures, and highest for long-term steady seepage (1.5).

6.3 Infinite Slope Analysis

For a slope of essentially infinite lateral extent (a long, uniform natural or cut slope with a slip surface parallel to the ground surface), the FOS reduces to a simple closed-form expression. For dry cohesionless soil, FOS depends only on the ratio of \(\tan\phi'\) to \(\tan\beta\) — failure is imminent exactly when the slope angle equals the friction angle. Seepage flowing parallel to the slope roughly halves this FOS for typical soils, since the destabilising pore pressure reduces the effective normal stress on the slip plane.

6.4 Swedish Circle Method (Fellenius, 1927)

The slip mass above a trial circular surface is divided into vertical slices; the FOS is computed as the ratio of the sum of resisting forces (from cohesion and friction on each slice base) to the sum of driving forces (the tangential component of each slice's weight). Fellenius's ordinary method of slices ignores the forces between adjacent slices entirely — this makes the method conservative (it underestimates FOS), with errors of up to 60% possible when pore pressures are high, but it remains useful as a quick, hand-calculable check.

6.5 Bishop's Modified Method (1955)

Bishop's simplified method improves on Fellenius by accounting for the normal (though not shear) forces between adjacent slices, giving a significantly more accurate FOS at the cost of requiring iteration — the FOS appears on both sides of the governing equation (through the term \(m_{\alpha i}\)) and converges after typically 3–4 iterations. The pore pressure ratio \(r_u\) conveniently expresses pore pressure as a fraction of the total overburden stress, ranging from 0 (dry slope) to about 0.5 (fully saturated, the worst case). Bishop's simplified method is standard for routine slope design; Spencer's and Morgenstern-Price's methods (which satisfy full force and moment equilibrium) are reserved for critical projects.

6.6 Taylor's Stability Chart (1937)

Taylor's method uses a dimensionless stability number to relate the critical height of a slope (the height at which FOS = 1) to cohesion, unit weight, slope angle and friction angle, all pre-computed into charts. For the special case of a vertical cut in saturated clay under undrained conditions (\(\phi=0\)), the stability number takes the well-known value 3.83, giving a simple direct formula for the critical height.

7.1 Types and Components of Earth Embankments

7.2 Terzaghi's Filter Criteria

A filter placed between fine core material and coarse shell material must satisfy two competing requirements simultaneously: it must be fine enough to physically retain the base soil's particles (preventing piping), yet coarse enough to remain far more permeable than the base soil (so it doesn't itself impede drainage). Where a single filter layer cannot bridge a very large size gap, a two-layer (graded) filter is used, applying the same criteria progressively between each pair of adjacent layers.

7.3 Seepage through Embankments — Phreatic Line

The phreatic line is the top flow line (free water surface) within an embankment dam, along which pore pressure is atmospheric. Casagrande's parabola construction gives an approximate but very useful geometric method for locating this line in a homogeneous dam, treating the downstream toe as the focus of a parabola. Where the downstream face is sloped, a correction (Schaffernak/Pavlovsky) accounts for the fact that the phreatic line exits partway up the downstream slope, above the toe, creating a visible "seepage face".

7.4 Compaction of Embankment Fill

Fill material is compacted near its optimum moisture content (OMC) to achieve maximum dry density (MDD), using either the Standard or Modified Proctor test as the laboratory reference. Field compaction is controlled by comparing the achieved field dry density to the laboratory MDD as a percentage — dam cores typically require 95–98% of Modified Proctor MDD, while the sand shell is instead controlled through relative density (targeting \(D_r \ge 70\%\), since the Proctor concept applies poorly to purely cohesionless material).

7.5 Critical Stability Conditions for Embankments

• End of construction (EoC): critical for the downstream slope; undrained condition in fine-grained fill since excess pore pressures have not yet dissipated
• Steady seepage: critical for the downstream slope; the long-term condition, analysed with effective stress parameters and the established phreatic line or flow net
• Rapid drawdown: critical for the upstream slope; the reservoir level drops faster than pore pressures inside the embankment can dissipate, and this typically gives the minimum FOS of all conditions
• Earthquake loading: analysed by pseudo-static methods (applying a horizontal seismic force), reducing FOS by roughly 25% from the static value, and separately checking liquefaction potential of any loose saturated fill

8.1 Classification of Dams

8.2 Forces on Gravity Dam

A gravity dam must resist self-weight, hydrostatic pressure, uplift, silt pressure, wave pressure, earthquake forces, and (where applicable) ice pressure and thermal effects. Uplift pressure deserves special attention: it varies roughly linearly from full hydrostatic head at the heel toward zero (or tailwater head) at the toe, and a drainage gallery is deliberately provided to cut this uplift distribution short, reducing the total uplift force substantially. Hydrodynamic (earthquake) pressure on the submerged upstream face is estimated by Westergaard's added-mass approach, on top of the ordinary pseudo-static horizontal seismic force on the dam body itself.

8.3 Stability Analysis of Gravity Dam

Every gravity dam section is checked against overturning about the toe, sliding along its base (and along weak planes in the foundation), and excessive or unbalanced base stress. The middle-third rule — requiring the resultant of all forces to fall within the middle third of the base — ensures no tension develops at the heel, which is critical because concrete and most rock/soil interfaces cannot reliably sustain tension.

8.4 Earth Dam Design Principles

• Seepage control: impervious core, cutoff trench extending to bedrock, grout curtain, and/or an upstream impervious blanket
• Slope stability: checked for end-of-construction, steady-seepage and rapid-drawdown conditions using Bishop's modified method; FOS ≥ 1.5 for the steady-state condition
• Freeboard: normal freeboard = wave height + 1.5 m; minimum freeboard = 0.9 m (IS 8237)
• Cutoff trench: extends to an impermeable stratum; minimum base width 3 m for access; side slopes 1:1 to 1.5:1
• Drainage system: a chimney drain, horizontal drainage blanket and toe drain combination is standard for zoned dams
• Instrumentation (IS 9527): piezometers (pore pressure), settlement gauges, seepage measurement weirs, and inclinometers

8.5 Spillway and Seepage Control

The spillway design flood is selected based on dam size and hazard classification — large dams are designed for the Probable Maximum Flood (PMF), while medium dams may use the Standard Project Flood (50–75% of PMF). Seepage through the dam body and foundation is governed by Darcy's law, and the exit gradient at the downstream toe must stay well below the critical hydraulic gradient (which triggers piping) — an FOS against piping of 4–5 is typically required, higher than the 3–4 used for ordinary seepage problems because dam failure by piping is catastrophic. Lane's weighted creep theory offers an alternative, purely empirical seepage-path-length design method.

9.1 Types of Retaining Structures

9.2 Lateral Earth Pressure Theories

Rankine's theory assumes a smooth, vertical wall with a horizontal backfill, giving simple closed-form active and passive pressure coefficients. For cohesive backfill, a tension zone develops near the top of the wall, down to a depth where the net active pressure becomes zero. Coulomb's theory is more general — it accounts for wall inclination, backfill slope and wall friction through a limit-equilibrium wedge analysis, and gives a resultant force direction inclined at the wall friction angle to the wall's normal, rather than the purely horizontal/vertical resultant of Rankine's theory.

9.3 Stability Checks for Gravity and Cantilever Retaining Walls

Every retaining wall must be checked for four failure modes: overturning about the toe (resisting moment from self-weight and passive resistance vs. overturning moment from active thrust), sliding along the base (base friction plus base cohesion vs. the horizontal thrust, with the weight of soil on the heel included for cantilever walls), bearing capacity of the foundation soil (no tension requires the eccentricity to stay within \(\dfrac{B}{6}\)), and overall slope stability of the wall, backfill and foundation treated together as a single slope.

9.4 Sheet Pile Wall Analysis

For a cantilever sheet pile in cohesionless soil, the required embedment depth is found by the free earth support method: setting net driving moment equal to net resisting moment about the point of zero net pressure, solving the resulting cubic equation for depth, then adding 20–40% extra depth as a practical safety margin. An anchored sheet pile additionally has a tie-rod providing horizontal support, reducing the required embedment depth compared to a pure cantilever wall of the same height. Rowe's moment reduction accounts for the flexibility of the sheet pile section itself, which redistributes bending moment away from the theoretical (rigid) value.

9.5 Reinforced Earth and Geosynthetic Retaining Walls

• Principle: frictional interaction between reinforcement strips (or geosynthetic layers) and granular fill increases the apparent cohesion of the composite mass, making the whole reinforced block behave like a gravity wall
• External stability checks: identical to a gravity wall — sliding, overturning and bearing capacity, treating the entire reinforced mass as one rigid block
• Internal stability checks: tensile force in each reinforcement layer must be checked against both tensile rupture and pullout resistance from the soil beyond the theoretical failure surface
• Geosynthetics (IS 14716): HDPE/PP geogrids or woven geotextiles, checked for tensile strength, pullout resistance, durability, and a creep reduction factor for long-term sustained load

10.1 Overview and Need

Ground improvement is needed when natural soil is inadequate for the intended load — low bearing capacity, excessive settlement, liquefaction potential, or expansive behaviour. Methods are broadly classified into mechanical (densification), hydraulic (drainage/consolidation acceleration), chemical/admixture (stabilisation, grouting), and reinforcement-based techniques (stone columns, soil nailing, geosynthetics).

10.2 Preloading with Prefabricated Vertical Drains (PVD)

Where drainage is primarily radial (toward closely-spaced vertical drains rather than only vertically toward the top/bottom of the layer), Barron's radial consolidation theory replaces Terzaghi's 1D theory. The spacing between drains, converted to an equivalent circular "unit cell" diameter, and the horizontal coefficient of consolidation (typically several times larger than the vertical \(c_v\) for soft clays, due to natural soil anisotropy and macro-fabric) together control how fast radial consolidation proceeds. The combined effect of simultaneous vertical and radial drainage is found by treating the two as independent, multiplicative processes.

10.3 Stone Columns (Granular Piles)

Stone columns are installed by vibro-replacement — a vibrating probe penetrates the soft clay and compacted gravel backfill is added in lifts to form a dense granular column. Because the stone column is much stiffer than the surrounding clay, it attracts a disproportionate share of the applied load (expressed by the stress concentration ratio), which both improves overall bearing capacity and reduces settlement (expressed by Priebe's settlement improvement factor). The typical failure mode is bulging of the column near the top, where lateral confinement from the surrounding soft clay is weakest.

10.4 Lime and Cement Stabilisation

10.5 Grouting Techniques

• Permeation grouting: low-viscosity grout injected into voids without disturbing soil structure; effective only in coarse sands and gravels
• Compaction grouting: very stiff mortar injected, displacing and densifying surrounding soil; used to correct differential settlement in loose fills
• Jet grouting (JSG): high-pressure water/grout jet cuts and mixes soil in place, creating "soilcrete" columns effective in almost any soil type
• Compensation grouting: injected between a tunnel and an overlying building to actively compensate for tunnelling-induced settlement

10.6 Dynamic Compaction

A heavy tamper (10–30 tonne) is repeatedly dropped from a great height (10–40 m) in a grid pattern, transmitting enough energy to densify loose fills, mine tailings, and other problematic granular deposits to significant depth. It is unsuitable for saturated fine-grained soils, since there is no time for excess pore pressure to dissipate between drops, and it must be kept well away from vibration-sensitive structures.

10.7 Soil Nailing

• Principle: steel bars (nails) are drilled and grouted into an existing slope at regular spacing, creating a composite reinforced soil mass through passive reinforcement (no prestress is applied)
• Design: checks both external stability (overturning, sliding, bearing capacity of the nailed block) and internal stability (nail tensile force, pullout resistance, and the shotcrete facing at the nail head)
• Advantages over anchors: no pre-stress loss, works well in existing fill, faster construction, lower cost, and a more flexible (gradual) failure mode

10.8 Vibro-compaction and Vibro-replacement

A. Key Values to Memorise

B. Exam-Angle Summary

C. Mnemonics & Memory Aids

D. Critical IS Codes — One-Line Summary

Practice Questions

Related Topics

Mini Quiz — Full Chapter Recap