Deep Foundations

When piles are used

A pile is a long, slender member that transfers load to deeper, stronger strata or distributes it through skin friction. Piles are used when:

• the soil near the surface is too weak or compressible for shallow foundations;
• loads are very heavy or concentrated (bridges, tall buildings, industrial plants);
• there are uplift or lateral forces (towers, offshore structures, retaining structures);
• scour may undermine shallow foundations (bridge piers);
• expansive or collapsible soils must be bypassed;
• future excavation next to the structure is expected.

Classification

Driven piles densify loose sands (increasing capacity) but cause noise and vibration and may heave clays; bored piles are quiet and suitable near existing structures and in clays, but may loosen sands.

Static capacity

Ultimate capacity = base (point) resistance + shaft (skin) resistance:

Piles in clay (undrained, total stress)

$\alpha$ = adhesion factor (about 1.0 for soft clays, reducing to about 0.3–0.5 for stiff clays; bored piles lower than driven); ${\bar{c}}_u$ = average undrained strength along the shaft.

Piles in sand (effective stress)

$K$ = earth pressure coefficient on the shaft (depends on installation: higher for driven displacement piles); $\delta$ = pile–soil friction angle. Field results show that base and shaft resistances do not keep increasing linearly with depth — beyond a critical depth (roughly 15–20 pile diameters) ${\sigma }_v^{\prime }$ is usually limited in these calculations.

Safe load: $Q_{safe}=Q_u/F$, with $F$ commonly 2.5 (IS 2911), or separate factors on base and shaft.

Dynamic formulas (driven piles)

Based on the energy of the hammer and the penetration (set) per blow at the end of driving:

Dynamic formulas are unreliable in clays (driving resistance differs greatly from static capacity because of pore pressures and remoulding) — use static analysis and load tests.

Pile load test (IS 2911 Part 4)

• Initial test — on test piles before the job, to decide design capacity (loaded to about 2.5 times the design load or to failure).
• Routine test — on working piles during construction (to about 1.5 times the design load).
• Loads applied in increments by a jack against kentledge or reaction piles; settlements recorded.

Other tests: cyclic load test (separates base and shaft resistance), lateral load test, pull-out test, dynamic pile testing (PDA), low-strain integrity tests.

Pile groups

Piles are used in groups under a pile cap.

• Minimum spacing (centre to centre): commonly about 2.5 to 3 times the pile diameter; wider for friction piles in clay to reduce overlap of stress zones; closer spacing permitted for end-bearing piles on rock.
• Stress zones of friction piles overlap, so group capacity may be less than the sum of individual capacities.

Group efficiency

Group capacity in clay

Check both:

1. Sum of individual capacities × efficiency (or simply $n{Q}_u$), and
2. Block failure of the group acting as a single large pier:

($N_c$ ≈ 9 for deep blocks, $B_g$, $L_g$ = plan dimensions of the group, $L$ = pile length).

The smaller value governs. In loose sand driven groups can have efficiency > 1 (densification).

Group settlement

Settlement of a group exceeds that of a single pile under the same load per pile (deeper stressed zone). For friction piles in clay, the group load is assumed to act on an equivalent raft at two-thirds of the pile length below the cap, spreading at 2 vertical : 1 horizontal; consolidation settlement is computed for layers below. For end-bearing groups, the equivalent raft is at the pile tips.

Well foundations and caissons

A caisson is a hollow box or cylinder sunk into place through soil or water and then filled with concrete to become a deep foundation. In India, large masonry or RCC open caissons used for bridge piers and abutments are called well foundations — the traditional choice for bridges over rivers with deep scour and alluvial beds (e.g. major Ganga and Brahmaputra bridges).

Why wells for bridges: large lateral and vertical loads, deep scour, need for a massive, rigid foundation below the maximum scour level; wells resist horizontal forces by soil passive resistance over a large embedded depth and are relatively easy to construct with local labour.

Types of caissons

Shapes of wells

Circular (most common — equal strength in all directions, easy sinking), double-D, twin circular, dumb-bell, rectangular and multi-dredge-hole shapes for large piers. The choice depends on the pier dimensions, flow direction and ease of sinking; circular and double-D wells are standard for Indian bridges.

Components of a well foundation

1. Cutting edge — steel edge at the bottom that cuts into the soil.
2. Well curb — tapered RCC ring above the cutting edge (inner slope commonly about 30°–37° to the vertical); transfers load and guides sinking.
3. Steining — the main wall of the well (masonry or RCC); provides weight to sink the well and resists earth and water pressures; thickness chosen to overcome skin friction and resist sinking stresses.
4. Bottom plug — concrete placed (usually under water, by tremie) at the base to seal the well and transfer load to the base soil.
5. Sand filling — inside the well above the bottom plug, to add weight and distribute load (sometimes left partly empty).
6. Top (intermediate) plug — concrete plug on top of the sand filling.
7. Well cap — RCC slab on top of the well supporting the pier.

Depth of well — scour and grip length

Scour depth

Lacey's regime depth of scour below the high flood level:

Grip length

The grip length is the depth of embedment below the maximum scour level that gives the well adequate lateral stability and bearing. Indian practice (IRC 78) requires the foundation level to be at least about one-third of the maximum depth of scour below the scour level (for wells in erodible strata), subject to stability calculations.