CELE Geotechnical Engineering — Foundations (Shallow and Deep)Summary

Shallow foundations rest on or just below the ground surface, typically within one footing width (B) of the surface. For isolated square or rectangular spread footings, the required area is calculated as A_req = P_service / q_a, where P_service is the factored or service load and q_a is the allowable bearing pressure (ABP) from soil investigation. The ABP is determined by bearing-capacity analysis (Terzaghi or Meyerhof equations with safety factors, typically FS = 2.5–3.0 on ultimate). Once A_req is known, check settlement (elastic and consolidation), especially in the soft clays prevalent in Philippine lowlands, and confirm that differential settlement remains within allowable limits (~25–50 mm, per NSCP 2015). A mat or raft foundation is used when individual footings would overlap (total area > 50% of building footprint) or when the soil is very weak, distributing load over a large area to reduce bearing pressure and settlement.

Piles (and drilled shafts) carry load through two mechanisms: (1) end bearing Q_p at the tip, resisting into firmer soil or rock, and (2) skin friction Q_s along the shaft, mobilized by relative movement between pile and soil. Ultimate capacity Q_u = Q_p + Q_s. For end bearing in clay, use Q_p = q_p × A_p where q_p = c_u × N_c* (N_c* ≈ 9 for deep piles, not the shallow value of 5.7). For skin friction, two empirical methods dominate: the α-method (clay) where f_s = α × c_u with α typically 0.5–1.0 depending on clay type and remolding, and the β-method (sand) where f_s = K × σ'_v × tan(δ) with K ≈ 0.5–1.0 and δ the pile-soil friction angle (~2/3 φ). Allowable capacity Q_a = Q_u / FS (FS = 2.5–3.0 typical in Philippine practice). This approach is universal in Southeast Asian engineering and directly referenced in NSCP 2015 Section 9 (Geotechnical Design).

In cohesive soils, the α-method assumes adhesion between pile and clay: f_s = α × c_u, where c_u is undrained shear strength (from laboratory triaxial tests or field vane shear) and α is an adhesion factor. The adhesion factor ranges from ~0.5 (for very stiff clays and long piles in which downdrag and/or stress relief reduce bond) to ~1.0 (for soft, normally consolidated clays with good interface contact). Total skin friction is Q_s = Σ(f_s × A_s) = α × c_u × π × D × L, where D is pile diameter and L is embedded length. In Philippine soft-clay regions (e.g., Bangkok-like clays in Metro Manila and coastal areas), α typically ranges 0.6–0.9 and must be justified by site-specific investigation. The α-method's simplicity and empirical support make it the default approach in NSCP-based practice.

In cohesionless soils, normal effective stress σ'_v and friction angle govern skin friction: f_s = K × σ'_v × tan(δ). The lateral-earth-pressure coefficient K varies with installation method (K ≈ 1.0–1.3 for driven piles due to soil densification; K ≈ 0.5–0.8 for bored piles with less soil disturbance). The pile-soil friction angle δ is typically estimated as δ ≈ (2/3 to 3/4) × φ', where φ' is the effective friction angle of the sand. Average σ'_v along the pile depth is used; greater depth increases Q_s due to higher overburden. In Philippine sandy sites (e.g., Quezon City soils, coastal Cebu), the β-method with site-calibrated K and φ' is essential. Overestimating K or δ leads to unsafe capacity estimates and is penalized in exams.

End bearing is the resistance mobilized at the pile tip. For clay, Q_p = q_p × A_p, where q_p is the bearing capacity at the tip and A_p = π D² / 4 is the tip area. The bearing-capacity equation for a deep pile in clay simplifies to q_p = c_u × N_c*, where N_c* ≈ 9 (not the shallow-footing value of 5.7) because the stress state, confinement, and shear-strain path at the pile tip differ from those in a shallow footing. Thus, Q_p = 9 × c_u × A_p. This value is conservative but widely accepted in Southeast Asia. If the pile tip is in sand or rock, different equations apply (e.g., q_p = σ'_v × N_q for sand). End bearing typically contributes 10–30% of total capacity in normally consolidated clays but becomes dominant in heavily overconsolidated clays or stiff soils.

Multiple piles acting together do not simply add their individual capacities. Overlapping stress zones and interaction between piles reduce the group's capacity. Group capacity is expressed as Q_group,ult = η × n × Q_single,ult, where η is the group efficiency (typically 0.70–0.90), n is the number of piles, and Q_single,ult is the ultimate capacity of a single pile. Efficiency depends on spacing: s / D > 3 generally yields η > 0.80; tighter spacing reduces η. In clay, also check block failure, where the group and surrounding soil act as a single large pier; if the block-failure capacity Q_block < η × n × Q_single, then Q_block governs and is often more conservative. For Philippine practice, typical group spacing is s = 2.5–3 D (center-to-center), and efficiency curves from Converse-Labarre or empirical methods (e.g., based on soil type) are used. The NSCP 2015 references the need to verify both individual-pile and group-failure modes.

Block failure occurs when the group of piles and the soil between them move as a single monolithic block, rather than each pile acting independently. The block is modeled as a large footing resting at the pile-group level, with dimensions slightly larger than the group's perimeter. The bearing capacity of this block is computed using standard footing equations: Q_block = A_block × q_p,block = A_block × (c_u × N_c + q' × N_q + ...). In clay, this simplifies to Q_block ≈ c_u × N_c × A_block. If Q_block < η × n × Q_single, the block failure controls and is the design capacity. Block failure is more likely in very closely spaced groups, short piles, or stiff clay. In Philippine soft-clay regions with consolidation settlement risk, block failure is often the governing mode and must always be checked; overlooking it leads to unsafe design.

Negative skin friction arises when the soil surrounding a pile settles more than the pile itself. As the soil moves downward relative to the pile, it exerts a downward (negative) drag force on the shaft, increasing the load on the pile rather than resisting it. This phenomenon is common in: (1) consolidating clay layers (e.g., reclaimed fill in Metro Manila, areas with ongoing land subsidence), (2) newly placed or compacted fills that continue to settle, and (3) sites with rising water tables that cause liquefaction or hydrocompaction. The downdrag force is modeled as an additional downward load Q_neg = f_neg × π × D × L_neg, where f_neg is the negative skin-friction stress (~0.5–1.0 × c_u in clay) and L_neg is the length of the consolidating zone. In design, Q_neg is added to the structural load (superstructure + pile weight), increasing the demand on the pile capacity. For piles in highly compressible soils or over old dump sites (common in Philippine reclaimed areas), negative skin friction can consume 20–50% of the pile's available capacity and must be explicitly calculated and accounted for in the design.

Allowable bearing pressure is the maximum load (per unit area) that can be safely applied to soil without inducing bearing failure or excessive settlement. It is derived from ultimate bearing capacity q_u (calculated using Terzaghi, Meyerhof, or Vesic equations) divided by a safety factor: q_a = q_u / FS, where FS = 2.5–3.0 for footings (2.5 typical for clays, 3.0 for sands due to higher uncertainty). ABP also accounts for settlement: elastic settlement is computed from soil modulus and footing dimensions; consolidation settlement is estimated from lab odometer tests and the effective-stress history (OCR). The NSCP 2015 (Section 9) provides ABP tables for common soil types but emphasizes site-specific investigation. In the Philippines, given variable soil conditions (from Holocene clays in Metro Manila to residual granites in Benguet), reliance on local ABP tables is dangerous; subsurface exploration with SPT/CPT, lab testing, and professional judgment are mandatory. The board exam often presents a simple ABP value and asks students to size a footing—recognizing this as a black-box input (even if physically derived elsewhere) is crucial.

After footing size is chosen from bearing capacity, settlement must be verified. Elastic settlement S_e ≈ (q_a × B / E_s) × I_s, where B is footing width, E_s is soil modulus (from lab tests or empirical correlations to SPT N-value), and I_s is an influence factor. Consolidation settlement S_c = Σ(m_v × Δσ' × Δh) occurs in compressible clays; m_v is the coefficient of volume compressibility (from odometer tests), Δσ' is the stress increase from the footing, and Δh is the layer thickness. Total settlement S_t = S_e + S_c; differential settlement (between adjacent footings) must be limited to ~L / 500 or ~25 mm to prevent structural distress (NSCP 2015). In Philippine soft-clay regions, consolidation settlement often dominates and can be large (100s of mm) if the clay layer is thick; mat foundations are preferred when S_c exceeds acceptable limits because they distribute load more evenly and reduce differential settlement. Careful site investigation, including consolidation testing, is essential.

A mat (or raft) is a thick, rigid concrete slab that covers the entire building footprint (or a large portion thereof) and distributes loads over a wide area. Use a mat when: (1) individual spread footings would overlap (total area > 50% of building footprint), (2) the soil is very weak and isolated footings would be uneconomical, or (3) differential settlement is a major concern and a rigid structure spreading load uniformly is advantageous. Advantages: reduced bearing pressure (distributed over large area A_mat), lower differential settlement (due to rigidity and load averaging), and robustness against local soil variability. Design involves computing the equivalent footing pressure q_mat = ΣP / A_mat, checking bearing capacity at this level, and then analyzing the mat as a thick beam/slab under the net uplift/downward pressure, accounting for 3-D load transfer. In the Philippines, mats are standard for medium-to-high-rise buildings on soft clays (e.g., Manila, Cebu) and reclaimed land; they are preferred for seismic resilience and to mitigate settlement-related damage.

The method used to install a pile affects soil disturbance, density, and the resulting capacity. Driven piles (impact hammer or vibratory) densify surrounding soil, increasing K and β values in sand but potentially remolding and softening clay (reducing α). Bored piles (auger, rotary drilling) disturb soil less, resulting in lower lateral stress and smaller K and β. Continuous-flight-auger (CFA) piles with simultaneous boring and concreting minimize disturbance. Jet-grouted and grouted piles increase α in clay by bonding and filling voids. In the Philippines, driven H-piles and bored piles (especially in sensitive Bangkok clays around Metro Manila) are common; CFA piles are increasingly used in urban areas with noise/vibration constraints. The NSCP and design standards expect engineers to account for installation method in estimating α and β; generic values are unsafe if the method is not appropriate to the soil. During the exam, if a problem specifies "driven pile" in clay, you might use α toward the lower end (e.g., 0.6–0.7); for bored in clay, a higher α (0.8–0.9) might be justified.

Once ABS is established (from bearing-capacity and FS), footing area A = P / q_a is determined. This size is then checked for settlement; if settlement exceeds limits, footing area must increase (reducing q_mat or q_footing) or type switched to mat.

Geotechnical site investigation yields undrained shear strength c_u (from triaxial, vane, or correlation with SPT) and friction φ' (from triaxial or lab testing). These parameters feed directly into Q_p and Q_s equations; poor investigation leads to unsafe or uneconomical designs.

Individual pile capacity Q_u is calculated first. Then, the number of piles n and spacing s/D determine efficiency η (from design charts or empirical formulas). Group capacity Q_group = η n Q_u is always less than n × Q_u due to stress-zone overlap. Block failure may further reduce effective capacity.

If clay (or fill) around the pile consolidates and settles faster than the pile itself, the soil surface drops relative to the pile shaft. This induced a downward drag force Q_neg that adds to external loads, reducing net safe capacity. Downdrag is especially significant in newly placed fills and reclaimed areas.

As building height and load increase, isolated footings become larger. When total footing area exceeds ~50% of building footprint, a mat becomes competitive or mandatory. A mat spreads the same total load over a larger area, reducing bearing pressure and differential settlement; trade-off is the cost and structural complexity of a thick, reinforced slab.

Driven piles densify sand but may remold clay, affecting the adhesion factor α. Bored piles minimize disturbance but have lower lateral stress. Engineers must justify their choice of α and β based on the expected installation method; generic textbook values are unsafe if method is not considered.

Ultimate (failure) capacity Q_u is calculated from soil mechanics (Q_p + Q_s). Allowable (design) capacity Q_a = Q_u / FS ensures safety margin. In design, external load must satisfy Load ≤ Q_a. Confusion between ultimate and allowable is a frequent exam error.

A 20-story residential tower in Metro Manila (typical Bangkok-like soft clay, c_u ≈ 60–80 kPa, φ' ≈ 25–28°) carries a total service load of 80 MN. Individual spread footings would be very large and uneconomical; a mat foundation is chosen. (1) Estimate ABS from bearing-capacity analysis with FS = 2.5, accounting for consolidation settlement via SPT-N and lab odometer tests; expect q_a ≈ 100–120 kPa for this soil. (2) Compute mat area A_mat = 80 MN / 0.12 MN/m² ≈ 667 m²; check that A_mat covers ~70% of footprint, confirming mat is appropriate. (3) Design mat slab thickness and reinforcement to handle differential pressure and 3-D load transfer (flexure and shear). (4) Estimate long-term consolidation settlement (300–500 mm typical); use mat's rigidity to minimize differential settlement. This is a classic Filipino high-rise foundation scenario.

A bridge crossing the Davao Gulf has piers on stiff clay (c_u ≈ 150 kPa, α ≈ 0.65) to 15 m depth, then sand (φ' ≈ 35°, K ≈ 0.9). Each pier carries 15 MN. A 2×2 group of 0.6 m dia, 25 m long piles is proposed (10 m in clay, 15 m in sand). (1) Compute end bearing at 25 m (sand): q_p,sand = σ'_v,sand × N_q ≈ 180 kPa × 35 ≈ 6300 kPa; Q_p ≈ 6300 × π/4 × 0.6² ≈ 1786 kN. (2) Skin friction: clay portion Q_s,clay = 0.65 × 150 × π × 0.6 × 10 ≈ 1840 kN; sand Q_s,sand = 0.9 × 180 × tan(35°) × π × 0.6 × 15 ≈ 3456 kN (using average σ'_v ≈ 90 kPa mid-sand). (3) Single-pile Q_u ≈ 1786 + 1840 + 3456 ≈ 7082 kN. (4) Group: η ≈ 0.80 (s/D = 3.0), so Q_group ≈ 0.80 × 4 × 7082 ≈ 22,662 kN; Q_a,group ≈ 22,662 / 2.5 ≈ 9065 kN, which exceeds 15 MN (OK). (5) Check settlement and confirm piles extend to sand to provide rigidity in seismic region. This illustrates mixed-strata design with two skin-friction methods and group efficiency.

A light-industrial warehouse on reclaimed fill in Manila Bay (recent dump, ongoing consolidation) carries 5 MN service load. Original fill (poorly compacted) is predicted to consolidate over 3–5 years, settling ~0.4 m relative to piles. (1) If shallow footings are used, differential settlement ≈ 0.4 m could crack the structure; piles are necessary. (2) Eight drilled piers, 0.5 m dia, 18 m long, reach stiff clay at 18 m. (3) Design must account for negative skin friction (downdrag): in the consolidating fill zone (upper 12 m), f_neg ≈ 0.5 × c_u,fill ≈ 0.5 × 40 ≈ 20 kPa; Q_neg = 20 × π × 0.5 × 12 ≈ 377 kN per pile. (4) Effective capacity of the group must accommodate this additional downward load, reducing the safe-load capacity by ~377 kN per pile or ~3016 kN total for 8 piles. (5) Design foundations for Q_structural + Q_neg, not just Q_structural. This is a real-world Philippine scenario (reclaimed areas in Manila, Cebu, Davao) where downdrag causes expensive foundation failures if overlooked.

A high-voltage transmission tower (light load, ~0.5 MN per leg) is founded on residual granitic soil with variable strength and rock outcrops. Soil profile: 3 m weathered granite (φ' ≈ 38°), then fresh granite (UCS ≈ 150 MPa). (1) Site investigation via drilling and RQD assessment determines shallow-rock level. (2) Shallow footing (1.5 m × 1.5 m) can be sized using ABS for the granitic soil or by bearing-capacity equations; q_a ≈ 300–400 kPa (residual is strong). (3) If rock is very close, a small pier or foundation anchored into rock may be cost-effective, avoiding the weight of a deep pile. (4) Seismic considerations: use higher FS (≈3.0) due to terrain variability and importance of tower stability. This problem showcases foundation selection in difficult terrain, common in Philippine upland regions (Cordillera, Mindanao highlands).