Foundation Engineering Report
Pulchowk Campus Area
This report has been prepared for the study of the subsurface conditions of the soil and to understand its features inside the Pulchowk Campus area.
Soil investigation is an essential phase in the design and construction of structural systems, as an entire site’s foundation relies on adequate subsoil conditions to safely transfer loads. Because exploring an entire site is cost-prohibitive, this investigation relies on targeted sampling to accurately estimate the site’s overall behavior.
To determine vital engineering parameters like soil strata thickness, groundwater condition, bearing capacity, shear parameters of soil, and expected settlement through a combination of field works and laboratory testing.
This investigation includes a preliminary phase (utilizing maps, ground reconnaissance, desk study) followed by a detailed exploration program specifically tailored to the project’s scale, structure type, and budget to ensure construction safety and efficiency. It also includes the engineering analysis of field and laboratory data to calculate the bearing capacity and soil properties.
The field work phase of the geotechnical investigation was conducted to directly observe the subsurface stratigraphy, determine in-situ soil properties, and obtain samples for laboratory evaluation.
Percussion boring is a geotechnical site investigation technique used to create boreholes in a wide range of ground conditions. The fundamental principle is to exceed the compressive strength of the ground material through the delivery of high-magnitude, repeated impacts from a heavy chisel bit dropped from a controlled height.
Mechanisms involved:
- Rock/Soil Fragmentation: The heavy chisel bit fractures the ground by crushing, chipping, and initializing tensile failure at the edge of the impact zone.
- Energy Transfer: Kinetic energy from the falling bit transmits through the cable and drill string to the bit-rock interface. The efficiency of this transfer governs the penetration rate.
- Bit-rock Interaction: Governed by energy transmission efficiency, rock hardness, and impact frequency. In soft/loose soils, fragmentation occurs readily.
- Rotation Between Strokes: The drill string is typically rotated slightly between drops to prevent the bit from impacting the same spot repeatedly, which optimizes fragmentation.
Types of Percussion Boring Systems:
| System | Mechanism | Depth Range | Typical Use |
|---|---|---|---|
| Cable Tool (Conventional) | Bit on cable drops from surface | Up to 40–50 m | Shallow water wells |
| Down The Hole (DTH) Hammer | Hammer mechanism at bit end | 30 m to 1000 m | Deep mineral / oil exploration |
| Top-Hammer (TH) | Surface hammer drives drill rod | 20–30 m | Mining / Construction |
At the site, the cable tool method was employed.
- Step 1 – Site Setup: A tripod rig was erected and stabilized over the borehole location.
- Step 2 – Drilling and Penetration: A chisel bit was repeatedly dropped to shatter the soil.
- Step 3 – Bailing and Casing Installation: A bailer was used and steel casing was installed in the upper loose soil to keep the hole open. The casing was advanced as needed based on ground conditions.
- Step 4 – Sampling and In-situ Testing: A soil sample was collected at a certain depth log, along with the Standard Penetration Test (SPT).
The Standard Penetration Test (SPT) is one of the most widely used in-situ geotechnical tests worldwide. It provides a rapid, cost-effective measure of the relative density of granular soils and the consistency of cohesive soils, and serves as the primary input for shallow foundation bearing capacity and pile capacity calculations. The test is governed by IS: 2131 (1981) and was also conducted in accordance with ASTM D1586 norms.
Equipment:
- Hammer: 63.5 kg (140 lbs), free fall through 760 mm (30 inches)
- Split-spoon Sampler (Split Barrel): 50 mm OD, 35 mm ID, 600 mm total length
- Drill rods connecting the sampler to the hammer system at the surface
- Cable/rope trigger system for consistent free-fall release
Test Procedure:
- Seating Drive (1st 150 mm): The split spoon is driven the first 150 mm to seat through any loose, disturbed material at the base of the borehole ($N_{1}$ blows).
- First Test Drive (2nd 150 mm): Blow count $N_{2}$ is recorded.
- Second Test Drive (3rd 150 mm): Blow count $N_{3}$ is recorded.
Observations
SPT Corrections
Overburden Pressure Correction ($C_{N}$): The SPT N-value is influenced by the effective overburden stress at the test depth. Higher stresses artificially inflate N values. The Liao and Whitman (1986) formula:
Dilatancy Correction [Terzaghi]: Very high N values may be caused by negative pore water pressure (dilation) during shearing rather than true density.
- If $N’ > 15$: $N” = 15 + 0.5(N’ – 15)$
- If $N’ \le 15$: $N” = N’$
SPT-Based Soil Classification:
| N-value | Relative Density | N-value (Cohesive) | Consistency | φ (deg) |
|---|---|---|---|---|
| 0–4 | Very loose | 0–2 | Very soft | < 30° |
| 4–10 | Loose | 2–4 | Soft | 30°–35° |
| 10–30 | Medium dense | 4–8 | Medium (firm) | 35°–40° |
| 30–50 | Dense | 8–15 | Stiff | 40°–45° |
| >50 | Very dense | >15 | Very stiff / Hard | > 45° |
Number of Blows
Overburden Pressure Correction ($C_{N}$):
- Depth of boring from ground level ($h$) = 6.598 m
- Weight of sample ($W$) = 164 gm
- Diameter of sample/sampler ($D$) = 35.5 mm
- Length of sample ($L$) = 8 cm
Dilatancy Correction: Since $N’ = 7 < 15$, $N'' = N' = 7$
Since the soil is cohesive clay: N-value = 7 (range 4–8), Consistency = Medium, $q_{u} = 50–100\ \text{kN/m}^{2}$
- Disturbed Sample: Natural structure partly or completely modified during sampling. Suitable for visual classification, grain size analysis, specific gravity, Atterberg limit testing, and chemical analysis.
- Undisturbed Sample: Natural structure and properties preserved during sampling. Used for consolidation, hydraulic conductivity, and shear strength tests.
A split spoon sampler was used during inspection, so a disturbed sample was achieved that restricts certain soil testing. All samples were transported to the laboratory in sealed plastic bags and tested within 7 days of collection.
- Wt of empty vessel: $W_{1} = 61.582\ \text{gm}$
- Wt of vessel + sample (dried): $W_{2} = 75.596\ \text{gm}$
- Wt of vessel + sample + water: $W_{3} = 171.000\ \text{gm}$
- Wt of vessel + water: $W_{4} = 162.201\ \text{gm}$
The clay was under the water table, so it was taken as 100% (1).
| Can No. | W₁ – Empty Can (gm) | W₂ – Can + Wet (gm) | W₃ – Can + Dry (gm) | Water Content w% | No. of Blows |
|---|---|---|---|---|---|
| 1 | 7.623 | 15.162 | 13.605 | 26.03 | 20 |
| 4 | 6.965 | 12.730 | 11.585 | 24.78 | 35 |
| 16 | 7.117 | 10.538 | 9.855 | 24.95 | 32 |
| 20 | 6.991 | 12.109 | 11.019 | 27.06 | 15 |
Liquid Limit = 25.61%
Flow Index ($I_{f}$) = $w_{2} – w_{1} = 28 – 22 = 6$
- Wt of moist soil ($W_{1}$) = 29.810 gm
- Wt of dry soil ($W_{2}$) = 21.790 gm
| W₁ – Can (gm) | W₂ – Wet soil + Can (gm) | W₃ – Dry soil + Can (gm) | PL (%) |
|---|---|---|---|
| 6.958 | 7.586 | 7.446 | 28.69 |
| 10.409 | 11.340 | 11.146 | 26.32 |
| 10.246 | 11.687 | 11.195 | 20.85 |
Plastic Limit = 20.85%
Soil was washed through a 75 μm sieve to remove fine particles and the retained soil was dried in an oven for sieve analysis.
| Sieve Size (mm) | Wt of Sample (gm) | % Wt Retained | Cumulative % Retained | % Finer |
|---|---|---|---|---|
| 1.700 | 415 | 24.4 | 24.4 | 75.6 |
| 0.425 | 330 | 19.9 | 44.3 | 55.7 |
| 0.106 | 370 | 21.8 | 66.1 | 33.9 |
| 0.075 | 325 | 19.1 | 85.2 | 14.8 |
| Pan | 260 | 14.8 | 100 | — |
- N-value = 7
- $q_{u} = 50–100\ \text{kN/m}^{2}$
- Undrained cohesion $c = q_{u}/2 = 25–50\ \text{kN/m}^{2}$
- For cohesive soils: $\phi = 0°$
From Terzaghi’s Bearing Capacity Theory with $\phi = 0°$: $N_{c} = 5.7$, $N_{q} = 1$, $N_{\gamma} = 0$
Upper boundary ($c = 50\ \text{kN/m}^{2}$): $q_{u} = 5.7(50) + 134.04 = \mathbf{419.04\ \text{kN/m}^{2}}$
Assumed pile data: Pile diameter $D = 0.5\ \text{m}$, Pile length $L = 6\ \text{m}$
End Bearing ($Q_{p}$) [for deep foundation $N_{c} = 9$]:
Skin Friction ($Q_{s}$) [for medium-stiff clay, $\alpha = 0.8$]:
Upper bound: $(Q_{p})_{upper} = 88.2\ \text{kN}$, $(Q_{s})_{upper} = 377\ \text{kN}$, $(Q_{u})_{upper} = \mathbf{465.2\ \text{kN}}$
For cohesive soils (IS 8009-parts): Total Settlement = Immediate Settlement + Consolidation Settlement
Immediate Settlement ($S_{i}$):
Primary Consolidation Settlement ($S_{c}$):
- $\sigma_{0}’ = 134.04\ \text{kN/m}^{2}$
- $e_{0} = \frac{w \cdot G_{s}}{S} = \frac{36.806 \times 2.69}{100} = 0.99$
- $C_{c} = 0.009(LL – 10) = 0.009(25.61 – 10) = 0.1405$
Percussion boring was carried out at the Pulchowk Campus site to a depth of 6.6 m using the cable tool (conventional) method. The field work was conducted in accordance with standard geotechnical investigation practices, and samples were sealed in plastic bags and tested within 7 days.
The SPT was conducted in accordance with IS: 2131 (1981) and ASTM D1586 norms. The observed N-value was 8 and corrected to 7. Based on the SPT soil classification, an N-value in the range 4–8 indicates medium consistency with $q_{u} = 50–100\ \text{kN/m}^{2}$ and $c = 25–50\ \text{kN/m}^{2}$, with $\phi = 0°$.
Laboratory test summary:
| Parameter | Symbol | Value |
|---|---|---|
| Specific Gravity | $G_{s}$ | 2.69 |
| Field Density | $\rho$ | 2.071 g/cc |
| Natural Water Content | $w$ | 36.81% [High] |
| Liquid Limit | $W_{L}$ | 25.61% |
| Plastic Limit | $W_{p}$ | 20.85% |
| Plasticity Index | $I_{p}$ | 4.76% [Low] |
| Degree of Saturation | $S$ | 100% |
| Initial Void Ratio | $e_{0}$ | 0.99 |
Wet sieve analysis shows 15.3% of the soil was retained in the pan, confirming a significant fine-grained fraction, consistent with a silty clay or clayey silt classification.
The natural water content (36.81%) exceeds the plastic limit (20.85%), placing the soil in a plastic state. The initial void ratio $e_{0} = 0.99$ confirms a loose, high void ratio structure.
Deep Foundation (0.5m dia × 6m pile): Capacity = 232.6 – 465.2 kN
Total Settlement: $S = 2.64\times10^{-5} q_{n}B + 0.0706\,H\,\log_{10}\!\left(\frac{134.04+\Delta\sigma}{134.04}\right)$
The site consists primarily of medium-consistency clay (silty clay). Key properties: Low to moderate strength with high void ratio and high natural water content.
⚠ Settlement Risk
The clay is highly prone to settling. To prevent the building from sinking, use pre-loading/surcharging and employ Prefabricated Vertical Drains (PVDs) to speed up water drainage from the clay.
⚠ Stability During Construction
Avoid piling up surcharge near open excavations, as the soft clay can suffer a sudden collapse. The site will likely need site improvement (such as preloading) or deep foundations for long-term stability.
To calculate the expected settlement, more information regarding the subsurface and project details (e.g., loads from the structure) will be required.
- Bhandari, S.K. and Bhatta, J. Introduction to Foundation Engineering. Kathmandu: Makalu Publication House.
- Arora, K.R. (2009). Soil Mechanics and Foundation Engineering. 7th ed. Delhi: Standard Publishers Distributors.
- IS Codes: IS 2131 (1981); IS Practice 8009-parts; ASTM D1586.
- Data from laboratory.
