Engineering Borehole Logs – Comprehensive Study Notes

Project Context & Methodology

The eight engineering borehole logs (BH-1 to BH-8) were drilled between 24 April 2025 and 18 May 2025 for the construction of Sekolah Menengah Kebangsaan Bandar Seri Putra (2), Hulu Langat, Selangor under JKR contract $\text{JKR.CKG.BST.CKP.800-8/1/511}$. All holes employed a YWE D-90R rotary-wash rig with $\text{NW}$ wireline casing. Core/sampling legends common to all logs are:

• P / SPT – Standard Penetration Test blow counts; expressed as $N = \frac{\text{blows}}{300\,\text{mm}}$ after seating blows.

• Rec (\% recovery) – ratio of recovered sample length to run length.

• D / UD / MZ / VS / C / W – disturbed, undisturbed, Mazier, vane shear, rock core and water samples respectively.

• Consistency descriptors for cohesive soils (Very Soft → Hard) and relative density descriptors for non-cohesive soils (Very Loose → Very Dense) follow JKR & BS 5930.

Drilling advanced in 1.5 m runs; every run ends with an SPT unless otherwise noted. Water levels were dipped repeatedly after completion; fluctuations illustrate delayed piezometric equalisation.

Borehole BH-1 (RL ${54.825\,\text{m}}$, finished depth ${12.285\,\text{m}}$)

• Static water table during drilling $\approx 9.90\,\text{m}$ below GL. Stabilised readings varied between $9.60$ and $11.20\,\text{m}$ over seven days.

• Entire profile consists of silts with intermittent sand lenses; colour grades from light orange at shallow depths to pale yellow/red deeper.

Significance: progressively increasing $N$ and low losses indicate a competent founding stratum below $6\,\text{m}$ for shallow foundations; however, plasticity shifts to high, warning of shrink/swell potential.

Borehole BH-2 (RL ${50.019\,\text{m}}$, finished depth ${7.648\,\text{m}}$)

• Water table shallow (initial $1.67\,\text{m}$; long-term $1.88\,\text{m}$). Expect perched or capillary rise affecting slab-on-grade designs.

• Strata uniformly hard sandy SILT with alternating pale grey, red & yellow hues; plasticity low–intermediate.

Depth-wise synopsis:

1. $0–1.50\,\text{m}$ Hard, low-plasticity: $N = 30 \rightarrow 50$.

2. $1.50–3.00\,\text{m}$ Hard, intermediate plasticity: $N \ge 40$.

3. $3.00–7.648\,\text{m}$ Sequence of hard sandy silts, colours pale red/yellow; $N$ values escalate to partial refusals (up to $N_{60}\approx 50$ by $7.5\,\text{m}$).

Implication: despite limited depth, strata are dense/hard enough for pad footing level at $\sim2–3\,\text{m}$ provided groundwater mitigation.

Borehole BH-3 (RL ${50.021\,\text{m}}$, finished depth ${15.155\,\text{m}}$)

Profile changes from firm gravelly CLAY/SILT near surface to hard silts at depth.

Key layers & observations:

• $0–1.50\,\text{m}$ Firm, very clayey gravel ($N=7$) – compressible.

• $1.50–4.50\,\text{m}$ Very stiff sandy silts, $N = 15–17$.

• $4.50–7.50\,\text{m}$ Hard silts, $N \approx 38–40$; signifying transition zone.

• $7.50–12.00\,\text{m}$ Hard–very hard silts of high plasticity, several partial refusals (blows per 60 mm $\rightarrow 50$).

• $12.00–15.155\,\text{m}$ Hard silts with greying at base; pronounced increase in $N$ (final test: $(25,32,18)$ → $N = 50$ after 180 mm) suggesting beginning of weathered bedrock or very dense residual soil.

Water table stabilised around $8.70–9.15\,\text{m}$, deeper than BH-2.

Borehole BH-4 (RL ${40.381\,\text{m}}$, finished depth ${13.785\,\text{m}}$)

Initial formation includes sandy CLAY lenses influencing lower recoveries (29–44 %).

Layer narrative:

1. $0–1.50\,\text{m}$ Firm sandy SILT, $N = 4$.

2. $1.50–3.00\,\text{m}$ Very stiff silty SAND ($N = 18$) + undisturbed clay sample UD1 at $2.25\,\text{m}$ (100 % recovery) – useful for triaxial testing.

3. $3.00–7.50\,\text{m}$ Very stiff to hard sandy CLAY / SILT, $N = 20–26$.

4. $7.50–13.785\,\text{m}$ Hard silts, increasingly high plasticity, frequent refusals; $N_{\text{max}}=50$.

Groundwater steady $\sim6.9–7.5\,\text{m}$.

Borehole BH-5 (RL ${46.480\,\text{m}}$, finished depth ${12.285\,\text{m}}$)

Closely parallels BH-3 but shallower.

Highlights:

• Near-surface ($0–1.50\,\text{m}$) firm silty soils ($N=6$).

• Rapid stiffening; by $3.0\,\text{m}$ $N=27$ and at $4.5–12\,\text{m}$ multiple hard silts with $N\ge30$ including three full refusals.

• Colour transition from light yellow to pale grey indicates oxidation gradient.

• Water table fluctuates $7.10–8.35\,\text{m}$.

Borehole BH-6 (RL ${46.487\,\text{m}}$, finished depth ${18.105\,\text{m}}$)

Deepest exploration; reveals thick very stiff to hard CLAY/SILT of very high plasticity down to $\sim10.5\,\text{m}$ followed by partially weathered sandy silts.

Chronological strata summary:

• $0–4.50\,\text{m}$ Very stiff clayey horizon, $N = 15–27$; lower recoveries (51–56 %) hint at fissuring.

• $4.50–9.00\,\text{m}$ Very stiff–hard silts, $N = 30$.

• $9.00–12.00\,\text{m}$ Hard high-plasticity silts, partial refusal ($N=50$ over <150 mm).

• $12.00–13.50\,\text{m}$ Pale red interbeds; a no-recovery zone at $13.5\,\text{m}$ suggests sand pocket or gravel seam; adjacent SPT P9 records $N=50$ after only 20 mm – near-refusal.

• $13.50–18.105\,\text{m}$ Hard sandy silts, three consecutive high-energy tests: $N = 50$ at $16.5$ & $18.0\,\text{m}$ illustrating potential founding stratum for piled solutions.

Water level stabilises at $9.00–9.60\,\text{m}$ (deeper than BH-5 despite similar RL due to drawdown during drilling).

Borehole BH-7 (RL ${56.270\,\text{m}}$, finished depth ${15.300\,\text{m}}$)

Soil sequence mirrors BH-1 but perched groundwater is higher (static $8.40–8.80\,\text{m}$). All materials are silts of varying plasticity with steadily increasing $N$.

Notable points:

• $N_0$ starts at 9 (stiff) then climbs to 34 by $6\,\text{m}$.

• Frequent refusals $N=50$ beyond $9\,\text{m}$.

• Overall recoveries good (58–97 %).

Borehole BH-8 (GL ${44.836\,\text{m}}$, finished depth ${15.265\,\text{m}}$)

Predominantly very stiff sandy silts with gravel in upper 6 m (low recoveries 29–44 % because of gravel) grading to hard silts below.

Sequence synopsis:

1. $0–1.50\,\text{m}$ Very stiff, $N=19$.

2. $1.50–7.50\,\text{m}$ Progressive stiffening, $N = 22–30$.

3. $7.50–15.265\,\text{m}$ Hard silts, partial refusals; final three tests yield $N_{60}=50$.

Water level ranges $7.60–8.92\,\text{m}$; perched conditions likely due to gravel seams.

Cross-Hole Comparison & Engineering Interpretation

1. Stratigraphic uniformity – All boreholes reveal a regional mantle of Quaternary/alluvial silt-dominant residual soils over weathered meta-sediments. CLAY pockets occur locally (BH-4, BH-6).

2. SPT trends – Sharp $N$ escalation at $\approx6–8\,\text{m}$ in every hole marks the transition from compressible near-surface soils to dense/hard substrata suitable for shallow or pile caps. Average refusal depth $\bar z_r \approx 9.5\,\text{m}$.

3. Plasticity grading – Upper soils vary from low to intermediate plasticity; deeper zones tend toward high plasticity which can shrink/swell. Foundation designs should incorporate $\Delta e$ predictions from Atterberg limits and suction tests.

4. Groundwater regime – Two distinct horizons:

   • A shallow water table $z_w \le 2\,\text{m}$ (BH-2) that threatens wet excavations.

   • A deeper piezometric surface $z_w \approx 8–10\,\text{m}$ prevalent elsewhere.
     Seasonal rise of $\pm0.3\,\text{m}$ captured in monitoring data.

5. Settlement outlook – Using Peck et al. correlations, soils with $N<15$ (upper 3 m, selected holes) may yield total settlement $s_t > 25\,\text{mm}$ under classroom column loads $\le 400\,\text{kN}$; hence raft slabs or excavation/replacement advised.

6. Bearing capacity indication – For hard silts ($N\ge30$) at $>5\,\text{m}$, Terzaghi’s equation gives ultimate capacity $q<em>u \ge 800\,\text{kPa}$; allowable $q</em>a$ with FS = 3 approximates $250\,\text{kPa}$, adequate for typical school structures.

7. Piling trigger – Where high plasticity and groundwater coincide (BH-1, BH-6, BH-7), driven or bored piles to $>12\,\text{m}$ may be more economical than subgrade improvement.

Practical & Ethical Considerations

• Health & Safety: High water tables demand safe excavation shoring. Rotary-wash returns must be treated to avoid turbidity runoff.

• Environmental: Excavated silt/clay spoils have low permeability; stockpile management prevents erosion.

• Quality Control: Lower recoveries in gravelly seams (BH-8) justify additional in-situ testing (CPTu) to reduce uncertainty.

• Educational Value: Logs illustrate the importance of correlating colour changes, plasticity and SPT N to infer depositional history—a vital lesson for engineering geology curricula.

Key Equations & Reference Values

1. SPT energy correction: $N<em>{60} = N \left(\frac{E</em>r}{60\%}\right)$ where $E_r$ is hammer energy ratio.

2. Terzaghi bearing: $q<em>u = cN</em>c + \gamma D<em>f N</em>q + 0.5 \gamma B N_\gamma$ (silts—treat as cohesive-frictional).

3. Immediate settlement of raft: $s<em>i = \frac{q</em>0 B (1-\nu^2)}{E<em>s} I</em>s$.

4. Empirical correlation (Peck et al.): $s<em>t \approx \frac{2q</em>0 B}{E<em>{50}}$ with $E</em>{50} \approx (2–5)N_{60}\,\text{MPa}$ for silts.

All numerical depths, RLs and water levels have been preserved from the logs; any omission should be cross-checked with original PDFs.