Geotechnical laboratory testing provides the measured soil properties that field tests can only estimate. Where the SPT gives a penetration resistance that must be correlated to friction angle, and the CPT classifies soil by mechanical behaviour rather than mineral composition, laboratory tests directly measure the fundamental parameters — grain size, plasticity, compaction characteristics, permeability, compressibility, and shear strength — that feed into the engineering calculations governing foundation design, slope stability, and earthworks specification.
This guide covers the principal soil mechanics laboratory tests used in geotechnical engineering practice: what each test measures, how the procedure works, which ASTM standard applies, how results are interpreted, and how specialist software automates calculations and professional report generation. The ten tests covered here correspond directly to those supported by DartiGeo and Dartis Soil Lab.
Selecting the right tests for your project #
Not every project requires every test. The appropriate test program depends on the soil types present, the engineering problem being solved, and the level of design detail required. The following table provides a general guide to which tests are typically specified for common geotechnical applications:
| Application | Tests typically required |
|---|---|
| Soil classification and description | Grain size distribution (sieve ± hydrometer), Atterberg limits, moisture content, specific gravity |
| Shallow foundation design | Classification tests + direct shear or triaxial (c’, φ’) for bearing capacity; consolidation for settlement in clay |
| Earthworks and compaction control | Proctor compaction (MDD and OMC), classification, moisture content |
| Settlement prediction in clay | 1-D consolidation (Cc, Cr, cv, σ’p), moisture content, Atterberg limits |
| Slope stability analysis | Direct shear or triaxial (c’, φ’), classification, moisture content |
| Dewatering and seepage assessment | Permeability (falling head for fines; constant head for sands), grain size distribution |
| Pavement subgrade assessment | Classification, Atterberg limits, Proctor compaction, CBR (not covered here) |
Classification tests — grain size distribution and Atterberg limits — are included in almost every site investigation program because they define the soil type and guide the selection of all subsequent tests and design approaches.
Grain size distribution — sieve and hydrometer analysis #
ASTM standards: D422 (particle size analysis), D7928 (hydrometer analysis)
Grain size distribution testing defines the proportions of different particle sizes present in a soil sample — the fundamental characteristic from which soil type, drainage behaviour, compaction properties, and susceptibility to liquefaction or frost action can be broadly inferred.
Sieve analysis (coarse fraction) #
A dried, disaggregated soil sample is passed through a nest of progressively finer sieves, from the coarsest opening at the top to the finest at the bottom. The mass of soil retained on each sieve is weighed and expressed as a percentage of the total sample mass. Standard sieve sizes used in geotechnical practice (ASTM) include: 75 mm, 50 mm, 37.5 mm, 25 mm, 19 mm, 12.5 mm, 9.5 mm, 4.75 mm (No. 4), 2 mm (No. 10), 0.85 mm (No. 20), 0.425 mm (No. 40), 0.25 mm (No. 60), 0.15 mm (No. 100), and 0.075 mm (No. 200).
The No. 200 sieve (0.075 mm) is the key boundary: material passing the No. 200 sieve is classified as fines (silt and clay). The percentage passing No. 200 is one of the primary USCS classification criteria.
Hydrometer analysis (fine fraction) #
Particles finer than 0.075 mm (silt and clay) cannot be separated by sieving — they would pass through the finest sieve as a group. Hydrometer analysis uses Stokes’ Law — which relates the settling velocity of a sphere in a fluid to its diameter — to determine the distribution of fine particle sizes. A soil-water suspension is prepared and a calibrated hydrometer measures the density of the suspension at timed intervals. As larger particles settle faster and the suspension becomes less dense, successive hydrometer readings allow the percentage of material finer than a given particle diameter to be calculated at each reading time.
Key output parameters #
| Parameter | Definition | Use in classification |
|---|---|---|
| D10 | Particle diameter at which 10% of the sample is finer | Effective size — controls permeability in sands |
| D30 | Particle diameter at which 30% of the sample is finer | Used in Cc calculation |
| D60 | Particle diameter at which 60% of the sample is finer | Used in Cu calculation |
| Cu | Coefficient of uniformity = D60 / D10 | USCS: GW requires Cu ≥ 4; SW requires Cu ≥ 6 |
| Cc | Coefficient of curvature = D30² / (D10 × D60) | USCS: well-graded soils require Cc between 1 and 3 |
| % passing No. 200 | Fines content by mass | Primary coarse/fine-grained boundary in USCS: >50% fines = fine-grained soil |
The output of grain size testing is the particle size distribution (PSD) curve — a semi-logarithmic plot of cumulative percentage passing against particle diameter. A steeply sloping curve indicates a well-graded soil; a nearly vertical section indicates a uniform (poorly-graded) grain size; a flat horizontal section (a gap) indicates a gap-graded soil.
Atterberg limits — liquid limit, plastic limit, and plasticity index #
ASTM standard: D4318
Atterberg limits define the moisture content boundaries at which a fine-grained soil transitions between different states of consistency — from liquid to plastic to semi-solid to solid. They are fundamental to the classification of clays and silts, and are widely used to infer engineering behaviour: swelling potential, compressibility, shrink-swell activity, and sensitivity to moisture change.
Liquid Limit (LL) #
The Liquid Limit is the moisture content (expressed as a percentage of dry soil mass) at which a soil transitions from a plastic to a liquid state. It is determined using either the Casagrande cup method (the traditional approach, in which a groove cut in a soil pat is closed by 25 blows of a standardised drop) or the fall cone method (more reproducible and preferred in European practice). The LL is the moisture content corresponding to 25 blows in the Casagrande method, determined from a flow curve plot of blow count against moisture content.
Plastic Limit (PL) #
The Plastic Limit is the moisture content at which a soil transitions from a plastic to a semi-solid state. It is determined by rolling a thread of soil between the palm and a glass plate: the PL is the moisture content at which the thread crumbles when rolled to 3 mm diameter. The test requires considerable operator skill and is more variable than the Liquid Limit test.
Plasticity Index (PI) #
PI = LL − PL
The Plasticity Index is the range of moisture content over which the soil behaves plastically. It is one of the most widely used single parameters in geotechnical engineering:
| PI value | Plasticity classification | Typical soil type |
|---|---|---|
| 0 (non-plastic) | Non-plastic | Silt (ML), fine sand |
| 1–7 | Low plasticity | Silt of low plasticity (ML), sandy clay (SC) |
| 7–17 | Medium plasticity | Clay of low plasticity (CL) |
| 17–35 | High plasticity | Clay of high plasticity (CH) |
| >35 | Very high plasticity | Fat clay, expansive clay (CH) |
Casagrande plasticity chart #
The Casagrande chart plots Plasticity Index (vertical axis) against Liquid Limit (horizontal axis). The A-line (PI = 0.73 × (LL − 20)) separates clays (above the A-line) from silts (below the A-line). The vertical line at LL = 50 separates low-plasticity soils (L suffix: ML, CL) from high-plasticity soils (H suffix: MH, CH). The chart is the definitive tool for USCS fine-grained soil classification when grain size analysis confirms more than 50% fines.
Specific gravity of soil solids #
ASTM standards: D854, C127
The specific gravity of soil solids (Gs) is the ratio of the mass of a given volume of soil solids to the mass of the same volume of water at 4°C. It is a fundamental parameter required to compute void ratio, degree of saturation, and unit weight relationships from moisture content and dry density measurements.
Gs is determined by the pycnometer method: a known mass of dry soil is placed in a calibrated pycnometer (volumetric flask), water is added and air evacuated by boiling or vacuum, and the mass of the flask filled with soil and water is measured. The difference in mass between the pycnometer full of water alone and full of water plus soil gives the volume of the soil solids.
| Soil type | Typical Gs range |
|---|---|
| Quartz sand | 2.65 |
| Most inorganic soils (clays and silts) | 2.60–2.80 |
| Calcareous soils | 2.70–2.85 |
| Organic soils | 1.30–2.50 (lower with higher organic content) |
| Peat | <1.50 |
For routine projects on inorganic soils, Gs = 2.65–2.70 is commonly assumed without testing. Testing is important for organic soils, calcareous soils, and any case where degree of saturation or accurate void ratio calculation is required.
Moisture content #
ASTM standard: D2216
The moisture content (or water content) w is the ratio of the mass of water in a soil specimen to the mass of dry soil solids, expressed as a percentage:
w (%) = (Mwater / Mdry) × 100 = [(Mwet − Mdry) / Mdry] × 100
The test procedure is straightforward: weigh the wet soil sample, dry it in an oven at 105°C for a minimum of 16 hours (or to constant mass), and reweigh. The difference in mass is the mass of water evaporated.
Moisture content is the most frequently performed soil test in any geotechnical laboratory. It is required as a standalone test to characterise the in-situ condition of samples retrieved from boreholes and as a supporting measurement in nearly every other laboratory test (compaction, consolidation, Atterberg limits, shear strength). Natural moisture content, when compared to the Liquid and Plastic Limits, also gives an immediate qualitative indication of the consistency of a fine-grained soil without further testing.
Compaction test — Standard and Modified Proctor #
ASTM standards: D698 (Standard Proctor), D1557 (Modified Proctor)
The Proctor compaction test determines the relationship between moisture content and dry density for a soil compacted at a specified compaction energy. It defines two key parameters used in earthworks specification and quality control:
- Maximum Dry Density (MDD) — the highest dry density achievable at the specified compaction energy
- Optimum Moisture Content (OMC) — the moisture content at which MDD is achieved
Standard Proctor vs Modified Proctor #
| Parameter | Standard Proctor (ASTM D698) | Modified Proctor (ASTM D1557) |
|---|---|---|
| Compaction energy | 600 kN·m/m³ (12,375 ft·lbf/ft³) | 2,700 kN·m/m³ (56,250 ft·lbf/ft³) |
| Hammer mass | 2.5 kg (5.5 lb) | 4.5 kg (10 lb) |
| Drop height | 305 mm (12 in) | 457 mm (18 in) |
| Number of layers | 3 | 5 |
| Blows per layer | 25 (standard mould) | 25 (standard mould) |
| Typical application | General earthworks, embankments, backfill | Highway subgrades, airfield pavements, heavily trafficked fills |
Test procedure #
Soil at a range of moisture contents (typically five to six specimens, bracketing the expected OMC) is compacted into a standard cylindrical mould using the specified hammer and drop height. The dry density of each compacted specimen is calculated from the wet density and moisture content. The dry density values are plotted against moisture content to produce the compaction curve — a characteristic curve with a distinct peak at the OMC.
Zero Air Voids (ZAV) line #
The ZAV line represents the theoretical maximum dry density at each moisture content if all air voids were eliminated (fully saturated soil). No compacted soil can plot above the ZAV line. The compaction curve always falls to the left of and below the ZAV line. The ZAV line is calculated as:
ρd,ZAV = Gs × ρw / (1 + w × Gs)
Plotting the ZAV line alongside the compaction curve is standard practice in reporting — it confirms the test data is internally consistent and shows the degree of saturation at and around the OMC.
Field application #
Earthworks specifications typically require compacted fill to achieve a minimum dry density expressed as a percentage of MDD — for example, “≥95% Standard Proctor MDD”. Field density testing (sand replacement method or nuclear density gauge) is used during construction to verify compliance.
Permeability test — constant head and falling head #
ASTM standards: D2434 (constant head), D5856 (falling head)
The permeability (or hydraulic conductivity) k of a soil determines how rapidly water flows through it under a hydraulic gradient. It is a fundamental parameter for seepage analysis, dewatering design, drainage system specification, and contaminant transport assessment. The appropriate test method depends on the expected permeability of the soil.
Constant head permeability test (coarse-grained soils) #
The constant head test is used for coarse-grained soils (sands and gravels) where permeability is high enough that steady-state flow through the specimen is achievable in a practical test duration. A constant water head difference (h) is maintained across a specimen of known length (L) and cross-sectional area (A). Once steady-state flow is established, the volume of water (Q) collected over a time period (t) is measured. Darcy’s Law gives:
k = (Q × L) / (A × h × t)
Falling head permeability test (fine-grained soils) #
For fine-grained soils (silts and clays) where permeability is low, the constant head method is impractical because flow rates are too small to measure accurately. The falling head test tracks the drop in water level in a standpipe connected to the soil specimen over time, from an initial head h1 to a final head h2. Permeability is calculated as:
k = (a × L) / (A × t) × ln(h1 / h2)
where a is the internal cross-sectional area of the standpipe.
Typical permeability values #
| Soil type | Typical k range (m/s) | Drainage classification |
|---|---|---|
| Clean gravel | 10⁻² to 10⁻¹ | Free-draining |
| Clean sand | 10⁻⁵ to 10⁻³ | Free-draining |
| Silty sand / Sandy silt | 10⁻⁷ to 10⁻⁵ | Poorly draining |
| Silt | 10⁻⁸ to 10⁻⁶ | Poorly draining |
| Clay | 10⁻¹⁰ to 10⁻⁸ | Effectively impervious |
1-D consolidation test #
ASTM standard: D2435
The one-dimensional consolidation test (oedometer test) measures the compressibility and time-rate of consolidation of a saturated fine-grained soil under incremental loading. It is the primary test for predicting primary consolidation settlement and its time dependence for structures founded on or within clay strata.
Test procedure #
An undisturbed cylindrical specimen (typically 63–75 mm diameter, 20 mm thick) is placed in the oedometer ring — a rigid metal ring that prevents lateral strain, enforcing one-dimensional compression — between two porous stones to allow drainage from both faces. Vertical loads are applied in increments (typically doubling: 12.5, 25, 50, 100, 200, 400, 800 kPa), and vertical deformation is measured with time at each load increment, typically over a 24-hour period. After the maximum load is reached, the specimen is unloaded in stages to assess rebound (swelling) behaviour.
Key output parameters #
| Parameter | Symbol | Definition and use |
|---|---|---|
| Compression index | Cc | Slope of the e-log σ’v curve in the normally consolidated range. Used to calculate primary consolidation settlement in NC clay: Δe = Cc × log(σ’f/σ’0). |
| Recompression index | Cr (or Cs) | Slope of the e-log σ’v curve in the overconsolidated range. Typically Cr ≈ Cc/5 to Cc/10. Used for settlement within the preconsolidated stress range. |
| Preconsolidation pressure | σ’p | The maximum past effective vertical stress experienced by the soil, determined by the Casagrande graphical construction on the e-log σ’v curve. Defines the boundary between OC and NC behaviour. |
| Overconsolidation ratio | OCR = σ’p / σ’v0 | Ratio of preconsolidation pressure to current effective vertical stress. OCR = 1 indicates normally consolidated; OCR > 1 indicates overconsolidated (has been subjected to higher stress in the past — e.g. by glaciation, erosion, or preloading). |
| Coefficient of consolidation | cv | Governs the time rate of consolidation. Determined from the deformation-time curve at each load increment using Casagrande’s log-time method or Taylor’s square-root-of-time method. Used with drainage path length to predict consolidation time: t90 = T90 × Hdr² / cv. |
| Coefficient of volume compressibility | mv | Volumetric strain per unit increase in effective stress. Used in direct settlement calculations and to derive cv from permeability: cv = k / (mv × γw). |
Direct shear test #
ASTM standard: D3080
The direct shear test determines the shear strength parameters of a soil — the effective cohesion intercept (c’) and the effective friction angle (φ’) — which are the primary inputs for bearing capacity calculations, slope stability analysis, and retaining wall design.
Test procedure #
A square or circular soil specimen is placed in a split shear box. A constant normal stress (σn) is applied vertically. The lower half of the box is then displaced horizontally at a controlled rate while the upper half is restrained, forcing failure along the horizontal plane between the two box halves. Shear stress (τ) is measured as horizontal displacement increases, until the peak (or residual) shear stress is reached.
A minimum of three specimens from the same sample are tested at three different normal stresses. The peak shear stress at each normal stress is plotted on a τ-σn diagram. The best-fit straight line through the three data points defines the Mohr-Coulomb failure envelope:
τf = c’ + σ’n × tan(φ’)
The intercept of the line is c’ (effective cohesion) and the slope is tan(φ’) (effective friction angle).
Drainage considerations #
The direct shear test does not control or measure pore water pressure. Drainage conditions during shearing depend on the displacement rate and the permeability of the soil. In practice:
- For coarse-grained soils (sands, gravels): any practical shear rate gives effectively drained conditions → results directly give c’ and φ’.
- For fine-grained soils (silts, clays): a very slow displacement rate is required to allow pore pressure dissipation and obtain drained effective strength parameters. Undrained shear strength testing of clays is better performed using the unconfined compression test or triaxial test.
Unconfined compression test #
ASTM standard: D2166
The unconfined compression test (UC test, also called UCS test) measures the unconfined compressive strength (qu) of a saturated cohesive soil — typically a stiff to hard clay or a cemented soil — under rapid axial loading with zero confining pressure. It is a quick, inexpensive alternative to triaxial testing for estimating undrained shear strength.
Procedure #
A cylindrical soil specimen (height-to-diameter ratio of 2:1, typically 38 mm × 76 mm or 50 mm × 100 mm) is placed between the platens of a load frame with no lateral support or confinement. An axial compressive load is applied at a controlled strain rate (typically 0.5–2.0% axial strain per minute) and both load and deformation are recorded until failure. Failure occurs when the axial load reaches a maximum, or when axial strain reaches 15%.
Undrained shear strength from UCS #
Under the assumption of a total stress Mohr’s circle with zero confining pressure (σ3 = 0), the undrained shear strength is:
su = qu / 2
The UCS test is classified as an undrained test because the rapid loading rate prevents significant drainage. However, since no confining pressure is applied, the test is not suitable for soft clays (which may deform under their own weight) or for soils with fissures (which can fail along fissures rather than through the soil mass, giving an anomalously low qu). For these cases, triaxial testing under appropriate confining pressure is preferred.
USCS and AASHTO soil classification #
Laboratory test results — primarily grain size distribution and Atterberg limits — are combined to formally classify soils according to standardised classification systems. Two systems are in routine use in geotechnical engineering:
Unified Soil Classification System (USCS) — ASTM D2487 #
The USCS is the international standard for geotechnical engineering classification. It divides soils into coarse-grained (more than 50% retained on the No. 200 sieve) and fine-grained (more than 50% passing No. 200) categories. Coarse-grained soils are further classified by gravel/sand content and by gradation (Cu and Cc). Fine-grained soils are classified using the Casagrande plasticity chart. The result is a two-letter group symbol (e.g. CL, SP, GM) and a group name.
AASHTO Classification System — AASHTO M 145 #
The AASHTO system classifies soils and soil-aggregate mixtures from A-1 (best subgrade material: well-graded gravel or stone fragments) to A-7 (worst: plastic clay). Fine-grained soils above the A-line on the Casagrande chart are classified as A-6 or A-7-6 (plastic clay); those below as A-4 or A-5 (silts). The Group Index (GI) is a numerical value computed from fines content, liquid limit, and plasticity index that further refines the classification within each group. The AASHTO system is predominantly used in highway and pavement engineering.
Both classification systems are automatically determined by DartiGeo and Dartis Soil Lab once grain size distribution and Atterberg limits data are entered.
How DartiGeo and Dartis Soil Lab handle laboratory testing #
DartisTech offers geotechnical laboratory test processing in two products: as the Laboratory Tests module within the comprehensive DartiGeo platform (which integrates field testing, laboratory testing, and foundation design in a single workflow), and as the standalone Dartis Soil Lab application for laboratories focused exclusively on soil mechanics test processing and reporting.
Both products support all ten test types covered in this guide and are built around ASTM standards throughout:
| Test | ASTM standard | Supported in DartiGeo | Supported in Dartis Soil Lab |
|---|---|---|---|
| Sieve analysis | D422 | ✔ | ✔ |
| Hydrometer analysis | D7928 | ✔ | ✔ |
| Atterberg limits (LL & PL) | D4318 | ✔ | ✔ |
| Moisture content | D2216 | ✔ | ✔ |
| Specific gravity | D854 / C127 | — | ✔ |
| Proctor compaction (Standard & Modified) | D698 / D1557 | ✔ | ✔ |
| Permeability (constant head & falling head) | D2434 / D5856 | ✔ | ✔ |
| 1-D consolidation | D2435 | — | ✔ |
| Direct shear | D3080 | — | ✔ |
| Unconfined compression | D2166 | — | ✔ |
After data entry, both products automatically perform all calculations (Cu, Cc, PI, MDD, OMC, k, Cc, cv, qu, and all derived parameters) and assign USCS and AASHTO classifications without manual computation. Professional reports — grain size distribution curves, compaction curves, consolidation plots, direct shear envelopes, and summary tables — are generated immediately and exported as PDF, Word, or Excel.
In DartiGeo, soil classification data from laboratory tests is linked directly to borehole layer descriptions, keeping the borehole log and laboratory results in a single consistent project file. There is no re-entry of data between modules.
Dartis Soil Lab additionally includes integrated borehole and sample management — assigning test results to specific boreholes and depth intervals — making it suitable for laboratories managing large multi-borehole investigation programmes.
Frequently asked questions #
What is the most common geotechnical laboratory test? #
Moisture content determination is the most frequently performed individual test — it is quick, inexpensive, and required by virtually every other test as a supporting measurement. Among classification tests, grain size distribution (sieve analysis) and Atterberg limits are included in almost every site investigation programme because they define soil type and govern the selection of all subsequent testing and design approaches.
What is OMC in soil testing? #
OMC stands for Optimum Moisture Content — the water content (expressed as a percentage of dry soil mass) at which a soil achieves its Maximum Dry Density (MDD) under a specified compaction energy. It is determined from the Proctor compaction test. Compacting a soil at a moisture content significantly below the OMC produces a dry, crumbly fill with lower density; compacting above the OMC produces a saturated fill with increasing air voids that decrease density. Earthworks specifications typically require field compaction to be performed within ±2% of laboratory OMC to achieve the specified percentage of MDD.
What does the plasticity index tell you about a soil? #
The Plasticity Index (PI = LL − PL) defines the range of moisture content over which a soil behaves plastically. A high PI indicates a soil that is highly plastic — it retains workable plastic behaviour over a wide moisture range and is generally more compressible, more prone to swelling and shrinkage with moisture changes, and more sensitive to moisture fluctuations in service. A PI of zero means the soil is non-plastic (a silt or fine sand) — it has no plastic behaviour at any moisture content. PI also feeds into the USCS classification (CL vs CH), the AASHTO Group Index, and several empirical correlations for compressibility and undrained shear strength.
What is the difference between the Standard and Modified Proctor test? #
The Standard Proctor test (ASTM D698) uses a compaction energy of approximately 600 kN·m/m³, representative of light to medium earthworks equipment. The Modified Proctor test (ASTM D1557) uses approximately 4.5 times more energy (2,700 kN·m/m³), representative of heavy compaction equipment used in highway subgrades and airfield pavements. For the same soil, the Modified Proctor produces a higher MDD and a lower OMC than the Standard Proctor. Specifying the wrong test standard for quality control — particularly using Standard Proctor targets for a highway pavement subgrade that will be compacted with heavy equipment — leads to incorrect quality assurance thresholds.
Why is the consolidation test important for clay foundations? #
The one-dimensional consolidation test is the only practical laboratory test that directly measures the compressibility parameters (Cc, Cr) and the time-rate of settlement (cv) of a clay. Without these parameters, primary consolidation settlement cannot be predicted reliably — which matters enormously for structures founded on or within soft to medium clay, where settlements of 100–500 mm or more are possible and may develop over decades. The test also determines the preconsolidation pressure (σ’p) and OCR, which defines whether the clay is normally consolidated (and therefore will undergo large primary settlement) or overconsolidated (and therefore relatively stiff).
What is the difference between a direct shear test and a triaxial test? #
Both tests measure the shear strength parameters c’ and φ’, but they differ in control and applicability. The direct shear test is simpler and faster — failure is forced along a predetermined horizontal plane and drainage is uncontrolled. It gives reliable results for drained conditions in sands and gravels, and for drained effective parameters in clays tested at very slow rates. The triaxial test provides better drainage control (undrained UU, consolidated undrained CU, or consolidated drained CD tests are all possible), applies confining pressure that more closely simulates in-situ stress conditions, and allows pore pressure measurement in CU tests to derive both total and effective stress parameters. For critical projects involving soft clays, sensitive soils, or situations where both undrained and drained parameters are needed, triaxial testing is the preferred approach.
Related documentation #
- Grain size distribution — sieve and hydrometer analysis explained
- Atterberg limits — liquid limit, plastic limit and plasticity index
- Proctor compaction test — Standard and Modified Proctor explained
- Soil permeability test — constant head and falling head methods
- Consolidation oedometer test — Cc, Cv, and settlement prediction
- Direct shear test — procedure and interpretation of c’ and φ’
- Unconfined compression test (UCS) for cohesive soils
- Soil moisture content test — ASTM D2216 procedure and results
- Which geotechnical lab tests to specify for your project
- Borehole logging — complete guide
- Standard Penetration Test (SPT) — complete guide
- Foundation design — bearing capacity and settlement guide
