Calculating and Correcting for Overburden Pressure in Soil Testing

Understanding Overburden Pressure in Soil Mechanics

Overburden pressure, also known as overburden stress, lithostatic pressure, or vertical stress, is the pressure caused by the weight of the overlying layers of material at a specific depth under the earth’s surface. This fundamental concept in geotechnical engineering and soil mechanics plays a critical role in understanding soil behavior, interpreting test results, and designing safe foundations and earthworks. The influence of this weight, known generally as overburden, causes a state of stress to exist, which is unique at that depth, for that soil. This state of stress is commonly referred to as the overburden or in-situ or geostatic state of stress.

Lithostatic pressure increases with depth. The magnitude of overburden pressure at any given point depends primarily on two factors: the depth below the ground surface and the unit weight (density) of the soil and materials above that point. Understanding how to accurately calculate and correct for overburden pressure is essential for obtaining reliable measurements of soil properties and making informed engineering decisions.

Overburden pressure influences numerous aspects of soil behavior, including consolidation characteristics, shear strength, compressibility, and the results obtained from various in-situ testing methods. Effective overburden stress can have a significant influence on cone penetration test CPT measurements. This influence can lead to an incorrect assessment of soil strength/resistance for such purposes as liquefaction triggering analysis. Therefore, proper calculation and correction procedures are fundamental to geotechnical practice.

The Fundamental Concept: Total Stress, Pore Pressure, and Effective Stress

To fully understand overburden pressure, it is essential to distinguish between three related but distinct concepts: total stress, pore water pressure, and effective stress. This distinction forms the foundation of modern soil mechanics and was formalized by Karl Terzaghi in his principle of effective stress.

Total Stress (Overburden Pressure)

Total stress is the total weight per unit area at a layer. Total pressure = unit weight x depth to consideration (adjust unit weight and thicknesses to stratigraphy). The total vertical stress at any depth represents the cumulative weight of all materials—soil, water, and any surface loads—above that point.

For a simple case with uniform soil conditions, the total vertical stress (σ_v) at depth h can be calculated as:

σ_v = γ × h

where γ is the unit weight of the soil and h is the depth below the surface. Thus, the total stress varies linearly with depth.

In stratified soil profiles with multiple layers of different materials, the calculation becomes a summation process. This means that for each soil layer, you multiply the density of the layer by its height, then add all the resulting weights together until the pressure at the desired depth is known.

Pore Water Pressure

Pore water pressure (sometimes abbreviated to pwp) refers to the pressure of groundwater held within a soil or rock, in gaps between particles (pores). The pressure of water in the pores of the soil is called pore water pressure (u).

The magnitude of pore water pressure depends on: the depth below the water table, the conditions of seepage flow. Under hydrostatic conditions, no water flow takes place, and the pore pressure at a given point is given by u = ɣw.h where ɣ_w is the unit weight of water and h is the depth below the water table.

Pore water pressure is ZERO above the water table and starts to have a value only BELOW the water table line. Below the water table, pore pressure is positive; at the water table, pore pressure is zero. However, in the unsaturated (“vadose”) zone, the pore pressure is determined by capillarity and is also referred to as tension, suction, or matric pressure.

Effective Stress: The Key to Soil Behavior

Effective stress is a fundamental concept in soil mechanics and geotechnical engineering that describes the portion of total stress in a soil mass that is carried by the solid soil skeleton, rather than the pore water. It is crucial for understanding the mechanical behaviour of porous media, as effective stress governs both their strength and volume change (deformation).

The principle of effective stress is fundamentally important in soil mechanics. It must be treated as the basic axiom, since soil behaviour is governed by it. The relationship between these three stress components is expressed by Terzaghi’s effective stress equation:

σ’ = σ – u

where σ’ is the effective stress, σ is the total stress, and u is the pore water pressure. Effective stress cannot be calculated directly but it can be calculated indirectly from total stress and pore water pressure i.e., by subtracting pore water pressure from total stress.

Effective stress is a measure of how much load a soil can carry. The strength and compressibility of the soil depend on the stresses within the solid granular fabric. These are called effective stresses. This is why effective stress, rather than total stress, controls soil behavior including shear strength, consolidation, and volume change.

Calculating Overburden Pressure: Methods and Formulas

Accurate calculation of overburden pressure requires careful consideration of soil stratigraphy, unit weights, groundwater conditions, and any surface loads. The calculation process varies in complexity depending on site conditions.

Single Layer with Uniform Soil

For the simplest case of a single, uniform soil layer, the total vertical stress at depth h is calculated using the basic formula:

σ_v = γ × h

where:

• σ_v = vertical total stress (overburden pressure) in kPa or lb/ft²
• γ = unit weight of soil in kN/m³ or lb/ft³
• h = depth below ground surface in meters or feet

The unit weight of soil varies depending on soil type, moisture content, and degree of compaction. Typical values range from approximately 16-22 kN/m³ (100-140 lb/ft³) for most soils, with saturated soils generally having higher unit weights than dry or partially saturated soils.

Multi-Layered Soil Profiles

Most real-world soil profiles consist of multiple layers with different properties. In such cases, the total stress at a given depth is calculated by summing the contributions from each layer above that point:

σ_v = Σ(γ_i × h_i)

where γ_i and h_i are the unit weight and thickness of each individual layer i. This summation accounts for variations in soil type, density, and moisture content throughout the profile.

Accounting for Groundwater

When groundwater is present, the calculation must distinguish between total stress and effective stress. Above the water table, soil unit weight is typically the moist or bulk unit weight. Below the water table, two approaches can be used:

Method 1: Calculate total stress using saturated unit weight, then subtract pore pressure

• Total stress: σ_v = (γ_moist × h₁) + (γ_sat × h₂)
• Pore pressure: u = γ_w × h_w
• Effective stress: σ’ = σ_v – u

Method 2: Use buoyant unit weight below the water table

• Effective stress: σ’ = (γ_moist × h₁) + (γ’ × h₂)
• where γ’ = γ_sat – γ_w (buoyant or submerged unit weight)

Both methods yield the same effective stress, which is the stress component that controls soil behavior. It is important to understand that the total stress and effective stress are EQUAL above the water table and that the effective stress is less than the effective stress below the water table (again, in most cases).

Including Surface Loads

When surface loads such as foundations, embankments, or stored materials are present, they contribute additional stress to the soil profile. For uniform, extensive surface loads (surcharge), the additional stress is simply added to the overburden pressure:

σ_v = q + γ × h

where q is the uniform surface load. For narrow surcharges, e.g. under strip and pad foundations, the induced vertical total stresses will decrease both with depth and horizontal distance from the load. In such cases, it is necessary to use a suitable stress distribution theory – an example is Boussinesq’s theory.

Determining Soil Unit Weight: Laboratory and Field Methods

Accurate determination of soil unit weight is crucial for precise overburden pressure calculations. The unit weight of soil (γ) is usually determined in the laboratory by preparing a representative soil sample and measuring its weight and volume. Several standardized methods are available for both laboratory and field determination.

Laboratory Methods

These test methods describe two ways of determining the total/moist/bulk density, dry density, and dry unit weight of intact, disturbed, remolded, and reconstituted (compacted) soil specimens. Common laboratory methods include:

Water Displacement Method: This method is suitable for cohesive soils. In this method sample of soil is trimmed into more or less uniform shape and weight(W1). Sample is the coated with paraffin wax (to prevent the entry of water into the soil) and is again weight (W2). The coated specimen of soil is immersed slowly in a container, that is completely filled with water and volume of water displaced by coated specimen is Vw. The unit weight is then calculated from the measured weight and volume.

Core Cutter Method: This method is suitable to be used for cohesive soils. This method is generally not suitable for gravel, sand and dry soil. It is generally used for soft silt and clay. Core cutter is a cylindrical tube type container which is opened from both top and bottom side. It is sharp at one side so that it can be plunged into ground to take out soil sample. The volume of the core cutter is known, and the weight of soil extracted is measured to calculate unit weight.

Direct Measurement Method: The dimensions and mass of a specimen are measured. This method is commonly used for cylindrical specimens obtained from sampling tubes or prepared in the laboratory for triaxial or consolidation testing.

Field Methods

In practice, bulk unit weight is often determined in the field using the sand replacement or nuclear density gauge, while the dry and saturated unit weights are usually derived from laboratory tests combined with moisture content measurements.

Sand Replacement Method: Since core cutter method in not suitable for hard & gravel soil, sand replacement method is used in this case. A small pit is excavated and the excavated soil sample is weighted. A calibrated cylinder containing sand is placed over the excavated pit and is filled with sand. volume of the pit is obtained from the calibrated cylinder.

Nuclear Density Gauge: This rapid, non-destructive method uses radioactive sources to measure soil density and moisture content in place. While convenient for quality control during construction, it requires calibration and trained operators.

Density is a key element in the phase relations, phase relationships, or mass-volume relationships of soil and rock. When particle density, that is, specific gravity is also known, dry density can be used to calculate porosity and void ratio. Dry density measurements are also useful for determining degree of soil compaction.

Overburden Correction in Cone Penetration Testing (CPT)

Cone Penetration Testing (CPT) is one of the most widely used in-situ soil investigation methods. However, CPT measurements are significantly influenced by overburden stress, necessitating normalization procedures to obtain meaningful soil parameters.

Why Overburden Correction is Necessary

Overburden stress can cause errors or drift in CPT measurements, creating the need for correction factors in deeper tests depths and soft or fine-grained soils. Since both the penetration resistance (qc) and sleeve resistance (fs) increase with depth due to the increase in effective overburden stress, the CPT data requires normalization for overburden stress to remove the influence of depth.

CPT measurements of tip resistance, sleeve friction and pore pressure tend to increase along with increasing depth and increasing overburden stress. Without normalization, comparing CPT results from different depths or different sites becomes problematic, as the measured values reflect both the inherent soil properties and the confining stress effects.

Normalization Procedures

For an accurate measurement of tip and sleeve resistance, unbiased by overburden stress, it is essential to normalize these index measurements appropriately. Presented herein is a comprehensive study reviewing all aspects of CPT normalization.

The normalized cone resistance is typically calculated using an overburden correction factor (C_N) or stress exponent (n). The general form of normalization is:

q_c1 = C_N × q_c

or

q_c1 = q_c × (P_a / σ’_v0)ⁿ

where q_c1 is the normalized cone resistance, q_c is the measured cone resistance, P_a is atmospheric pressure (reference stress, typically 100 kPa or 1 atm), σ’_v0 is the effective vertical overburden stress, and n is the stress exponent that varies with soil type.

Interpretation of experimental and field cone penetration test (CPT) data from across a broad range of stress conditions requires defining the dependence of the measurements on overburden stress and other influencing factors. The stress exponent n typically ranges from about 0.5 for clays to 1.0 for sands, though more sophisticated approaches use variable exponents based on soil behavior type.

Applications of Normalized CPT Data

To ensure that your data is consistent, it is important to use these parameters in deep tests and in soft, fine-grained soils. Normalized CPT parameters enable:

• Consistent soil classification across different depths and sites
• Reliable correlation with soil properties such as friction angle, undrained shear strength, and relative density
• Assessment of liquefaction potential
• Determination of bearing capacity and settlement characteristics

In addition to normalized CPT parameters, overburden pressure allows us to understand and calculate the following engineering parameters: Effective overburden stress: the effective stress on the soil skeleton, which is calculated by subtracting the pore pressure from the overburden stress

Overconsolidation Ratio and Its Relationship to Overburden Pressure

The overconsolidation ratio (OCR) is a critical parameter in soil mechanics that relates directly to the stress history of a soil deposit and has important implications for soil behavior.

Definition and Significance

Over consolidation ratio: the ratio of past maximum effective overburden stress to present effective overburden stress. The overconsolidation ratio (OCR) is a qualitative indicator of this densification or stiffening of the soil, and it is defined as the ratio of the maximum overburden stress ever experienced by the soil (i.e., with the ice sheet on top) to the present overburden stress (i.e., without the ice sheet).

OCR = σ’_p / σ’_v0

where σ’_p is the preconsolidation pressure (maximum past effective stress) and σ’_v0 is the current effective overburden stress.

A soil that is currently under its maximum effective overburden stress is said to be normally consolidated and has an OCR of 1. A soil that once experienced a greater pressure (for example, if it was once under a glacier) is considered over consolidated and will have a higher OCR.

Causes of Overconsolidation

Several mechanisms can cause a soil to become overconsolidated:

Change in total stress due to removal of overburden can cause preconsolidation pressure in a soil. For example, removal of structures or glaciation would cause a change in total stress that would have this effect.

Change in pore water pressure: A change in water table elevation, Artesian pressures, deep pumping or flow into tunnels, and desiccation due to surface drying or plant life can bring soil to its preconsolidation pressure.

Change in soil structure due to aging (secondary compression): Over time, soil will consolidate even after high pressures from loading and pore water pressure have been depleted.

Chemical weathering: Different types of chemical weathering will cause preconsolidation pressure. Precipitation, cementing agents, and ion exchange are a few examples.

Impact on Soil Behavior

The OCR significantly affects soil engineering properties. The OCR governs the response of the soil to loading. Soils with low OCR (typically NC to LOC) tend to contract when subjected to shearing forces, and expel the water during shear. Soils with high OCR (typically MOC to HOC) tend to expand or dilate when subjected to shearing forces, which causes water to be drawn into the soil.

Overconsolidated soils generally exhibit:

• Higher shear strength compared to normally consolidated soils at the same current stress level
• Lower compressibility and settlement potential
• Dilative behavior during shearing
• Higher lateral earth pressure coefficients
• Greater resistance to liquefaction

Practical Considerations in Overburden Pressure Calculations

While the theoretical framework for calculating overburden pressure is straightforward, practical application requires attention to several important factors that can significantly affect accuracy.

Variability in Soil Unit Weight

The unit weight, , will vary with the water content of the soil. Moisture content variations can cause significant changes in unit weight, particularly in fine-grained soils. Since soil volume shrinks with drying of swelling soils, total density will vary with water content. Hence, the water content of the soil should be determined at the time of sampling.

The unit weight of soil is affected by several factors, including the soil type, moisture content, and consolidation, and is an essential parameter in geotechnical engineering and construction applications. Representative sampling and testing at appropriate depths and locations are essential for obtaining reliable unit weight values.

Groundwater Level Fluctuations

The position of the groundwater table has a profound effect on effective stress calculations. Changes in water level below ground (water table changes) result in changes in effective stresses below the water table. Seasonal variations, pumping activities, or changes in precipitation patterns can cause the water table to fluctuate, altering the effective stress distribution in the soil profile.

Engineers must consider both current groundwater conditions and potential variations when calculating overburden pressures for design purposes. In some cases, worst-case scenarios (highest or lowest anticipated water table) should be evaluated.

Stratification and Heterogeneity

Real soil deposits are rarely uniform. In practice, the exact height and density of the soil layers at the test site are usually not known, so you may have to determine an average density based on what you do know about the geology of the area. Detailed subsurface investigation through borings, test pits, and in-situ testing is necessary to characterize soil stratigraphy accurately.

Thin layers with significantly different properties can be particularly important. For example, a thin clay layer within a sand deposit may create a perched water table, or a thin sand seam in clay may provide drainage paths that affect consolidation behavior.

Seepage Conditions

When water flows through soil, the pore pressure distribution deviates from hydrostatic conditions. In conditions of seepage in the ground there is a change in pore pressure. Upward seepage reduces effective stress, while downward seepage increases it. The hydraulic gradient must be considered when calculating pore pressures under seepage conditions.

The seepage force can be significant in excavations, beneath dams, or around sheet pile walls, and failure to account for it can lead to instability problems such as piping or bottom heave.

Equipment Calibration and Quality Control

Regular calibration and validation of measurement equipment help ensure reliable results. The quality of the result produced by this standard is dependent on the competence of the personnel performing it and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection/etc. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results.

Quality control measures should include:

• Regular calibration of testing equipment
• Verification of calculations and data reduction procedures
• Cross-checking results with alternative methods when possible
• Proper sample handling and storage to minimize disturbance
• Documentation of testing procedures and conditions

Advanced Topics: Stress-Dependent Soil Behavior

Soil behavior is inherently stress-dependent, and understanding this relationship is crucial for advanced geotechnical analysis and design.

Influence on Shear Strength

The shear strength of soils is directly related to effective stress. For cohesionless soils (sands and gravels), the Mohr-Coulomb failure criterion relates shear strength to effective normal stress:

τ = σ’ tan φ’

where τ is shear strength, σ’ is effective normal stress, and φ’ is the effective friction angle. For cohesive soils, an additional cohesion term is included. The effective stress, which depends on overburden pressure and pore pressure, thus directly controls the shear strength available to resist failure.

Consolidation and Settlement

When loads are applied to saturated fine-grained soils, the initial response is an increase in pore water pressure. When a load is applied to soil, it is carried by the water in the pores as well as the solid grains. The increase in pressure within the porewater causes drainage (flow out of the soil), and the load is transferred to the solid grains. The rate of drainage depends on the permeabilityof the soil.

As excess pore pressures dissipate through consolidation, effective stress increases and the soil compresses. The magnitude and rate of settlement depend on the change in effective stress (related to overburden pressure changes), the compressibility characteristics of the soil, and the drainage conditions.

Liquefaction Potential

Laboratory tests have shown that the cyclic resistance ratio (CRR), a normalized measure of a soil’s cyclic resistance to liquefaction, decreases as the effective confining pressure increases. When effective confining pressure increases, multiple factors influence liquefaction resistance. First, soils become more contractive (or less dilative); therefore undrained cyclic resistance tends to decrease.

The overburden correction factor K_σ is used in liquefaction analysis to account for the effect of overburden stress on cyclic resistance. This factor adjusts the cyclic resistance ratio measured or correlated at one stress level to the stress level of interest, typically normalizing to a reference stress of one atmosphere.

Stiffness and Modulus

Soil stiffness, as characterized by elastic modulus or shear modulus, increases with increasing effective confining stress. This stress-dependency must be considered in deformation analyses, particularly for problems involving significant stress changes or large structures where stress levels vary considerably with depth.

Small-strain shear modulus typically varies with effective stress according to a power law relationship, with the exponent depending on soil type and stress history. This relationship affects ground response analysis for seismic loading, foundation settlement calculations, and soil-structure interaction problems.

Case Study Applications

Understanding how to apply overburden pressure calculations in real-world scenarios is essential for practicing engineers. Consider several common applications:

Foundation Design

For foundation design, accurate determination of in-situ stress conditions is fundamental. The bearing capacity of shallow foundations depends on the effective overburden stress at foundation level, which provides confining pressure that enhances soil strength. Settlement calculations require knowledge of the initial effective stress state and the stress increase caused by foundation loads.

Deep foundations such as piles derive shaft resistance from the lateral effective stress, which is related to the vertical effective overburden stress through the lateral earth pressure coefficient. End bearing capacity similarly depends on the effective stress at the pile tip level.

Retaining Wall Design

The in situ lateral pressure of soil is called earth pressure at rest and it is generally calculated by the product of the overburden stress times the coefficient K0; the latter is called the coefficient of earth pressure at rest. Lateral earth pressures acting on retaining walls, basement walls, and braced excavations are directly proportional to the vertical effective stress.

Active and passive earth pressures, which represent limiting stress states, are calculated based on the vertical effective stress distribution. Groundwater conditions significantly affect these pressures, as pore water pressure reduces effective stress and also exerts direct hydrostatic pressure on the wall.

Slope Stability Analysis

Slope stability depends critically on the effective stress distribution within the slope. Without appropriate drainage, rainfall can saturate the clay and increase its pore water pressure. This, in turn, reduces the soil’s effective stress, results in a loss of strength and can potentially lead to failure.

Changes in groundwater conditions, whether from rainfall infiltration, irrigation, or changes in external water levels, alter pore pressures and thus effective stresses. This can trigger slope failures even without any change in geometry or external loading. Proper calculation of initial stress conditions and potential changes is essential for reliable stability analysis.

Excavation Support

Excavations alter the stress state in surrounding soils by removing overburden. This stress relief can cause bottom heave in soft clays, lateral movement of adjacent ground, and changes in groundwater flow patterns. The magnitude of these effects depends on the initial overburden stress that is removed and the properties of the affected soils.

Dewatering systems used to control groundwater during excavation lower the water table, increasing effective stresses and potentially causing settlement of adjacent structures. Careful analysis of stress changes is necessary to predict and mitigate these effects.

Common Errors and How to Avoid Them

Several common mistakes can lead to significant errors in overburden pressure calculations:

Confusing Total and Effective Stress

One of the most fundamental errors is using total stress when effective stress is required, or vice versa. Remember that soil behavior—strength, compressibility, and volume change—is controlled by effective stress, not total stress. Always clearly distinguish between these quantities in calculations and ensure the appropriate value is used for each application.

Incorrect Unit Weight Selection

Using the wrong unit weight for soil conditions is a common source of error. Above the water table, use moist or bulk unit weight. Below the water table, use saturated unit weight when calculating total stress, or buoyant unit weight when calculating effective stress directly. Never use dry unit weight for in-situ stress calculations unless the soil is actually dry.

Neglecting Groundwater Effects

Failing to account for groundwater, or assuming it is at a certain depth without verification, can lead to serious errors. Always investigate groundwater conditions through observation wells or piezometers, and consider seasonal variations. Remember that perched water tables can exist above the regional water table in layered soils.

Oversimplifying Soil Stratigraphy

Assuming uniform soil conditions when significant layering exists can produce inaccurate stress distributions. Adequate subsurface investigation is essential to identify all significant soil layers and their properties. Thin layers with contrasting properties may be particularly important for certain applications.

Ignoring Stress History

Treating all soils as normally consolidated when overconsolidation may exist can lead to unconservative designs. Overconsolidated soils behave differently from normally consolidated soils, exhibiting higher strength and lower compressibility. Laboratory consolidation tests or correlations with in-situ tests can help identify overconsolidation.

Software Tools and Computational Methods

Modern geotechnical practice increasingly relies on software tools to perform overburden pressure calculations and related analyses. Spreadsheet programs can efficiently handle multi-layer stress calculations, while specialized geotechnical software packages offer more sophisticated capabilities.

Commercial geotechnical software typically includes modules for:

• Calculating stress distributions in complex soil profiles
• Performing CPT and SPT data normalization and interpretation
• Analyzing consolidation and settlement
• Evaluating bearing capacity and slope stability
• Designing retaining structures and excavation support systems

While these tools are powerful, users must understand the underlying principles to input appropriate data, select suitable analysis methods, and critically evaluate results. Software cannot replace engineering judgment and understanding of soil mechanics fundamentals.

Future Directions and Research

Research continues to refine our understanding of stress-dependent soil behavior and improve methods for calculating and correcting for overburden effects. Areas of ongoing investigation include:

• Advanced constitutive models that better capture stress-dependent stiffness and strength
• Improved normalization procedures for in-situ tests across a wider range of soil types
• Better understanding of aging and cementation effects on stress history
• Development of more reliable methods for determining preconsolidation pressure
• Integration of geophysical methods for characterizing stress states in situ
• Application of machine learning and artificial intelligence to predict soil properties from test data

These advances promise to enhance the accuracy and reliability of geotechnical analyses that depend on proper characterization of overburden pressure and effective stress.

Conclusion

Calculating and correcting for overburden pressure is fundamental to geotechnical engineering practice. Accurate determination of total stress, pore water pressure, and effective stress is essential for interpreting soil test results, predicting soil behavior, and designing safe, economical foundations and earthworks.

The basic principles are straightforward: total stress increases with depth according to the weight of overlying materials, pore water pressure depends on groundwater conditions, and effective stress—the difference between total stress and pore pressure—controls soil behavior. However, practical application requires careful attention to soil stratigraphy, unit weight determination, groundwater conditions, and stress history.

Normalization of in-situ test results for overburden stress effects enables meaningful comparison of data from different depths and sites, and provides the basis for correlations with soil properties. Understanding overconsolidation ratio and its effects on soil behavior is crucial for many applications.

By following established procedures, using appropriate testing methods, maintaining quality control, and applying sound engineering judgment, geotechnical engineers can reliably calculate overburden pressures and correct test results to obtain the soil parameters needed for safe, effective designs.

Additional Resources

For those seeking to deepen their understanding of overburden pressure and related topics, the following resources provide valuable information:

• ASTM International – Standards for soil testing and classification
• Geoengineer.org – Educational resources and technical articles on geotechnical engineering
• Federal Highway Administration Geotechnical Engineering – Manuals and guidance documents
• International Society for Soil Mechanics and Geotechnical Engineering – Technical committees and publications
• ScienceDirect – Research articles and technical papers on overburden pressure

Continued study of soil mechanics principles, combined with practical experience and attention to detail in calculations and testing, will enable engineers to master the essential skill of calculating and correcting for overburden pressure in soil testing and analysis.