Tank Settling / Foundation Fundamentals

1. Settlement Overview

Storage tank settlement is the downward movement of the tank and its foundation caused by the consolidation of underlying soil under the weight of the tank, stored product, and foundation itself. All tanks settle to some degree after construction and hydrostatic testing; the engineering challenge is determining whether the magnitude and pattern of settlement are within acceptable limits for continued safe operation.

Settlement evaluation is a core component of the API 653 in-service inspection program. API 653 Section 12.3 provides criteria for evaluating settlement that has occurred since the tank was constructed or since the last survey. The evaluation considers several distinct settlement patterns, each with different implications for tank integrity. Uniform settlement is generally benign if connected piping can accommodate the movement. Differential settlement, however, introduces bending stresses in the shell and bottom plates and can cause seal malfunction in floating-roof tanks, weld cracking, and even catastrophic bottom plate failure in extreme cases.

Why Settlement Matters

2. Settlement Types

Tank settlement is classified into several distinct patterns based on how the settlement varies around the tank circumference and across the bottom. Each type has different causes, consequences, and remediation approaches.

Uniform Settlement

Uniform settlement occurs when the entire tank and foundation settle downward by approximately the same amount at all locations. This happens when the underlying soil is homogeneous and the tank load is evenly distributed. Uniform settlement is the most benign form because it does not introduce any bending stresses in the tank shell or bottom plates. The primary concern with uniform settlement is the effect on connected piping, which must have sufficient flexibility to accommodate the downward movement without overstressing. Most tanks experience 1 to 4 inches of uniform settlement during the initial hydrostatic test, with additional settlement of 0.5 to 2 inches during the first few years of operation. Uniform settlement up to 6 inches is generally considered acceptable for most tank installations. Beyond 12 inches, the piping flexibility analysis should be carefully reviewed, and settlement monitoring frequency should be increased.

Planar Tilt

Planar tilt, also called rigid-body tilt, occurs when one side of the tank settles more than the opposite side, causing the tank to tilt like a coin on a table. The settlement pattern around the circumference follows a sinusoidal curve when plotted against angular position. Planar tilt is caused by non-uniform soil conditions, asymmetric loading (such as a heavy nozzle or piping connection on one side), or foundation construction inconsistencies. The effect of planar tilt on a fixed-roof tank is primarily cosmetic and operational: the liquid level becomes uneven, affecting level measurement accuracy and potentially causing overflow on the low side. For floating-roof tanks, tilt is more problematic because the floating roof must remain level regardless of shell tilt, which creates uneven gaps between the roof edge and the tilted shell, degrading the seal performance.

Differential Settlement

Differential settlement is the difference in settlement between two adjacent measurement points. It is the most direct indicator of localized foundation distress. Unlike planar tilt, which is a smooth gradient, differential settlement represents abrupt changes in the settlement profile that create localized bending stresses in the shell and bottom plates. API 653 limits differential settlement to 1 inch per 10 feet of arc length for rigid-bottom tanks on ringwall or slab foundations and 2 inches per 10 feet for flexible-bottom tanks on gravel pads. When differential settlement exceeds these limits, it indicates that the foundation is unable to uniformly support the tank load, and remediation should be investigated.

Edge Settlement

Edge settlement refers to the localized downward movement of the tank bottom near the shell-to-bottom junction. This type of settlement is particularly insidious because it concentrates stress at the critical junction between the shell and bottom plates. Edge settlement is often caused by soil consolidation directly under the tank shell where the contact pressure is highest, or by erosion of the foundation pad near the shell. The bottom annular plates, which are designed primarily for membrane stresses from liquid weight, experience significant bending stresses from edge settlement. If the edge settlement exceeds approximately 2 to 3 inches relative to the tank center, fatigue cracking at the bottom plate lap welds may develop. Edge settlement is measured by surveying the tank bottom profile along radial lines from the shell to the center.

Out-of-Plane Settlement

Out-of-plane settlement is the deviation of each measurement point from the best-fit tilted plane through the settlement data. After removing the uniform and tilt components from the measured settlements, the remaining deviations represent true localized distortion of the tank shell. This is the most meaningful metric for assessing foundation adequacy because it isolates the settlement that actually causes structural stress, as opposed to rigid-body movement. A high out-of-plane index at a specific location indicates a localized foundation problem such as a void under the pad, a soft soil pocket, or washout of foundation material.

3. Measurement Methods

Accurate settlement measurement is essential for reliable evaluation. The most common method is surveying permanent benchmarks welded to the outer surface of the tank shell at equally-spaced intervals around the circumference. These benchmarks are surveyed using precise optical or digital levels referenced to a stable benchmark outside the tank's zone of influence.

Benchmark Installation

Settlement benchmarks are typically small steel brackets or lugs welded to the tank shell at a consistent height, usually 12 to 18 inches above the bottom of the lowest shell course. The benchmarks must be installed at uniform spacing around the circumference. API 653 recommends a minimum of 8 equally-spaced points for tanks up to 150 feet in diameter and 16 points for larger tanks. The points are numbered sequentially starting from the north position and proceeding clockwise. For initial baseline surveys on new tanks, benchmarks should be installed before the hydrostatic test so that test-induced settlement can be measured.

Survey Methodology

Settlement surveys should be performed using precision surveying equipment capable of measuring elevation differences to within 0.01 inches. The reference benchmark must be located at least 1.5 times the tank diameter away from the tank to be outside the zone of soil influence. Surveys should be performed under consistent conditions, preferably with the tank at a consistent liquid level, because temperature-induced shell expansion and liquid head can affect benchmark elevations. When comparing surveys from different dates, the liquid level at the time of each survey should be recorded and corrections applied if necessary.

Survey Timing and Frequency

4. Settlement Analysis

Settlement analysis follows a systematic procedure to decompose the raw survey data into meaningful components and compare each against the applicable API 653 limits. The modern approach uses Fourier decomposition to cleanly separate the planar tilt from localized distortion.

Step 1: Compute Uniform Settlement

The uniform settlement is simply the arithmetic mean of all the circumferential readings. This represents the average downward movement of the tank as a whole. Subtracting this value from each reading centers the data around zero, making the tilt and distortion patterns easier to visualize.

Step 2: Fourier Decomposition for Tilt

The settlement readings around the circumference are treated as a periodic function of angular position. The first-order Fourier coefficients (cosine and sine terms) capture the planar tilt component. The Fourier analysis provides both the magnitude and direction of the tilt without requiring any iterative calculations. This method is mathematically rigorous and handles any number of equally-spaced measurement points. The amplitude of the first-order harmonics gives the tilt amplitude, and the phase angle gives the direction of maximum settlement.

Step 3: Out-of-Plane Deviations

The out-of-plane deviation at each measurement point is the difference between the actual measured settlement and the value predicted by the best-fit plane. These residuals represent the true localized distortion of the shell perimeter. The out-of-plane index is the maximum absolute deviation divided by the arc length between adjacent points, normalized to inches per 10 feet. This normalization allows direct comparison between tanks of different diameters and different numbers of measurement points.

Step 4: Differential Settlement

The differential settlement between each pair of adjacent points is computed as the absolute difference in settlement values, divided by the arc length, and normalized to inches per 10 feet. This calculation is performed on the raw data (not the residuals) because the API 653 limit applies to the actual differential between adjacent shell portions. The maximum differential value is compared against the limit of 1 inch per 10 feet for rigid foundations or 2 inches per 10 feet for flexible foundations.

Step 5: Assessment

Each computed metric is compared against the applicable API 653 limits. A tank passes if all metrics are within limits. If any metric exceeds its limit, the tank fails and engineering evaluation is required to determine corrective action. A marginal status (approaching limits within 75-100% of the limit) triggers increased monitoring frequency. The settlement rate, computed from the change between successive surveys, provides information about whether settlement is ongoing or has stabilized.

5. Foundation Types

The choice of foundation type directly affects the settlement behavior and allowable limits for a storage tank. API 650 Appendix B provides design recommendations for the three primary foundation types used in the petroleum industry.

Ringwall Foundation

The ringwall foundation is the most common foundation type for large aboveground storage tanks. It consists of a reinforced concrete ring wall, typically 12 to 24 inches wide and 24 to 48 inches deep, that supports the tank shell at the bottom of the lowest course. The interior of the ring is filled with compacted granular fill material (typically crushed stone or sand) that supports the tank bottom plates. The ringwall transfers the concentrated shell load to the soil while the interior fill supports the distributed bottom plate load. This design accommodates the different loading intensities and settlement characteristics under the shell versus the center. The concrete ring provides a rigid, level surface for shell erection and helps resist differential settlement at the shell perimeter. Ringwall foundations are appropriate for soil bearing capacities above 2,000 psf and tank diameters up to 300 feet. For very large tanks on marginal soils, pile-supported ringwalls may be necessary.

Mat/Slab Foundation

A full reinforced concrete mat or slab foundation extends under the entire tank bottom. This type provides the most uniform support and is most effective at minimizing differential settlement. Slab foundations are used for critical-service tanks, tanks on weak or variable soils, and tanks where settlement must be minimized. The slab acts as a rigid diaphragm that distributes the tank load over a large area, reducing contact pressure on the soil. The thickness of the slab depends on the tank diameter, soil bearing capacity, and reinforcement design; typical thicknesses range from 12 to 36 inches. Slab foundations are significantly more expensive than ringwalls but provide superior settlement performance. They are also required for tanks in seismic zones where rocking resistance is needed.

Gravel Pad Foundation

Gravel pad foundations consist of a compacted layer of crushed stone or gravel placed directly on the prepared subgrade. The gravel pad may be a full pad extending under the entire tank bottom or a ring pad supporting the shell perimeter only. Gravel pads are the simplest and least expensive foundation type, commonly used for small field tanks (under 50 feet diameter) and temporary installations. The compacted gravel provides drainage, prevents bottom plate corrosion from soil contact, and distributes the tank load to the underlying soil. Because gravel pads are inherently more flexible than concrete foundations, they tolerate larger differential settlements without structural damage to the foundation itself. API 653 reflects this by allowing 2 inches per 10 feet of differential settlement for gravel pads compared to 1 inch per 10 feet for concrete foundations. However, the higher flexibility means that the tank shell must accommodate more distortion, which may not be acceptable for all tank configurations.

Foundation Design Considerations

6. Remediation Methods

When settlement exceeds API 653 limits, several remediation options are available. The choice of method depends on the severity of settlement, the cause, the tank size, the operational requirements, and the cost-benefit analysis of each option.

Shimming and Grouting

For moderate differential settlement (1 to 3 inches above the limit), the gap between the tank bottom and the foundation can be filled by injecting cementitious grout or compaction grout beneath the tank bottom. This is performed without lifting the tank by pumping grout through small-diameter injection ports drilled through the tank bottom or inserted through the foundation edge. The grout fills voids and provides additional support under areas that have settled excessively. Shimming involves placing steel shims between the shell and the ringwall to restore a level bearing surface. Both methods are relatively low-cost and can be performed while the tank remains in service at a reduced liquid level.

Mud Jacking and Slab Jacking

Mud jacking (also called slab jacking or pressure grouting) involves pumping a controlled-density slurry beneath the tank foundation to raise the settled areas back toward their original elevation. This technique requires careful monitoring and control to avoid over-lifting the tank or creating hydraulic fractures in the soil. Modern polyurethane foam jacking provides more precise lift control and faster curing than traditional cement-based grout. Mud jacking is effective for settlements of 2 to 6 inches and is best suited for uniform or planar tilt correction rather than highly localized settlements.

Foundation Replacement

For severe settlement that cannot be corrected in place, the tank may need to be taken out of service, cleaned, and lifted from the foundation for complete or partial foundation replacement. This is the most expensive and time-consuming option but provides the best long-term solution for foundations that have fundamentally failed. Foundation replacement typically involves excavating the failed sections, improving the subgrade soil (compaction, soil stabilization, or pile installation), and constructing new foundation elements. The tank bottom may also need repair or replacement if settlement has caused bottom plate damage.

Operational Mitigation

In some cases, operational changes can mitigate the effects of settlement until permanent repairs can be made during a scheduled turnaround. These include reducing the maximum fill level to decrease hydrostatic loads, adjusting the fill and drain rate to minimize dynamic soil loading, increasing the settlement monitoring frequency to track progression, and performing a fitness-for-service assessment (API 579-1) to establish safe operating parameters with the existing settlement. Operating restrictions should be clearly documented and reviewed by the tank inspector and responsible engineer.