Corpus sync: neutrality remakes, note hygiene, datasheet relocation, transformer-split cross-refs
---
title: Ground Improvement (Aggregate Piers and Soil Mixing)
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### A pre-construction geotechnical investigation conducted in accordance with IBC Section 1803 shall provide subsurface data sufficient to support the ground improvement design, including borings or cone penetration tests to a depth below the deepest element and laboratory classification and strength testing of the affected strata.
−### The geotechnical investigation underpinning the ground improvement design must be specific to the site and adequate in depth and density; existing borings that are too sparse or too shallow are not acceptable as the sole basis of design, because element type, depth, and spacing all derive from the site soil profile. {note}
+### The geotechnical investigation underpinning the ground improvement design must be specific to the site and adequate in depth and density; existing borings that are too sparse or too shallow are not acceptable as the sole basis of design, because element type, depth, and spacing all derive from the site soil profile.
### Ground improvement under IBC Section 1808 is an alternative foundation system with no prescriptive code path: there is no table to size elements from, so a project-specific geotechnical report and engineer approval are mandatory rather than optional. {note}
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### Settlement of the improved ground under design service loads shall not exceed the specified total and differential settlement limits.
−### Settlement limits are commonly expressed two ways: a total settlement cap (often about 1 inch for slabs and 0.5 to 1 inch for spread footings) and a differential settlement ratio between adjacent supports (commonly L/480 to L/240 for structural frames), with the structural engineer setting the governing values for the specific frame. {note}
−
```datasheet
label: Maximum total settlement under design load
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- 0.5
- 0.75
− - 1.0
+ - 1
- 1.5
+ - 2
default: 1.0
```
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```
+### Settlement limits are commonly expressed two ways: a total settlement cap (often about 1 inch for slabs and 0.5 to 1 inch for spread footings) and a differential settlement ratio between adjacent supports (commonly L/480 to L/240 for structural frames), with the structural engineer setting the governing values for the specific frame. {note}
+
### Post-improvement settlement under design load shall not exceed the fraction of the unimproved settlement assumed in the design.
−### Granular reinforcement typically reduces post-improvement settlement to about 30 to 60 percent of the unimproved settlement; the design report shall state the settlement reduction the layout is intended to achieve so that it can be verified against load test results. {note}
+### Granular reinforcement typically reduces post-improvement settlement to about 30 to 60 percent of the unimproved settlement; the design report shall state the settlement reduction the layout is intended to achieve so that it can be verified against load test results.
## Liquefaction Resistance {toc}
### Where the design intent includes mitigation of liquefiable soils, the improved ground shall achieve the specified post-treatment density or penetration resistance, and that resistance shall be verified by post-treatment testing rather than assumed from the pre-treatment design.
−### Specifying aggregate piers in liquefiable granular soils on the strength of the pre-treatment design alone is a recognized pitfall: densification and drainage performance vary with as-installed conditions, so post-treatment CPT or SPT verification is required to confirm the improvement actually occurred. {note}
−
```datasheet
label: Liquefaction mitigation required
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```
+### Specifying aggregate piers in liquefiable granular soils on the strength of the pre-treatment design alone is a recognized pitfall: densification and drainage performance vary with as-installed conditions, so post-treatment CPT or SPT verification is required to confirm the improvement actually occurred. {note}
+
# Method Selection {toc}
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### Method selection is the first and most consequential decision: cohesive versus granular soils, the depth to groundwater, overhead and lateral access limits, and the governing performance target (bearing, settlement, or liquefaction) each push toward a different family of methods, and the wrong choice cannot be corrected by adjusting spacing. {note}
+### The ground improvement method shall be specified, selected based on the soil type, groundwater depth, access constraints, and governing performance target.
+
```datasheet
label: Ground improvement method
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### Vibro compaction densifies clean loose granular soils with a vibrating probe and does not add aggregate or form a column; it is a densification method, not a reinforcement method, and is ineffective in cohesive soils. {note}
−### Vibro stone columns shall not be specified in stiff cohesive soils with undrained shear strength above approximately 50 kPa (1,000 psf) without a constructability review by the specialty contractor, because dense or stiff soil may resist the lateral expansion the column needs to mobilize its capacity. {note}
+### Vibro stone columns shall not be specified in stiff cohesive soils with undrained shear strength above approximately 50 kPa (1,000 psf) without a constructability review by the specialty contractor, because dense or stiff soil may resist the lateral expansion the column needs to mobilize its capacity.
## Cementitious Stabilization Methods {toc}
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### Element diameter, design depth, grid spacing, and area replacement ratio shall be as shown on the accepted treatment layout for each foundation and slab area.
−### Element diameter is commonly 18 inches for rammed aggregate piers (the most frequent size), increasing to 24 to 30 inches for heavier loads and 30 to 48 inches for vibro stone columns; the design depth extends to a competent stratum or refusal and at least 1.5 times the footing width below bearing elevation. {note}
−
```datasheet
label: Aggregate pier / stone column diameter
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```
+### Element diameter is commonly 18 inches for rammed aggregate piers (the most frequent size), increasing to 24 to 30 inches for heavier loads and 30 to 48 inches for vibro stone columns; the design depth extends to a competent stratum or refusal and at least 1.5 times the footing width below bearing elevation. {note}
+
### The area replacement ratio (ratio of element cross-sectional area to tributary area) shall be as required by the design to achieve the specified bearing and settlement performance.
−### Area replacement ratios commonly run 10 to 35 percent, with a design optimum often near 20 to 25 percent for soft clay sites; the ratio is the principal lever the designer uses to trade element count against performance. {note}
−
```datasheet
label: Area replacement ratio (As/A)
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```
+### Area replacement ratios commonly run 10 to 35 percent, with a design optimum often near 20 to 25 percent for soft clay sites; the ratio is the principal lever the designer uses to trade element count against performance. {note}
+
### Aggregate for piers and stone columns shall be crushed stone graded in accordance with ASTM D6913/D6913M, of nominal 0.75 to 1.5 inch size.
### Aggregate shall contain no more than 5 percent passing the No. 200 sieve.
−### Fines content must be controlled because the column's drainage function (especially for liquefaction mitigation and for stone columns below the water table) depends on the aggregate staying free-draining; excess fines clog the column and defeat the drainage path. {note}
+### Fines content must be controlled because the column's drainage function (especially for liquefaction mitigation and for stone columns below the water table) depends on the aggregate staying free-draining; excess fines clog the column and defeat the drainage path.
### Aggregate shall have a Los Angeles abrasion loss of no more than 50 percent when tested in accordance with ASTM C131/C131M.
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### Soil-mix and jet grout column diameter, design depth, and grid or panel layout shall be as shown on the accepted treatment layout.
−### Wet-method single-axis soil-mix columns are most commonly 24 to 36 inches in diameter; multi-axis soil-mix walls run 36 to 60 inches; single-fluid jet grout columns run 18 to 36 inches and triple-fluid jet grout columns run 48 to 78 inches. {note}
−
```datasheet
label: Soil-mix / jet grout column diameter
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step: 6
setpoints:
+ - 18
- 24
- 36
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```
+### Wet-method single-axis soil-mix columns are most commonly 24 to 36 inches in diameter; multi-axis soil-mix walls run 36 to 60 inches; single-fluid jet grout columns run 18 to 36 inches and triple-fluid jet grout columns run 48 to 78 inches. {note}
+
### The binder type and dosage shall be selected for the in-situ soil chemistry and the target unconfined compressive strength, and shall be confirmed by trial-mix or bench-scale testing before production.
−### The binder is commonly Portland cement (Type I/II, or Type III where high early strength is needed), sometimes blended with slag or lime; in-situ soil chemistry governs the choice because organics, sulfates, or low pH can inhibit cement hydration and, if unaddressed, cause complete column failure, which is why trial-mix testing is mandatory rather than advisory. {note}
−
```datasheet
label: Binder type
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```
+### The binder is commonly Portland cement (Type I/II, or Type III where high early strength is needed), sometimes blended with slag or lime; in-situ soil chemistry governs the choice because organics, sulfates, or low pH can inhibit cement hydration and, if unaddressed, cause complete column failure, which is why trial-mix testing is mandatory rather than advisory. {note}
+
### Soil-mix and jet grout columns shall attain the specified unconfined compressive strength at the specified age when tested in accordance with ASTM D4832.
−### Target unconfined compressive strength is commonly 50 to 150 psi (345 to 1,034 kPa) for settlement control in soft clay and 100 to 300 psi (690 to 2,069 kPa) for structural load transfer, with environmental containment work often requiring at least 50 psi; the design report shall state the governing target and age. {note}
+### Target unconfined compressive strength is commonly 50 to 150 psi (345 to 1,034 kPa) for settlement control in soft clay and 100 to 300 psi (690 to 2,069 kPa) for structural load transfer, with environmental containment work often requiring at least 50 psi; the design report shall state the governing target and age.
```datasheet
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### A compacted granular load transfer platform shall be placed over the improved zone where required by the design to redistribute structure loads across the element grid and the intervening soil.
−### Omitting the load transfer platform is a common pitfall: the granular working mat (often with geogrid reinforcement) is what spreads load from the structure across the stiff elements and the softer soil between them, and without it most of the composite-system benefit is lost. {note}
−
```datasheet
label: Load transfer platform thickness
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```
+### Omitting the load transfer platform is a common pitfall: the granular working mat (often with geogrid reinforcement) is what spreads load from the structure across the stiff elements and the softer soil between them, and without it most of the composite-system benefit is lost. {note}
+
# Installation {toc}
−## General {toc}
+## Installation shall follow the accepted ground improvement design, treatment layout, and quality control plan.
−### Installation shall follow the accepted ground improvement design, treatment layout, and quality control plan.
+## A preconstruction survey of adjacent structures, slabs, and utilities within the zone of influence shall be performed and documented before installation begins.
−### A preconstruction survey of adjacent structures, slabs, and utilities within the zone of influence shall be performed and documented before installation begins.
+## Aggregate pier, soil mixing, and jet grout installation can cause ground heave, vibration, and lateral soil displacement; a documented preconstruction survey of adjacent foundations and buried utilities establishes the baseline needed to detect and attribute any damage. {note}
−### Aggregate pier, soil mixing, and jet grout installation can cause ground heave, vibration, and lateral soil displacement; a documented preconstruction survey of adjacent foundations and buried utilities establishes the baseline needed to detect and attribute any damage. {note}
+## The contractor shall stop ground improvement work and notify the engineer when hazardous soil or groundwater is encountered, and shall not resume until abatement clearance is obtained under [[sync/hazardous-material-abatement]].
−### The contractor shall stop ground improvement work and notify the engineer when hazardous soil or groundwater is encountered, and shall not resume until abatement clearance is obtained under [[sync/hazardous-material-abatement]].
−
## Aggregate Pier and Stone Column Installation {toc}
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### Soil-cement spoil and grout returns shall be contained, characterized, and disposed of in accordance with the accepted spoil management plan.
−### Wet soil mixing and jet grouting generate significant volumes of soil-cement spoil and grout return; disposal must be planned and permitted before mobilization, because on contaminated sites the returns may be a regulated waste and an unplanned spoil stream can halt the work. {note}
+### Wet soil mixing and jet grouting generate significant volumes of soil-cement spoil and grout return; disposal must be planned and permitted before mobilization, because on contaminated sites the returns may be a regulated waste and an unplanned spoil stream can halt the work.
### Overlap between adjacent columns in walls, grids, and mass-mix blocks shall be maintained within the tolerance required by the design to ensure continuity of the treated mass.
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### A minimum waiting period shall elapse between element installation and footing excavation or load application, as required for the method and binder.
−### Premature loading before strength gain is a recurring field problem; aggregate piers can typically accept footing construction within about 24 hours, while soil-mix and jet grout columns require 7 to 28 days of curing depending on the binder and target strength before load is applied. {note}
−
```datasheet
label: Minimum waiting period before load application
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```
+### Premature loading before strength gain is a recurring field problem; aggregate piers can typically accept footing construction within about 24 hours, while soil-mix and jet grout columns require 7 to 28 days of curing depending on the binder and target strength before load is applied. {note}
+
# Testing and Verification {toc}
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## Ground improvement bearing capacity and settlement assumptions shall be coordinated with the shallow foundation design under [[sync/shallow-foundations]] and confirmed before installation. {note}
−## Mass earthwork, site grading to working-platform elevation, and engineered fill above the improved zone are specified under [[sync/earthwork]] and shall be coordinated so the working platform is in place before installation and the load transfer platform is built on a prepared subgrade. {note}
+## Mass earthwork, site grading to working-platform elevation, and engineered fill above the improved zone are specified under [[sync/earthwork]] and shall be coordinated so the working platform is in place before installation and the load transfer platform is built on a prepared subgrade.
## Performance monitoring of the improved ground during and after construction (settlement plates, inclinometers, piezometers) is specified under [[sync/geotechnical-instrumentation-and-monitoring]]; this standard's testing requirements cover construction verification, not long-term monitoring. {note}
## Where the project also includes thin-layer chemical or mechanical subgrade stabilization for pavement, that work is specified under [[sync/soil-stabilization]] and is distinct from the deep ground improvement covered here. {note}
−## Sequencing with hazardous material abatement under [[sync/hazardous-material-abatement]] shall be documented where soil mixing or jet grouting is performed on a contaminated site, so that spoil and grout-return handling, permitting, and clearance are resolved before and during installation. {note}
+## Sequencing with hazardous material abatement under [[sync/hazardous-material-abatement]] shall be documented where soil mixing or jet grouting is performed on a contaminated site, so that spoil and grout-return handling, permitting, and clearance are resolved before and during installation.
# Warranty {toc}
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## The warranty shall remain in effect notwithstanding the owner's acceptance of load test and verification results.