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CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
Forward: This report is a new and revised version of the recommendations published in the RFG No. 111 in 2005. It takes into account both the experimental results and observations of the French "Union Syndicale Géotechnique" (USG) in the interest of better understanding soil-structure interactions. This version also includes chapters on design based on laboratory and cone penetration tests (CPT). It presents the guidelines to be followed by all building contractors in France (according to Article R111-40 of the French Building and Residential Construction Rules) and oversight entities dealing with the design, calculation, installation and quality control of stone columns and their interaction with buildings and settlement- sensitive structures.
The following participated in drafting this document:
P. Aguado (Apave)
P. Berthelot (Bureau Veritas)
L. Carpinteiro (Socotec)
F. Durand (Fugro Geotechnique)
M. Glandy (Soletanche Bachy Pieux)
P. Liausu (Menard)
B. Pezot (Menard)
C. Poilpre (consultant)
S. Lambert (Keller Fondations Speciales)
J. P. Volcke (Franki Fondation)
G. Billoet (Keller Fondations Spéciales) and J. R. Gauthey (Spie Fondations) also participated in discussions.
This document has been approved by the Union Syndicale Géotechnique (USG, the French geotechnical union association) and the Comité Français de Mécanique des Sols (CFMS, the French society for soil mechanics, ISSMGE member society).
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
1 INTRODUCTION
(1) The columns referred to in these guidelines are vertical piles made of cohesionless granular materials that are driven into the soil and then compacted by repetitive action.
Comment #1: These columns do not contain any particular type of binder at any level. In this document, the term “stone column” is limited to the definition above.
(2) The columns can be constructed in regular or variable grids in rows or groups, or even in isolation.
(3) Their design takes into account the type of structure to be built, the kind of load, tolerances, and requirements regarding absolute and differential settlement, as well as the nature and characteristics of the soil to be treated.
(4) The purpose of stone columns is to give the soil under the structure to be built new general and/or local characteristics so that the structure’s various infrastructure elements (isolated or strip footing, raft foundations, concrete slabs, embankments, etc.) will demonstrate predictable and justifiable behavior consistent with the regulations and tolerances that apply to the structure of the building and its intended use.
(5) Treating soil with stone columns involves a combination of the following actions, of which one or more may be intended:
Improving bearing capacity (^) Reducing settlement Improving uniformity of geotechnical characteristics Increasing the consolidation rate by creating drainage elements Increasing equivalent ground mass characteristics (horizontal shear strength, angle of internal friction and deformation parameters).
(6) A stone column is a soil improvement measure. It is neither a foundation element, nor a deep foundation element.
(7) The foundation of a structure built on soil treated by stone columns is always superficial (isolated or strip footing, raft foundations, concrete slabs, etc.). It can also be part of the “foundation” of an embankment.
(8) The objective is to control the behavior of these structures, and in particular to reduce the settlement.
Comment #1: Depending on the existing grid density and geotechnical conditions, this coefficient is usually between 1.5 and 4 under distributed loads.
(9) The need for soil treatment should be analyzed by the project’s geotechnical engineer, who should provide and approve the calculation hypotheses.
Comment #2: A reminder: the stone column geotechnical study should examine not only the soil to be treated, but any soil that could potentially settle under the treated layer.
Comment #3: The contractor must involve the geotechnical engineer in the design and the execution of the project in compliance with standard NF P 94-500 concerning geotechnical procedures in France.
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
2 FIELDS OF APPLICATION
2.1 By type of structure
(1) The most common uses of stone column treatment involve structures with concrete slabs or raft foundations with surface loads sensitive to settlement:
Hangars and warehouses Industrial and commercial buildings (^) Silos and tanks of all kinds Watertight hydraulic structures (tanks, sewage treatment plants, etc.)
(2) By extension, they can be used under other types of structures, too, as long as the residual deformations of the treated soil and underlying layers are consistent with the structure in question and with associated technical measures:
(^) On-shore civil engineering works (roads, embankments, bridges, retaining walls), and maritime structures (seabed, lake and riverbed reinforcement), Under shallow building foundations.
(3) They can also be used for stable heterogeneous embankments where systematic treatment with an appropriate and regular grid can help improve and/or homogenize the soil characteristics to make it suitable for surface construction.
2.2 Use in seismic zones
(1) It is also possible to use stone columns in seismic zones where they may reduce the risk of soil liquefaction and increase shear resistance.
(2) Refer to the guidelines entitled “ Procédés d’amélioration et de renforcement de sols sous sollicitations sismiques ” published by the French Association of Earthquake Engineering (AFPS).
2.3 Limitations of use
(1) The lateral earth pressure exerted by the surrounding soil is a determining factor in column construction, behavior and capacity.
(2) Therefore:
Stone columns must not be used in soil that exhibits a potential risk of a loss in volume and/or mechanical soil characteristics, especially waste dumps and peat deposits. More generally, stone columns should not be used in soils with a loss on ignition of more than 5% as specified by standard XP 94-047.
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
Los Angeles test loss < 35% Micro De-val test loss < 30% Combined Los Angeles + Micro D-val test loss < 60%
Comment #1: Los Angeles test, standard DIN EN 1097-2; Micro De-val test, standard NF EN 1097-1.
(4) The particle size test depends mainly on the backfill material. Bottom-feed vibrators are more sensitive and an unsuitable particle size can plug the pipe.
(5) The following indicative values can be used as a reference:
(^) Bottom-feed vibrators with a material transfer pipe on the side: the most commonly used particle size is 8/ Other methods: the most commonly-used particle size is 20/
(6) The standard for particle homogeneity: less than 5% particles smaller than 80 Pm.
4 CONSTRUCTION PROVISIONS
4.1 Diameter of stone columns
(1) The diameter of the stone columns depends on:
the device used and its suitability to the soil at the site, the layers of soil penetrated and their characteristics, (^) the total amount of energy used (power output, vertical force and running time).
(2) The diameter of the column may vary over its length, depending on the resistance of the different layers penetrated.
Comment #1: The diameter of most dry method columns is between 50 and 80 cm.
(3) All things equal, the diameter of wet method columns is usually greater than dry method columns due to the soil extraction caused by water jetting.
4.2 Interface between the treated soil and the building structure
(1) If the foundation element does not have enough inertia to distribute load bearing in a homogeneous way on the initial column grid, it is necessary to add a load transfer platform between the foundation elements and the treated soil. The purpose of this platform is to improve the load bearing distribution.
(2) If the stone columns are being used for their draining properties, a drainage layer (with outlet) should be added at the top of the columns.
Comment #1: In practice, isolated and strip footings and raft foundations do not require a load transfer platform, as opposed to concrete slabs (reinforced or not) which do.
(3) With few exceptions, in general, soil treatment with stone columns does not in and of itself improve the behavior of surface soil between columns where there is no increase in the values for either the EV2 modulus or the Westergaard coefficient.
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
4.3 Installation of footings and raft foundations
(1) Footings and raft foundations must be constructed in the same way as on untreated soil. They can be cast directly into the trench or into a formwork.
(2) When the footing base is less than 50 cm below the work platform level, the head of the column must be re-compacted to ensure the characteristics specified in the calculations.
Comment #1 : This re-compacting can be done in conjunction with platform re-compacting.
4.4 Installation of a load transfer platform
(1) The minimum thickness for a gravel load transfer platform to distribute the load is 40 cm.
(2) In the case of concrete slabs, the top layer of gravel should at least have the characteristics of subgrade as defined by DTU 13.3 and Guide GTR92.
Figure #1: Load Transfer Platform
Comment #1: For the purposes of this document, the following should be noted for the subgrade layer:
(^) Class F materials are not permitted (according to standard NF P 11-300) The minimum thickness is 25 cm The modulus of elasticity EV 2 is greater than 50 MPa
(3) It is up to the soil treatment designer to define the thickness and the minimum characteristics of the platform according to the nature of the treated soil.
Comment #1: Bear in mind that the criteria applied to gravel layer calculations are its punching resistance (if necessary), its thickness and the modulus of elasticity.
(4) The concrete slab designer must verify that the load transfer platform’s thickness and characteristics are sufficient to resist the pressure exerted by the concrete slab, including the punching resistance, and that the values remain above the minimum concrete slab requirements (standard NF P 11-213 ref. DTU 13.3 on concrete slabs).
(5) The load transfer platform can be partially or entirely installed before the stone columns and therefore can be used as a work platform.
DISTRIBUTION LAYER
TOP LAYER
CONCRETE SLAB
TREATED GROUND
LOAD TRANSFER PLATFORM
Thickness Q 25 cm
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
Figure #2: Case of columns lying outside of footings
(4) For columns that are not aligned, it is recommended to adopt the same configuration (footing edge equals at least the installation tolerance). Otherwise, the column settlement and stress must be verified if the columns do not conform to the theoretical layout.
Comment #1: Appendix I includes some standard configurations.
Comment #2: Under strip footings, placing columns in alternate rows limits the incidence of column deviation.
4.6 Maximum reference grids:
(1) For soil to be considered treated by stone columns, and whatever the intended objective, the maximum reference grid should be 9 m² and the substitution rate should be greater than 3%.
(2) For strip footings with a single row of columns and no load transfer platform, the maximum grid spacing without specific justification is 2.5 m.
4.7 Minimum reference grids:
(1) The minimum reference grid should be 2.25 m².
(2) For strip footings and groups of two to five columns, the spacing between the columns axes cannot be less than 1.5 times the diameter of the stone column, or 1.20 m.
Comment #1: A high substitution rate can lead to a risk of raising the platform.
Comment #2: To address specific problems (if part of structure is too small, for example), it is possible to reduce the spacing in (1) and (2) by conducting a feasibility study.
Planned position
Part of column that does help increase the footing bearing capacity (case of lightly-loaded footings)
e = position tolerance (usually 20 cm)
Part of the column that does not contribute to increasing the footing bearing capacity.
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
Comment #3: For columns executed with the wet method, a process rarely used for earthen sites due to the soil extraction required, it is possible to use larger diameter stone columns (1.00 to 1.20 m) more closely spaced than those installed using the dry method.
4.8 Stone columns under an embankment supporting a foundation
(1) It is possible to treat all types of soil in the scope of application with stone columns, and then build a substitute or raised embankment on the improved soil using fine, quality- controlled materials and demonstrating the utmost care during installation and strict attention to compaction, for example according to the 1980 recommendations by COPREC LCPC.
Figure #3: Installation of stone columns under an embankment supporting a foundation
(2) The configuration of the columns takes into account the superposition of stresses generated by the embankment itself and also the various structural elements built into the fill thickness. This configuration requires justification in particular if the height of the embankment is less than half of the reference grid measurement.
4.9 Tolerances
4.9.1 Columns in grid networks
(1) Due to their own characteristics, stone columns can provide new characteristics to soil that are equivalent to soil’s basic lattice structure located at its center.
(2) In this case, the concept of column installation deviation does not apply, though it is applicable for localized foundation elements.
4.9.2 Columns under footings
(1) The isolated and strip footings installed on treated soil must be considered foundation elements.
(2) The tolerance for stone columns is therefore +/- 20 cm.
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
(2) In general, the justifications that need to be provided in the calculations are based on these two criteria:
The overall allowable bearing capacity for the improved soil after treatment and justification of the different types of foundations regarding failure, and The absolute settlement of the various structural aspects for each construction and justification of the differential settlement for the structure, or between the structure and the concrete slab according to the allowable tolerances for each construction and any relevant regulations in effect. (3) When there are other objectives set, the following additional information must also be provided:
In the case of soil liquefaction, it must be shown that the columns will minimize this risk. If the columns are intended to act as vertical drains to accelerate consolidation, the consolidation time is calculated. In the case of slope stabilization, the calculation compares the factor of safety to the circular failure rate.
5.3 Hypotheses
(1) The building construction constraints (maximum bearing capacity and allowable deformation) must be specified in the documents specific to the project.
(2) The soil behavior hypotheses are the geotechnical characteristics identified from soil tests: stratigraphy, soil composition, hydrology, rheological and mechanical properties (Young’s modulus, Poisson’s ratio, lateral earth pressure, compression index Cc, void ratio, etc.) for all soil layers.
Comment #1 : When the data result from correlations, it is best to obtain written consent from the geotechnical engineer attesting to the validity of the hypotheses under consideration.
(3) The hypotheses concerning the stone columns, especially in terms of their modulus of deformation, diameter and length depend in large part on the material used and its performance.
(4) Common values for non-geometrical parameters used in the calculations for the backfill materials described in § 3.3 and correctly employed are as follows:
Young’s modulus: average over column volume Ecol = 60 MPa
Inter-granular internal angle pea gravel (^) ’c = 38 degrees
crushed stone gravel (^) ’c = 40 degrees
Poisson’s ratio (^) col = 1/
Unit weight of saturated material in place (^) col = 21 kN/m 3
Table #1: Common column characteristics
Comment #1 : Higher or lower values are possible, but must be explained by appropriate tests.
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
5.4 Maximum allowable stress for stone columns
(1) To calculate the maximum allowable stress, first determine the vertical stress-rupture point qr for an isolated column based on the characteristics of the column and of the soil after treatment, and according to the possible modes of failure illustrated below (Figures a, b and c; see Soyez 1985):
Bulging failure (a criterion often used to determine size) General shear failure (failure is rare, occurring when columns are short) Punching effect shear failure (“floating columns”)
Figure #4: Failure modes
5.4.1 Bulging failure
(1) By analogy with the triaxial conditions, the effective lateral expansion failure stress qre is given according to the depth and radial stress r by:
qre = r. tan^2 (/4 + ’c /2) Greenwood (1970)
(2) The radial stress value results from the geotechnical context and is estimated based on laboratory tests (triaxial tests) or in situ tests (pressuremeter, vane tests and cone penetration tests) as part of the project’s geotechnical study.
Comment #1 : For example, for the pressuremeter, the radial stress is estimated as follows for each soil layer:
r # ple*
If in addition, ’c is around 38 degrees, then:
qre # 4. ple*
Where ple*^ is the equivalent net maximum pressure calculated according to the following formula:
p (^) le*^ = min (ple*^ [z] over the length of the column at each layer with ple*^ [z] = geometric average of pl*^ values over a length of 2 SC centered on z, but limited to 1.5 times the lowest pl*^ value for a length of 2 SC.
a) column bulging failure b) general shear failure c) punching effect failure (floating column)
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
(4) The cohesion values (C (^) u , Cup and Cum) result from the geotechnical context and are estimated based on laboratory tests (triaxial tests) or in situ tests (pressuremeter, vane tests and cone penetration tests) as part of the project’s geotechnical study.
Comment #1 : For example, for the pressuremeter the following can be used:
C (^) u # pl*^ /5.5 when pl*^ < .3 MPa, Cu in MPa C (^) u # pl*^ /10 + .025 when pl*^ G 0.3 MPa, Cu in MPa
Comment #2 : For example, for CPTs the following can be used:
C (^) u # (qc - po ) /15 where po is the total vertical stress for the level in question
Comment #3 : Under the footings, the columns are always considered non-floating according to the following criterion.
A non-floating column is defined as one that stops within a horizon characterized by C (^) up greater than or equal to 150 kPa (or approximately p (^) l*^ G .8 MPa or qc G 2.5 MPa) or such that 9. C (^) up > qr
Comment #4 : In all cases, it is necessary to calculate and take into account the settlement of the soil underneath the columns.
5.4.4 Stresses within the columns
5.4.4.1 Failure stress
(1) Vertical failure stress qr within the column is equal to:
q (^) r = min(qre ; qrp ; 1.6 MPa)
5.4.4.2 SLS stress
(1) At the service limit state (SLS), the allowable vertical stress qaSLS within the column is obtained by applying a safety factor of 2 to the vertical failure stress q (^) r :
qaSLS = qr /2 = min(qre/2 ; qrp /2 ; 0.8 MPa)
5.4.4.3 ULS stress
(1) The maximum stress for calculation qaULS within the column is obtained by applying a safety factor of 1.5 on the vertical failure stress qr :
qaULS = qr /1.5 = min(qre/1.5 ; qrp /1.5 ; 1.06 MPa)
Comment #1 : This means applying a coefficient of 1.33 to the allowable stress at the ULS:
^ qaULS = 1.33. qaSLS
5.5 Evaluating stress and settlement at the service limit state (SLS)
(1) The methods described below (§ 5.5.1 and 5.5.2) are usually only valid if:
The additional load on the soil between the columns (calculated according to the aforementioned methods) remains inferior to the allowable stress for the untreated soil The column bases rest on a more compact ground layer.
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
Comment #1 : A layer is considered more compact when it is characterized by C (^) up greater than or equal to 150 kPa (approximately pl*^ G 0.8 MPa or qc G 2.5 MPa) or such that 9.Cup > q (^) r.
(2) They are based on the soil elasticity hypotheses and on the conservation of flat, horizontal sections and depend on the stiffness of each element (soil and columns).
Comment #1 : As part of pressuremeter and cone penetration test methods, to remain within the elastic range under the foundation element (raft foundation, concrete slab, footing, etc.) it must be verified that in the SLS:
qsol < kp. pl /2 + q’ 0 for pressuremeter tests q (^) sol < kc. qc /2 + q’ 0 for CPT
where qsol is the maximum stress transferred to the soil between the columns.
(3) Other methods are also available. It is necessary to use the orders of magnitude obtained under special conditions as follows.
Comment #1 : These calculation methods should make it possible to verify the bearing capacity and settlement criteria described in § 5.5.1, 5.5.2 and 5..
5.5.1 Homogenization method for concrete slabs and raft foundations subject to uniform, infinite loads
(1) After constructing the columns, the settlement for each layer i at the center of the building is expressed as follows:
hi. t wi = ---------------------------------------------------- ai. Ecol + {(1- ai ). Esi. (1-si )/(1-si -2si ²)}
and the value of the stress within the column at the layer i (ci ) can be expressed as:
Ecol. t ci = ---------------------------------------------------- ai. Ecol + {(1- ai ). Esi. (1-si )/(1-si -2si ²)}
where:
ai : the replacement ratio for layer i Ecol : Young’s modulus for the column Esi : Young’s modulus for layer i si : Poisson’s ratio for layer i t : Average vertical stress exerted by the building hi : thickness of layer i
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
The above formulas become:
. hi. t
wi = ---------------------------------- ai. Ecol + {(1- ai ). (^) c. qci }
and the stress within the column at the layer i ( ci )can be expressed by:
. Ecol. t
ci = --------------------------------- ai. Ecol + {(1- ai ). (^) c. qci }
Comment #4 : Since pressuremeter and CPT correlations can be difficult to make for certain types of soil (saturated soft clay, unsaturated soil, etc.), oedometric moduli measured in a laboratory may be preferable.
(4) The stresses should remain below the maximum allowable values specified in 5.4.4.2:
ci < qai
The total settlement (Zwi , increased if necessary by the settlement of layers under the bases of the columns) should remain below the values set by the operating conditions.
Comment #1 : If the load varies from one area to another and/or if the stratigraphy is not uniform, it is necessary to also check if the differential settlement values are acceptable.
Comment #2 : If the column bases do not rest on a more compact layer, a specific explanation is offered.
5.5.2 Columns under footings with vertical and centered load bearing
5.5.2.1 SLS loads
(1) The steps for calculation are the following for a given footing (whose surface S (^) s = B. L) supported on n columns (with area S (^) col ), bearing surface load q (^) SLS.
Comment #1 : In all cases, it is necessary to calculate and take into account the settlement beneath the columns.
CONSTRUCTION AND QUALITY CONTROL OF STONE COLUMNS
UNDER BUILDINGS AND SENSITIVE STRUCTURES
Step General principle Special cases: Based on pressuremeter tests and/or static CPT results 0 First check the condition: {n. Scol. q (^) a + (Ss – n. Scol). q’u/3} > q (^) SLS. Ss with q (^) a maximum allowable stress within the colum (see §5.2) and q’u soil failure stress before improvement under a centered load. n = number of columns, Ss = surface of the footing.
Comment #1 : This value q’ (^) u must be provided or approved by the geotechnical engineer.
1 Calculate the settlement ws without treatment according to the rules in effect. Thus the following can be determined: ks = q (^) SLS /ws
Based on a pressuremeter test: ws = q (^) SLS (B. As / E (^) c + Ad. B /E (^) d) with As =. c / Ad = 1.2 (d/0,6) / 9 and where E (^) c and E (^) d are the equivalent pressuremeter moduli corresponding respectively to the areas of spherical and deviatoric stress. Based on a CPT: ws = C. q (^) SLS. B / E (^) sol with E (^) sol = (^) c. q (^) c where (^) c see § 5.5. and C = 0.5 for isolated footings and 1.1 for strip footings. 2 The settlement equation for a column wcol with stress at the head, q (^) col wcol = '. q (^) col. H / E (^) col where H is the level at which the settlement is calculated ' is a ratio that shows that there is a distribution of stress from the column to the soil. Comment #1 : In practice, H = min(2.5B; Lc ) is used since over 85% of soil settlement occurs between 0 and 2.5B Comment #2 : As an initial approach, use ' = 1 (no distribution); when the distribution can be calculated, there is ' = q moy /q col where q moy is the average of stresses in the column. For a homogenous soil over 2.5 B, 'mini = 0. 3 The column stiffness can be deduced and expressed as: kcol = q (^) col /wcol = E (^) col / ('. H) 4 The stiffness of the entire “footing + column” structure at the height in question: k = {ks. (Ss – n. Scol ) + (n. k (^) col Scol )}/ (B. L)
