IAEG2006 Paper number 240

Improvement in bearing capacity of soil by geogrid - an experimental

approach
A. RAHMAN AL-SINAIDI1 & ASHRAF HASSAN ALI2
1
GOTEVOT. (e-mail: alsinaidi@hotmail.com)
2
MOE. (e-mail: ashrfhi@hotmail.com)

Abstract: When designing structures that will impose a significant load over a large area, such as buildings,
tanks, walls, slopes or embankments, geotechnical engineers must address the following situations, especially
when dealing with weak foundation soils: bearing capacity failures, intolerable total and differential
settlements, large lateral pressures and movement, and slope instability. The construction of reinforced soil
foundations to support a shallow spread footing has considerable potential as a cost-effective alternative to
conventional methods of foundation support. With this technique, more than one layer of geogrid reinforcement
is placed within layers of engineering fill material under the footing to create a composite material with
improved performance characteristics.
Here an attempt is made to present the details of an investigation of the performance of geogrid
reinforcement in soil. For this purpose, model isolated footing load tests were conducted on soil with and
without multi-layers of geogrid at different depths below the footing. The load settlement characteristic for
each soil-geogrid configuration was observed. The influence of various selected parameters on the load
settlement behavior were studied and critically appraised for their practical significance.
The paper discusses the mechanisms of this system using a large-scale model footing for a case studies in
which geogrid-reinforced soil footing is used for a schools project in Saudi Arabia. That site has a very soft
silty clay/clayey silt soil, and the large-scale plate loading test was tested according to the procedure of ASTM
D1194.
Also, the paper presents the successful application in the use of geogrid-reinforcement. The field
observations proved that the geogrid-reinforced system creates an enhancement to the very soft/soft soils and
minimizes the differential settlement. The geogrid-reinforced system is more economic and attractive and
demonstrates superior performance compared with most other ground improvement techniques and is optimal
for rapid construction and/or strict total and differential settlements of the structure and/or a thick and newly
placed fill.

Résumé: Lors de la conception des etablissements comme: Batiments,Chateaux.deau,murs soutiens , pentes ,

barrages , cela suppose que les charges importantes se repartient sur une grande superficie (surface) et
specialement les ingenieurs geotecniciens quand ils traitent le sol de faible fondation, doivient traiter les cas
suivantes: Cas dechec de support du sol par decapage, descente totale, difference d`abaissement differential,
Pressions – laterals, Grands Mouvements, Instabilite des inclinaisons des pentes. L`applicatain de la technique
de sol arme et consolide sous les foundations superficielless et peu profondes a de grandes potentialites
considererables. C`est une alternative economique pour les methodes traditionnelles dans la consolidation des
fondations , Par lutilisation de cette technique plusieurs couches de renforcement de grille Geogride sont mises
a linterieur des couches de remblai construcrives sous les fondations pour former une couche de sol fonde qui a
des caracteristiques d`execution ameliore. C`est une tentative pour presenter des details experimentals
appliques par l`ulilisation de Geogride comme moyen d`armament et d`amelioration du sol , et pour ce but on a
executer tsois (3) experience comme modele reel pour tester le chargement du sol: Le premier test a ete
effectue sur un sol consolide par plusieurs couches de grilles de Geogride aux profondeurs differentes sous la
base (foundations), Un autre test a ete execute sur un sol naturel sans amelioration, Le troisume test pour un sol
ameliore par lutilisation de systeme de colnnes pierreuses, ces deux dermiers test sont effectues au voisinage du
premier test.
On a observe et enregistre les resultas de la caracteristique d`abaissement du sol sous l`effet de la charge , et
ceci pour le sol ameliore de Geogride et pour le sol naturel. L`influlence des divers parametres choisis et
emouvants sur les valeurs d`abaissement sous l`effet de charges a ete etudie et a ete evaluee dune maniere
critique pour leur signification pratique. Ce papier discute les mecanismes d`execution pratique pour cette
exeperience , par lutilisation de grande base de chargement comme modele pour etudier le comportement du sol
ameliore par lemploie de grille de Geogride.
La compraraiossn entre sol ameliore par Geogride , sol naturel sans amelioration et sol ameliore par
l`utilisation de colomnes pierreuses sous les memes charges. Cette esperience de comparaison a ete faite dans
une situation d`un projet d`ecole en cours de construction en Arabie Saoudite. Le sol de de ce site est un sol de
limon argile/ argileux tres souple ( doux). Par la des procedures dexcperience et de test de chargement ont ete
fait par l`emploie de grande base de chargment selon les caracteristiques Americaine ASTM 1194.
Aussi ca papeir presnte l`application reussites par l`usage de Geogride comme armature el renforcement du
sol. Les resultats des experiences sur champ ont prouve aussi que le systeme de sol arme de grille de Geogride
cree une ameliorations du sol doux / tres doux et diminue les valeurs de descente totale et de descente
differentille.
Le systemc de sol arme de grille Geogride et un systeme tres economique et pratique et demontre quil est
superieur compare a la plus part des autres techanique utilisees dans l`amelioration du sol. Aussi Cest une
methode ideale pour la construction rapide et diminue la descente totale et l`abaissement differentiel des
fondations du batiment a cause des charges subites..

Keywords: settlement, strength, in situ tests, layered materials, load tests.

© The Geological Society of London 2006 1

 IAEG2006 Paper number 240

INTRODUCTION
Rising land costs and decreasing availability of areas for urban infill have established the situation that previously
undeveloped areas are now being considered for the siting of new facilities. However, these undeveloped areas often
possess weak underlying foundation materials – a situation that presents interesting design challenges for geotechnical
engineers. To avoid the high cost of deep foundations, modification of the foundation soil or the addition of a
structural fill is essential.
Binquet & Lee (1975a and b) investigated the mechanisms of using reinforced earth slabs to improve the bearing
capacity of granular soils. They model tested strip footings on sand foundations reinforced with wide strips cut from
household aluminium foil. An analytical method for estimating the increased bearing capacity based on the tests was
also presented. Fragaszy & Lawton (1984) also used aluminium reinforcing strips and model strip foundations to
study the effects of density of the sand and length of reinforcing strips on bearing capacity. Several authors also
studied strip foundations but reinforced with different materials such as steel bars (Milovic 1977, Bassett & Last 1978,
Verma & Char 1986), steel grids (Dawson & Lee 1988, Abdel-Baki et al. 1993), geotextiles (Das 1988) and geogrids
(Milligan & Love 1984, 1985, Khing et al. 1993, Ismail & Raymond 1995). Other researchers adopted circular (Rea &
Mitchell 1978, Haliburton & Lawmaster 1981, Carroll et al. 1987, Kazerani & Jamnejad 1987), square (Akinmusuru
& Akinbolade 1981, Guido et al. 1985, 1986, 1987, Guido & Christou 1988, Adams & Collin 1997) or rectangular
footings (Omar et al.1993, Yetimoglu et al. 1994).
All of these researchers concluded that reinforcement increased the bearing capacity and reduced the corresponding
settlement of the foundations compared to the unreinforced soil. However, it was also realized that an initial horizontal
and vertical movement of the reinforcement is needed to mobilize the reinforcing strength. Hence, the ultimate
bearing capacity of the reinforced earth would be increased but the initial settlement at small loads still could not be
avoided. This is important as the design of foundation systems are usually controlled by limiting the expected
settlements, which are generally about three to five percent of the settlement corresponding to the ultimate bearing
capacity. Within this range, the traditional reinforced methods cannot develop their strength sufficiently and,
consequently, the observed improvement in performance has been limited. For example, Adams & Collin (1997)
showed that using a single layer of reinforcement, the pressure producing a settlement of 0.50% of the footing
diameter, B, is between 92% and 119% of that for the unreinforced case.
The interaction between the geogrid and soil is very complex. Jewell et al. (1985) identified three main
mechanisms of interaction between soils and geogrid: (1) soil shearing on plane surfaces of the grids, (2) soil bearing
on lateral surfaces of the grids, and (3) soil shearing over soils through the apertures of the grids. The first two are the
skin friction and passive pressure resistance of the contact area between soils and geogrid. The third is the interfacial
shear on the surface of a rupture zone created during shearing.

MATERIAL PROPERTIES

Geotechnical specifications
The experiment was conducted at a school project’s site. The school project was located at the eastern region of
Saudi Arabia close to the Arabian Gulf and the soil investigation found the soil is very soft/soft, light grey/grey sandy
-4 -4
silty clay, with shells, with a thickness of 20 m. The soil had low permeability (K = 1.92 x 10 ~ 3.27 x 10 cm/sec)
and the ground water level was observed at a depth of 1.5 m.

Geogrid
A geogrid is defined as a geosynthetic material consisting of connected parallel sets of tensile ribs with apertures of
sufficient size to allow strike-through of surrounding soil, stone, or other geotechnical material (Koerner 1998).
Existing commercial geogrid products include extruded geogrid (Geogrid – Tenax LBO 330 SAMP) (which was used
in the experiments), woven geogrids, welded geogrids, and geogrid composites. Extruded geogrid are formed using a
polymer sheet that is punched and drawn in either one or two directions for improvement of engineering properties.
Extruded geogrids have shown good performance when compared to other types for soil reinforcement applications
(Cancelli et al. 1996, Miura et al. 1990, Webster 1993). Most geogrids are made from polymers, but some products
have been manufactured from natural fibres, glass, and metal strips. This paper, however, will focus exclusively on
polymer-based geogrid.

EXPERIMENTAL SET UP
Two bearing capacity tests using large-scale plate load tests were conducted on unreinforced ‘natural’ soil and on
compacted soil reinforced with geogrid. Load verses settlement curves were produced for each test. The variation in
bearing capacity over increase in settlement is represented by bearing capacity verses settlement curves.
The loading tests where conducted on two stages. In the first stage the load was increased incrementally until
reaching the load of failure; after that unloading was conducted until zero load according to ASTM - D1194. In the
second stage a quick loading was performed until reaching the load of failure followed by quick unloading reaching to
zero load according to DIN - 18134 1993. The field tests were performed at the site of the school project with isolated
square footings of a size 1 m x 1 m and 0.4 m deep (Figure 1). Testing was carried out with a Universal Testing

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 IAEG2006 Paper number 240

Machine. For accurate measurement of the load, it was applied through a jack pressure of capacity 200T (=2000 KN)
with least count 10T (=100 N). Settlement was recorded using four dial gauges of least count 0.01 mm.

Figure 1. Full scale loading test

Loading test setup on natural soil

• The dimension of the excavation was 2.5 m x 8 m and 1 m depth
• The loading continued until failure load (settlement was 25 mm)
• Maximum stress was 0.6 Kg/cm, and the settlement was 27.75 mm

Loading test setup on compacted soil reinforced with geogrid

• The dimension of the excavation was 6 m x 6 m and 3 m depth
• No compaction was performed on the natural layer of soil
• A cover of woven Geotextile was used on the top of the natural soil
• First layer of Geogrid was used directly on the top of the Geotextile
• The total depth of the construction fill was 2.0 m
•
st
The construction fill on the top of 1 layer of Geogrid was 200 mm
• The construction fill on the top of the next four layers was 400 mm each
• The total number of Geogrid layers was 5
• The loading isolated footing was on the final layer of construction fill
• Maximum stress was 4.2 Kg/cm, and the settlement was 28.85 mm

INTERPRETATION OF RESULTS AND DISCUSSION

Various useful graphs have been prepared based on the observations of the experiment. Following interpretations
have been drawn:

• The graph of load verses settlement of unreinforced ‘natural’ soil indicated failure of the soil for a settlement
2
of 27.75 mm at a stress equal 0.6 Kg/cm (Figure 2).
• Improvement in bearing capacity of reinforced soil over that of unreinforced soil was observed for all five
reinforcing layers
• The load settlement curves for the reinforced soil test continued to rise beyond the failure point of
unreinforced ‘natural’ soil at a settlement of 27.75 mm. This indicated the contribution of reinforcement in
resisting bearing pressure
• Reinforced soil is better than natural soil by 7 times and better than stone column by 1.75 times (Table 1).

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 IAEG2006 Paper number 240

Table 1. Comparison between natural soil, reinforced soil and soil enhanced by stone column
Soil situation qu qall Total settlement (mm) at Safety factor Note
Kg/cm2 Kg/cm2 maximum load
Natural soil 0.6 0.2 27.75 3 Immediate
settlement
Soil reinforced by 4.2 0.8 28.85 3
Geogrid
Soil enhanced by Stone 2.4 0.8 21.56 2 Data from
column parallel test

Figure 2. Relation between stress and settlement

CONCLUSIONS
• Considerable improvement in bearing capacity was observed in the reinforced soil compared with the
unreinforced soil.
• Cost has been reduced to be 1.5% of the total cost.
• Construction time for the geogrid was less than construction time for stone columns.
• Geogrid is expected to be more stable with time than stone columns.
• Technical work for stone columns needs proficiency but Geogrid method does not.
• Quality control and quality assurance are easier to satisfy than for stone columns.

Acknowledgements: The authors acknowledge the support provided by Ministry of Education. Thanks are extended to Eng.
Nageeb Bozgandah for his assistance.

Corresponding author: Dr A.Rahman Sinaidi, GOTEVOT, Paly, Riyadh, 11544 P O Box 55713, Saudi Arabia. Tel: +966
505467687. Email: alsinaidi@hotmail.com.

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