Publisher’s version / Version de l'éditeur:
Roads and Engineering Construction, 94, 7, pp. 92, 94, 96, 121-26, 1956-11-01
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
NRC Publications Archive
Archives des publications du CNRC
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Techniques of road construction over organic terrain
MacFarlane, I. C.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=09411e9d-2c9b-4f2a-9d6e-48bbd74a7688
https://publications-cnrc.canada.ca/fra/voir/objet/?id=09411e9d-2c9b-4f2a-9d6e-48bbd74a7688
Ser
THT
N21t2
a o . h 5
e . 2
BI,DG
NATIONAL RESEARCH
COUNCIT
CANADA
Techniques o,
Rosd Construction Over
Orgonic Terroin
A N A L Y Z E D
By l. C. ltocForlone
TECHNICAL PAPER No. 45
of lha
DIYISION
OF BI'ILDINE RESEARCH
BUILDING
RESEARCH
- LlPr'4 PY '
iAN I
Iq57
TIATOITAT RESEARCH qOUNGII:
Reprintcd frorn
"loodr and Engineering Coarlruclion" Vol. 94, No. 7, July, 1956
PRICE IO CTNTS
PRINTED IN CANADA
N l c 4 t 2 7
This publication is being distributed by the Division
of Building Research of the National Research Council
as a contribution towards better building in Canada. It
should not be reproduced in whole or in part, without
permission of the original publisher. The Division would
be glad to be of assistance in obtaining such permission.
Publications of the Division of Buildins Research
may be obtained by mailing the appropriat'e remittance,
(a Bank, Express, or Post Office Money Order or a cheque
made payable at par in Ottawa, to the Receiver General
of C;anada. credit National Research Council) to the
National Research Council, Ottawa. Stamps are not
acceptable..
A coupon system has been introduced to make pay'
ments for publications relatively simple. Couponb are
available in denominations of 5, 25, and 50 cents, and
may be obtained by making a remittance as indicated
above. These coupons may be used for the purchase of
all National Research Council publications including
specifications of the Canadian Government Specifications
Board.
Techniques
ol
Roqd Construction Over
Orgonic Terroin
BY lvon C. llqcForlqne
Secrefqry, Muskeg Subcommitlee, Associote Commiltee on Soil qnd Snow lilechonics' Nqtionsl Reseorch Council. Ottowo, Onf'
This is a review of the methods used by various countries and
organizations in overcoming the peculiai problems of buildjng
,oid,
".rors
what is ambiguo-usly termed mt'ts[<eg,
peat bog, meadow
bog, swamp, marsh, fenr-heath, barren, moor - to mention a few
of iire mori-polite iet-r.
The article comprises- a. paper.presented
at the Eastein Muskeg Research Meeting, held in Quebec, Que.,
early this year under the auspices of the Associate Committee on
Soil'and Snow Mechanics of-the National Research Council.
For engineering purposes "muskeg" (or "organic terrain" as it has now become knolvn) may be roughly de-fined as "that terrain which is made up of a living organic mat of mosses' sedges and/or grasses (with or with-out tree growth) underlain by an ex-tremely compressible mixture of partly disintegrated and decomposed organic material." It is'characterized by low bearing capacity and abnormally high water content. The depth of such a deposit of vegetable matter (or "peaty material") may vary trom a few inches to many feet. The mineral soil sub-stratum is usually a clay, silt, or a silty clay, but in some instances is a sand or gravel.
The Muskeg Problem
The word "road" is used here in the popular sense of the term, mean-ing "any 'track that is used for travel, conveying goods, etc." It applies
equal-ly well to the bush road as to a Part of the Trans-Canada Highway system. Considering the definition of organ-ic terrain (muskeg) and knowing its nature, it is apparent that this terrain is highly unpredictable and generally cannot be expected to furnish a very firm or dependable base for a road. Muskeg is normally unstable under ap-plication of load so that normal means of road construction (e.g., earth or rock fill dumped on the ground sur-face and spread evenly) are usually inadequate for organic terrain areas.
In general, there are two types of failure of roads built over organic ter-rain: (a) failure by lateral flow (or shear), and (b) failure by compression
(excessive se,tdement).
Lateral flow failure is the more likely type in very wet muskeg. The gravirational force of the fill placed on the surface of the organic deposit is transmitted laterally through the
sub-surface (organic) material, thereby "squeezing" this material out from beneath the fill. The fill is caused to subside and heaving of the surface to the sides of the embankment fre-quently results. This effect is sketched in Fig. 1.
In drier muskeg areas, failure maY be caused by the fill, because of its weight, penetrating into the muskeg, thereby causing subsidence of the em-bankment as A result of the consolida-tion of the organic material and some-times of the substratum as well.
The choice of a particular method of construction will depend a great deal on local conditions, so that con-struction should always be preceded by an accurate surveY of the area. The depth of the oiganic deposit should be carefully determined, and a profile of lhe contours of the substratum should also be plotted. Ttris cannot be over-ernphasized as a means of achieving a design for the proposed road that will reduce the possibility of later difficul-ties.
There are three main ways to con-struct a road over organic terrain:
l. Floating the road on the muskeg; 2. Removing the unstable material
and replacing it with fill; or
s-.(
a
\
O R G A N IC
. M A T E R I A L
\ . 1 J\ ,
-t{--\
. t . \, \
S - \ 4
t
M I N E R A L S O I L B A S E
Fig. l-Efrect of fill on organic mat€rial (latenal flow failurc)'
Rocd Construction Clver Orgonic Torrain
Excav-ation- of muskeg and replacement Construction in muskeg unavoidable Pile construction llnderfill blasting: Relief method Floating rmd on muskeg Muskeg stabilization Spreadfoundation Othermaterials:
Peat Straw Wire mesh Lateral support
I
Chemical stabilizationI
Sand drainsI
Natural drainage( c )
as ev€nly as possible over the muskeg and thus reduce differ' ential semlement; and
3. To prevent the fill material from penctrating the surface of the muskeg and sinking into it. Considerable labour is involved in making and laying corduroys, so the method is cosdy, and its use for major roads is decreasing. Normally it is used only on minor roads, but this method of construction was often uscd on the Alcan Highway whcre muskeg was cncoumercd.
Materials other than logs and brush-wood have bcen used to increase the
( D )
Fig. 2-Operation of undenfrll blasting method.
buoyancy of thc surface mat of the muskeg. In the U.S.A., wire mesh has been used occasionally for this purpose. Dried bundlcs of peat have becn used as a base for the fi'll in Scandinavia and Holland. and in G,reat Britain bundles of straw have been used.
Lqfcrol Support
When roads that must sustain heavy traffic loads are floated on muskeg, it
-.nl
t t
P E A T , .
I.T
( B )
1
^l
may be nccessary to provide lat€ral sup-port to prevent failure of the subsur-face material. This can be accomplish-ed by buildiqg countcrvveight fills on either side of the roadway to stabilize the unstable organic material beneath the main cmbankment. An dternative m€thod - used in Scodand - is to provide a counterweight by digging a trench on either side of the roadway embankment and filling it with rock.
. 1 . " \
PEAT
. )
t . (
M A I N
t' P E A T
f f i l r j t '
\.
. . f
)
-f
/-it I
t t .(
: Y
1
fli
PEAT
\ \ t
' 1
P E A T .
( \ ,
Ia
-J
,r
Ercovotion ond Replocement by Stoble toteriol
The most obvious approach to the stable material and to backfill with whole problem of construction over mineral soil. This method is considered organic terrain is to exavate the un- by many engineers to be the orily
F I L L
{'n
t
i _:"1j::'
. ^ 1 +
P E A T
T'- |
. \ /
)
Fi,g. &-Toc ohooting method of blasting.
i
S U B S I D
I A R Y C H A R G E S
( A )
P E A T T C . i
z t( A )
( B )
really dependable way of building a perman€nt road over muskeg. How-ever, it can be economically feasible only for shallow depths. Opinion dif-ters as to what the maximum cconom-ic depth should be: it varies from 6 to 12 ft. in the United States; 18 ft. is recommended in Germany and 12 ft. in Holland. Excavation may be carried out most efiiciently by mechanical means (dragline), but it is difficult because the trench fills with water and the sides slough in. Another method of excavation is to use explosives in the so<alled "trencih meth,od of blast-ing". This usually entails blasting a trench (or ditch) about 50 ft. in length and immediately backfilling it with stable material. Trench blasting is recommended when the muskeg is quite firm and will not slip, and where the organic material is not more than 12 ro 15 ft. deep.
Pal{,isl Excqv,ofion
In this method only part of the un-stable material is excavated. The amount removed may vary from a removal of the surface mat to a fairly deep excavation. Such a procedure is sometimes followed in fairly shallow deposits where the type of constructron does not warrant the cost of a com-plete excavation. The same methods for removing the organic material may be used as in total excavation.
Regardless of the method of excava-tion, the fill material should be a porous soil, such as sand or gravel, to provide adequate drainage. Clay fill is not recommended. Since the fill does not setde to the bottom of the organic deposit, the road is actually floating in it. The embankment is supported by the side and upward pressure of the surrounding peaty material. The fili will continue to sink un'til ,pressure is equalized and a certain degree of stab ility is reached.
Displocement lllethods
Grovity Displccement
When an earth fill is placed on the surface of an organic deposit, it will inevitably setde under either equil-ibrium is reached or the fill reaches the solid substratum. This may take years. The natural period of settlemenr
due to gravity may be accelerated in a number of different ways, all of which involve displacing the uns'table organic material from beneath the em-bankment after it has been placed. A surcharge of 15 to 20 ft. in excess of the fill may be used to accelerate settle-ment. The excess is later removed when the fill has setded adequately. If the peaty material is too sti{f to be easily displaced by the weight of the embankment and a surcharge alone, it can be softened by impregnating it with water, a process known as "jet-ting". After an embankment has been built across a muskeg area, water iets are directed through the fill into the organic material below. These iets are sunk rapidly to the bottom of the or-ganic deposit, then slowly withdrawn. The increase in water content con-siderably reduces the stability o{ the organic material so that the embank-ment displaces it and the fill settles to the bottom. This method is consider-ed by some American experts to be especially suitable for displacement of deep deposits of fairly soft organic material.
The Michigan State Highway De-partment uses a water jet method ex-tensively, but the principle is different from that oudined above. The em-bankment is constructed across the muskeg, and about l0 ft. of surcharge is added. This fill is then saturated to a n€ar "quick" condition by proper arrangernents of the water jets. The increased weight of the embankment due to the water displaces the peat and causes the fill to settle. The surcharge is then removed and the fill brought to grade.
Bog Blosfing
The use of explosives is a means of displacing the organic material from beneath the embankment where other methods are unsuitable. There are two main variants to the blasting method: underfill blasting, and the toe shooting method.
Underfill Blosting
Before the embankment is placed, it is usual to break up the surface mat by light charges. This prontotes an even settlement of the fill. After the required quantity of the granular fill
material has been placed on the sur-face of the muskeg, explosives are placed beneath the embankr.nent at or near the bottom of the organic deposit. This may be accomplished by water iets or by the use of casings, driven down through the fill and the organic material. The amount of explosives used and the num,ber of holes will depend upon the depth of the muskeg and the height of fill. The usual pro-cedure is to have one or more rows of charges along the center-line of the embankment and a further row or rows of subsidiary charges just outside both sides of the fill. To permit a more effective displacement, the outside charges are detonated a moment before the main center charges. The force of the explosives is confined from above by the fill and from below by the hard bottom, so it follows the path of least resistance - to the sides. The blast displaces the organic material un-der the fill to one side, liquefies the surrounding material, and creates a cavirybeneath the embankment, allow-ing it to settle rapidly to solid bottom. This method of displacement is said to {rc effective for depths of unstable material up to 30 ft. It has been used successfully in the United States (espe-cially Michigan), Eire, and Germany.
Fig. 2 is a series of diagrammatic sketches to illusrate the underfill blast, ing method.
The relief method of blasting is a combination of the underfill and trench systems of accelerated settle-ment. The fill is placed on the surface of the muskeg (the surface mat being broken up as before). Ditches are then dug on both sides df the fill, either by mechanical means or the trench blast, ing method. These side ditches relieve the lateral pr€ssure so that the weight of the fill can more easily push out the underlying soft material ro rhe sides. Setdement can be allowed to take place naturally or may be acceler-ated by detonation of light charges under the fill and beneath the ditches to aid in liquefaction of the organic rnaterial.
Toe Shoofing Method
In the toe shooting method of blast-ing, t'he unstable ,material ahead of the
advancing fill is 'blasted: More fill is then dumped into the cavity left by the blast and is advanced with a "V" point until it forces the peat up in a wave ahead of it. A surcharge oi fill is add-ed and charges are placadd-ed around the toe of the fill. These are detonated and the organic mater;al is blown out ahead of the fill. The fill is built up again and the process repeated. This mothod is used for soft peats up to a depth of 20 ft. Fig 3 illustrates the toe shoo'ting method.
For deeper deposits (up to about 50 ft.) a similar procedure is followed, known as torpedo blasting. Instead of the whole charge being at the bottom of the organic deposit, sticks of ex-plosives are tied at various points on a long pole about 10 ft. in length. Sev-eral of these "torpedoes" are placed upright in the unheaved peat at the end of the fill, and are then detonated. The resu,lts are similar to the toe shooting method.
Pile Construction
Pile founda,tions transrnir the weight of trhe road directly to tlre firrn strarum underlying the muskeg. By this method, all setdement is eliminated and there is a minimum of disrturbance of the peat during construction. Its use has been limited chiefly to roads in built-up areas where other methods of construction are inconvenient. This method is very expensive and its use has been restricted fairly well ro Eur-ope, particularly Holland, although the method has been used in the United States. One case is recorded of creo-soted timber piles being driven through the organic material and then capped by longitudinal reinforced-con-crete beams carrying qhe pavement slab.
Stqbilizotion of tuskeg
Sur{oce DroincAe
Drainage of a muskeg area is usually extremely dif{icult and often imposs-lble. Much of the literature on this important aspect of organic terrain concerns the drainage of bogs for reclamation of the peat as a fuel. Road engineers can learn much albout drain-age methods from the peat industry and agriculturalists, Flowever, even
when circumstances are such that sorne drainage can be carried out, usually only the surface can be drained. Be-cause of the way in which the water is retained, the water content of the peat is affeoted for only a very short distance away from the drainage ditches.
A number of factors contribute to the difficulty of draining an organic deposit by normal methods of dminage such as ditching and well-points. Fre-quendy the deposit is in a lowJying area and there is no ou,tlet for the drainage ditches. The most important factor of all to consider, however, is the nature of the material itself. Of particular relevance to the problem of drainage is the manner in which the peat holds water. Even when there is an oudet for the ditches, drainage can at best remove but a fraction of the water in the organic material.
There are various com,binations of water in peat; a brief review of these will illustrarte the difficulties involved in trying to drain this material. Water is held in five combinations:
I. Water of occlusion (water present in the larger cavities in the peat, as in a sponge; can be parrtially re-moved by drainage, especially if there is a load on the muskeg to squeeze out the water);
2. CapiLlary water (water present in the flbres and tissues drat make up the organic material, as in a blottcr);
3. Colloidally bound wa,ter (water present in the cellulose gels); 4. Osmotically bound water; 5. Chem'ically 'bound water (water
of hydration).
In drainage techniques all but (l) and (2) are of academic interest only. The pro,blem is to remove the water held by those two com,binations.
Attempts are frequendy made to drain an area of organic terrain prior to construction. espocially when the flotation method is contemplated. There are two schools of thought on the matter. One recommends that the area be drained as well as possible be-fore construction, hut on the embank-ment (or other form of road construc-tion) that rhas been floated on the muskeg no further attefixpts at drainage
should be made, excopt to i€lrriove excessive sulface water d,ue to rainfall.
The other ,point 'of vievr' advocates no drainage at ai'I, except to drain exc€ssive surface water.
The first opinion seems to be the more prevalent at the present time. In Great Britain and Canada it is a fairly common prac,tice to dig quite deeP ditches bes'ide roads which are floated on organic terrain. Although the up-per layers of the organic material are drained thereby and the shear strength is increased (and therefore the stabil-ity), there are certain disadvantages. The drainage is an extremelY slow procedure and involves considerable volurne shrinkage of the peat and sub-s€quent settlement of the road. Also, by digging a ditch adjacent to the road, much of the lateral supPort of the peat is removed, increasing the danger of failure.
A compromise solution is the use of double drainage. A ditch about 50 ft. to either side of the road is used to drain the excessive surface water. The ditches adjacent to the roadway then have a fairly constant depth of water which minimizes al'ternate swell-ing and shrinkage of the peat under the road.
Sond Droins
The sand-drain method of sta'biliz-ation was developed specifically to accelerate settlement and consolidation of marshy areas and has been used extensively for silty soils. Its use has m,any'advocates in the constrtlction of roads over certain types of organic terrain. The principle is very simple. The time required for consolidation of a soil varies as the square of the leng'th of the escape path of ttre soil moisture. The purpose of vertioal sand drains is to reduce the distance the water has to travel by permitting it to travel horizontally as well as vertically, thus shortening the period of consolidation. Vertical hoiles are dug 'into the soft foundation and are backfilled with a cloan sand. To allaw a horizon'tal flow of water as well as a vertical flow, 'a sand iblanket, 2 to 8 ft. thick, is spread over the sur+face of the ground. The embankment is then built on top of this sand blanket. The weight of the
fill squcczes the watcr from thc organ-ic material and into the vertorgan-ical sand columns, where it rises, theoretically, to the level of the sand blanket and drains away. The sand columns also serve to increase the stability of the organic material in the initial sages of consolidation, preventing it from failing in lateral shear due to the weight of the embankment.
The sand-drain method has been used chiefly in the U.S.A., bu,t has also been used to a ,limitod ex't€nt in Eur-ope and South America.
Chemicol Stobilizqlion
Considerable success has been achiev-cd in stabilizing mineral soils by using va,rious types of stabilizers such as ce-ments, resins, chemical hardening, bitumens, and freezing. It is undeter-mined whether such methods would be of value in stalbilizing organic soils
as these soils are chiefly vcgetablc in character and likely would not rcspond to the above-noted treatments as do mineral soils. Although from timc to time one hears of a chemical or other means of muskeg stabilization, to detc there does not appear to be any ade-quate solution to this problem.
Conclusion
This brief paper has atrcmpted to present a rdsum6 of the currcnt meth-ods of constructing roads across or-ganic terrain. Only the morc im-portant poin'ts of each method have been described. Pe.rhaps nothing new has been presented for thosc who are thoroughly familiar with all aspects of highway construction, but it is hoped that ii has been informativc to thosc who have not been too closely asso-ciated with this aspect of organic ter-rain exploita,tion.
A list of publications of the Division of Building
Research can be obtained on application to the
Pub-lications Section, Divisio*n of Building Researctu
National Research Council, Ottawa, Canada.
This papcr is a contribution frorr thc Division of Building Research, National Rcscarch Council of Canada, and is published with the approvd of the D.irector.
Refcrenecs
A Preliminary Annotated Bibliography on Muskeg comtpitled lby I. C. I\4"9-Farlane, Bibliqgraphy No. ll, Div,i-sion of Building Resarch, Na'ti,onal Researdh Council, Ottarva. S.ptott-ber 1955. 'Secitions "F" and "G". Field Manud of Soil Engineering.
(Third Edlition). lvfichigan Statc Highway Dept., 'Lansing, Mich. August 1954. Oharptor 4.
Soil Mechanics for Road Enginecrs Her Majesty's Stationery Offrcc, Lon-don. 1952. C[rapter 25.
Road Crosses Swamp on Timber Pjle Foundation. Engi,noeri,ng and Con-rtrracrting, 70:l:15,15. |'anuarry 1931.
