48.-A Reinforced Brick Masonry Retaining Wall with Reinforcement in Pockets

by C. R. ABEL and M. R. COCHRAN Brick & Tife Service Inc., Greensboro, North Carolilla

ABSTRACT

The design and construction of a reinforced b,ick masonry retaining wall wilh reinforcement in pockets is described. Ali example of a complele design is gh'en. The wall is a cGntile)'er­lype structure consisling of a vertical stem rigid/y connected to a canven­liollal reinforced concrete base. The s/em is so/id brick masonry and pro­\lides pockets, 011 lhe tellsion side, which contai" lhe I'ertieal reinforcemen' and lhe bonding e/ement, concrele grau/o

Since much relailling wall con­Slfuelion is performed by lhe general building contraclor, ir is particular/y adVQnfageous to specify materiais already foulld 011 lhe job in associaled Conslruelioll. /1 is equal/y imporlant to propose a sysrem that al/ows simple cons/ruetion sequences, easi/y per­formed by lhe Iradesmell flormally employed by lhe buildillg COlllraclor. The sysrem is considered economic anel allractil'e to lhe briek industry, general contractor, designer and owner.

Mur de Soutenement en Maçonnerie en Briques à Renforcement du Type HPoche"

L'étude et la constructioll (/'UII mur de soutenement en maçonnerie en brique avec renforcemelll en "poclles" so"t déeriles. Le mur est une eon­s/ruelion de Iype en eneorbellement consistant en un tronc vertical relié de façol1 rigide à une base en béton armé traditionnelle. Le trone est en maçomlerie en briques pleines et forme des poches, dll côté de la (raelion, qui C011liemlent le renforeemelll vertical el l'élémellt liant, le coulis de bétoll. Puisqu'ul1e grande pOrfie de la con­struetion du mur de soulenement est réalisée par l'entrepreneur principal de la cOl1truction, il est parriculiere­ment al'antageux de spécifier des marériaux déjà uti!isés sur le chanfier dans une conslruction voisine. /I est également important de proposer UII

sysfeme qui permelle des successiolls de construc!iolls simples, aisément réalisables par la main-d'oeullre em­ployée habituel/emenl par I'enlre­preneur de bâtiment. Le systeme ap­paraít économique el illtéressant pour l'industrie de la brique, aussi bien pour I'enlrepreneur que pour I'archi­tecte et même le propriétaire.

Eine Stützmauer aus bewehrtem Ziegelmauerwel'k mit Taschen fü,· den Baustahl EntlVlIrfwld KOllstrukrioll einer SlülZ­mauer aus belVehrtem Ziegelmauer­werk mit Taschen für den Baustalzl sind besclzrieben. Ein Beispiel zeigl die vollsliindige Planung. Die frei­!ragende Mauer besteht aus einem senkreehlen Syslem, das fest mil einem Fundament aus herkommlichem Stalzlbeton I'erbunden isto /n dem massil'en M auerwerk sind auf der ZugspallllLmgsseile Taschen ausges­part, in denell die senkrechte Beweh­rllng mil GujJbeton eingebullden ist. Da schon )liele Stülzmauer-Konstruk­tionen durell Bauunternehmer ausge­fülrrt lVurdell, isl es angebrach!, die beim Arbeiten mil Verbul1dkol1sfruk­rionen I'erwendelen Materialien eill­zeln zu nellllen. Gleich wichtig iSI der Vorselllag für ein System, das eine einfache Bauweise erlaub!, die für den Halldwerker leiclzt GlIsführbar ist wu! von Bauunlernehmern einheitlich angewendet wird. Das System wird l'on der Ziegelindustrie, den Bauun­terne/unern, Bauingenieuren und Bauherrell aIs wirtschaftlich und in­teressanl angesehen.

1. INTRODUCTlON

Relaining walls have for many years been used only when any allernative Solulion proved impractical. The basic reason is simply cost. In many instances buying additional land lo widen culS and fills was cheaper, bUI with lhe increasing cost and decreasing availability of land, retaining walls are becoming more necessary to civil engineers in urban highway and railway projects, and to architects who rely on walls lo allow land lo be used which was once rejected for topographical reasons.

While this continuous grout core wall was widely acclaimed by the design profession and construclion industry, the brick industry felt an even more orderly system should be developed to furlher si mplify assembly of the wall and to keep each craftsman confined more c10sely to his normal work processes. The pocket-type RBM retaining wall evolveu frum lhe ensuing 4-year study, where calculations and pilot construction projects followed each worthwhile idea.

Brick engineers in North Carolina therefore seI out to simplify retaining wall construction and capture this lucrative market for bricks. A cantilever retaining wall system was developed using a conventional reinforced concrete footing and brick masonry stem, reinforced and grouted in a continuous collar joint or grout core. Con­struction of Ihis reinforced brick masonry (RBM) wall was easily accomplished by ma sons, sometimes already at work on the building projecl site. When the mason had 'topped out' his wall, no further stripping, facing, finishing, or painting was required.

295

The highest cantilevered retaining wall of this type so far conslructed , and probably the maximum height that can be economically used is 24 ft. The ribs were spaced aI 4-ft centres.

2. DESIGN

Earth-retaining walls of RBM have been used for many years in the USA. In general, they are economically com­petitive and , as a by-producl of their function, produce a structure of outstandingly pleasing appearance.

Figure I shows a Iype extensively used and well estab­lished as a practical method. Figure I(a) outlines the

296 A Reinforced Brick Masonry Retaining Wall with Reinforcement in Pockets

12' o·

6'

(a) ~

(b)

4'

~

FIGURB l-Details of (a) cross·section and (b) plano

general approach to cross-seclional configuration. Figure I(b) shows, in plan, the relative position and spacing of the reinforcement pockets. Vertical bars are placed on a bevel, sloping from an effective depth (d,) of 5·7 in. at the base, to an effective deplh of 4 in. at the topo In this instance, the reinforcement is placed in two rows with the dowel bars, in one row, having an effective deplh (dI) of 6·7 in. The pockets are filled with a concrete grout, thereby effeclively compleling lhe helerogeneous slem assembly.

The slem reduces in thickness (Figure la), from 12 in. at the base, to 8 in. aI a point 3 fI 4 in. above lhe base. This provides a minimum of 2 in. of cover for lhe re­inforcement and is consistent with the effort to use the leasl praclieable amounl of masonry.

In Ihis paper il is assumed lha I lhe back-fill malerial is coarse-grained and permeable wilh provision for drainage. For this ideal condilion lhe horizontal earlh pressure has been delermined by a semi-empirical melhod, oUllined by TERZAGHI and PECK,I where the total earth pressure,

P=tkH' . (1)

k = 30 Ibf/ftz per ft and H = height of backfill. Figure 2 shows lhe appropriale diagrams for earth pressures aeting horizontally on lhe stem, Figure 2(a), and on the tolal strueture, Figure 2(b). The weighl of the backfill is assumed to be 100Ib/fI J• The values Ihus oblained c10sely

T T H

1 H

1 ~ ~

fo-i--.I r.30H P·15H 1

H:height M~

(a) (b)

FIOUll 2-Diagrams of horizontal pressure acting 00 (a) the stcm Ilnd (b) the total structure.

agree with those given by Rankine where

p= - H2 K=lan2 45 0 __ KW (<lJ)

2' 2 . (2)

and <lJ (angle of internaI friction) = 33 0 40'. Figure 3 shows lhe resultant forces, due lO vertical

" ~

W" 6270 IM 1ft t

P-2534lbf/ft

4·33'

1"':'Ü'IJ§l ~

FIoultE 3---Earth pressure 00 fooUng.

loads and horizontal force, aCling against lhe reinforced concrete base.

Figure 4(b) graphically iIIustrales lhe vertical re­inforcement-size, lenglh and number of bars-and relates it to the analysis, Figure 4(a). To meet code requirements, lhe top portion of the wall is provided with not less than 0'19 in2/ft of reinforcemenl.

The design conforms to the SCP! requirements2 and the American Concrete Institute code requirements for re­inforced concrete. Design factors and stress allowances are based on an ultimate compressive stress of3000 Ibf/in2 for both brick masonry (r.,) and reinforced concrele (f' J.

CriticaI slresses in the stem occur where H = 12 fI. Assuming a slraight-Iine distribution, Figure 5 shows the maximum flexural compressive stress, fcom the trans­formed area, to be 12101bf/in ' in lhe masonry. This

Or 'T 12'0'

:~ ~ ~

~r M [bf /In] 4H A, • n210o'j[4~ d

~III~ H~~:t \1 M [!>f/nJ' 5H' IT2]

d nn]-S+o. 1417H , n ] 4tO'I4I7H d2 n .•

71- \

2'8 5'l! :t ~ f.i-'1111 2'91 1130

"" "'Y9'!

" 12 O .. ....L..

(a) (b)

FIotJlU! 4--Vcrtical relO o . fi rcement.

C. R. Abel and M. R. Cochran 297

io'!oi

rrr: A,

b-04S" t .. 12-d'67" n "9,2 A.·3-58~

M'414720 irHb kd" 2-42" jd 'SIl9'

C·T~410Ib 1 ... ~ ,. 12!O lbtfjn 1

1, - 1; .. 19 6501bt/ ln'

FIOURE 5- Stress diagramo

exceeds, by a negligible amount, the code allowance of 0·40 f' m~ 1200 Ibf/in'. The tensile reinforcement stress of 19650 Ibf/in' is within the code allowance of20ooo Ibf/in' for steel having a yield stress of 40000 Ibf/in'.

It appears reasonable to assume that the design results in a structural unit with the capacity to sustaio the pre­scribed loading within the area of safe and prediclable slress leveIs. Loading applied to the solid brickwork 3 ft O in. in length, is assumed to be transferred to th~ grout pockels by shear. At this inlerface of brick and concrete graut, the brickwork presents a rough surface which results in a very strong rnechanical bando However. the natural allowable chemical bond is sufficient to resist the average unil shear stress (see Figure 6) of 5·6Ibf/in'. It is probable that Ihis section will enler inlo

Vfr-l-IZ'1l ' 2 160 [4} - 8640lb Unit Sh~a Stnl:s!> " v .~ .. 2 7tbO'inX07.ff.,. 38 ~

Unit 80nd Stnzss • u • 1~ ,~1S'e~167j _109lbf{1"I1<!tO

(al

I~ ~ IHIHHtI Appliq{j l..Ood " 360 Ib/tt

V· 360-1-- ,.401b

f ... !~ .. 11 bf/lnl

Unrt Avtlragoz She.ar StrfSS" ~ .5·6Ibf/lrk3S'S

(bl FIOUlll! 6-Bond Ind shear stress.

arch 3ction3,4 between graut pockets. In tbis instance (Figure 7) the resistanee to bending is Iimited by the eapadty of the brickwork in compression. The !'in. conlmuous bars are placed horizontally in the face side of lhe wall (Figure I) primarily lo minimize the develop­menl of vertical cracks due lo shrinkage and changes of temperalure.

v

~ !-f.

ttttt!ttt Applle'd Lood

f ... O·40f~

F-f~ M-F-il

PIOva! l - Arch actton.

This design can easily be modified to meet other criteria. An increase in stem thickness would permit less reinforcement and allow some increase in the spacing of grout pockets. By increasing the length of grout pockels. ali reinforcement could be placed in one plane, thereby resulting in a grealer e/fective depth (d,l.

3. CONSTRUCTION 3.1 Foolings The reinforced concrete footings used are similar to those found in conventional cantilever construction except for the spacing of footing-stem dowels. Rather than relying on earlh edging to restrain fresh green concrete, footings should be formed, and a framework supplied to support lop reinforcing steel and especially to hold dowels securely. Dowels must be set to exact horizontal and vertical dimensions. AIso a continuous slringer musl be seeurely seI to form lhe shear key and fix horizontal alignment of the masonry stem. A forming system is invaluable in positioning and securing these items (Figure 8).

FIGURE 8-Reinforced concrete footings.

3.2 Masonry Slem Providing the footing is clean and free of laitence, the masonry slem may be slarted within 24 h of eomplelion. The layout of the stem is sim pIe since the wall face is previously located by lhe shear key formed into the fooling and lhe position of the pockets are shown by lhe loeation of the dowel clusters.

As is required in RBM conslruction, physical pro­perlies of briek must meet ASTM specificalions, and mortar musI conform to ASTM proportions. Brick should be checked for initial rate of absorption and the necessary corrective measures taken.

Ali mortar joinls, collar, bed and head joints muSI be complelely filled. It is permissible lo ' hand Iay' lhe ouler shell of bricks and 'float' core brick into plaee. This aIso assures complele filling of inlerior or core joinls. Convenlional bonding aI six-course intervaIs is required in ali brickwork belween pockets.

Afler bricklayers have buill masonry above the heighl of lhe doweIs, no further main reinforcing sleel is encountered, only No. 2 hars muSI be placed as required in bed joinls on the wall face . Bricklayers may conlinue

298 A Reinforced Brick Masonry Retaining Wall with Reinforcement in Pockets

10 build lhe wall as Ihey would any convenlional wall, with no responsibililies for tying steel, setting wall lies or grouling as was the case in lhe original type wal l.

When the mason has completed his work, the wall face is completed, with a smooth appearance from lowest lo highest sections of lhe wall (Figures 9(a) and (b)).

3.3 Reinforcing Sleel Placemenl and Forming

Afler lhe masonry is completed and pro per setting time elapsed, other workers (not bricklayers) may c1ean mortar droppings from pockets, straighten dowels and tie 00 vertical bars. While no horizontal bars are required, it may be necessary to faslen at leasl one bar lo vertical 5teel to serve as spacer. Oiled forms are lhen set at pockets and secured inlo position wilh lemporary bracing or wiring (Figure 10).

3.4 Grou! Orout should conform to ASTM specilicalions and con­tain Portland cemenl, sand and medium-size aggregate. Water content musl be carefully designed and conlrolled to ensure satisfactory bonding. Lifts should be limiled to 8 fi under normal condilions and lhe grout should be vibrated.

3.5 Drainage and Damp-proofing

Weep holes must be provided at regular intervals or alternatively, a parallel drainage syslem may be placed on lop of the fooling heel. This syslem should be con­nected into a storm sewage struclure and properly back­filled with graveI. The whole back surface of the wall should be damp-proofed with a bituminous material , to discourage efflorescence.

3.6 Backfilling

A minimum thickness of 12 in. of coarse graveI musl be placed againsl lhe wall as backfilling progresses from footing to a point near finished grade.

4. SUMMARY AND CONCLUSIONS

Design and construction of relaining walls has long been considered a specialized field by many in the construction industry. The new pocket-type RBM wall is stiU an engineering design, but conslruction has been simplified so that it can be built by the heavy or engineering con­tractor, or by a general or masonry contractor already working on the site.

lt is easy for concrete craftsmen to build concrete foot­ings; bricklayers need no extra skills in building the masonry stem; less-skilled workers can fix vertical bars, place rough forms, and place grout in pockels.

Orderly construction procedures spell economy for owner, designer and contraclor. With this RBM wall , structural excellence and beauty are al1 added banus to lhe Qwner.

REFERENCES I. TERZAGHI, K. and PECK, R. S., Soil Mechanics In Engineering

Practice. New York and London. Wiley, 1955. 2. STRUCTURAL CLAY PRODUCTS INSTlTUTE, Recommendcd

Building Code Requirements for Engineered Srick Masonry. Washington D.C., SePI, August. 1969.

3. MONK. C. B. JR .• Resistance of Structural Clay Products to Dynamic Forces. Washington D.C., StruclUral Clay Pro­ducts Research Foundation, Report No. 7, November, 1958.

4. PLUMMER, H. c., Brick and Tile Engineering. Washington D.C. , Structural Clay Products Institute, 1962.

(a)

(b) FIGURE 9- Reta ining wall: (a) low section ; (b) high section.

FIGURE lO- Oiled forms secured by bracing.