P. H. Smith, B.Sc.,A.M.I.C.Err A.M.Inst.H.E.Technical Director, Stephenson Clarke (P.F. Ash Development) Ltd.(Powell Duffryn Group)

The use of P.F.A. ingrouting andfoundation work

Most of the modern power stationsin the U.K. which have coal-fired fur-naces are few with pulverised coal andfrom the flue gases of these power sta-tions is removed pulverised fuel ash.The current annual production of P.F.A.in the U.K. is approximately 8 milliontons and the predicted increase in an-nual output is such that P.F.A. willcontinue to be a readily available en-gineering material over most parts ofthe country in the foreseeable future.

Properties influencing the use of P.F.A.in Grout

During the last five years, P.F.A. hasbeen increasingly used in the slurriedform both neyt, or mixed with cement,to form a grout and P.F.A. is rapidly re-placing the use of sand in this field ofengineering. Some of the properties ofP.F.A. which have influenced its grow-ing use in grout are as follows: —<»

Possibly the most important factor isthe spherical particle shape of P.F.A.(Fig. 1) This particle shape gives alubricating action to the grout ensuringgood flow properties. In other words,the particles of P.F.A. emulates theaction of ball bearings and this greatlyfacilitates pumping.

The fineness of the particles ofP.F.A. is another important factor as atleast 75 per cent and in some cases 95per cent of the material passes a B.S.200 sieve. The fineness of particle sizetends to improve pumping conditionsby keeping the grout in suspension andhelps to reduce sedimentation. It alsomeans that P.F.A. grout can penetratethe small interstices as would be foundsay, in old sand mortar.

Another important factor is the lowspecific gravity of P.F.A. The apparentspecific gravity of P.F.A. averages 2.1 asagainst an average of 3.14 for cementand 2.7 for sand so P.F.A. will settleout but not as quick as either cementor sand. This indicates that P.F.A.grout can be pumped far longer dis-tances than grout incorporating sand.

Uses of P.F.A. GroutBy far the most important facet of

P.F.A. grout is its use either neat orwith a small cement addition to fillabandoned sewers, mine shafts etc.which require filling usually to prevent

jig,

the settlement of surface works. Forbulk filling projects P.F.A. is suppliedin a lightly moistened condition in tip-pers. Usually, the material is thenmixed with water to form a slurry andmost types of mixers can be used forthis purpose although purpose-mademixers, such as those having verticalpaddies, are superior to traditional con-crete mixers of the "free fall" type. Inmany instances the slurry is placed bygravity and this of course, is by farthe most economical way of placinglarge quantities.

Gravity filling is frequently aided bysluicing large quantities of water intothe voids along with the P.F.A. slurry.After the P.F.A. has settled, the surpluswater used to "carry" the P.F.A. isthen removed either down existingdrains, by pumping or by filtrationthrough the sides of the voids beingfilled. Depending on the efficiency ofdrainage therefore the settled P.F.A.will self-harden because of uniquepozzolanic properties and in time can

simulate a soft rock in ideal drainageconditions. Cement and lime can ofcourse be added to the P.F.A. slurryshould higher strengths be requiredwithin a shorter period. For site con-venience the cement content is usuallyspecified as being a proportion of vol-ume of the P.F.A. content and for mostprojects requiring high early strengthsthe proportion is of the order of 1cement: 6 P.F.A.

Pre-mixed blends of P.F.A. and cem-ent supplied dampened in bulk are nowbeing produced by P.F.A. BasemixLtd., a Powell Duffryn subsidiary Com-pany. The first plant now in productionis located at Brighton and the secondplant located at St. Helens Lanes. willbe in operation by the end of 1968, fol-lowed by a third plant at West Thur-rock, Grays, Essex. These pre-mixedblends termed Basemix have beenfound ideal for bulk filling of abandonedsewers, etc. where better strengths arerequired.

Frequently however, gravitational

Fig 2; Model D.D.B double drum Colcrete mixer used for P.F.A./cement mixes.

Ground Engineering

filling is not feasible and pumpingequipment is used by the specialistfirms employed. Various types ofpumps are suitable amongst which theColcrete mixer (fig 2). Mono screw-feed pump, "Boogee" type pump andpneumatic concrete placer appear tobe the most widely used.

Apart from its bulk filling use, P.F.A.is being increasingly used as either anaggregate or partial cement replacer inthe higher quality, richer grouts used inmore specialist uses such as the fillingof the overbreak behind tunnel liningsand the filling of rock fissures adjacentlarge engineering works such as dams.

It has become increasingly evidentthat from both an economical and prac-tical standpoint, contractors operatingin shafts, tunnels and confined spacesprefer to have their grouts pre-weighed,pre-blended and bagged in $ cwt papersacks, or delivered in bulk pressuretankers to operating silos. Furthermore,more sophisticated grout mixes are be-ing called for, comprising blends ofportland cement with PFA and benton-ite mixes containing Kaolinite, Lapon-ite (a synthetic gelling clay similar toan enhanced bentonite); gypsum addi-tions; plasticisers etc. all in combinationwith PFA and cement. Since thesemixes have varying properties of vis-cosity, thixotropy and strength, theselection of PFA becomes of paramountimportance and only Rio tested Pozzo-lanic ashes are used. Since this exten-sion of PFA usage lies outside themarketing scope of PFA agents, thesemixes are being produced and sold ex-tensively by the Pozament Cement Co.Ltd., of Ferrybridge, Yorkshire, who alsoproduce from PFA a low heat cementfor use in mass concrete to reduce theeffects of the heat of hydration. Poza-ment grouts go under the trade nameof lntegrout, whereas the low heatcement is referred to as PozamentCement.

Because of its fineness, P.F.A. hasbeen found to be extremely useful asan aggregate in the grout used to sta-bilise decayed mortar in old buildingsand other structures and to this endis being used extensively in the remed-ial work to Norwich Cathedral, YorkMinster and Winchester Cathedral. Forthis remedial work the P.F.A./cementgrout is mixed in small vertical paddlemixers and pumped into the mortarusing small hand operated pumps ofthe Reader Nevander type. These smallhand operated pumps are used becauseexcessive pressures would have a detri-mental effect on the structures.

Extremely quick-setting grouts andmortars incorporating P.F.A. are nowin use and these are applied to suchjobs as re-pointing the masonry in rail-way tunnels, and so on, where remedialwork has to be accelerated because ofthe limitations on time available to car-ry out the work.

Self Hardening properties of P.F.A.Suitably moistened P.F.A. adequately

compacted in shallow layers exhibitsreasonably high shear strength proper-ties. The material has both high co-

hesion and a high angle of shearing re-sistance both of which increase con-siderably with age. Table No. (1)ceillustrates this increase in shear strengthwith age for a number of pulverisedfuel ashes obtained from differentsources. These results were obtainedfrom undrained triaxial compressiontests carried out in the laboratory onsamples compacted to B.S. Compac-tion Test maximum dry density.

Table No. (2) illustrates the increasein unconfined compression strengthwith age for samples of Scottish P.F.ashes made up in the laboratory againto B.S. Compaction Test maximum drydensity. These results constitute partof the extensive research work onScottish P.F. ashes recently carried outby the University of Glasgow.<'>

From the figures established in TableNo. (1) the average cohesion after oneday would be in the order of 6

Ib/in'0.42

kg/cm') and the average angleof shearing resistance at the sameperiod would be in the order of 33deg. In the field however, the densityachieved would be about 90 per centof the B.S. compaction test maximum.In other words, the actual shearstrength would be somewhat less thanthe laboratory figures above and from

vestigate the bearing capacity of a stripfooting founded in P.F.A.

Ultimate bearing capacity= qr= cNc+>DrN,+v(B/2JNv

where Nc, N, and Nv are parametersdependent on the angle of shearing re-sistance of the soil.

B=width of footing.Dr=depth of footing.

Designing again for P.F.A. immediatelyafter laying at 90 per cent relative com-paction and assuming a 3 ft wide foot-ing founded at 3 ft. below the surface,we have

C=4 Ib/sq.inv=83 Ib/cu.ft if'6.5 degB=3 ft D,= 3ft

Using tabulated values of the bearingcapacity coefficients:—

Mean ql —4X144X28+83X3X14+83x (3/2) x 12 Ib/sq.ft

7.2 +1.55 +9.67 tons/sq.ft

9.42 tons/sq.ft

Using a factor of safety of 3 this givesa safe bearing capacity of 3.15 ton/

experience in the field the cohesionwould be approximately two thirds thelaboratory figure and the angle of shear-ing resistance would be reduced byabout one fifth. Therefore, taking thelaboratory averages given above, theaverage field figures immediately aftercompaction would be in the order of:—

Cu=4 Ib/in'(0.28 kg/cm')@

=26.5'oundations

constructed on P.F.A.Compacted partially saturated P.F.A.

is a material possessing both cohesionand a marked angle of shearing resis-tance and can be expected to have ahigh bearing capacity. Terzaghi's equa-tion for the general shear failure ofshallow foundations can be used to in-

sq.ft. This value can again be expectedto increase with time. It would appearreasonable therefore to design narrowfootings founded on thick beds of well-compacted P.F.A. to carry design loadsof the order of 3 ton/sq.ft without riskof undue settlement.

This safe bearing capacity is basedon ideal conditions using freshly pro-duced P.F.A. compacted to 90 percent relative compaction in a thickbank well above the water table. Forany particular design, therefore, con-sideration will have to be given toother factors such as sub-soil condi-tions below the PFA, density of thePFA and the height of the water table.

Another advantage of PFA structuralfill is that the material is ideal for

Fig 1: Ash particles from e Pufverised Fue/ Boiler X160.

22

trenching as because of the self-harden-ing properties, deep, neat trenches canbe excavated usually without timber.(Fig 3) P.F.A. digs like a hard cheeseand can be excellently trimmed whererequired. The latter characteristic makesPFA an ideal material for cutting outholes for pad foundations, strip foot-ings, and man-holes. As PFA has aslight sulphate content the usual prac-tice is to protect concrete footings bylaying polythene sheeting in the trenchexcavated through the P.F.A. prior toconcreting.

P.F.A. below water tableThe ideal load bearing fill applica-

tion for P.F.A. is fill in thick banksabove the water table. P.F.A. is not freedraining and if it's moisture content iswell above the optimum the materialhas reduced shear strengths because itcannot be satisfactorily compacted,hence it is not generally good practiceto place P.F.A. below the water tablefor load bearing purposes unless it isstabilised with cement. Pre-mixedcement stabilised P.F.A. such as Base-mix is therefore coming more into usefor load bearing fill below water tablelevel.

In the foundation design work forstructures to be constructed on wetpits back-filled with P.F.A. proper pre-cautions must therefore be taken. Platebearing tests may indicate high loadbearing capacity, but trial bores shouldbe taken to establish the height of thewater table. The shear strength of theP.F.A. below and immediately above thewater table will be reduced and thehigh load bearing capacity establishedby the Plate bearing test on the surfacewould apply possibly only to the crustof self-hardening P.F.A. lying above thewater table.

Furnace bottom ash which has a

grading similar to coarse sand is freedraining and can bP used more suc-cessfully below the water table as it isstable when tipped into water.

It has been reported that P.F.A. be-low the water table in lagoons can bestabilised by the "Vibroflotation"method and on some sites small areasof P F.A. which have become unstablehave been completely stabilised by theinsertion of a poker vibrator for a shortperiod. The displaced water is thenled away by grips.

Pi%'ng through P.F.A.Because of its self hardening proper-

ties it has not been found practical inmany cases to drive piles throughP.F.A. banks more than 4 ft (1.21 m)thick, constructed above ground, andthe usual practice is either to use boredpiles or bore first and drive the piles inlater.

When friction piles are driventhrough certain sandy soils the vibra-tion of the pile driving tends to com-pact the soil which then settles and atthe same time "tightens" itself aroundthe pile. The soil in settling, tends to"drag down" the pile so causing nega-tive friction thus reducing the bearingcapacity of the pile. P.F.A. on the otherhand, because of its self-hardening pro-perties, will not settle or "tigthen up"around the pile owing to vibration asmuch as soils so there will be lessnegative skin friction.

Rafts of P.F.A.Over low bearing capacity ground

such as areas of peat or compressableclay, P.F.A. rafts having a thickness ofnot less than 4 ft (1.21 m) have beenextremely successful as working plat-

forms or sub-bases to access roads.Unlike hardcore which is partially lost

in the poor ground, P.F.A. forms astrong, monolithic, lightweight raft andalthough its strength at water taLlelevel may not be very high, the hardcrust on top is sufficiently strong tosupport heavy plant and road bases.

REFERENCES(1) Smith, P. H.—Fly Ash in Grout. Civil En-

gineering. London, Nov. sr Dec. 1962.(2) Raymond, S.—Pulverised Fuel Ash in High-

way Engineering snd as Structural Fill.Paper read to British Geotechnical Society,Inst. Civil Engineers, London, March, 1SI 6.

(3) Sutherland, H. B., Finlay. T. W. —A La-boratory Investigation of the Age Harden-ing Characteristics of P.F. Ash. The Uni-versity of Glasgow, Department of CivilEngineering, Dct. 1964.

ls

I Ãi)~

ir

ilkFig 3: P.F.A. digs very cleanly and can beexcavated without timber.

P.F.A. Source

TABLE No. (1)«I —Increase in shearstrength with age

Elapsed Time at Test Days

14 56

AgecroftBatterseaBoldDunstonSkelton GrangeWestwood

Cu(Ib/in') (kg/cm')

6.5(0.45)7.0(0.49)5.0(0.35)6.0(0.42)4.0(0.28)5.0(0.35)

Cu(degs) (Ib/in') (kg/cm')

33.638.034633.634.331.8

30.0(2.10)12.0(0.84)34.0(2.39)8.0(0.56)

16.0(1.12)7.0(0.49)

TABLE No. (2)(s) —Increase inunconfined compression strength

with age.

Cu(degs) (Ib/in') (kg/cm')

34.6 30.0(2.10)36.5 16.0(1.12)38.6 40.0(2.81)34.2 10.0(0.70)40.0 26.0(1.82)31 8 12.0(0.84)

(degs)

37.036.538.834.740.036.5

P.F.A.

28

Elapsed Time

91

at Test Days

182 371 1230(3.4 years)

Unconfined compression strength (Ib/in') (kg/cm')

BaronyBraeheadKincardinePortobello

45(3.16)34(2.39)52(3.65)50(3.51)

127(8.92)122(8.57)53(3.72)75(5.27)

144(10.12)140(9.84)47(3.30)92(6.46)

171(11.95)157(11.03)58(4.07)101(7.10)

130(12.65) 189(13.28) 246(17.29)171(12.02) 185(13.00) 189(13.28)64(4.49) 74(5.20) 81 (5 69)111(7.80) 115(8.08) 124(8.71)

346 (24.32)300(21.09)117.5(8.25)138(9.70)

24