Ground movements associated withtrench excavation and their effecton adjacent servicesby P. B. RUMSEY", BSc(Eng), MSc(Eng), MICE IIL I. COOPER>, BSc, MSc, MIMM

IntroductionWHEN A TRENCH is excavated move-ments occur in the adjoining ground. Theamount of movement depends on the geo-metry of the trench, the type and proper-ties of the soil, the location relative to thetrench, methods of excavation, groundsupport, standard of construction and levelof inspection.

At present the influence of these factorscannot be reliably assessed due to lackof field data on ground movements due totrenching (Symons, 1980; O'ourke, 1980).A pipe buried in ground affected by trench-ing is likely to be subjected to both trans-ient and long-term flexural strains whichcould damage the pipe.

(b) To build up a series of case histor-ies on ground movements due totrenching in a range of ground condi-tions, and

(c) To develop a reliable method of pre-dicting the effect of trenching onadjacent buried services.

Previous workAn empirical method of predicting

Strain gauges

ground movement due to trenching hasbeen proposed by Crofts et al (1977) whoalso described a method of predictingstrain in an adjacent pipe using an elasticmodel consisting of a beam embedded inan elastic foundation. More recently adesign procedure based on finite elementanalysis has been derived by Kyrou (1980)which allows fundamental soil properties(E and K,) to be employed in the predic-

To datum 20m

Water Research Centre programmeA major research programme has been

initiated in this area by the WRC Engin-eering Centre to enable the water industryto assess the likely effect of sewer trench-ing on adjacent buried services and tospecify amendments to trenching practiceto limit ground movements where this isrequired.

The aims of this programme may bebroadly defined as follows:(a) To establish the scale of the prob-

lem,

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*Geotechnics Section Leader, and )Manager, En-ineering Services, WRC Engineering Centre,windon, Wilts. Fig. f. Detailed trench experiment typical plan layout

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Fig. 2. Site 1iinclinometer tube profiles28 Ground Engineering

Fig. 3. Site 2; inclinometer tube profiles

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Fig. 4. Site 3;inclinometer tube profiles Fig. 5. Site 4;inclinometer tube profiles

tion of pipe strain adjacent to a trench.Both methods have yet to be checkedagainst field measurements.

Records of ground movements availableto date are limited to those published byTRRL (Symons, 1980) and British Gas(Howe et al, 1980) from which the fig-ures presented in Table I have been ex-tracted.

Movement due to trench excavation onany one contract can vary over small dis-tances along the length of the trench evenwhere the factors controlling the movementare apparently similar. Hence in practice,statistical analysis of a large sample offield data, and development of an empiricalmodel of ground movement, may providethe best approach to a practical methodof prediction.

Field experimentsIn order to substantially increase the

available data on ground movements in-duced by current trenching practice, mea-surements of surface and subsurfacemovement are currently being carried outat a large number of Water Authority sitesthroughout England and Wales. The fieldexperiments take two forms; simple ex-periments in which horizontal and verticalground surface movement are recordedalong a line perpendicular to the directionof a passing trench, and more detailed ex-periments in which ground movements atdepth are monitored using inclinometersand settlement gauges, and pipe strain ismeasured by means of strain gaugesmounted on adjacent mains (Fig. 1). Inboth cases the instruments are read priorto, during, and, where possible, for a longperiod (up to 2 years) after excavation.

The results of up to 50 simple experi-ments are expected to be available by theend of the programme. A total of six ormore detailed experiments in a variety ofground conditions will be carried out. Fourof the latter have been carried out to date,the results of which are summarised

below.

Site 1At this site a sewer was under construc-

tion in a rural area. The trench was 4.5mdeep by a nominal 1.5m wide. Groundconditions comprised silty sands and gra-vels to a depth of 3m and silty sandy claybelow 3m. Groundwater level was about2m below ground level. The trench supportsystem consisted of 3m long by 2.4m hightrench boxes.

Maximum horizontal and vertical move-ments of the order of 7mm and Smmrespectively were recorded immediatelyfollowing excavation and reinstatement.Movement during Sq months followingreinstatement has been very small (1-2mmor less).

The maximum recorded change in strainon a 100mm dia. main parallel to the trenchand 1.8m distant was 75 microstrain. Thisstrain has decreased significantly (possi-bly as a result of seasonal temperaturevariation) since trench reinstatement.

Profiles of inclinometer tubes locatedalong a line perpendicular to the trenchwali, nearly six months after excavationand reinstatement of the trench, are pre-sented in Fig. 2.

Site 2A 3.7m deep by 1.3m wide trench in

sand was excavated at this site in a minorurban road. Groundwater level was about2m below ground level. Trench sheetswere used as support.

Maximum horizontal and vertical move-ments were consistently small (generally1-2mm) during excavation and for 3months following reinstatement.

Maximum recorded change in strain in100mm and 150mm diameter cast ironmains parallel to the trench at distancesof 0.5m and 2.5m was 80 and 55 micro-strains respectively. This strain has de-creased significantly (possibly as a res-ult of seasonal temperature variation)

since one month following trench rein-statement.

Profiles of inclinometer tubes, threemonths after excavation and reinstate-ment, are presented in Fig. 3.

Site 3Excavation of a 4.3m deep by 1.2m

wide trench, supported by close sheeting,was carried out in a dense sand in a minorurban road. Groundwater level was about3.5m below ground level.

Maximum horizontal and vertical move-ments were very small (about 1mm) dur-ing excavation and up to one month afterreinstatement.

Strain was recorded at three locationsin a 100mm diameter cast iron water mainrunning parallel to the trench at a dis-tance of 1m. The maximum measuredchange in strain immediately following ex-cavation was 65 microstrain. Each set ofgauges showed a strain "wave" travellingalong the pipe with the advance of thetrench head, the effects of which appearedto dissipate after reinstatement.

Profiles of inclinometer tubes, one monthafter excavation and reinstatement, arepresented in Fig. 4.

Site 4A 3.8m deep by 1.2m wide trench was

excavated in firm to stiff sandy silty clayin a minor rural road. The trench was sup-ported by a 3m long by 2m high trenchbox. Groundwater level was about 1.5mbelow ground level.

Maximum movements towards thetrench recorded by inclinometers at dis-tances of 0.5m, 1.9m, 4.2m and 7.1m fromthe trench wall were 17mm, 8.5mm, 5mmand 1.5mm (horizontal) and 6mm, Omm,1mm and 1mm (vertical) respectively,three weeks after excavation and reinstate-ment.

Strain gauges were installed some dis-tance from the inclinometer line on an80mm diameter cast iron main running

March, 1982 29

parallel to the trench and within 0.5m ofthe trench wall.

The maximum changes in strain re-corded by gauges mounted on the pipe attwo locations were 110 and 50 micro-strains. Both sets of gauges showed astrain "wave" effect, with strains tendingto return to values recorded prior to thetrench being excavated.

Profiles of inclinom etc r tubes, twomonths after excavation and reinstate-ment, are presented in Fig. 5.

DiscussionPublished data on ground movements

and pipe strain associated with trenchexcavation are summarised in Table I.

There is at present no general agree-ment on allowable strains in cast and duc-tile iron pipes due to trenching takinginto account such factors as installation,environment changes and traffic loading,nor on the relationship between move-ment and pipe strain. Hence no attemptwill be made to draw conclusions fromthe records of measured strain presentedin Table I, and elsewhere in this Paper.However, work currently being carried outat the WRC Engineering Centre suggeststhat the measured strain associated withthe installation of new pipes, and, indeed,the strain resulting from instrumentationof a buried pipe for experimental pur-poses, may well be of the same order as,or more than, the pipe strain resulting fromtrench excavation.

Ground movements measured at Sites1 to 4 are generally significantly less than

those presented in Table I, particularly inthe case of the trenches excavated in sandor sand and gravel. Peck (1969) conclu-ded, in connection with deep excavation,that "the minimum settlements that canbe expected corresponding to the bestopen-cut construction practice, vary con-siderably with the type of soil. They arelikely to be negligibly small adjacent tocuts in dense sands and relatively stiffcohesive granular materials"... but..."excessive adjacent to cuts in soft plas-tic clays". The measured horizontal move-ments (which are in all cases greater thanthe vertical downward movements) atsites 1, 2 and 3 where ground condi-tions fall into the former category are,indeed, very small. Further experimentsare programmed for soft clay sites.

The site for most trenching projectsis generally of considerable length. Fur-thermore, it is unusual for excavation tobe restricted to one soil type only. Thelength of the trench and the cost of theproject generally dictate that informationon soil and ground water conditions onany one section is of a general natureonly since the cost of ground investiga-tion and testing to provide sufficient in-formation to derive reliable predictions ofground behaviour along the full lengthof the trench would be prohibitive. Con-struction procedures and workmanship arealso important, and often unpredictable,parameters in assessing likely groundmovement. Consequently, a statistical ap-proach to the prediction of ground move-ments may constitute the most realistic

solution to providing information forindustry.

In current practice, large ground move-ments causing significant damage to struc-tures are the exception rather than therule where workmanship is of a high stan-dard. The evaluation of a large number offield experiments will highlight those areaswhere damaging movements may occurand enable special precautions to be for-mulated and applied, reducing the fre-quency and extent of damage to adjacentservices.

References1. Symons, I. F. (1978): Discussion of "Lateral

displacement of shallow burled pipelines dueto adjacent deep trench excavations", (Crofts,J, E., Msnzies, 8, K. & Tarzi, A. I.) Geotech-nique 18, No. 2.

2. O'ourke, T. D. (1978): Discussion of "Lateraldisplacement of shallow buried pipelines dueto adjacent deep trench excavations", (Crofts,J. E., Menzies, B. K. & Tarzi, A. I.) Geotech-nique 18, No. 2.

3. Crofts, J. E., Menzies, B, K. & Terzi, A. I.(1977): "Lateral displacement of shallow buriedpipelines due to adjacent deep trench excava-tion". Geotechnique 17, No. 2, 161-179.

4. Kyrou, K, (1980): "The effect of trench excava-tion induced ground movements on adjacentburied pipelines". PhD Thesis (unpublished).Department of Civil Engineering, University ofSurrey.

5. Symons, I. F. (1980): "Ground movements andtheir influence on shallow buried pipes". ThePublic Health Engineer, Vol. 8, No, 4, pp. 149-153, 172.

6. Howe, M. P., Hunter, P. & Owen, R. C. (1980):"Ground movements caused by deep excava-tions and tunnels and their effect on adjacentmains". Second Conference on Ground Move-ments and Structures, Cardiff.

7. Peck, R. B. (1969): "Deep excavations and tun-nelling in soft ground", Proc. 7th InternationalConference Soil Mechanics and Foundation En-gineering, Mexico 225-290.

TABLE I. RECENT PUBLISHED RECORDS OF GROUND MOVEMENT AND PIPE STRAIN NEAR TRENCHES

Source

Site 1tTrench/geometry Main'round Trench support5.5m deep 230mm (4.7m) Soft sandy Hydraulic3.5m wide Parallel to trench clay shoring (failed)

'Max measuredmovement

120mm H230mm V

(1.5m)

Max measuredstrain in main

Site 2'.0m deep3.6m wide

Site 3'.0m deep0.76m wide

Site 4'nknown depth

305mm(2.7m) Parallel totrench

100mmDuctile iron crossingtrench

305mm cast ironcrossing trench(supported)100mm steel crossingtrench

Stiff sandyclay

Firm clay

Unknown

Unknown

Close trench sheets

Nil

Unknown("minimal" )Unknown("minimal" )

70mm H170mm V

(1.5m)43mm H

115mm V("close" )22mm H

Unknown

230jts

175p,s

1 000jzs

Site 5t

SiteA'm

deep4.0m wide

3.3m deep1.5m wide

100mm cast ironcrossing trench

Unknown Nil

Clay over Steel platesgravel

Unknown

50mm H25mm V(1.5m)

Main fractured

Site B'.3m deep1.9m wide

1.5m fillover sandwith gravel

Steel sheets 100mm V(0.5m)

Site C'm deep1m wide

Nil London clay Hydraulic shoringunits

30+mm H(1.3m)

Site Ds 5m deep1m wide

Nil London clay Steel sheets 30mm H20mm V(0.5m)

Howe et el, 1980Symons, 1980

~ Figures in brackets given distance of measurement point or main from edge of trenchH HorizontalV vertica I

30 Ground Engineering