6_Chapter 6 Stabilizing and Investingating Landslips

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CHAPTER SIX

Stabilizing and investigating landslips

Remedial measures for stabilizing landslipsOverview Many general reviews of methods of slope stabilization have beenmade. Some of the more recent are by Root (1953), Baker and Marshall(1958), Brawner (1959), Zaruba and Mencl (1982), Duncan (1971, 1976),Schweizir and Wright (1974), Smith (1974), and Broms (1969, 1975).Hutchinson (1978, 1982, 1983, 1984a, 1984b) has made outstanding con-tributions in this area and the authors have drawn freely on his work inpreparing this chapter. Professor Hutchinson's kind permission to dothis is gratefully acknowledged.

The main methods of slope stabilization are described in the sectionsthat follow. They fall into the following categories:

. drainage

. changing the slope geometry

. earth retaining structures, including the use of anchors

. miscellaneous methods.

It may be advantageous on occasions to use a combination of some ofthese methods to obtain an optimum solution.

Drainage

Overview Drainage can be a most effective form of slope stabilization provided thatthe drains are properly maintained but this rarely occurs in practice.Drainage channels should be inspected on a regular basis and kept freeof debris and drainage layers should be carefully designed using thewell known ®lter rules to prevent clogging up with time. Weep holes inwalls should be cleared on a regular basis and a properly designed drain-age blanket should be placed behind the wall.

A slope protected by drainage measures should be monitored bypiezometers and other indicators to check that the drains are functioningproperly.

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It is common practice in earth dam engineering to monitor slopes on aregular basis to check that all is well. This is not usually the case withnatural or man-made slopes, even if failure could result in damage toproperty or loss of life. In the latter cases, the authors believe that stabili-zation works should include a programme of monitoring and maintenanceon a regular basis.

Surface drainagePonds formed on a slope or behind the crest should be drained and thewater should be conducted away from the site and discharged at a safelocation.

Tension cracks should be dealt with to prevent the build-up of waterpressure in them. They are often sealed but this is likely only to beeffective in the short term, as the seal is usually broken as a result offurther down-slope movement. Drainage of tension cracks should beconsidered.

Cut-off trench drains are often placed behind the crest of a motorwaycutting to intercept ground water ¯ow. The location of such cut-offtrenches should be chosen with care to avoid the trenches acting as poten-tial tension cracks in subsequent landslides. It is advisable to cover thebottom of a cut-off trench with an impermeable ¯exible lining to minimizethe risk of a trench acting as a source of water at another point in thecutting.

An interesting situation developed in a long slope in the London clay atWhitstable. Although the slope was long and fairly uniform, instabilitywas only evident at one location. Careful examination revealed that theground surface at the top of the slope behind the failing masses slopedalmost imperceptibly towards the unstable section thus leading surfacewater directly to the slip. There is no doubt that this exacerbated analready critical situation.

Erosion control

Toe erosion If the toe of a landslide is located under water, in the sea or ariver for example, it is essential to prevent erosion at this critical point.Rock armour, cribwalls or gabions, can supply effective protection.

Surface erosion This can be controlled by establishing vegetation on theslope, using berms to lead water safely away, or providing shallowherringbone drainage systems. In parts of the world where rainfall ishigh, e.g. in Hong Kong and Malaysia, a thin layer of soil cement is some-times used to provide not only erosion control, but also to prevent rainfallfrom entering the slope. These soil cement layers require drainage behindthem to prevent water pressure building up. Short lengths of pipe are

CHAPTER 6 STABILIZING AND INVESTIGATING LANDSLIPS

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often inserted through the soil cement layer for this purpose although theeffectiveness of such drainage pipes may be doubtful.

Seepage erosion A potentially dangerous situation is illustrated in Fig.6.1 and is not uncommon. If left untreated, the superincumbent stratacan be undermined and the underlying clay stratum may be softened oreroded leading to stability problems. Seepage erosion may be controlledby placing inverted ®lters over the area of discharge (Terzaghi and

Peck, 1967), taking care that the materials used satisfy the well known®lter criteria, or by intercepting the seepage at some distance back fromthe face with wells or sand drains.

Deep drainage trenchesCase study: University of Surrey, Guildford UK An interesting caserecord where deep drainage trenches were used to stabilize an unstableslope in London clay arose when the University of Surrey was constructeddirectly over part of an old landslip at Stag Hill in Guildford, Surrey, in theUK (Simons, 1977).

That such a suitable site as this could be found undeveloped so close to acity like Guildford, is due entirely to a geotechnical reason and that is thata landslip had taken place on the slopes of Stag Hill probably some timeduring the 19th century and, following this, the site was blighted andnot developed.

Boreholes and trial pits were put down, the distribution of pore waterpressure throughout the slopes, i.e. at different depths and locations,was measured, a comprehensive laboratory investigation was carriedout on undisturbed samples recovered from boreholes and trial pits, and

stability calculations in terms of effective stress were performed. Figure6.2 shows the site in 1952 before construction, and Fig 6.3 shows thesite during construction in 1969. In both aerial photos, the extent of theslip can be clearly seen.

The site: topography and geology: The site is shown on the site plan ofFig. 6.4 and the slopes on which the main part of the university is builtare remarkably uniform varying from about 8 8 to 10 8. The slopesbecome gentler moving further to either the east or the west.

The whole site is underlain by London Clay and, according to geologicalrecords, this is followed by the Woolwich and Reading beds, and then the

Clay

Sand under water pressure

Clay

Seepage

Fig. 6.1 Seepage erosion

SHORT COURSE IN SOIL AND ROCK SLOPE ENGINEERING

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Chalk, but none of the boreholes put down on the site penetrated throughthe London Clay. The London clay is an over-consolidated jointed ®ssuredclay of Eocene age.

Towards the top of Stag Hill, undisturbed brown London Clay overlying

the blue London Clay, extends to the ground surface. Close to the toe ofthe slip and further north, the undisturbed clay is overlain by up to 6 mof brown and yellow mottled sandy and silty clay. It is probable that thisupper zone is a redeposited mixture of London Clay and the more sandyClaygate beds which overlie the London Clay.

Con®rmation that this upper zone was redeposited was obtained from atrial pit more than half-way up the slope where fragments of chalk werefound at a depth of 1.5 m.

Fissures and joints were observed throughout the London Clay, and inthe redeposited material in the trial pits, and in the samples taken fromgreater depth.

Fig. 6.2 Aerial view of the University of Surrey site in 1952 before construction of the

university commenced. Guildford Cathedral is partly built. Slip scarps can be seen to theleft of the Cathedral boundary hedgerow. Note the concentration of vegetation in theslip scarps


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Piezometers: To establish thepore water pressure distribution in theclay,piezometers were installed. These consisted of a porous pot surroundedby a plug of sand in a borehole with a small diameter pipe or tube beingconnected to the pot and taken vertically up the borehole to the groundsurface, the space above the sand plug being back®lled with a bento-nite±cement grout, to prevent leakage down the borehole, thus ensuring

that the measurements re¯ect correctly the pore water pressure aroundthe sand plug. The dimensions of the sand plug and of the internaldiameter of the connecting tube were so chosen with respect to the perme-ability of the clay, that a suitably short response time was achieved, i.e. thata change in pore pressure was followed reasonably quickly by acorresponding change in water level in the tube.

In October and November 1965, 19 piezometers were installed (the 100and 200 series), and records were taken, but most of these piezometerswere soon lost during construction of the Phase 1 buildings.

In December 1966, six additional piezometers were installed, A, B, Cand D on a line down the slope west of Wates House and through what

Fig. 6.3 Aerial view of the University of Surrey site in 1969. The scarp and toe of the slipcan be seen in the right hand corner of the photograph


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is now the Leggett Building, and 201 and 204 were reinstalled. Piezo-meters A, C, B and D were in ground not affected by drainage measuresat that time, so that the readings here could be compared with thoseaffected by the drainage system.

In March 1969, 17 new piezometers were installed (the 400 series) andregular records have been taken since then. At the present time, of the 42

piezometers originally installed, only six appear now to be functioningproperly and this kind of wastage is by no means unusual. Loss of piezo-meters due to subsequent building and to vandalism is only too common.

It was decided that the only practicable and economic way of stabilizingthe slope would be to reduce the water pressures in the slopes perma-nently by installing a system of deep, gravel-®lled, drainage trenches.This system had successfully been proved previously on London Clayslopes. Gregory (1844) described the use of gravel-®lled counterfortdrainage trenches to stabilize cuttings in the London Clay on the line ofthe London and Croydon Railway. A number of recommendations werethen made for Stag Hill as follows.

205

204412

1 8 0

1 9 0

200

2 2 0

2 3 0

2 5 0

2 4 0

2 1 0

Toe

D

C

B

A

Backscarp

0 10 20 30 40 50 mScale

BatterseaCourt

111 101102

104402 403

103404

405

406

106105107

108

109110

112

401

Library

AC AB

AA

Lecturetheatres

413410

411203

202 206

408407201

419

Trialdrain

2 5 0

2 4 0

2 3 0

2 2 0

21 0

2 0 0

1 9 0

1 8 0

1 7 0

1 6 0

1 7 0

SurreyCourt

414

416415

417

N

Deep drainage trenches

Fig. 6.4 Site plan of the University of Surrey (contours in feet above sea level)


157



. A trench drainage system to be installed with drains nominally 5 mdeep at 15m centres.

. Foundations for heavy buildings to be piled transferring loads belowthe level of the slip surface.

. Low rise buildings applying a gross foundation pressure of not morethan 37 kPa at 1.5 m below existing ground level, could be constructedwithout piling. Taking account of the weight of excavated soil thismeans that the net increase in load on the ground at foundationlevel was about 8.6 kPa.

In December 1965, a trial slope drain was constructed, and betweenMarch and June 1966 the main Phase I slope drainage system was con-structed, bored cast in situ piling was then installed and construction ofthe buildings followed.

In September 1965, the total cost of making the site available for theproposed University development was estimated as follows:

Land price £350,000Stabilizing drainage system £153,000Extra foundations £100,000Total £603,000

This gave a cost per acre of £7,100 that compared with a cost of £30,000per acre quoted for another new university being developed in an urbanenvironment.

So, in spite of the fact that additional costs were required so that the Uni-versity of Surrey could be safely constructed on an existing landslip, therewas no doubt whatsoever that the development should proceed, and infact the university was fortunate in acquiring so appropriate and valuablea location for its development.

Results of piezometer observations Because of space limitations, only themain points arising from the observations can be discussed.

(a) Initial distribution of pore water pressure with depth . There is evi-dence that a classical under-drainage system exists on the site.Figure 6.5 shows the results of pore water pressure observationsmade on piezometers prior to the introduction of the drainagesystem, and it is clear that the deeper a piezometer is installed, thelower the piezometric level. This is due to under-drainage into theunderlying chalk, resulting from wells having been sunk in thechalk for water supply purposes, which has led to a progressiveunder-drainage in the London Basin. Figure 6.6 shows the reductionin water head in the chalk for a point located in Hyde Park (Wilsonand Grace, 1942). This has resulted in a settlement in the London


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clay, which varies depending on the thickness of the London Clay atany point, and the amount of water level reduction, and has amaximum value of about 0.3 m for the period 1865 to 1975.

This is of crucial importance in slope stability problems, since theactual distribution of pore water pressure with depth will governthe factor of safety of the slope and also the depth of the failure sur-face. In cases where the effects of under-drainage are apparent in theLondon Clay then the depth to the failure surface will be signi®cantly

P205 P201 P104 P107 P102

3 · 1 m

4 · 7 m

7

· 7 m

1 3 · 5 m 8 ·

1 m

1 · 6 m

0 · 4 m

1 8 · 2 m

1 0 · 4 m

Readings taken on4 January 1966prior to effects ofdrainage measures

Fig. 6.5 Piezometric levels showing under-drainage, University of Surrey

30

–30

–60

–90

O.D.

m

Water levelin the Chalk

1820 1843 1878 1895 1911 1936

Drift and gravel

London Clay

Woolwich andReading bedsThanet Sand

Chalk

Fig. 6.6 Water levels in the chalk in Central London, after Wilson andGrace (1942)


159



less than if a hydrostatic pore water pressure distribution is assumed.Similar distributions of pore water pressure with depth in the Londonclay, at other locations, have been reported by Butler (1972). It istherefore essential when investigating the stability of a Londonclay slope that piezometers be installed at varying depths, and ofcourse at different locations, so that a reliable knowledge of thepore water distribution throughout the slope is obtained.

(b) Effect of drain installation ± initially. The effect of the trial drain

installed in December 1965 is shown in Fig. 6.7 and a signi®cantdraw-down in water pressure can be seen. It should be pointedout, however, that this draw-down is also partly due to the seasonaldrop in water pressure, which must be expected during the spring±summer period. Furthermore, the construction of the trench drainsinvolves cutting deep slots into the London clay thereby for a shorttime reducing the horizontal total stress on the sides of the trenchto zero. This temporary stress reduction would also lead to a reduc-tion in pore water pressure.

Most of the piezometers were then lost during building construc-tion, and it is necessary to turn to the piezometers installed in 1969for further information. Figure 6.8 shows the observations for piezo-meter C, in an undrained area, and for piezometer 404 installed some1.5 m from a drain, at the same surface ground level. It can be seenthat the drainage system has depressed the pore water pressures inthe slope, in particular in the winter months where a reduction inhead of about 2.7 m has been achieved. If the average depth of theslip surface is 6.1 m this means an increase in the factor of safetyagainst sliding of some 42%, which is appreciable! There are,therefore, no grounds for concern as to the safety of the university

1965 1966

N D J F M A M J J A S O N D0

1

2

3

D e p t h

b e l o w g r o u n d s u r f a c e : m

Piezometer P106Depth 6 ·1 m2·4 m from of drainDrains installedMarch / June 1966

LC

Fig. 6.7 Effects of trial drain, University of Surrey


160



buildings. Figure 6.9 shows the variation in pore water pressure forpiezometer 404 over a number of years.

Figure 6.10 indicates the distribution of pore water pressure acrosstwo drainage trenches and although the draw-down towards thedrains is perhaps somewhat less than might have been hoped for,the bene®cial effects of the drainage system are obvious. A wordof warning must, however, be inserted here. While deep drainshave been shown to lower the water pressures throughout theslopes here at Guildford, it does not necessarily follow that a similarreduction can be expected for all London clay slopes.

Buckthorp et al . (1974) report a case where deep drains did nothave the desired effect and this was because the site had previouslybeen wooded, and the trees were removed immediately prior to thedrains being installed. The destruction of the root system offset anyreduction in pore water pressure which the drains otherwise wouldhave had.

19701969

M A M J J A S O N D J F M A M J J0

1

2

3

D e p t h

b e l o w g r o u n d s u r f a c e : m

Outside drained areaPiezometer CDepth unknown

Inside drained areaPiezometer 404Depth 5 ·7 m

Fig. 6.8 Comparison of piezometers C and P404, University of Surrey

2

3

D e p t h

b e l o w s u r f a c e : m

1969 1970 1971 1972 1973 1974 1975 1976

Piezometer P404 – 2·1 mfrom centre line of drain

Fig. 6.9 Readings of piezometer 404, University of Surrey


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Weeks (1969) described the effects of counterfort drains at Sevenoaks,near London, where an average reduction in pore water pressure headof 2 m was achieved at the depth of the slip surface.

The results of the pore water pressure observations at the University ofSurrey site con®rm the original design decisions and show that majorstructures can be safely founded directly on an old landslip, provided aproper drainage system is installed and provided heavy building loadsare taken down below the level of the slip surface. Due attention must,however, be paid to the actual pore water pressure distribution through-out the slope, and to the effects of removal of vegetation.

In¯uence of barometric pressure It is interesting to note that when

measuring pore water pressures on the Stag Hill site, the observationsre¯ected changes in barometric pressure. Ko Èhler and Schulze (2000)measured barometric variations in pore pressure in a cut constructed inthe 1920s in Lias clay close to Lu Èhnde (Germany) that they instrumentedin 1997. From inclinometer readings they found that slope movementscould be correlated with major barometric pressure drops (up to 5 kPabelow mean barometric pressure).

Three-dimensional design considerations The three-dimensionaltrench problem is highly complex, taking into account several factorsincluding:

P404 P403 P402

2·1 m

1·5

3·0

4·5

6·0

2·1 m6·1 m5·2 m

0

D e p t h : m

5·6 m

15 ·5 m

4·7 m

5·6 m5·2 m

4·7 m 4·5 m

5·7 m

Drains installedMarch / June 1966Piezometers installedMarch 1969

21 May 196914 December 1976

Fig. 6.10 Results of observations on piezometers P404, P403 and P402


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. non-homogeneous and anisotropic permeability varying with effectivestress,

. partial clogging of the sides of the trenches (smear)

. variable in¯ow of ground water from upslope

. intermittent rainfall, involving saturated and partially saturated ¯ow.

A simpli®ed solution based on two-dimensional ¯ow has been presentedby Hutchinson (1978) and by Bromhead (1984) and provides a basis for thedesign of a trench drainage system. Drain widths on 0.5 m to 0.9 m, depthsof 3.5m to 5.0 m, and spacings of 10m to 20 m are common.

Bored piles and counterfort drainsCase study: Gypsy Hill, London UK The use of bored piles together withcounterfort drains to stabilize a major landslip in a clay slope at Gypsy Hill,South London, has been described by Allison et al . (1991). Counterfortdrains are trench drains that also act as a structural element strengtheningor `buttressing' the slope.

In June 1980, a row of Victorian detached houses in Victoria Crescent onthe west side of Gypsy Hill in South London was demolished to make wayfor a new housing development. The rear gardens of the houses slopedupwards, away from the houses, at a steep angle and were retained atthe toe of the slope by brick walls up to 2.6 m in height. The slope angleimmediately behind the walls was about 19 8.

The height of the retaining wall was reduced to approximately 0.5 mto improve daylighting to the rear of the new properties. This wasachieved by providing a new reinforced concrete wall at the toe ofthe slope, and regrading the rear gardens by steepening them over adistance of some 20 m to join the existing slope line near to the uppersite boundary.

Work proceeded on the new three-storey townhouses and the roofconstruction was nearing completion when, in February 1984, after aperiod of heavy rain, major downhill slip movement took place in the

rear garden slope. Saturated soil ¯owed over the top of the new reinforcedconcrete retaining wall and piled up against the rear wall of the newbuildings.

The movement took place across practically the full width of the site,and extended beyond the upper site boundary, causing severe disruptionto an access road and partly exposing the foundations of a three-storeyblock of ¯ats. The area of the slip within the site boundary is shown inFig. 6.11.

Site investigations revealed that the slip had taken place on a pre-existing shear plane at a depth of about 4 m to 5 m below the groundsurface. This shear plane was found to be the interface between relatively


163



intact London clay and an overlying mantle of colluvium. Piezometersinstalled immediately after the slip indicated a ground water surfaceapproximately parallel with the slope, about 0.5 m below ground level.The movement was occurring on an approximately planar surface withan average slope of about 9 8.

In the absence of remedial measures, continued downhill movement ofmaterial on the slope would result in the following.

0 5 10 15 20 25 m

ScaleN

P10

P15

P14P13

P2

P6

P8P9

P7

P3(S)

P11(S)P12(D)

P4(D)

P1

I4 I5I8

I27

I14

I21

I12I10

I17

I2

33– 39 V ict or ia Cr e s ce nt ( unde r cons t r uct ion)

Existing rear retaining wall

Phase 2Counterfortdrains

Lateral extentof slip

Y

X D o w n s l o p e

d i r e c t i o n

Inclinometerreadingdirections

Lateralextent

of slip

Phase 1piles

Access RoadSite Boundary

Exis ting 3 s tore y block o f fla ts

600 mm dia. bored pilecontaining 305 × 305 × 118 UCInclinometer and numberPiezometer and numberShallow

Deep

(S)

(D)

Note: Reinforced concrete retainingwalls and associated worksomitted for clarity

Fig. 6.11 Site plan, Gypsy Hill, after Allison et al . (1991)


164



. A build-up of saturated soil against the rear of the new properties,leading to penetration of door and window openings and structuraldamage to the walls.

. Uphill regressive movements leading to the undermining of thefoundations of the existing three-storey block of ¯ats to the upslopeside of the site boundary.

The need for rapid action was a matter of over-riding importance. Afterconsidering various options, it was decided to install 30 No. 600 mmdiameter concrete bored piles about 10 m long in 3 rows, 9 piles in thetop row, 7 piles in the middle row, and 14 piles in the bottom row asshown in Fig. 6.12. Each pile was reinforced by a 305 Â 305 Â 118kgsteel universal column. The piles were designed following Viggiani(1980). Inclinometers were installed in 10 piles to monitor performance.

It was recognized that the factor of safety would be greatly reduced if

the ground water should subsequently rise to near the ground surface.Accordingly, a system of counterfort drains was adopted and designedfollowing Bromhead (1986). The system comprised four counterfortdrains running the full length of the slope. The drains were located withan average spacing of 4.5 m, a minimum drain depth of 3.5 m, and adrain width of 0.5 m.

Inclinometer readings showed that pile deformations stabilized shortlyafter pile installation and prior to construction of the counterfort drains.Only very minor movements were recorded subsequently. The piles inthe upper row had a maximum deformation of 37mm, while the deforma-tions of the piles in the middle and lower rows were up to 15 mm.

P1 Piezometer tip and number

33 –39 VictoriaCrescent

1

1

2

2

3

3

4

4

Flats

Access

Spur drain

Outline of counterfort

P1P10

P12P15

P9

P11

P4

P2P3P5P6

P7P8

P13P14

Existing retaining wall

Original ground level

Final ground level

Post-slip ground level

Rows of piles

Fig. 6.12 Section through site, Gypsy Hill, after Allison et al . (1991)


165



Twelve piezometers were installed in the slope and piezometer read-ings for the deeper section of the slip zone indicated that the averagelevel of the water table was at least 2.5 m below ground level. For thepurpose of slope stability calculations, the average draw-down wasassumed to be 2.0 m.

This is an important case record showing that bored cast-in-place con-crete piles, reinforced with heavy structural steel sections, proved to be asatisfactory means of arresting an active shallow landslide within a shorttime period, on a site with very limited access. The provision of slopedrainage in the form of counterfort drains resulted in a signi®cant lower-ing of the piezometric levels and greatly enhanced the stability of theslope. The installation of instrumentation, comprising inclinometersembedded in the piles and also piezometers installed within and adjacentto the counterfort drains, provided useful data which generally con®rmedthe original design assumptions and provided con®dence in the satisfac-tory performance of the remedial works.

Reference may also be made to works by Hong and Park (2000), Ergun(2000) and Nichol and Lowman (2000).

Pumping from wellsAn interesting example of slope stabilization involving pumping fromwells is given by Pilot et al . (1985). The building of the A8 autoroute insouth-eastern France near the Italian border involved the construction

of a large cutting with a maximum depth of 40 m. The ground conditionsare marl, which when intact is very hard but is highly sensitive to theaction of water, in contact with which it readily decomposes forming aplastic material in which the slip surfaces encountered during the workswere found.

Drainage was provided by installing eight pumping wells 125 mm indiameter and up to 15 m long, each with a pump in the bottom of thewell. The operation of the pumps is discontinuous and is controlled bythe water levels in the wells which were taken down to the underlying

intact marls. Additional drainage measures included sub-horizontaldrains up to 80 m long with a slope of 20%, and some run-off ditches.It was reported that all of the arrangements worked well, and piezo-

meters indicated that the ground water table was lowered practically tothe bottom of the decomposed marl.

If a clay slope is underlain by a more permeable material, for examplethe chalk or a sand layer, and the water head in such a permeable layeris below the ground water table in the clay, the stability of the clayslope can be improved by installing a system of vertical sand drainsthrough the clay layer and terminating in the underlying material. Thisis by no means an uncommon situation that can be used with advantage


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to provide an economical and effective method of lowering ground waterpressures.

Reference may also be made to the works of Maddison and Jones (2000)and to Maddison et al . (2000).

Horizontal drainsOverview These are usually drilled into a slope on a slightly risinggradient and provided with perforated or porous liners (Root, 1958; Tongand Maher, 1975). They are often 60 m to 100 m in length, thus providingdrainage deep into the slope. They are quick to install and can bringabout an improvement in slope stability in a short period of time whichcan be important if nearby structures are endangered. In cold climates, itmay be necessary to protect the drain outlets from freezing.

Case study: Lyme Regis, Dorset, UK Lyme Regis is situated on anactively eroding stretch of the West Dorset coast and has thereforealways faced considerable challenges from coastal erosion and landslip-ping. Although the earliest known coast protection structure, the Cobb,dates back to the 13th century, many of the town's sea walls are relativelyrecent. It was not until around 1860 that the Marine Parade sea wallwas successfully completed and the East Cliff sea wall was built only inthe 1950s. Prior to the construction of these defences, the coastlinewould have been actively retreating in a similar way to the unprotected

parts of the coast today, and there is strong historical evidence toindicate that a large part of the original mediaeval town has been lost tothe sea.

Problems arising from coastal landsliding have been particularly seriousduring the 20th century. Some ®fteen individual properties have beendestroyed and many more severely damaged. There have been severalmajor sea wall breaches along the main frontage, frequent substantialdamage to Cobb Road and the complete loss of the main coastal road toCharmouth.

The Lyme Regis Environmental Improvements were initiated by WestDorset District Council in the early 1990s, with the principal aim ofimplementing engineering works to help ensure that the integrity of thetown's coast protection is maintained in the long term and to reduce thedamage and disruption caused by coastal landsliding.

The construction work for Phase 1, which comprises a new sea wall androck armour adjacent to the mouth of the River Lim, was completed in1995. Since then, West Dorset District Council has been carrying out aseries of preliminary studies to gain information for the conceptualdesign of economic and environmentally acceptable coast protectionworks for the remaining areas of the town.


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The coast protection works, when implemented, are likely to comprise acombination of the following broad elements:

. slope drainage

. slope strengthening

. provision of new foreshore structures

. strengthening and refurbishment of existing sea walls

. beach replenishment.

The stability of the landslides at Lyme Regis has been shown by bothongoing monitoring and the preliminary stability analyses to be verysensitive to ground water levels and seasonal ground water variations(Fort et al ., 2000). By reducing ground water levels down to or belowdry `summer' levels, the stability of the active landslides should be signi®-cantly improved. This has lead to preliminary proposals for the installationof a network of sub-horizontal drain arrays positioned at various eleva-tions within the landslide systems at both Lyme Regis town and EastCliff. These would be a technically feasible, environmentally acceptableand cost effective technique of reducing ground water levels, thus improv-ing stability and reducing ground movements. They have major potentialadvantages over conventional trench drains in that they have a relativelylow construction and environmental impact, can be installed at signi®-cantly greater depths below existing ground surface, and can be designedto allow for longer term maintenance.

In essence the drains should satisfy a number of basic requirements toenable them to perform successfully, as follows:

. The size should be adequate to carry the maximum water ¯ow withoutdisturbance to the adjacent ground or development of excessive out-¯ow pressures.

. There should be no signi®cant loss of ¯ow by re-in®ltration into theground along the drain length.

. Any liner or pipe should be suf®ciently strong and rigid to be easilyinstalled to the design length and orientation, and capable of support-ing the borehole without signi®cant deformation or collapse. In addi-tion the liner or pipe should be able to accommodate some groundmovement without failure.

. The slotted or perforated length of any liner should be formed so as toprevent soil ingress, or it should be provided with an appropriate ®lter.

. In the long term the drain should function satisfactorily without clog-ging and with the minimum of maintenance.

The ground conditions at Lyme Regis comprise an interbedded sequenceof mudstones and clays with layers of limestone. Landslide deposits com-prising largely cohesive materials of variable thickness mantle the in situ


168



geology. Figure 6.13 shows a typical ground model section based onobservations (Fort et al ., 2000). Measured ground water levels within

the landslide materials are close to the ground surface. Pressures recordedwithin the in situ geology are generally less than hydrostatic, suggestingunder-drainage.

A preliminary proposal is to conduct trials that are located at twopositions within Lyme Regis town to test the application within twoareas dissimilar in topography, elevation, geology and landslide system.They would also be positioned so that it would form part of the futurestabilization works at Lyme Regis. Possible drain array locations andcon®guration are shown in Fig. 6.14. Installation of the drains would be

carried out by drilling plant of the type shown in Fig. 6.15.

Changing the slope geometryRegrading a slope may be used to increase stability and this may becarried out as follows:

. the slope is made ¯atter

. a toe berm is constructed (see Fig. 6.16)

. material is removed from the top of the slope to reduce the overallheight.

Site conditions often dictate the most advantageous solution.

0 10 m

Scale

F is h B e d

Gr ey Le dge

T able Le dge

Seacliff

Seawall

Beach

Mudslide

Loading at headfrom landslide above

1998 clifftop postion

1995 clifftop postion

Active slip surface observedapprox. 2 m above Grey Ledgein exposure above seawall

Denotes moving landslide

Position of shear surfaceconfirmed by inclinometeror visual observation

Fig. 6.13 Typical ground model section, Lyme Regis, after Fort et al . (2000)


169



4 5

4 0

3 5

3 0

2 5

2 0

E l e v a t i o n m A O D 4 5

4 0

3 5

3 0

2 5

2 0

E l e v a t i o n m A O D 4 5

4 0

3 5

3 0

2 5

2 0

E l e v a t i o n m A O D

P 2

P 3 P

6

P 4

P 5

P 1

S e c t i o n

A

S e c t i o n

D

S e c t i o n

F

N

D r a i n

d r i l l e d t o g r o u n d s u r f a c e

D r a i n

t e r m i n a t e s w

i t h i n t h e g r o u n d

N o t e : d r a i n

l e n g t h s a n d p o s i t i o n s a r e p r o v i s i o n a l

B i r c h i B e d

E x i s t i n g p i e z o m e t e r .

( P r o p o s e d a d d i t i o n a l

p i e z o m e t e r s n o t s h o w n )

P r o p o s e d l o c a t i o n s o f p n e u m a t i c p i e z o m e t e r

D

C

E

F

B

A

F i g . 6 . 1 4

P r o p o s e d l o c a t i o n o f d r i l l e d d r a i n a g e a r r a y t r i a l s , L y m e R e g i s ( p e r m i s s i o n o f W e s t D o r s e t D i s t r i c t C o u n c i l a n d

H i g h - P o i n t R e n d e l )


170



When ¯attening a slope, the possibility of reactivating the old deepseated slip surface should be considered, as illustrated in Fig. 6.17 (Hutch-

inson, 1978, 1983, 1984).When considering cut and ®ll solutions, the neutral line theory of Hutch-inson (1978) is helpful. Figure 6.18 illustrates a simple example. A ®llplaced on a slope will have two effects.

(a) It changes the net disturbing moment which, depending on thelocation of the ®ll with respect to the centre of rotation of a potentialslip surface, may have a positive or a negative effect.

(b) It will increase the effective stress acting on a slip surface but onlyafter dissipation of excess pore water pressures has occurred. In a

saturated clay slope, any ®ll placed will not result in an increase ineffective stress in the short term.

Fig. 6.15 Drilling of sub-horizontal drain arrays, Robin Hood's Bay,Scarborough, UK (permission of Mr Keith Cole, West Dorset DistrictCouncil)

Removal of soil

Toe, berm preferably free draining

Fig. 6.16 Cut and ®ll to improve slope stability


171



Conversely, if a clay slope is regraded to a ¯atter angle, the possibility ofa reduction in effective stress and hence strength with time as the clayswells should be taken into account.

Great care must be taken to ensure the correct positioning of a toe berm.It must not, of course, initiate a local slide and may not be bene®cial withtranslational slides where there may be a potential for an over-ride slide tooccur (see Fig. 6.19).

A large-scale example of the use of a toe berm to improve slope stabilityis found at Folkstone Warren on the south coast of England. Here amassive landslip occurred during the First World War that affected thetransport of men and equipment to the Western Front. Extensive ®llingwas placed on the foreshore as a toe load (Viner-Brady, 1955). Althoughwell positioned, the toe berm improved the overall factor of safety byonly 3.5% to 5%.

Centre of slip circle

Removal of soil

Deep seated circular slip surface

Fig. 6.17 Flattening a slope may lead to reactivation of a deep-seated slide,after Hutchinson (1983, 1984b)

Fills alwaysadvantageous

Fills alwaysunfavourableFills adverse in

short term, butbeneficial in

long term

Centre of rotation

α

Drained neutral pointα = φ′ mobilized

Undrained neutral pointof landslide α = φ,immediately undercentre of rotation

Fig. 6.18 The neutral line theory, after Hutchinson (1978)


172



Earth retaining structuresDrainage and changing the slope pro®le are usually the ®rst choice forstability improvement schemes. If, however, space is restricted,then earth retaining structures, if properly engineered, can play a usefulrole.

Retaining walls of various types have been used in slope stabilizationworks, e.g. gravity structures including cribwalls and gabions, as well asreinforced concrete walls, steel sheet pile walls, and large diameterbored piles. They can be free standing or anchored. The use of steepreinforced slopes has greatly increased in recent years and reinforcedearth structures can be found in many parts of the world. Soil nailingcan be a viable solution.

The design of such structures is outside the scope of this Short Coursebook. Reference can be made to works by Clayton et al. (1991), Hoek

and Bray (1977), Ingold (1982), Littlejohn et al . (1977), Grant et al .(2000), Martin and Kelly (2000) and Jamaludin and Hussein (2000).

Miscellaneous methodsElectro-osmosisOverview L. Casagrande introduced electro-osmosis into civil engineer-ing in the mid-1930s and its application has been demonstrated on anumber of occasions (Casagrande, 1947, 1952, 1953). In most cases,electro-osmosis was used in silty soils to produce a temporary stabiliza-

tion, for instance, during excavation. A famous case was the stabilizationof slopes for the construction of U-boat pens in the very soft Trondheimclayey silt (Casagrande, 1947).

In the electro-osmosis system, direct current is made to ¯ow from anodeswhich are usually steel rods driven into the soil, to cathodes which can alsobe steel rods or slotted pipes. The electrical gradient produces a ¯ow ofpore water through the soil from anode to cathode where it can beremoved, for example, by pumping from pervious cathodes. The increasein shear strength obtained was primarily due to the increase in effectivestress due to the ¯ow of water. When the current is switched off, thepermanent increase in shear strength is small.

Potential over-rider slip

Toe berm

Translational slip

Fig. 6.19 Translation slip stabilized by a toe berm, illustrating a potential over-rider slip


173



Figure 6.20 shows a typical arrangement to stabilize the face of acutting. The ¯ow of water is directed away from the face of the cutting,from anode to cathode.

Case study: A Ê s, Norway At one stage it was considered that the use ofelectro-osmosis to increase the shear strength of clays was economicallyimpractical. There are now cases where electro-osmosis has been success-fully applied to that end, see e.g. Casagrande (1953) and Bjerrum et al .(1967) who describe an important and extremely interesting case whereelectro-osmosis was used to obtain a permanent increase in shear strengthof a Norwegian quick clay so that two excavations for a sewage treatment

plant to a depth of about 4.5 m could be safely carried out.The site is located at A Ês, about 30km south of Oslo on the eastern side of

the Oslofjord. The ground conditions are shown in Fig. 6.21 and a cross-section across the site is shown in Fig. 6.22. The undrained shear strengthnear the base of the excavation was as low as 6 kPa. It was clear that thedanger of a bottom heave failure (Bjerrum and Eide, 1956) was greatand it was decided to use electro-osmosis to increase the shear strengthof the quick clay.

The electro-osmosis installation used reinforcing bars of 19 mm diameter,

10 m long, as electrodes that were pushed 9.6m into the ground in 10 rows2.2m apart and at a spacing of about 0.6 m in each row. In total, 190 barswere used and the area of the installation was 200 m 2 .

No pumping devices were installed since it was expected that the waterexpelled at the cathodes would be capable of forcing its way up throughthe clay and this is in fact what happened. Small trenches were dugalong the cathodes to allow the expelled water to drain away, and insuch a way, the water was collected and the discharge measured.

The installation was monitored by 14 piezometers, one precision settle-ment gauge, and 11 Borros settlement gauges that measured the settle-ment at various depths. Measurements of voltage applied and current

Cathode, can be a perforated tube

Original water table

Anode, usually a steel rod

Water flows fromanode to cathode

Fig. 6.20 Simpli®ed electro-osmosis installation


174



consumed are shown in Fig. 6.23 which also gives the observed settlementat three gauges.

After the electro-osmosis was complete, the cathodes were easy toremove and, not being corroded, were later used as reinforcing steel.The adhesion developed between the soil and the anodes preventedremoval of the anodes. During excavation, however, some pieces of the

anodes were recovered and it was found that corrosion of the anodesamounted to 37% of the original weight.The total amount of electricity consumed was 30 000 kWh and the total

weight of electrode steel was 4.5 tonnes. Altogether, 100 m 3 of water wasexpelled and a volume of about 2000 m 3 of clay was stabilized, i.e. 17 kWhof electric power and 2.25 kg of steel were necessary to stabilize 1 m 3 ofclay.

The settlement observations given in Fig. 6.24 show that the groundsurface within the treated area amounted to about 500 mm for theperiod of treatment of some four months. At a distance of only 2 m fromthe outside rows of electrodes the settlement was negligible.

D e p t h : mDescrip-

tion ofsoil

Water content: %

10 20 30 40

Unitweight:kN/m 3

Shear strength: kPa

10 20

Sensi-tivity

Vertical effective stress: kPa

50 100Weath-eredcrust

5

Quickclay

10

15

w Pw

w = Water contentw

L = Liquid limitw P = Plastic limit

w L

19 ·3

19 ·6

20 ·0

19 ·5

19 ·5

Vane boring I

50

80

80

7070

10

>100

>100

>100

Effective pressure

Vane boringUnconfined compression testCone test

Pre-consolidationpressure observedin consolidation test

Fig. 6.21 Ground conditions of the electro-osmosis installation, A Ê s, after Bjerrum et al .(1967)


175



10+

8+

6+

4+

2+

1–

3–

5–

7–

9–

Electrode row no:

N

–+

B

P12

P14

P11

P13

P10P2 P4P1 P5

P3P6P8

P9P7

S1

S2

S3

S4

B r o o k

Excavation

BV7

V3

V4V1V5V6

V2

I5

67

8

9

10

1112

Section B –B

S1

S4

S3

P6

P12

P13

P14

P7 P9

P11 P10P1 P2

P3P4

P8

P5

0 5 10 m

Scale

Bedrock ≈ 28 m

Excavation

5 678 9101112

Weatheredcrust

Quickclay

Piezometer installations P1 –P14

Precision settlement gauge S1Borros settlement gauge S2 –S12

Vane boring I before electro-osmosis

Vane borings V1 –V5 after 30 days ofelectro-osmosis

Vane borings V6 –V7 after 103 days ofelectro-osmosis

Fig. 6.22 Plan and cross-section of the electro-osmosis installation, A Ê s, after Bjerrum etal .(1967)


176



0

0

0

0

20

20

40

40

60

60

8080

100

400

300

200

100

120Time: days

Current reversed

V o l t a g e b e t w e e n e

l e c t r o d e s : V

C u r r e n t : A

5000

10 000

15 000

20 000

25 000

30 000

P o w e r c o n s u m e d : k W

h

0

10

20

30

40

50

S e t t l e m e n t : c m

April May June July August1964

S 3 ( d e p t h = 8 · 0 m )

S 2 ( d e p t h = 4 · 0 m ) S 1 ( d e p t h = 1 · 0 m )

Fig. 6.23 Electricity consumption and settlements, A Ê s, after Bjerrum et al .(1967)


177



A total of 14 conventional NGI-type piezometers were installed. These

consist of a porous bronze tip 300 mm long, with an outside diameter of30 mm, ®xed to a 6 mm inside diameter copper tube from the tip to thetop of the pipes. Copper tubing was used instead of nylon in anticipationof negative pore pressures. It would be expected that electro-osmosiswould radically alter the pore pressure conditions. As water is removedfrom the clay next to the anodes, suctions will develop in these areaswhile at the cathodes the pore pressure will increase. It quickly becameobvious that a production of gas took place in the piezometers, whichwould have made the readings unreliable. Bubbles rising from the holesleft by some borings performed along the cathodes indicated that thegas formation was considerable. Ignition of the gas by a match ¯amedemonstrated the presence of hydrogen. Bjerrum et al . (1967) point outthat piezometers in electro-osmotic installations should consist of non-conducting materials such as ceramic ®lters and plastic tubing to minimizeproblems of gas formation due to electrolysis.

The following main conclusions may be drawn from this extremelyimportant case record.

. Electro-osmosis is a useful engineering procedure for improving the

engineering properties of a limited volume within a large deposit offsoft clay.. Due to electro-osmosis, the undrained shear strength increased from

an initial value less than 10 kPa to a maximum value of 110 kPa nextto the anodes.

. During the period of treatment an unexpected temporary reduction inundrained shear strength was observed, in spite of the fact that waterwas being removed from the clay and surface settlement was occur-ring. This reduction in strength occurred not only within the treatedclay volume but also to a large depth in the clay beneath the treatedarea. Bjerrum et al . (1967) believed that the reduction in strength

0

0 5

200

400

600 S e t t l e m e n t : m m P12 P11 P10

10 m

Scale

S1

S1 –S11

1–

2+

4+

6+

8+

10+

3–

5–

7–

9–

Electrode row no:

Original ground surface

Fig. 6.24 Settlement observations, A Ê s, after Bjerrum et al . (1967)


178



within the treated clay volume was due to shear deformations but noexplanation can be offered for the reduction observed in the claybeneath the treated volume.

. This case record emphasizes the necessity for carrying out wellplanned control measures including observations of settlement, porewater pressure and, in particular, undrained shear strength at varioustimes during the electro-osmosis treatment. Failure to do so couldresult in disaster.

. The response of soil to electro-osmosis is not fully understood butclearly changes in a soils fundamental physical±chemical propertiesoccur leading to surprising results.

Another unexpected result was reported by Eide and Eggestad (1963) inconnection with an excavation carried out in Oslo for the foundations for

the new headquarters of the Norwegian Telecommunications Administra-tion. Electro-osmosis was used to stabilize the very soft highly sensitiveclay. It was noted that signi®cant increases in undrained shear strengthoccurred together with a settlement of the ground surface, even thoughthe pore pressures showed a slight increase!

GroutingInjection grouting as a treatment for stabilizing clay embankment slipshas been known for over 50 years and has found particular application

to railway embankments.Controlled hydro-fracture systems are used. A grid of injection points isset up with the base of the points below the slip plane, and a quantity ofgrout is speci®ed for injection into the various points based upon experi-ence, the local conditions, and depth of injection. More than one level ofinjection may be required. A 30% sand±cement grout that has beenaerated by 15% to 25% to increase the viscosity and ¯ow properties hasbeen found to be suitable (Ayres, 1985) and at a cost of one third of alter-native conventional methods. Ayres' paper gives two case records wheregrouting was successful. Grouting has also been successfully used tostabilize cutting slopes in stiff clays (Ayres, 1961). If done without care,however, grouting can increase pore pressures and trigger a slide.

VegetationThe bene®cial effects of vegetation on slope stability develop graduallyover a period of time that may be weeks or months in the case of grassesand herbaceous vegetation or several years for shrubs and trees. Theadverse effects of sudden removal of vegetation on slope stability arewell known and have lead to slope instability problems on a number ofoccasions, e.g. Gray (1977) and Wu (1976).


179



The bene®cial effects include

. reduction in pore water pressures caused by evaporation and tran-spiration

. reinforcement of the soil by root systems

. anchoring, arching and buttressing by tap roots.

Adverse effects include

. down-slope wind loading from large trees

. additional down-slope forces caused by the weight of large trees.

It should be noted that vegetation will result in a reduction of surfacerun-off and a corresponding increase in water in®ltration but the overalleffect on pore water pressures is bene®cial, i.e. it reduces them.

While it is dif®cult to assess with any precision the degree of bene®t thatvegetation can have on slope stability, attempts have been made toquantify such effects, e.g. see works by Coppin and Richards (1990), Wu(1994) and Operstein and Frydman (2000).

It would seem sensible for design purposes to ensure that any slope has afactor of safety above unity without taking into account the effects of vege-tation, and then to consider that any vegetation is an added insurance.

Species of trees which may be suitable for improving the stability ofslopes because of their extensive root systems, would be willow, poplar,oak, elm, horse chestnut, ash, lime and sycamore maples.

FreezingThis is an expensive technique and is rarely used to stabilize slopes. A dis-advantage is that the whole process of drilling holes, installing the plant,and freezing the ground may take several months. Freezing certain typesof ground can cause severe heaving.

Ground freezing involves sinking boreholes in the area to be treated at1.0 m to 1.5 m centres. Concentric tubes are inserted in the boreholes andrefrigerated brine is pumped down the inner tubes and rises up the annu-

lar space between the inner and outer tubes or casings. It then returns tothe refrigeration plant via the return ring main. Usually it takes from sixweeks to four months to freeze the ground. A well known application isin temporarily stabilizing a ¯ow of silt during construction of the GrandCoulee Dam (Gordon, 1937). A number of examples of ground freezingin civil engineering works have been given by Harris (1985). A generalreview of the technique is provided by Sanger (1968).

Combining stabilizing methodsWhen using more than one method of improving slope stability, the possi-bility of progressive failure should be considered. If the various methods


180



employed invoke different stress±strain relationships from the soil and from

the materials and/or techniques involved, one method may be approachingfailure before the another technique can make a signi®cant contribution tothe overall stability. In this way, the design intention, of each of the methodsincreasing stability at the same time, might not eventuate.

Grant et al . (2000) used a combination of regrading, bored horizontalunder-drainage, and ground anchors to give an acceptable factor ofsafety of a slope that had been oversteepened and had a history of instabil-ity (Fig. 6.25). A particular feature of the works was the way in which theseparate and combined actions of each of the methods were considered.The overall stabilization scheme was designed to give a factor of safetygreater than 1.5 assuming the worst likely conditions. The contributionsof the methods of stabilization, however, were balanced in such a waythat even if any two of them should fail to perform as intended, then theslope would still remain with a minimum likely factor of safety of atleast 1.1 under worst case conditions.

Investigating landslipsOverview The remedial methods for stabilizing landslips outlined in this chapterrequire that a proper and thorough site investigation be carried out in

Slip surface

Ground water level

Pre-works cross-section

Post-works cross-section

Crib wall

Ground anchors

Drainage

Regraded ground profile

Made ground

Laminated clay

Unshearedlaminated clay

London Clay

Fig. 6.25 Pre-works and post-works cross-sections showing stabilization by regrading, bored horizontal under-drainage and ground anchors, after Grant et al . (2000)


181



advance. The techniques available for site investigation generally are wellknown and have been described in some detail by Clayton et al . (1995), forexample, and need not be repeated here. There are some particularaspects of site investigation, however, that need to be emphasized.These aspects are covered in the following sections and include

. surface mapping

. distribution of pore water pressure

. location of slip surfaces

. early warning systems.

Surface mappingThe desk study and walk-over survey are two essential components of anyground investigation but are of particular importance when slopes are

being assessed. They can provide a great deal of information at littlecost. The site itself and the following sources of information should beconsulted:

. topographic maps

. stereo aerial photography

. geological maps and publications

. geotechnical and engineering geology journals

. previous ground investigation reports

. well records

. meteorological records

. newspapers.

The value of aerial photographs cannot be overemphasized. Aerial photostaken at different times of the year and at different times of the day can bemost revealing and can indicate down-slope movements that may not beobvious to the naked eye at ground level (e.g. see Figs. 6.2 and 6.3).

When carrying out the walk-over survey, the following features shouldbe looked for:

.

tension cracks. toe bulges. lateral ridges. uneven ground. graben features. back-tilted blocks. marshy ground. streams. springs and ponds. cracked roads. inclined trees


182



. breaks and cracks in walls

. distorted fences

. drainage paths, both natural and man-made.

Some of these features are indicated in Fig. 6.26.Walk-over surveys, carried out during and immediately after heavy

rainfall, can be most revealing and can show adverse drainage conditionsthat may not be apparent under dry weather conditions. Weep-holes inwalls should be carefully studied to see whether they are functional orblocked up, as is often the case.

Distribution of pore water pressuresOverview An absolutely essential part of an investigation into the stability of a slopeis the determination of the distribution of pore water pressures, the impor-tance of which cannot be overemphasized.

The basic principle of operation of all piezometers used for measuringpore water pressures is that of a water-®lled porous element placed inthe ground so that the soil water is in continuity with the water in theporous element ± this may be saturated at the time of installation or theground water may be allowed to ¯ow directly into the porous element.

Hanna (1973) has de®ned the requirements of any piezometer as:

. to record accurately the pore water pressure in the ground

. to cause as little interference to the natural soil as possible

. to be able to respond quickly to changes in ground water conditions

. to be rugged and remain stable for long periods of time

. to be able to be read continuously or intermittently if required.

Tension crack verticaloffset and stretchedroots

Arcuate scars indicating rear scarp

Counterscarps

Pond

Cracked and bulged toe

Backtilted i.e. upslope tiltwith deep movements

Trees, fences, etc. tilted down-slopedue to shallow movementstree growth may compensate

Fig. 6.26 Surface indications of instability, after Bromhead (1979)


183



Because some ¯ow of water from the spoil into the piezometer isrequired for any piezometer to be able to record pore water pressurechanges, a time lag exists between a change in pressure and the recordingof that pressure change by the piezometer.

Standpipes and standpipe (Casagrande) piezometersThe simplest form of pore pressure measuring device is the observationwell or standpipe (see Fig. 6.27). This consists of an open-ended tubethat is perforated near the base, and is inserted in a borehole. The spacebetween the tube perforations and the wall of the borehole is normallypacked with sand or ®ne gravel, and the top of the hole is then sealedwith well tamped puddle clay or bentonite grout or concrete to preventthe ingress of surface water.

Ventilated plastic cap

Protective metal cover

Concrete plug

20 mm IDplastic pipe

Gravel orsand backfill

Perforatedplastic pipeat base

10 –20 mm IDplastic pipe

Low air entry

porous ceramic

Sand filter

Compacted backfill

Compactedbackfill

Standpipe Standpipe piezometer

Cement andbentonitegrout seals

Fig. 6.27 Standpipe and standpipe piezometer


184



Measurements of water level in the standpipe are made by lowering anelectrical `dipmeter' down the open standpipe, giving a visual or audiblesignal when the water level is met.

The standpipe is very simple to install but, unfortunately, suffers fromconsiderable disadvantages. First, no attempt can be made to measurepore water pressure at a particular level, and it has to be assumed that asimple ground water regime exists with no upward or downward ¯owbetween strata of differing permeability. A second major disadvantageis due to the considerable length of time required for equalization of thelevel of the water in the standpipe with that in the ground in soils of lowpermeability.

To overcome uncertainties connected with the standpipe the mostcommon practice is to attempt to determine the water pressure over alimited depth by sealing off a section of the borehole. The system com-monly used is termed a standpipe piezometer or a `Casagrande' piezo-meter and is illustrated in Fig. 6.27. The sand ®lter is sealed above andbelow with grout, often a well mixed cement/bentonite seal in proportionsof 1 : 1, and the mix should be as stiff as is compatible with tremie pipeplacement at the base of the hole or by bentonite balls or pellets. Thecement/bentonite seal is typically 2 m long. Vaughan (1969) has examinedthe problems of sealing piezometers installed in boreholes, when the groutseal is extended up to the ground surface, and concluded that for a typicalinstallation the permeability of the seal can be signi®cantly higher than

that of soil surrounding the piezometer tip without serious errors arising.This illustrates the value of back®lling the entire hole with grout ratherthan using relatively short seals.

In order to speed equalization between ground water pressures and thelevel in the standpipe, it is important to ensure that the sand ®lter issaturated. The time required for equalization of pressures may be com-puted if the `¯exibility' of the piezometer system and the permeabilityand compressibility of the soil is known. Solutions for the time lag havebeen presented by Hvorslev (1956) for incompressible soils, and by

Gibson (1963, 1966) for compressible soils.Porous metallic tips are in extensive use and the Geonor piezometer tipis shown in Fig. 6.28. It comprises a 30 mm diameter ®lter cylinder con-nected to a central shaft by top and bottom pieces. The bottom piece iscone-shaped, the top piece being threaded to steel tubes. The piezometerelement is ®tted with a small diameter plastic riser pipe, usually nylon. Insoft ground the piezometer can be installed by driving; pre-boring may berequired in harder ground.

It is possible to measure the pore water pressure at different depths atany one location by using more than one porous elements carefully iso-lated from each other. This is not easy to achieve in practice and because


185



of this, when measurements of pore water pressure are required at differ-ent depths, separate boreholes are often used with only one ®lter elementin any one borehole.

Pneumatic piezometersThe pneumatic piezometer tip consists of a ceramic porous stone, behindwhich is mounted an air-activated cell (Fig. 6.29). The tip is connected toinstruments at the surface via twin nylon tubes and these are connected in

Plastic tube

Metal pipe

Central shaftwith holes

Porous bronzefilter

Solid conicalend piece

Fig. 6.28 Geonor piezometer tip

Measuring system

Pressure gauge

Airflowindicator

WaterPiezometer tip

Nitrogen pressurevessel

Twin plastic tube

Air activatedhydrostatic pressure cell

Flexible membrane

High or low air entryceramic

Fig. 6.29 Pneumatic piezometer equipment


186



turn to a ¯ow indicator and a compressed air supply and pressure measur-ing apparatus. When the pore water pressures are measured, air or nitro-gen is admitted to one line but is prevented from ¯owing up the other lineby a blocking diaphragm in the tip. When the air pressure reaches thepore water pressure, the diaphragm is freed away from the inlet and theoutlet tubes in the tip; air returns up the vent line and a visible signal ofthis is given by air bubbles in the air ¯ow indicator. When the return airceases to ¯ow, the pressure in the feed line is equal to the pore waterpressure.

Typically, the amount of water displaced by the diaphragm is very small(< 100 mm 3) and the total ¯uid volume of the tip is low, and therefore thetime required for equalization between the ground water pressure and theair line pressure is very small.

The pneumatic piezometer is considerably more expensive than asimple standpipe piezometer and the system requires a more sophisti-cated read-out unit.

It has been suggested that in the long term some pneumatic piezo-meters become unreliable and, probably because of this, their applicationto slope stability problems has been limited.

Electrical piezometersThe principle of the electrical piezometer is that of a diaphragm de¯ectedby the water pressure acting against one face, the de¯ection of the

diaphragm being proportional to the applied pressure. This pressure ismeasured by means of various electrical transducers. Such devices havevery small time lags and are sensitive but, unfortunately, their long termreliability is open to question. One of the authors once spent an entiresummer installing vibrating wire type piezometers in a major earth damonly to have the instrument hut struck by lightning in the followingwinter. Because the hut did not have a lightning conductor, the electricalpiezometers were destroyed! Fortunately, we had also installed hydraulicpiezometers which survived the lightning strike and these functioned

successfully ± a classic reminder that not only do instrument huts requirelightning conductors, but also that we always need backup systems.

Hydraulic piezometersThe closed hydraulic piezometer was developed at the Building ResearchStation in the UK and is widely used in unsaturated earth ®ll applications.When using the closed hydraulic piezometer, measurements of waterpressure are made at a point remote from the piezometer tip. The tubingconnecting the tip to the measuring device must be ®lled with a relativelyincompressible ¯uid. To achieve this, two tubes connect the tip to the mea-suring point at the ground surface and then are ¯ushed with de-aired


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water, or an antifreeze solution in cold climates, before taking readings(Fig. 6.30).

When planning a piezometer installation, thought must be given toprotecting the piezometers from vandalism. This has cost implications.Furthermore, provision must be made for piezometers to be read on aregular basis. It is important that the readings are taken during and imme-diately after periods of heavy rain so that critical pore water pressures areobtained.

Location of slip surfacesAn important part of the investigation of a landslide is the determination ofthe depth and three-dimensional shape of the slip surfaces that character-ize it. Knowledge of these features are needed:

. in order to enable piezometers, inclinometers and other instrumenta-tion to be correctly placed

. to provide a guide for sub-surface sampling techniques

. to help with the carrying out of back-analysis

Pressure gauge

Pressure tank

Supply tank

Rubber bladder

Valve key

To piezometers

Vacuum gauge

Return tank

Water trap

(c) Panel mounted de-airing unit

Grout

Bentonite plug

Sand

Tapered tip

(a) Piezometer tip

Piezometer tip

A

H

B

Header tank

(b) Header tank with mercury manometer forpressure measurement

Manometer

Fig. 6.30 Twin tube hydraulic piezometer equipment


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. to permit the design of appropriate stabilization or control measuresand long term monitoring.

It is helpful to locate the slip surface or slip surfaces before deciding onthe position and depths of piezometers but the possible existence of an

undiscovered slip should not be overlooked. Particularly if major struc-tures could be involved in a landslip or there is a possibility of loss oflife, it would be prudent when assessing the stability of a slope toensure that it is stable no matter where a slip surface could be locatedand with the design using the residual strength. This would require know-ing the distribution of pore water pressure with depth and this could meanat any one location measuring the water pressure at two or three depthintervals.

The ®rst approach to locating slip surfaces is to observe them in test pits

or in borehole samples. Strict safety precautions should be observed wheninspecting test pits, bearing in mind that the inspection may be time con-suming and collapse of the pit sides may occur. In addition, it is easy tomiss a slip surface if the inspection is carried out by staff that are notexperienced. The location of slip surfaces from borehole samples requires,ideally, continuous sampling or two boreholes located close together withsampling depths chosen so that the complete depth range is covered. It isalso important to remember that multiple slips may well exist and to makequite sure that the investigations locate the lowest of these.

Inclinometers are frequently used to monitor movements in earth ®llsand slopes. Most of these instruments consist of a pendulum-activatedelectrical transducer enclosed in a waterproof `torpedo'. The torpedo orprobe is lowered down a near vertical guide casing installed in theground. The inclination of the casing from the vertical is measured at pre-determined intervals along the casing, often 0.5 m, and the pro®le of theshape of the casing is obtained by integration of the observed slopevalues from a ®xed point either by taking the casing into rock or someother stable material at depth where it can be assumed that there is notranslation, or using the top of the casing as the datum, ®xing its positionby survey methods.

It is convenient to use a casing with diametrically opposite grooves, asshown in Fig. 6.31, down which the probe can run on wheels.

A crude type of inclinometer is a ¯exible tube installed in a borehole,down which steel rods of increasing length are lowered in turn and therod length which is just able to pass a given point gives a measure ofthe curvature of the tubing in the vicinity of that point. Alternatively, arod can be lowered to the bottom of the tube and then pulled up at inter-vals. If slip movements suf®cient to ¯ex the tube have occurred, the rod

will jam in the lower part of the ¯exure.


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In a test to failure in a trial bank on soft clay near Bangkok, Eide andHolmberg (1972) pushed 10 m long brittle wooden sticks into theground (inside a metal tube that was withdrawn after installation) adjoin-ing the proposed bank, on lines following cross-sections of the anticipatedfoundation failure. When this occurred, the sticks were broken at the levelof the slip surface. By pulling out the upper broken-off portions of thesticks, the depth to the slip surface at each point could be determined.

Fig. 6.31 Inclinometer equipment


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A check on the location of a slip surface may be made if cusps in thedistribution of moisture content with depth can be detected. In a dilatantsoil that expands on shearing, for example an over-consolidated clay, anincrease in the moisture content would be expected and such increaseshave been reported by Henkel (1957), Skempton (1964) and D'Appoloniaet al . (1967). In a contractant soil, for example like a normally consolidatedclay, local decreases in the moisture content in the vicinity of the slipsurface would be expected (see Fig. 6.32).

Other methods, perhaps used less frequently, for detecting slip surfaceshave been described in detail by Hutchinson (1983).

Early warning systemsWhile early warning systems are highly desirable, they generally becomeneglected or damaged as a result of vandalism. Piezometer and inclin-ometer readings can give early warning of impending instability, butthe readings need to be taken on a regular basis and then assessed byan experienced geotechnical engineer or engineering geologist.

Vertical and horizontal deformation observations taken on ®xed points,and measurements of changes in the width of tension cracks can be most

Moisture content

Moisture content

D e p

t h

D e p t h

Slip surface

Slip surface

(a)

(b)

Fig. 6.32 Moisture content cusps at slip surfaces in (a) over-consolidatedclay and (b) normally consolidated clay


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helpful. Ranging rods placed in straight lines across a slope can give visualindication of down-slope movements, but are subject to vandalism.

If early warning systems are installed on a slope, it is absolutely essen-tial that they are maintained and read and assessed on a regular basis andprovision must be made for the cost involved.


6_Chapter 6 Stabilizing and Investingating Landslips

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Transcript of 6_Chapter 6 Stabilizing and Investingating Landslips