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TRACKS: EXPERIENCES WITH HEAW EXCAVATING EQUIPMENT Dirk H.R.

Spennemann The following is a report on wor...

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OF WHEELS AND TRACKS: EXPERIENCES WITH HEAW EXCAVATING EQUIPMENT Dirk H.R.

Spennemann

The following is a report on work experience with heavy excavating equipment gained during salvage excavations conducted in February 1989 in Tonga.

Since the experience was quite different from expectations,

BACKGROUND Expansion of factory development resulted in the need to construct a new sewage treatment plant at the Small Industries Centre,

located at the outskirts of Nuku'alofa,

Tongatapu,

Kingdom of Tonga.

I was asked by the developer (L.E.

Belz,

RE.) to conduct salvage excavations in the area prior to construction.

No legal requirements exist in Tonga requiring a developer to conduct an environmental or an archaeological impact assessment.

Salvage work depends entirely on goodwill and personal interest of the developer.

f he soils of the development site are derived from ash falls which most likely originated from one of the periodically surfacing submarinevents.

The A-horizon usually consists of O.3Om to 0.40 m of friable,

slightly sticky garden soil without any stones or inclusions.

The B-horizon(Fahefa soil) is unweathered ash which was found to dry out and harden very quickly.

Cracks and fissures were common after one or two hours of exposure to the sun or half a day in the shade.

When detailed work was required,

frequent watering of the surface was unavoidable.

During the excavation there were several heavy to torrential downpours of short duration.

At the beginningthese were a help as they permittedthe excavationof driedout soils.

However,

after several downpours connected with a tropical depression,

which hit Tongatapu on February 12 and brought 117 mm of rain in 24 hours,

This literally drowned the low-lyingsoutheast part of the site.

Since

the sheet-flooding fdlowed some days of rain,

the ground in this part of the site was already saturated with moisture and the water did not drain from it.

Eventually,

The ground remained too wet for manual,

let alone machine work for another two days.

Extent of Work Since a total area of 70 m X 25 m was to be affected by the construction work,

excavation by hand would have taken more than the two weeks available.

Previous fieldwork experience on Tongatapu (1985-88) had shown that topsoil was heavily affected by gardening and does not contain any archaeologicalinformation.

However,

could be expected in the subsoil.

Thus it was decided to use machinery to strip off the topsoil to a depth of 0.30 m,

drawing on experience gained in both research and salvage excavations in Germany.

A MATTER OF CHOICE Under ideal conditions,

one would have chosen a large,

excavator with an articulated arm and a two metre wide,

Operating backwards,

this would have left a level and cleanly scraped area without any tooth,

In addition,

its entire weight would have been on ground protected by topsoil which was to be removed later.

An experienced and well motivated operator can remove the topsoil of an area 50 m X 20 m in between four and five hours,

without the assistance of dump trucks,

and leave behind an area which is ready for plan drawing and detailed excavation.

Given the unavailability of such machinery,

other equipment had to be used.

The following types,

all manufactured by Caterpillar,

were available: * a D8L bulldozer fitted with a blade (or bucket if so wished)

a front-end wheel loader (model 950) graders (model 12E)

What should one take

The bulldozer fitted with a bucket and the front-end loader could excavate the soil and pile it at the sides of the working area.

The grader could run across the site scraping the soil to the sides.

Thus,

the aim was to select the equipment which would cause the least damage to the sub-surface features or their contents.

The advantage of the bulldozer and front-end loader over the grader was that the latter would require more runs across the site,

thus possibly musing greater compaction of the soil.

The main advantage of a grader over the other two appearedto be in its lesser overall weight (11 metric tonnes versus 13 tonnes and 45 tonnes),

which in addition is distributed over six tyres.

On the same line of argument a font-end loader wourcl be considerabty lighter than a heavy bulldozer.

Experimentswith bulldozers (as opposed

Spennemann 71

to excavators with articulated arms) in Germany have shown that the tracks compress and compact the underlyingsoil to such an extent that subsequent excavation by hand becomes difficult.

It was decided to remove the top 0.25 m using a front-end loader and to pile the soil up on the eastern end of the area.

During this manoeuvre an attempt was made to keep the traversing of the cleared area to a minimum.

The remaining f i e centimetres were scraped off with a grader,

thus reducing the number of times the grader had to run over the site,

and at the same time providing a clean surface.

COMPARISON OF IMPACT Just before the start of this excavation work an adjacent area had been bulldozed with the D8 bulldozer and several archaeological features exposed,

This allowed a comparison betweenthe impact of the two machinetypes.

The bulldozer had left a marked surface consisting of parallel rows of tiny ridges,

The remaining area was relatively smooth,

save for the tooth-marks of the bucket.

This area needed to be cleaned by hand,

to make the features visible for drawing and excavating.

Rubber-tyred machines,

after the grader had been run over it.

Because the grader does not scoop the soil away,

it created some blurring of features,

where the softer soil of the feature infilling was smeared over the harder subsoil.

This does not occur when excavators with articulated arms are used.

However,

both types of rubber-tyred machinery,

tended to skid on the moist ground,

especially if it was pushing a heavy load.

This compacted the surface of some of the soil,

making excavation by hand tedious.

Bothtypes of equipment created as much work for the manual phase,

so no preference can be advanced on these grounds.

The main factor was the vertical pressureon the ground.

Unfortunately no comparison between the two machine types could be made.

After some rain,

but prior to the tropical depression,

It was observed that the soil showed a slight vertical movement underneath the wheels of a front-end loader.

This made no difference to any of the structures save for one.

Here,

An earth oven contained an unexpected burial.

The top of the burial (head and feet) was about 0.20 m

All bones were broken in small,

apparentlyduetovertical pressurefrom the machine moving across above (see below).

THE RIGHT CHOICE

the question arose whether this had been the rigM choice of machine,

or whether one should have selected a grader for the entire process.

As the compaction of the soil and the damage incurred were due mainly to the weight of the machinery,

a detailed assessment of the ground pressure was conducted.

The overall operating weights and the ground pressures (in pounds per square inch [psi]) of the earth-moving machinery employed in the excavationsare shown in Table 1.

The figures dearly indicate that rubber-tyred machines,

be they graders or font-end loaders,

exert a much greater pressure on the ground than tracked machines.

MACHINE

SPECIFICATION

Bulldozer

Bulldozer

Wheel-loader Grader

GROUND PRESSURE (psi)

Wheel-loader .

OVERALL OPERATING WEIGHT (t)

"45.00

17.83

12.93

108.40

Psi of rubber-tyred machinery calculated from tyres,

assuming standard tyre pressure and a full bucket.

Table 1 Variations of weight and ground pressure of earth-moving machinery (all Caterpillar) employed at the Small Industries Centre,

Nuku'alofa,

Kingdom of Tonga.

(Information courtesy of Caterpillar and Bridgestone Earthmover Tyres,

At first sight there is a contradiction in the fact that a 13 tonnes front-end loader with tyres pumpedto standard pressurehas a higher ground pressurewith an empty bucket than with a full bucket,

when it weighs about 18 tonnes.

In fact the additional vertical weight on the front axle (distributed to the back axle at a ratio of 78:22%) causes a compression of the tyres and an increasein their footprint from 790 cm2in the unloaded to 1,300 cm2 in the loaded state.

CONCLUSIONS Clearly,

the decision to use machinery was the right one.

My excavation experience in Tonga had shown that four motivatedworkers can strip off 100 m* in three days and leave a clean area behind.

Stripping off 1,750 m2 represents about 210 persondays of work,

Even with an army of 50 workers the job would havetaken over four days.

The financial savingswere also enormous: 210 persondays

Spennemann 73

in Tonga are equivalent of about Aus$1,470,

while four hours worth of loading and grading came to about Aus$300.

In hindsight,

the choice of equipment was wrong.

One should have chosen the bulldozer and then cleaned up by hand.

The disfiguration of the surface by the track marks would have been outweighed by the greatly reduced ground pressure.

The misconceptionwhich I and others with whom I discussed the matter have laboured under,

is that lighter machinery means the same or lesser impact.

The 'soft' rubbertyred machinery is in fact more damaging.

It should be mentionedthat ultra-widetyres with low tyre pressure exist,

which have a wide footprint and thus a low ground pressure.

These are used in sensitive forest and swamp areas.

The same is true for bulldozers,

for which ultra-wide tracks can be obtained.

Often,

such machinery is unavailable and one has to make do with what is available.

It is worth taking a close look at the ground pressure rates before any choice of equipment is made.

Alele Museum RO.

Box 629 Majuro Republic of the Marshall Islands 96969

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