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Logging of Soil Macrofabric

Introduction

Macrofabric logging consists of a detailed description of all aspects of the soil. It is an important and extremely cheap means of better understanding the soils at a site. Macrofabric logs should be used to

a) interpret the depositional environment of the soil unit, which will allow a better understanding of the likely soil properties.

b) Identify marker horizons in the soil unit, which can be matched to similar horizons over possibly large distances.

c) Identify relatively small differences or variations in a soil unit, and allow selection of a range of representative soil samples for laboratory testing.

Macrofabric logging requires good understanding off geology and geotechnics to be done properly. The information obtained is of considerable value in understanding the likely engineering behaviour of soil. Macrofabric logging should be undertaken preferably by the Project Geotechnical Engineer, or in research projects by the Senior Researcher.. It should not be delegated to a junior or inexperienced engineer, geologist or technician.

Material for Logging

Any type of relatively undisturbed material can be used for fabric logging. Often undisturbed samples are used, but it is also possible to use borehole core.

Since it is common to core between sample locations in boreholes in Indonesia this will often form the majority of the macrofabric log. The quality of the core depends on the skill of the boremaster. Most core is never inspected and the boremaster may be unaware that there is any need to undertake reasonably careful coring. The most important aspect is the need to limit the amount of flushing water in the coring, to stop excessive wetting up of the core. If it is found impossible to split the core because it is too soft then this is likely to be because of poor coring practice.

Logging Procedure

The following procedure applies to cores. A similar technique can be used for undisturbed samples after they are extruded from the sampling tube.

1 Select a single core run; note the core run details on the wrapping and check against the drillers log that this was a single core run1.

2 Unwrap the core from its protective plastic.

3 Estimate the total core recovery. The ends off the core may be extended and off smaller diameter so this must be taken into account in estimating recovery length. Record this amount in the Log in centimetres.

1 Many site investigations fail to record such information. If the information is not available this should be noted in the Note section on the Macrofabric log.

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4 Using a pair of steel rulers or a palette knife or a stiff bladed knife, cut along the axis of the core to about half its depth.

The actual depth to be cut will depend on the strength of the clay and should be found by experience for each soil horizon. For stiffer clays it will be necessary to cut the core up into short lengths before splitting them along the axis. Lengths as short as 8cm will be found necessary in hard clays.

When cutting, take a mental note of the feel of the material: smooth for clay, rough for sandy layers and lumpy for gravel or wood inclusions.

5 For undisturbed samples one half of the split sample may be set aside and sealed for further laboratory testing. Core samples are suitable for Atterberg Limit, Clay Content, Organic Content and other index tests butt are not suitable for moisture content determination or any other tests requiring undisturbed samples.

6 Inspect the split core run and make a preliminary decision on how many different Units to log from a first visual inspection. It may be useful to insert a small wooden marker at each visible change; these horizon changes can be moved as more information is collected in the log.

7 For each Unit note the colour of the freshly split sample. Ideally a colour chart should be used. Otherwise a consistent colour description should be adopted using three descriptors:

Tone Light (Medium) Dark (Medium is not used but implied if light or dark are not used)

Modifier Greenish; brownish; greyish etc

Basic Colour Brown; Grey etc

After logging a series of sections, check back that there has not been a gradual colour change which has been missed in the log. This must be done quite quickly as the initial colour after splitting can change after only a few minutes. In particular grey marine clays are often slightly greenish on first splitting but this disappears rapidly.

8 Note the consistency: Very soft though hard using the standard field logging method of measurement using finger pressure. If unsure of the procedure then this can be supplemented by a pocket Penetrometer, though the standard Penetrometer is very insensitive to variation at the lowest clay strengths.

The consistency should be that of the actual core. If it is believed that the sample is disturbed, from other evidence of the fabric log, then the consistency should be modified to, for example Very soft (disturbed) ..

9 Describe the particle size range of the dominant part off the Unit eg silty CLAY. The dominant particle size is written in Capitals.

10 Identify subsidiary or minor horizons, dealing first with laminations or lenses of material.

Laminations should be described by the material making them up, by the thickness off the individual laminations and the spacing off the laminations. If the

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laminations are unclear or not persistent through the core then this should be noted. If the laminations recur at intervals it may be appropriate to log the dominant part and the laminations as one Unit.

eg Very soft dark greyish green silty CLAY with 1mm lamination of brown fine sand at 4.05, 4.25-4.30 spacing 2 to 5mm.

The example above shows that the laminations were identified in relation to the depth of the borehole. If the core recovery is less than 100% then this requires some interpretation. Generally it can be assumed that

Some core loss will occur at the form the top of the core run.

A fixed amount of core loss can occur at the bottom of the core run if the boremaster discards the material from the cutting shoe; this can be checked on site.

Where there is substantial loss of recovery, look for a sand layer. This is likely to be the area of most loss.

11 Next record inclusions. In marine and estuarine clays these are most likely to be coral, shells, wood.

Many commercial borehole logs record such inclusions as gravel. This is highly misleading. Remember that in a sedimentary depositional environment clay and gravel will never be being deposited at the same time. If gravel exists, and it is most unlikely that a mineral soil of gravel size will be present in any quantity, then it marks a major short term \change in depositional environment such as a major landslide or other catastrophic event.

Remove sufficient of the fragments to achieve an accurate determination off the material. Wash the fragments and describe

Eg with occasional coarse gravel size fragments of coral.

Where organic material is found then it should be logged in accordance with the procedures for logging peat. It is off importance to identify the amount of organic material as a general statement with some humus (an actual example from a commercial log) could easily mislead the Geotechnical Engineer into assuming significant geotechnical problems. The macrofabric log for a nearby borehole however reads with occasional leaf fragments and traces of wood.

If sufficient organic material is present it is helpful to try to identify the type of material; the difference between for example Nipah Palm and mangrove swamp seed pods gives information about how the clay was being deposited.

In volcanic area the presence of volcanic ash may be expected in the clay horizons. In some cases this can provide a clear marker horizon. One form of volcanic ash, found on the north Java coast in the Older Alluvium is described as follows:

With fine gravel sized nodules of black and brown silt.

Few people are sufficiently knowledgeable to identify different types of shell, and in general such detail is not used. However in special circumstances a palaeontological analysis of the soil horizons has been found useful. This may include micropalaeonological (pollen) studies and can only be undertaken be experts in such a field.

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Structure

Soft clays are unlikely to have an evidence of structural disturbance. Stiff to hard clays may contain slickensiding (polished surfaces resulting from tectonic movement or landsliding activity). However it is necessary to inspect such surfaces closely as core drilling can result in polished, usually conical, surfaces which may be mistaken for geological features.

Disturbance

Both undisturbed samples and core will have suffered some disturbance. This will be most visible in thinly laminated deposits; expect to see laminations bent up at the edge of the sample, as a result of the drilling or sampling activity.

Relogging at Time Intervals

The initial log should be made immediately the sample is split. However some of the features in the core may become clearer as the sample dries out. This is particularly the case when there are laminations of sand or silt in clay; the sand will dry more quickly and change colour.

Therefore the cores should be reinspected at intervals, and any additional features should be added to the logs.

The time of inspections will depend on the particular features and also the environment in which the core is stored. It is recommende3d that a selection of core be placed in a gentle drying environment and studied at frequent intervals to assess the best time to relog the remainder. The final log should record the logging undertaken, the drying environment, and make a general statement about what additional features became visible at different times, to assist in future similar logging exercises.

Macrofabric Log

The initial log should include individual descriptions for each element of core logged. A summary log can be prepared after all the logging is completed, grouping the descriptions together into a smaller number of Soil Units which will relate to the overall geotechnical design. However the initial macrofabric log is the output deliverable from the Macrofabric logging process.

The log should include . [example log]

[Note that IGMC Stage 1 standard log contains stuff I dont understand, and no explanations if the experienced Macrofabric Logger in IRE can explain it then he can upgrade this method statement to make use of that log.]

[Note: a number of papers by McGown deal with fabric logging, and should be cross referenced; they have not been referred to in this preliminary draft]. AJB/ab/Dell5000/W:\Geoguides Drafts and Inputs\Soil fabric logging.doc/10 Oct 2000