Running a Trial Pit Site Investigation
A trial pit (also known as a test pit or inspection pit) investigation is a highly effective way of obtaining data on the subsurface soil and rock conditions which underlie a site. It allows for the various soils and rock types to be logged, the soils to be sampled and a preliminary assessment of the groundwater regime to be made.
However, to be effective the process must run smoothly. In a nutshell the procedure is to hire a TLB (Tractor-Loader-Backhoe) from a plant hire company, rendezvous with the machine on the agreed date, dig, log, sample and backfill as many pits as necessary to provide enough data to characterise the geology of the site and then to return to the office to process the data and write up the geotechnical report. It sounds simple in practice but there are a myriad of considerations which, if not attended to, can seriously derail the process. Set out below are some guidelines and sage advice to assist you in getting going and keeping your stress levels to a minimum.
Hiring the TLB
The plant hire companies need some lead time when it comes to booking a machine. Generally give yourself a week if you can – a phone call to the company will tell you if they have one available. It may take a dozen calls to track down a machine if there is a crunch on. Try and find a plant hire company as close to the site as possible – 30 km being probably about the limit that a TLB can economically travel and bearing in mind the time constraints imposed by a machine that travels at speeds of less than 60 km/h. When phoning around for a machine, ask for their ‘wet’ rate – this means they will keep the machine supplied with fuel. You don’t want to be involved in tracking the machine to filling stations and paying to pump diesel into someone else’s equipment. Bear in mind that your hourly cost exceeds the hourly rate of the plant and it is important that you use your time as efficiently as possible. Confirm your booking with a fax or email, along with your contact number, agreed hire rates and order number, and it is worth while drawing a sketch with directions to the site. You might also be required at some stage to work with a 20 or 30 tonne excavator, particularly when investigating dam sites or looking for borrow materials. The procedures are the same, but you will have to pay for ‘establishment’ whereby the machine is brought to site on a low bed.
Spend a little time the day before getting all your equipment together. Here is a list:
- Geological pick
- Log sheets/field notebook
- Site Plans
- Pens, highlighters, coloured pencils, erasers etc.
- Indelible Marker
- Exam Pad
- 30 m Tape Measure
- 5 m Steel Tape Measure
- Pocket knife
- Bread knife for undisturbed sampling
- GPS Batteries
- Scale Rule
- Bulk Sample Bags
- Indicator Sample Bags
- Cling film (unperforated) for undisturbed sampling
- Cable Ties
- Dayglo spray paint
- Ladder (optional)
- Logging guidelines
- First aid kit
It is worth purchasing a large, lockable tool box with various compartments, into which all the above can be stowed and kept in an orderly manner and prevents the equipment from rolling around in the back of the vehicle when on a bumpy road. A good sized pencil case is also useful for the same reason.
This should cover most eventualities and allow you to obtain good, reliable data from the test pits. However some additions to the above list will make your life a good deal easier, particularly if the day turns out hot or long. A cooler box to keep lunch and drinks cold will be a huge morale booster in the heat of the day. Keep the sun off – nothing worse that getting sunburned, particularly if you have several days of test pitting ahead of you. And besides, 20 years of prolonged exposure to the South African sun is going to give you skin cancer and with that in mind, a long sleeved shirt and wide brimmed hat is recommended. I gave up on shorts a long time ago too – long trousers stop soil getting into your boots, your shins from getting torn when pushing through brambles and of course they keep the back of the legs from sunburn. So here is my list:
- Cooler box
- Long-sleeved shirt
Other considerations are:
- a dynamic cone penetrometer (DCP), spare rods, grub screws, Allan keys and spanners.
- plastic water containers for percolation tests
- wages for field staff
- petty cash for tolls, refreshments and general expenses
- hazard tape
- first aid kit
- The phone number of the local municipality’s technical services department.
If you have employed casual labour you will undoubtedly need to feed them. Often they haven’t eaten in the morning, and come lunch time you won’t get any more work out of them until they have been fed, so be prepared to drive to the nearest shop to bring in some supplies. This can be disruptive to say the least and it is best to deal with these issues before leaving home. This all might sound inconsequential now, but can seriously impact on the smooth running of your day.
You will have agreed on a time and meeting place with the plant hire company. They are usually good at getting their man to site in the dawn, so you should try to get to site early too. Murphy’s Law being what it is, you will have occasions when you have to wait for the TLB. Once you have collected the TLB there may still be some travelling to do to get to the site, so you will have to proceed at the pace of the TLB to guide him to the correct place. The chances are that you will not be au fait with the site either and there is nothing wrong with leaving the operator while you go out and carry out a quick reconnaissance – perhaps to find exactly where the site is, or maybe to assess access routes.
Once on site, quickly decide where you are going to dig your first pit, set the operator up and let him get on with it. Instruct him to dig a trench 1 bucket wide and as deep as he can go. Get the operator to place the topsoil on one side and the subsoils on the other side of the pit. When it is time to backfill, the material goes back in the same order in which it came out, with the topsoil tidying things up at the end and making conditions easier for the re-establishment of the vegetation. Once the TLB is digging you will now have a chance to catch your breath, sort out sample bags, GPS the position, and generally get organised. While the operator digs you can also decide where you are going to excavate your next pit.
Once the pit has reached a suitable depth – it might have ‘refused’ meaning it can’t penetrate any deeper into the bedrock or it may have attained the maximum reach of the backhoe. Do not accept anything less than 3 m deep holes in soils – all the current machines can attain at least this depth. Get the operator to dig a sloping face at the machine end of the pit. This may take a little more time, but it makes for easy access and improves safety.
Which brings me to an important aspect of any trial pit investigation. There have been a number of deaths in test pits over the years. If a pit were to collapse there is very little chance of rescue. It is illegal in Europe and the UK to enter any pit deeper than 1.2 m without the pit being shored. Conditions are generally more onerous there due to high groundwater levels, but as I write this the director of a firm of consulting engineers is being prosecuted for the death of one of his geologists in a test pit. He may face a life prison term if found guilty. Read more here. Do not underestimate the danger of working in excavations. A cubic metre of soil can weigh anything from 1.5 to 2 tonnes and suffocate you to start with. Digging you out with a TLB is going to take off arms, legs and your head, so to reiterate then, great care must be exercised in and around excavations and in this light the following precautions are recommended.
- Dig an entrance ramp with the TLB. A ladder is also useful for getting out of a pit rapidly.
- Observe the operation from the front of the pit furthest away from the TLB. Standing on the long side of the pit is hazardous due to potential collapse and possible sideswipes from the excavator boom.
- Log the holes immediately after they have been excavated. Don’t go into holes dug the day before, or accept offers from the client to dig the holes before you arrive on site
- The holes should be dug at right angles to the contour. If this is impossible to avoid, then don’t allow the operator to put the soil on the upslope side of the pit
- Log the pit from the ladder or from halfway down the ramp – preferably with your torso above the ground level. Spend as little time in the base of the pit as possible.
- If there is water flowing into the pit, avoid going in.
- Pits excavated in old landfill sites may become hazardous due to accumulations of methane gas and should not be entered.
- Get the TLB operator to clear rocks and clods away from the edge of the pit.
- Wearing a hard hat in the pit is a good idea.
- Riding in the excavator bucket is an option for deep pits – although you have to have faith in the operator’s skills. You will soon gauge his level of competence just by watching how he places the bucket and how fast he digs the pit.
- When in doubt, stay out.
Dig yourself a ramp at the TLB end of the pit as per the Figure above.
It may be necessary to log the pit from the surface if it is considered that it isn’t safe to enter. If this is the case then you need to be on the ball during the excavation phase. As each bucket of material comes out, grab a handful and log it. When the material changes, get the operator to stop while you measure the depth of the change. Although not as accurate as logging the pit in situ it is better than trying to ascertain the geology at say 3 m by peering into the gloomy depths.
Note: These are guidelines only and are not prescriptive. The risks or entering a unshored excavation lie entirely with your employer and you and GeoZone will not accept any liability or claim based on the above advice. It is given in good faith and has been included to highlight the need for extreme caution when working in excavations.
Logging the Pit
The logging should be carried out according to the procedures outlined in a paper by Jennings, J.E., A.B.A. Brink & A.A.B. Williams 1973 and titled ‘Revised guide to soil profiling for civil engineering purpose in South Africa’. This paper has been expanded by the SAIEG, AEG and SAICE and can be accessed here. It is not my intention to expand any further on this, except to say that I would rather log rock as per the International Society of Rock Mechanics (ISRM) criteria – certainly when it comes to the discontinuity descriptions at least. Pay attention to the origins of the materials – transported soils will behave differently to residual soils and it is important to separate these out in your logs.
If there is something you aren’t sure about, describe it in your own words in the notes section, bring home a sample, sketch it and photograph it. Draw attention to potential issues in your logs with a high-lighter or bold lettering. For instance if I see loads of slickensides in a horizon I will draw a skull and crossed bones in my notes and write “danger’. This might sound silly, but back in the office, with dozens of identical looking log sheets to work through, those pits which I have designated as dangerous then leap out at me.
Some people log in a field note book. I am not a fan of this, particularly when you are starting out. Spend some time with Excel and build yourself a log sheet. Filling in the boxes will force you to make choices and record depths, samples, water levels, and of course soil descriptions. These checks are not to be found in a field notebook.
It is worth taking to the field some crib sheets – printed out and placed in a plastic sleeve or laminated to keep them in reasonable condition. Once in the pit have a good look around to see if there is anything untoward going on. Have a hack at the pit sidewalls all around the excavation, looking for fissuring, pinholing and slickensiding. Look for disturbed bedding planes or strange geology – accumulations of rock in a matrix of clay for instance, or mixed up geology or large influxes of groundwater. Once you are satisfied that the soil profile within the pit is not disturbed, then it is a simple matter of logging a representative face. What I do is hack my way down the entire profile of the pit sidewall, then score out horizontal lines with the point of the pick where I see changes in the profile – changes may be difference in moisture content, colour, consistency, structure, or soil or rock type. There are going to be gradational changes so some arbitrary boundaries will need to be set, but literally the line has to be drawn somewhere. Once the profile has been divided up, I measure the depth to the various lines I have scored and then I describe each layer. At the end of the logging process I have:
- a description of all the soil and rock types from ground level to the base of the pit.
- A record of rock head depth and refusal depth
- A list of where the samples were taken
- A record of where groundwater seepage was recorded.
Take note of the geology and conditions in the vicinity of the test pit. Surface seepage, boulder and rock outcrop, erosion gullies, the location of other features including existing buildings, fences, roads, pipelines and so forth are all valuable data which will assist greatly in writing the report and may in fact be crucial when it comes to recommendations. Indeed as you walk/drive to the next test pit position it is important to mark on the map the occurrence of wet areas, rock outcrop and boulder fields.
Once all the samples are out of the pit and I am satisfied that I have all the data I need, it is time to backfill the pit and move on to the next. Aim for at least two pits per hour, although this may vary depending on the operator’s skills, access and sampling requirements.
Write legibly too – you probably wont have to type up the logs and spare a thought for the person who does.
There will be a budget for laboratory testing in the proposal and sufficient samples must be taken to be able to satisfy the requirements of the job. The job will undoubtedly require at least indicator samples, with bulk samples also a distinct possibility. At the start of the investigation build up a ‘feel’ for the geology of the site before sampling, and it is recommended that sampling be started no earlier than the third test pit. Take Indicator samples at depths of 1.0 to 1.5 m as this is where most of the activity will take place in terms of foundations soil heave and settlement. Bulk samples, on which Modified AASHTO dry density tests and California Bearing Ratio (CBR) tests are carried out, are generally taken to determine the engineering properties of the soils which may be used as subgrade for roads and paved areas.
In this light sample only the upper 1.0 to 1.5 metres, anything below this is often beyond the range of the proposed earthworks. That said, it may be that the site is steep and will be subjected to large cut and fills and an understanding of the properties of the deeper soils may be required. Bear in mind also that it is far better to take extra samples than it is to under sample and have to return to the site to take additional samples. The best way to get the bulk samples is to instruct the digger operator to place a small pile of each geological horizon on one side of the pit away from the spoil pile. This is best supervised, particular at the outset of the job, but saves an enormous amount of heartache and hard labour by removing the necessity of having to hack the sample out of the pit sidewall with a spade once the pit has been excavated.
It is important to keep track of depths of the various sample piles as they come out – these can be measured with the tape measure and entered directly onto the log as the machine excavates the hole. Bag filling and label writing can then be carried out by your assistant while the pit is being logged. The depth of sample, description and pit number must be recorded on both the sample label and the pit log. Take some time during the course of the day to assess what samples have been taken and what is outstanding. A quick sample list on the back of a log sheet often is useful in deciding what is still required.
For bulk sampling, fill two large fertiliser bags to give you at least a 30 kg sample. If the materials are to be stabilised with lime then fill 4 bags. Twelve to 15 shovels full of soil should suffice for each bag. Put the label inside and close up the bag – best done with cable ties. Indicator samples should be approximately 1 kg of earth – fill a small sample bag and don’t forget the label. Help the laboratory by writing on the outside of the bulk samples the same details as what went onto the label – this makes for easier handling and sorting on the laboratory side – otherwise they have to delve into every bag to look for a soggy tag – not an easy job, particularly in view of the fact that not every sample will be tested.
Pay great attention to the labelling. Murphy is always out there waiting to bring you down. Professor Scoggings of the Survey Dept of the old University of Natal used to hammer this into us. He added to the whole Murphy’s Law thing when he asked us if we knew O’Reilly’s Law? O’Reilly’s Law states that Murphy is an optimist. I digress, but the point is, be methodical, careful, tick the boxes, make careful notes and get the labeling right. Use a ball point pen or indelible marker to write on the label – no pencils or water soluble ink. This sounds obvious but there are many instances where this advice has been ignored with predictable results. What goes onto a label? The company name, the job number, the job description, the test pit number, the depth of the sample, the type of sample, the date, your name, and how many samples there are – S1/2 indicates that there are two sample bags and that this particular bag is the first of two. If it read S2/2 then we are looking at the second sample bag. If there are three sample bags then they would be designated S1/3, S2/3 and S3/3. Fairly self explanatory.
Below is a example of a sample label.
Occasionally undisturbed samples are required from a pit. Use the TLB as much as possible in getting the samples out. Ask the operator to dig an excavation two buckets wide and down to the depth at which the sample is required. Once the approximate depth has been reached, carefully cut downwards and around the smple with the spade and bread knife. The aim is to cut out a block of soil at least 30cm x 30 x 30 cm. This process is not to be rushed, and it is fairly easy to cut the side and top of the sample to the required dimensions. The trick is to separate the underside of the sample from the soil beneath, but before this is carried out it is best to swathe the sample in cling film to add strength to the block.
It is important that the block is not stressed or allowed to crumble, as this will affect the laboratory test results. Once wrapped it is time to work underneath the block with the spade and bread knife until such time as it can be separated from the substrate below. Once loose further wraps with the cling film are called for until it is considered safe to lift out of the excavation. The sample should then be labeled, with the top of the block indicated by arrows and the word ‘top’. It cannot be reiterated enough that to try and rush the process and lever out the sample will only cause it to break and make it necessary to begin the process again.
Many sites are underlain by services – electric cables, water pipes, fuel pipelines and telecom lines. These are to be avoided at all costs. That said, the time will come when you will break a water main or electric cable. These things are stressful and potentially expensive. If you go back to the equipment lists above you will see that I have included the phone number of the local municipality. This is useful when that water pipe is first ruptured by the TLB bucket; you are going to need to get hold of someone fast, and it is always good to have the necessary phone numbers to hand.
That said, it is perhaps best to avoid breaking the services in the first place. The best way to do this is to get the services drawing from the client or the municipality showing where the various cables and pipelines run. Local knowledge is also good – there may be someone who knows where the services are located – worth while tracking down if you can. Telkom will provide you with drawings of where their fibre optic and other cables run but it needs two to three weeks lead time for them to provide this data. A third way is to hand dig a test pit to a depth of at least 1 m before enlarging the pit with the TLB. This is useful in highly sensitive areas but of course entails a good deal of extra work and effort.
The above options are not always available to you, so a bit of common sense is called for when arriving on any site. Often service drawings are not always accurate anyway, so look for manhole covers and other evidence of services. Pipelines generally run between manholes so stay away from any line running between two covers. Similarly pipelines are often located along servitudes occupied by existing overhead cables or telephone lines, so don’t dig beneath these lines. When working in suburban or peri-urban areas, avoid digging on a line between the taps in the front gardens and the road, as there will generally be an offtake leading from the mains to the house supply. Try and determine which side of the road the main water pipeline runs and avoid working in these areas.
Then watch carefully when excavating the pit. Pipes and cables are generally bedded in clean sand and if you encounter a sudden, artificial looking sand horizon, stop digging immediately and make sure that you aren’t about to do dome serious damage. In some instances hazard tape is laid along the top of the sand bedding and gives the first warning of an underlying service.
Also be careful of overhead power lines and low hanging telephone cables – it is very easy to snag these lines with the TLB boom.