How to do backfill density test on remote sites?

Introduction

When you build important infrastructure like telecom towers or broadcasting towers in remote areas, it is crucial to verify that all design parameters are met at the site with our design. In this post I'll show you how I preformed soil density tests in several sites in Zimbabwe. The result will demonstrate that why these kind of tests are important and how they can prevent future failures. In some areas where construction works are less disciplined, shortcuts can show up in many forms by many actors. One of them I saw is to use organic soils for backfill instead of good quality, compact-able soil. The structures under the investigation were 4-legged, angular 112m lattice towers designed by myself in 2015. The site visits were carried out in and of May, 2016.

The Methodology

There are several methodologies out there when it comes to on-site measurements or tests in the geotechnical field. A civil engineer always need to choose the most reliable but "performable" procedure taking into account several parameters, like location, access to geotechnical labs, costs of equipments, type of soils under investigation, client's budget, etc. In this particular case I chosen the sand replacement method for the soil density test of the backfills of single pad foundations. This method gives a good feedback about not only density of the soil but also the compaction of the backfilled material.

When we design footings with slab and we rely on the soil weight as to encounter our uplift forces by the tower legs, then we have to make sure that the soil density is matching with our calculations. The verification of the degree of compaction can be determinate on site by a simple procedure consisting essentially in removing and weighing a part of compacted soil and replacing in the hole with sand by a simple apparatus recording the volume of sand and then calculating the density of the removed soil. The tests have been carried out by the 35-T0129 6.5” dia. (165.1 mm) model from Controls Group (Milan, Italy). The sand replacement method is used to determine the density and water content of compacted soils placed during the construction of earth embankments, road fill, and structural backfill. It often is used as a basis of acceptance for soils compacted to a specified density or percentage of a maximum density determined by a test method, such as the test methods ASTM698 or ASTM1557.

What you will need?

To perform a sand-replacement test you might chose from different procedures. In my case the ASTM D1556 was an obvious choice as the tower was design according to American standards, like TIA-222-G, ACI318-05M, etc.  To perform a sand replacement density test you will need the following apparatus:

• sand-cone with the valve, as per the chosen standard
• sand-container connected to the cone for holding the sand
• base plate designed for the sand-cone
• high precision scale (5g precision is the requirement, and it should be able to measure up to 20kg)*
• soil container for soil removal
• tools for soil preparation and removal (scraper, spoon, chisel, hammer, etc.)
• sand-calibration container
• standard sand 0.3 to 0.6 mm grain size
• hand calculator (or smart phone)
• form to fill-in the field measurement data

The standard recommend a scale with the capacity of up to 20kg. In my case to fit with the schedule I was able to pick up a scale with a capacity of 10kg with 1g resolution. This did the job for me, but with two drawbacks. One, the price tag on it was high (around 1200EUR) and two, I was not able to fill-in the sand-container with it's full 5 liters. These two you can easily avoid if you have a chance to plan your field-work in advance. The price is only an economical question, but with the volume you should not be constrained by a small scale. However, my procedure was still compliant to the standard as it met the requirements for the minimum soil volumes as listed below.

Before testing

We are not carrying out a full-scale geotechnical investigation but it is always important to collect data about geology of the soil we are about to investigate. This is site and country specific to Zimbabwe, so if you want to skip this part you may jump to the procedure right away.

Relief

The landscape is characterised by extensive outcroppings of Precambrian rock, which is between about 570 million and 4 billion years old. The most ancient part of this rock formation, known as the basement complex, covers the greater part of the country. About four-fifths of the basement complex consists of granite; the Matopo (Matopos) Hills south of the city of Bulawayo are formed from prolonged erosion of an exposed granite batholith. Some of the hills are surmounted by formations, known as balancing rocks, that have been eroded by wind and water along regular fault lines, leaving some blocks precariously balanced upon others. Elsewhere are found innumerable small rounded granite hillocks known locally as kopjes. Belts of schist in the basement complex contain the veins and lodes of most of the country’s gold, silver, and other commercial minerals.

Drainage and soil

Major faulting from southwest to northeast formed the middle Zambezi trough, which is now partially flooded by the Lake Kariba reservoir. Other faulting episodes affected the depressions of the Sabi (Save) and Limpopo rivers. Except for a small area of internal drainage in the dry southwest, these three rivers carry the entire runoff of the country to the Indian Ocean via Mozambique. The central ridge-line of the Highveld is the major divide separating Zambezi from Limpopo-Sabi drainage. The light, sandy soils found in most parts of Zimbabwe are residual soils developed largely from the granite parent material. They are highly weathered and leached, even in the areas of lower rainfall, and do not easily retain water because of their coarse texture. Outcrops of basement schists give rise to rich red clays and loams—some of the country’s best soils—but their extent is limited. Since most rain occurs in heavy showers during a few months of the year, rapid runoff and high rates of erosion are common. The meagre mineral reserves in most soils imply an inherently low fertility; under cultivation, productivity drops rapidly after a few years. The difficulty of cultivating these lighter soils is greatest in the black farming areas, where population pressure no longer allows land to be temporarily abandoned to rejuvenate after cultivation; black farmers, because of a lack of capital, are also less able than white farmers to maintain the mineral fertility with manure and chemical fertilisers.

A short description of the procedure

A test hole is hand excavated in the soil to be tested and all the material from the hole is saved in a container. The hole is filled with free flowing sand of a known density, and the volume is determined. The in-place wet density of the soil is determined by dividing the wet mass of the removed material by the volume of the hole. The water content of the material from the hole can be also determined and the dry mass of the material and the in-place dry density are calculated using the wet mass of the soil, the water content, and the volume of the hole.

Calibration

Before start testing, we have to calibrate both the cone apparatus and the bulk sand we are using for the test. The density of the standard soil can vary based on atmospheric pressure, humidity, etc. I have chosen Method A. as calibration method based on the ASTM D1556 and I will show you how I implemented into my procedure. In this way I got a precise bulk density of the standard sand every time I performed a density test and I made it as a routine, implemented into the standard procedure. I have also created a form for my procedure which I filled in on the sites. You can download it from here.

The Procedure

Step 1. Select the location which is representative to the area under your investigation.

Step 2. Check the apparatus and valve if they are free to any damages, fill the sand-container with the standard sand and measure the total weight. The valve may stuck sometimes, so try to open and close it several times.

Step 3. Before we do the actual test we have to calibrate the sand-cone apparatus. I have chosen Method A as per the standard, so to determine the mass of the sand which is need to fill in the cone and base plate.

• Fill the apparatus with sand that is dried and conditioned to the same state anticipated during use in testing.
• Determine the mass of the apparatus filled with sand, in g.
• Place the base plate on a clean, level, plane surface. Invert the container/apparatus and seat the funnel in the flanged center hole in the base plate.
• Open the valve fully until the sand flow stops, making sure the apparatus, base plate, or plane surface are not jarred or vibrated before the valve is closed.
• Close the valve sharply, remove the apparatus and determine the mass of the apparatus and remaining sand. Calculate the mass of sand used to fill the funnel and base plate as the difference between the initial and final mass.
• Repeat this procedure minimum of three times.

Step 4. Prepare the surface of the location to be tested, so the place should be flat. You can use the tools, scrapers but I found it easy to just use the base-plate itself and level the ground with it by pushing and moving around. This depends on your soil type obviously. The surface should be smooth as possible and free of voids or holes in it.

Step 5. Sit the base-plate on the ground as above and place the cone on it. Make sure that the plate is not moving. You may use some nails around the plate but you can ask a colleague of you to keep it firm.

Step 6. Dig a whole through the base-plate until a minimum depth same as the size of the cone. A minimum volume of the hole should be as per Table 2. as shown above. The hole should be kept as free as possible of pockets. Place all excavated soil, and any soil loosened during digging, in a moisture tight container that is marked to identify the test number. Take care to avoid losing any materials. Protect this material from any loss of moisture until the mass has been determined and a specimen has been obtained for a water content determination.

Step 7. Clean the flange of the base plate hole, invert the sand-cone apparatus and seat the sand-cone funnel into the flanged hole at the same position as marked during calibration. Open the valve and allow the sand to fill the hole, funnel, and base plate. Take care to avoid jarring or vibrating the apparatus while the sand is running. When the sand stops flowing, close the valve. By the standard, the plate should come with a small edge around it, so to give a nice and smooth connection between the apparatus and the the base-plate.

Step 8. Determine the mass of the apparatus with the remaining sand, record, and calculate the mass of sand used.

Step 9. Determine and record the mass of the moist material that was removed from the test hole.

Calculation

By now we have all the data we need for the calculation. If you downloaded the form from above you may record your measurements in that form. For the standard-sand you also have to carry out a calibration  for its density. I will not go through those steps here in this post, but in short, you will need a container with a known volume where you can flow the sand in a similar fashion as during the test. These steps are described in the standard at Annex 2.

The mass is in $g$ and the volumes are in $cm^3$. Calculatet the mass of the test hole:
$$ V = (M_1 - M_2)/\rho_1 $$
where:
$V=$ the volume of the test hole, $cm^3$
$M_1=$ mass of the sand used to fill the test hole, funnel and base plate, $g$
$M_2=$ mass of the sand used to fill the funnel and base plate, $g$
$\rho_1=$ bulk density of the sand, $\frac{g}{cm^3}$
Calculate the in-place wet density of the material tested as follows:
$$ \rho_m = \frac{M_3}{V} $$
where:
$M_3=$ moist mass of the material from test hole, $g$
The calulation is the same for the the dry density in case you can follow a standard procedure to determine the water content from the preserved soil.

The standard describes how to report the results and what to include in the report. If you downloaded the form from the above link, then you should be good to go. But please kindly double check if I missed something from the field form.

In my mission I have been visited 5 different sites in Zimbabwe's remote areas including lots of travels to the each locations. The country is huge and the roads are sometimes very challenging. Some locations were a real test of hill climbing by the 4x4, 6 cylinder, 3.0 liter engined SUV and even some of the last 100 meters we had to cover by foot.

Photo gallery

Summary and results

During the site visits I have encountered many different soil types and conditions. The sites only partially passed the backfill density test, and 2 of them were rejected. During the foundation designs we have followed 19kN/m3 soil density for the backfills. This unfortunately haven't met in some sites, and I have found values low as 16.2kN/m3. When we design optimal structures and utilise uplift as much as possible, these error ranges are significant. In one site the low density was due to organic content and thus the density was not a surprise as a low value.

On those sites where the foundation backfill were already done but the density found to be too low, I have recommended soil replacement by imported soil with required density. Anyhow, there should not be circumstances when organic soil are used as backfill for foundations of any permanent structures, above all not a 112m broadcasting tower. This is one of the shortcuts some contractors do that instead of importing proper soils, they use the excavated organic soils from the top layers. Basic mistakes (shortcuts) can bring much more extra cost afterwards. On one of the site the tower has been already erected. This gave a challenge to the soil replacement.

After some calculations and re-run of the analysis of the 112m tower with lower basic wind speed than the design wind speed, the following recommendation was made.

The backfill removal and replacement has to be done in portions, i.e like the below drawing. The existing backfill should not be removed entirely at any time. The slab has to have at least 50% counter weight by the soil compared to what was originally designed. The colours below shows the replacement at the same time.

The backfilling has to be carried out uninterruptedly, favourably in more shifts. The weather forecast has to be checked at all the time and period with very low wind speed has to be chosen. The wind speed should not be more then 20km/h! In this way the foundation will still be capable to fulfil enough safety against overturning during the soil replacement.

Site 1.

Site 2.

Site 3.

Site 4.

Site 5.