A project site was used as a case study mainly to highlight the importance of anticipating the presence of hard materials on site and the associated problems if hard materials are detected during construction work.
The study was undertaken alongside the remained hilly terrain.
The original hills have been leveled down to the required reduced level and the site is now bounded by half completed cut platform.
Within the excavation site, the dig-able and rip-able material of the hills cover was seen to have been stripped off and removed, leaving exposed the rock mass.
From visual observation, the project site contains many protruded coarse-grained granite stones. The protruded ground face formed an outcrop of boulders of grey color coarse-grained to medium biotite granite.
Most of it is located along the hill slopes. The surrounding earth is made up of reddish brown, sandy silt with some gravel.
With reference to past experience and information derived in the literature study, the project site is believed to contain granite bedrock at depth.
Types of data obtained from field
This case study involved a number of field investigations and measurements as mentioned below:
- Total Station survey for existing ground profile and topography.
- Mackintosh Probe is used to provide a profile of penetration resistance with depth to give an assessment of the variability of in-situ materials on site.
- Seismology instruments equipped with wave detection i.e. geophone, wave recorder i.e. for displaying seismograph, a sledge hammer and steel plate for beating. These instruments are meant for seismic refraction tests to detect the presence of hard materials at depth below the ground surface.
- A Percussion Rig Boring (Wash Boring) consists of a derrick, power-winch and a set of drilling tools are used to drive through the overburden soil layers and coring bits are being used during core-drilling to recover the rock cores.
Total Station Instrument
Total Station instrument is being used to carry out the existing ground profile and cross section survey.
Total Station instrument consists of the prism, tripod and level staff.
Plate 2 indicates the Total Station instrument used for this case study.
Mackintosh Probe
One of the most common types of probing is Mackintosh Probe. The Mackintosh prospecting tool consists of rods which can be threaded together with barrel connectors and which are normally fitted with a driving point at their base, and a light hand-operated driving hammer at their top.
The tool provides a very economical method of determining the thickness of soft deposits such as peat. Probing is carried out rapidly, with simple equipment.
It produces simple results, in terms of blows per unit depth of penetration, which are generally plotted as blow-count/depth graphs.
Seismic Refraction Test Equipment
In order to detect the presence of hard materials at depth below ground surface, seismic refraction tests are being adopted. Plate 3 depicts the seismograph equipment for seismic wave test. Among the equipment used to carry out these tests are as follows:
- Seismograph set for the recording of seismic wave data.
- Geophone for detecting the seismic waves.
- Sledge Hammer and metal plate for the beating to create shock waves.
- Measurement tape for measuring distance.
Boring Tests
A Percussion Rig Boring (Wash Boring) consists of a derrick, power-winch and a set of drilling tools are used to drive through the overburden soil layers and coring bits are being used during core-drilling to recover the rock cores.
Surveying Works Procedure
The longitudinal profile and cross section for existing ground levels were taken at project site.
Survey work is carried out to determine the limit of case study. Besides this, survey work is carried out to determine the datum for the whole area in preparing contour mapping of area involved.
This will make it easier to draw the cross section of case study area. During surveying work, it is bound to come across obstacles and thus shifting of alignment could not be avoided.
The total station equipment’s are being used to carry out surveying work at project site to determine the coordinates and reduced levels of each designated tests.
In determining the horizontal profile, surveying work using trevass method has been adopted to produce the bearing and distance of case study area. The reduced levels are determined by use of trigonometry method.
For this case study, total station equipment used is TOPCON type where the data collected are using TOPCON FC5 Data Collector.
Determining the presence of Hard Material and Overburden
In order to determine the volume of sub-surface soil and presence of hard material underneath, data from the measurement of cross section survey needs to be analyzed and plotted first.
To obtain a more accurate volume, a few cross section survey needs to be carried out. The more cross section survey is taken, the more accurate will be the volume.
The datum and coordinates for the respective bore holes carried out shall form part of grid survey for more accuracy in determining the calculation of volume and reduce having to resort to interpolation.
From the site investigation carried out i.e. using seismic reflection and wash boring methods, the thickness of sub-surface soil and presence of hard material at respective tests could be determined.
For calculation in terms of volume for the sub-surface soil and hard material of whole area, two methods being frequently used are Trapezoidal method and Simpson method.
Prior to calculating the volume, the area for each cross section profile must be obtained first. Hence, each cross section survey shall be in uniform distance apart.
This is essential in order to apply the Trapezoidal or Simpson formulas to obtain the volume directly. The calculation of respective cross section survey will determine the thickness of sub-surface soil.
Simpson’s Method
Area, A = a/3[Y1 + Y7 + 2(Y3+Y5) + 4(Y2+Y4+Y6)]
Trapezoidal Method
Area, A = ½(Y1+Y2) x a + 1/2(Y2+Y3) x a + 1/2(Y3+Y4) x a +……+1/2(Y6+Y7) x a
Upon obtaining the area of respective cross section, the volume for the designated compound could be determined.
Take an example, if the place for investigation has 5 cross sections and every section has area of A1 till A5.
Distance in between respective cross sections is d, then;
Volume, V = d/3[A1 + A5 + 2 x A3 + 4 x (A2 + A4)] – Simpson’s Method
Volume, V = d [1/2(A1 + A5) + A2 + A3 + A4] – Trapezoidal Method
By using these two equations, the answer obtained always varies slightly. However, the difference is minor and it is frequently used to estimate the quantity of earthwork in large scale and estimate of presence of hard materials underneath the earth.
Mackintosh Probing
During Mackintosh Probing, the driving point is streamlined in longitudinal section with a maximum diameter of 27mm.
The drive hammer has a total weight of about 4kg. The rods are 1.2m long and 12mm dia.
The device is often used to provide a depth profile by driving the point and rods into the ground with equal blows of the full drop height available from the hammer: the number of blows for each 150mm of penetration is recorded.
When small pockets of stiff clay are to be penetrated, an auger or a core tube can be substituted for the driving point. The rods can be rotated clockwise at ground level by using a box spanner and tommy bar. Tools can be pushed into or pulled out of the soil using a lifting/driving tool.
Because of the light hammer weight the Mackintosh probe is limited in the depths and materials it can penetrate.
Seismic Refraction Tests
Seismic Refraction tests are possibly the most important and commonly used supplementary methods in site investigation. The purpose is to detect the presence of hard materials at depth below the ground surface.
Preparation of Seismic Refraction Tests
Amongst the equipments used for seismic refraction tests are:
- Seismograph for measuring seismic waves
- Geophone for detecting seismic waves
- Sledge Hammer and metal plate for knocking and producing source of energy.
Works to affix the geophone for the transmission of seismic waves were carried out upon completion of surveying works.
This is to ensure convenience in installing the equipment in straight alignment and also the results obtained could be plotted in straight line as well.
During these periods when tests are being carrying out, the equipment’s are required to be fixed at the designated locations determined earlier by the surveying works.
For this purpose, the geophone is fixed at the designated locations at a distance of 5.0m c/c along the alignment of case study area.
A total of 14 geophone equipments were used and placed alongside the alignments mentioned above. All the geophones were connected to the seismograph with wiring whereby the seismograph will read, measure and plot out the waves detected by the geophones.
The geophones used in this case study are fixed vertically upright above firm ground levels. However, at areas where the ground conditions are weak and when strong wind is encountered, geophone equipment’s were planted a few centimeters below ground surface. This is to avoid unnecessary disturbance during the event of testing being carrying out.
The geophone equipment is also forbidden to be placed on the surface containing roots. In surrounding the geophone equipments, ensure it is kept clean from grass, soil and sand for reasons to avoid disturbance against the wave signals received by the geophones as mentioned above.
During the course of carrying out tests, the seismograph equipments are to be placed as far as possible apart and away from the geophone locations for effective results.
In order to produce seismic waves, a sledge hammer is used to generate energy. The sledge hammer is to knock onto a piece of metal plate on the ground surface to generate energy or wave noise.
The wave will move through layers of soil beneath the earth surface and subsequently refracted back to earth surface whereby it is detected via geophones. The sledge hammer is connected to seismograph with wiring.
The maximum distance apart between sledge hammer and last geophone equipment (nearest to the knocking metal plate) is between 3 to 5 times more than the depth of hard materials beneath the location of knocking.
A metal plate for knocking to produce energy or wave sound, measuring in size of 180mm x 180mm and thickness of 15mm is suitable.
A smaller plate size will not be effective as it will sink into earth surface upon the impact of knocking to dissipate energy distribution. Moreover, smaller late is difficult to carry out knocking.
Implementation of Seismic Refraction Tests
Upon fixing the geophone and seismograph, the metal plate for knocking is placed on firm earth surface approximately 5.0m away from the nearest geophone no.1. Weak ground surface and wild grass surrounding the plate for knocking must be removed earlier to produce maximum energy impact when knocking by the sledge hammer carried out.
Switch on the seismograph equipment and reset the readings to zero prior to carrying out the test. It is utmost important to ensure no disturbance to the geophone and no one else walking past or cause any movement adjacent to testing location except the knocking sound produced by the sledge hammer only.
This is due to geophone equipment is very sensitive and it captures any reading of any kind be it produced by human movement or whatever it is. The readings produced will be affected by the wind, passing traffic and miscellaneous.
Thereafter, instruct the operator to carry out knocking with the sledge hammer on the metal plate. During the process of carrying out knocking with the sledge hammer, ensure that the operator knocks the metal plate accurately and also to make sure the sledge hammer does not knock the metal plate more than once which is as required.
In order to obtain a good result, the more test taken the better result will be. However, time and cost is another contributing factor to limit the number of tests taken. Therefore, a total number of 14 seismic refraction tests were conducted.
Percussion Rig Boring Test
The equipment for Percussion Rig Boring (Wash Boring) consists of a derrick, power-winch and a set of drilling tools. A percussion method is used, whereby the tool assembly is raised by the winch to about 1 m above the bottom of the hole and then allowed to fall under its own weight, thus driving the cutting tool into the soil.
When the tool becomes full of soil, it is raised to the surface, where disturbed samples may be taken from its contents.
The most usual borehole diameter is 150mm, but others up to 300mm can be drilled; the maximum depth of exploration, although dependent on soil type to some extent, is around 50-60 m.
In compact cohesion-less soils, or where boulders or cobbles are encountered, the chisel is used to break up hard materials; fragments and slurry are then removed using the bailer.
In wet conditions and in loose soils, and for very deep holes, a casing must be installed near the surface. This usually consists of steel tubes, screwed together in as many lengths as appropriate, and jacked or knocked into the drilled hole as drilling proceeds.
They can be hauled out after completion of drilling or left in place if further observations are required.
In stiff soils and rocks power-operated core-drills are used, consisting of small-diameter hollow tube, fitted at the lower end with a coring bit. The core barrel is rotated at speeds ranging between 600 and 1200 rpm, a controlled pressure applied and water circulated through the bit.
The fragments removed in the annular cut are brought to the surface with the circulating water as the core fills the barrel. A drilling run of 1-3 m is usually made before raising the barrel and removing the core.
The more usual standard sizes of core barrel used in site investigation range between 30 and 100 mm (hole diameter), although larger-diameter equipment is available for special uses.
Recovered Core Samples
The presence of discontinuities reduces the overall strength of a rock mass and their spacing and orientation govern the degree of such reduction. Hence, the spacing and orientation of the discontinuities are of paramount importance as far as the stability of structures in jointed rock masses is concerned i.e.
Samples Observations
- Fresher core samples with higher RQD values could be recovered at depth, especially in zone where less fracture occurs.
- Fractures are caused partly due to the wobbling drilling rods and core barrel alignment and in most cases the enormous pressure concentrated on the drilling surfaces.
- Most of the fractured surfaces observed showed prominent secondary infilling of joint, failures or minor faulted zones.
- The granite is homogeneous throughout the depth of boreholes.
- A deeper coring using a higher speed and bigger capacity drill plant could penetrate and recover better RQD.