|April 3rd 2018, Tuesday|
Rock Slope Design and Remedial Measures
- Tarun K.Raghuvanshi
The slopes which are susceptible to failure by various modes like, plane, wedge or circular mode of failures can be stabilized by various methods. These methods could be as simple as altering the geometry of the slope or comparatively complicated and difficult like providing reinforcement and retaining structures. The important factors which influence the stability of the slope are; i) slope angle ii) dip of the failure plane or plunge of the line of intersection of two wedge forming planes iii) shear strength parameters, cohesion and angle of internal friction and finally the water saturation condition.
For the stability condition either of the above-mentioned factors may have comparatively high or low influence. Some of the factors may be responsible in inducing the forces for sliding where as, the others may contributes in providing the resistance to the sliding. The net result of these factors will defines whether the slope is going to slide, under the given conditions, or it will be stable. The relationship of the factors responsible for inducing sliding and the factors responsible to provide resistance to the sliding is defined by the limit equilibrium. Factor of safety can be defined as the ratio of the total forces available to resist sliding to the total forces available to induce sliding. If the factor of safety is less than unity, it implies that the resistive forces are less than that of the forces which induce sliding. Thus, for the condition when the FOS is less than 1.0 the slope may fail, provided it is subjected to a triggering factor, natural or man made. The factor of safety of a potential unstable slope may be improved by providing several methods such as, safe slope design by altering the geometry of the slope, reinforcement by providing rock bolts, retaining structures, shotcreating etc.
Remedial Measures for Slope
A considerable stability can be achieved by making concave slope face. The safe slope angles for a given slope can be obtained by adopting technique proposed by Hoek and Bray, 1989. In this technique, the factor of safety is obtained by assuming different slope angles and slope heights. The factor of safety, as calculated for each combination of slope angle and slope height, is marked over a graph sheet by taking slope angles on X-axis and height on Y-axis. Later, a contour curve corresponding to a factor of safety equal to 1.2 is drawn. Thus, the safe slope angles for different slope heights are obtained from this graph. Based on these safe slope angles a slope cross section is prepared in which height of the slope is considered from top to bottom.
Rock bolts are used to reinforce jointed rock much as reinforcing bars supply tensile resistance in reinforced concrete. In rock slope, tunneling and underground mining, steel rod inserted in a hole drilled in the face of a rock formation to support the sides or roof of the excavation.These are equally effective in natural and cut slopes, as these rock bolts act as a binding tools between the two rock blocks on the either sides of the discontinuity plane. The bolt may be provided with an expanding device to grip the rock at the far end of the hole or may be bonded to the rock by means of expanding cement. Rock bolts may be used singly or in rows.There are static and tensioned rock bolts. Tensioned rock bolts should be used only where a force is needed to counteract the forces making the structure unstable..
In most cases static bolts should be used. The logic behind a static bolt is that if the structure is safe enough to drill into and install rock bolts, it already has an inherent factor of safety. If the stability of the structure is adversely affected in the future the static bolt will automatically go into tension with the exact amount of force and in the exact location that it should.
Improving Drainage of the slope
When there is an influence of water saturation the slope section becomes unsatable for static and dynamic conditions. Therefore if somehow, the drainage is controlled the slope section may demonstrate stable condition. In order to improve the drainage conditions horizontal collection drain in the crest region on upper slope may be provided which may collect the rain water and drain it away from the slope face. In addition to this on the upper slope surface and on the slope face shotcreting with wire mesh and random Rock bolts, may also be provided. In addition to this it is suggested to provide perforations in between the shotcreting surface, so that the water if any trapped, in between the shot creating layer and the slope face, be drained out.
Remedial Measures for Slope Section
Having Rotational Mode of Failure
In case of slopes which has a potential for rotational mode of failure demonstrates great challenges for the geotechnical engineers and engineering geologists. Such slopes which are mostly heterogeneous in nature, pose problems while estimating the shear strength properties for the material. The cohesion for soil mass under saturated conditions may approach zero value under such condition the slope will be unstable for all static and dynamic condition, though practically it would be very rare chance that cohesion becomes zero. For such slopes the most feasible remedial measure is to improve the slope geometry and to provide the proper drainage system. The retaining wall may be provided at the toe of slope with perforated weep holes, this may help in draining any excess water trapped in between the wall and the soil face, which may generate pore water pressure. In addition to this along the toe, just at the junction of the wall, a lined drain may also be provided. This will help in draining the water which may damage the foundation of the retaining wall.
Quality Control In
Quality control during construction of embankment dam is an important factor in deciding its performance and stability. To control the seepage and migration of soil particles various forms of precautions are designed in the form of impervious core, drainage blankets, chimney drains etc., which are protected by filters. Performance of these transition filters is based on the type of construction material and grain size. Various design criteria are adopted in designing of these filters. If the appropriate construction material as per the design is not selected or if there is any carelessness during the construction stage, it may lead to;
Therefore, successful performance of the structure, as envisaged by the designer, entirely depends on the extent to which the design calculations and assumptions are actualized in the field. Quality control aims to ensure that the plans and specifications of the design are actually implemented at the site. These include,
i) Borrow area
ii) Placement in the embankment
Borrow Area Control
During the exploration stage suitable borrow areas have to be located to obtain the sufficient quantities of the construction material for the different zones. The design and material exploration are interdependent. In fact the designer has to design the structure as per the availability of construction material within the economic distance. The designer should utilize the material, if suitable, from the compulsory excavations of the foundation, abutments or the spillway channel. The extent of the area from which material of specified characteristics for a particular zone is available has to be determined by laboratory tests and marked out at site. The depth upto which acceptable material is available is determined from exploratory pits. A number of well distributed pits over the borrow area will be an assurance of consistent quality and adequate quantity of material. Control in the borrow area includes control over;
Stripping is required to eliminate soil containing quantity of organic material. Organic matter imparts a darkish colour to the soil, but colour alone should not be criteria for the limit of the stripping. The percentage of the organic matter at various depths should be determined by the laboratory tests and based on the laboratory results the stripping limit should be decided. It should be ensured that only material from specified locations and depths goes into a particular zone of the dam. In case of varied stratified deposits blending may be done after proper laboratory tests.
It is more economic and more effective to do uniform mixing to adjust deficient moisture content in the borrow area rather than at the fill site. The moisture deficiency due to marginal losses during transportation can be compensated at fill site by sprinkling and mixing. Borrow pit area irrigation in advance of excavation is an economic method to get the desired moisture in the fill material. Borrow pits are irrigated by pounding water on the surface by low dykes or by a pressure sprinkling system. The length of time in getting desired moisture content may vary from few days to weeks as it will depend on the permeability of the soil deposit and the depth of the soil deposit. After irrigation a curing period is required to allow the added water to be uniformly absorbed in the soil. For flat areas and low permeability soil pounding is more effective method to get the desired moisture content. Whereas on sloping areas or for shallow depth wetting on a large area, pressure sprinkling would be more appropriate method. If the soil is over wet it may be dried by ripping and plowing. By excavating and heaping the soil in wind rows – part of water drain out by gravity and part of it evaporates.
Compaction of Cohesive Soils
- Role of Moisture In Compaction
The water plays an important role in the compaction of fine grained soils. The water acts as a lubricant and facilitate in expulsion of air from the voids and movement of the soil grains in the voids. If water is more it hinders the movement of soil grains as it occupies the voids. Therefore, the moisture content should be optimum so as to obtain the maximum benefits of compaction. Cohesive soils follow the principles of `optimum moisture content’ as described by proctor. The compacted effort is measured by the energy imparted to a unit volume of the soil. In the laboratory it is controlled by the free fall under gravity of a rammer of known weight through a specified height and a specified number of times on a given soil volume.
In the field compaction is imparted by a number of passes of a roller of specified type, weight and dimensions on a soil layer of specified thickness. The mode of compaction and type of rolled used, exert an appreciable influence on the `optimum moisture content’ and the maximum dry density even with the same compacted effort
The standard proctor compaction has been found to approximate to the actual field compaction achieved by 12 passes of the standard 20 tonne dual drum tampina roller on 20 to 22.5 cm loose layers compacted to about 15 cm thickness.
Compaction of Non Cohesive Soils
Cohesionless soils falling under the classification GW, GP, SW and SP do not respond to conventional rolling. Vibrations are the most effective method of compaction for these soils. Maximum effect is obtained if the frequency of vibrations is close to the natural frequency of the sand layer. Vibratory smooth drum rollers, 5 to 15 tonnes in weight, with a vibration frequency of 1100 to 1500 pulses per minute are commonly used for field compaction of sand and gravel. 2 to 4 passes of the roller traveling at a speed 2.5 km/h are adequate to compact layers of 30 to 35 cm compacted thickness. As an alternative, but less effective method is to water then makes a few passes with a heavy crawler tractor. Tests by Lewis on compaction of fine, medium and coarse sands by vibration at various moisture contents indicate that approximately the same degree of compaction could be achieved in either a dry or fully saturation condition.
Compaction of Cohesive Soils In Field
After the excavated material is brought from the borrow area and placed at its proper location, it is spread to the desired loose thickness by bulldozers. The next stage is rolling to achieve the specified dry density. For rolling compaction two types of rollers are used;
1) Sheep foot rollers
2) Pneumatic rubber tyred rollers
Sheeps Foot Rollers
Sheeps foot rollers can either be towed by crawler or be self propelled. The basic feature of sheeps foot roller is the `feet’ or prismatic attachment welded to the cylindrical drum of the roller. Two drums are mounted on the frame side by side. The outside diameter of each drum is 5 ft and length is 5 to 6 ft. The weight of the roller is 6000 kg/m of drum length. The feet are uniformly spaced 9 in center to center. The feet extend 9 – 11 from the drum surface.
Pneumatic Rubber Tyred Rollers
These consist of an assembly of tyred wobble wheels on two axles, front and back staggered so as to cover the full width. The wheels are mounted to have freedom in dependent vertical movement. A cart supported on the axles can be loaded with stones to give the required loading. Common type of Pneumatic roller used for compaction in embankment dams are 50 tonnes roller with a tyre pressure 80 to 100 psi. 4 to 6 passes of the roller on layer 22.5 cm compacted thickness are usually adequate to obtain the specified compaction.
Control of Compaction –Impervious Zone
An embankment compacted at a moisture content wet of optimum will have lower permeability, high flexibility or capacity to deform without cracking and lesser compressibility on saturation. USBR practice is to place the fill at 1 to 3% below optimum. It was found that this limited the construction pore pressure to not more than 30% of the weight of the overlying fill, which resulted in appreciable economy. However, for small dams (<15 m height) this consideration is not true, therefore for small dams compaction on slightly above `OMC’ was recommended. As per US Army Corps Practice Compaction is done at or above optimum moisture content. According to Middlebrooks the core material should satisfy the following criteria;
Placement Control In The Field
The placement control in field is concerned with;
a) Properties of the material before compaction
b) Uniformity and correctness of water content
c) Compacted layer thickness
d) Dry density attained after compaction
Control on placement is very important as the matter from the borrow area for a specific zone should be placed in the same zone. For this an expert supervision is required. The expert supervision involves visual observations by the colour of the material or by feel of running through fingers.
Field Tests: Visual checks must be well supported by the field tests so as to ensure the quality control in construction. The laboratory testing is time consuming as for moisture content and dry density, require drying of soil sample in an oven at 110oC for 24 hours. By the time results will come the layer will be buried by other layers. Therefore some rapid methods are required.
Field Density Tests: The field density specified is taken for the top most layer immediately after its compaction. The subsequent density can be measured by taking samples from boreholes or by the nuclear density meter. The compacted density of the top layers is measured by taking a known volume of the soil and determining its weight and moisture content. For rapid determination of moisture content in the field, a portion of the sample is weighed dried in a pan, cooled and weighed again. Another method is taking a sample of specified weight of moist soil and mixing it with specified quantity of calcium carbide. The quantity of acetylene gas produced gives the measure of moisture content.
Nuclear Method: Over the past few years, nuclear density meter and moisture meters have been developed and tested in field. These are consistent and reliable and can prove very useful in construction control. A density meter depends on the scattering of `’ rays by the soil particle. Two types of arrangements are employed, one for surface measurement and other for measurements in borehole.
Nuclear Moisture Gauge : This is based on the ability of the hydrogen nucleolus to slow down fast moving neutrons. The number of hydrogen nuclei depends on the moisture content.