Design & Planning of Excavation Sequence & Blasting Techniques for Caverns

The cavern construction technology governs many important aspects of design, and the design processes continue to the end of the construction phase. A reasonable excavation sequence and control blasting is one of the key techniques for construction of large underground powerhouses.

A 26.2m wide, 57.3m high, and 202.9m long cavern of 1000MW Tehri pumped storage project is under construction with top heading & benching method to accommodate four reversible turbines of 250MW each. The geology encountered was very poor and many large openings were there, so excavation of the cavern was very challenging which required special excavation techniques. Due to the vicinity of an underground cavern close to the under-operation cavern of 1000 MW Tehri HPP, control blasting technique was adopted to maintain the design PPV and minimize over excavation of the cavern with heading & benching.

The project is in an advanced stage of construction and the powerhouse excavation is now complete. Concreting of all four units is in progress with a target to commission the project by June.

Figure 1: Arrangement of Powerhouse Cavern

Introduction

The construction of the Tehri PSP is under progress by THDCIL, which envisages large underground machine hall cavern housing 4 reversible pump turbine units, each of 250 MW capacity. Interestingly, the underground Tehri PSP, which is in an advanced stage of construction, is in the same hillocks as the Tehri Hydro Power Project (HPP). The major project components of the PSP are 26.2m (W) X57.3m (H) X202.9m (L) size machine halls to accommodate 4 nos. turbine of 250 MW each, 22m dia. 02 nos. upstream surge shafts with surge chambers, 77m (L) X24m (H) X10m (W) size BVC, 81m (L) X20m (H) X13m (W) PAC and 18m dia 02 nos. downstream surge chambers, 02 nos. of 1081m & 1176m TRTs.

Large underground cavern of Tehri PSP executed with Drill & Blast method (DBM). It was anticipated that the blasting operation undertaken during excavation of PSP components may adversely affect safety of the hydro-mechanical installation of the nearby HPP project. Therefore, it was important that the blasting operation be carried out in a scientific manner and continuous vibration monitoring be carried out for the assessment of damage potential of blast induced ground vibration during excavation in the underground cavern.

Figure 2: Geological Model of Powerhouse cavern

Blasting operation was optimized taking into consideration proximity of various structures susceptible to damage, prevailing rock mass conditions, and design rock mass support system. Optimization of drilling and blasting practices in PSP project is carried out using emulsion explosives and non-electric initiation system. An emulsion explosive is a better product in respect of safety, quality, and productivity, as compared to slurry explosives. During optimization of the blast design, maximum charge per delay plays a great role in vibration control.

During initial field investigation, a comprehensive database of vibration was generated and a model of the vibration attenuation characteristics was evolved. Maximum charge per delay is fixed for each blasting pattern as per the distance and rock mass condition.

Geological Setup

Figure 3: Power House Cavern (26.2m (Width) X57.3m (Height) X202.9m (Length)

Project site is within the Lesser Himalaya which lies tectonically between the Main Central Thrust (MCT) and the Main Boundary Thrust (MBT) respectively. The former separates Meta Sedimentary sequence of Lesser Himalaya to the north from Crystalline rocks of Higher Himalaya, and latter disjoin the lesser Himalayan sequence from molasses sediments of Frontal Fold Belt (FFB), in the south. Phyllites of Chandpur formation of Jaunsar group and Quartzites and Meta basics of Garhwal group are exposed along the project. Rock units are part of the low-grade metamorphic rock that has been thrusted, folded, and deformed. There are two main tectono-stratigraphic units: the Krol Super Group and the Gharwal Group (R. Shankar et.al.1989). This former Super Group is subdivided into the Jaunsar Group and Krol Group. Jaunsar Group is found at the Tehri site and includes quartzites and phyllites in varying proportions that have undergone various phases of deformation leading to development of numerous tectonic dislocations, sheared zones, seams and joints of different scales and categories.

Type of lithology categorized in the Tehri site have been classified by Geological Survey of India. The nomenclature has been based on variable proportions and quality of quartzite and Phyllite include: PQM –Phyllitic Quartzite Massive; PQT – Phyllitic Quartzite Thinly Bedded; QP – Quartzitic Phyllite; SP – Sheared/Shattered Phyllite.

PQM and PQT are more quartzite (arenaceous) and rarely micaceous in composition and are coarser in grain size. These rocks are grey, dark grey, brownish grey, grayish grey and green in colour. It is mainly comprised of quartz, feldspar, and oriented laths of micaceous minerals. QP is more areno argillaceous in composition, fine grained and dark coloured. SP comprises of argillaceous and deformed variants of PQM and PQT rock, formed in sheared zone area which has weak rock mass characteristics.

Figure 3: Top heading CENTRAL GULLET blast design & firing sequence

Excavation Methodology of Cavern

The Excavation of Powerhouse cavern is proposed with construction of Pilot tunnel (central gullet) at top of cavern, including side slashing and various stages of benching. The top heading is planned with pilot tunnel using tunnelling techniques. This gives easy access to the cavern roof for installing support works. The supported roof gives safe working conditions for the excavation of the lower levels of the cavern. The lower levels excavated used benching techniques. The size of top heading finalized based on several factors and divided into three sections including pilot tunnel, left side slashing and right side slashing. These three sections of top heading finalized based on the following factors:

1. Reach of drilling jumbo
2. Rock quality
3. The area of unsupported roof that can be exposed at any one time
4. The presence of weak rock zone which limits the area of unsupported excavation face because of instability
5. Limitation on the quantity of explosive discharged in a round given by blast vibration acceptance criteria
6. Practical depth of blast holes, which is 3 to 4m depending on the cross- sectional area of top heading.

Considering all these above factors, 5m diameter pilot tunnels (central gullet) with 3m pull length and two part of side slashing with 3m pull length adopted for top heading of cavern. For excavating the roof vault, a central gullet was excavated first along the entire length of the powerhouse cavity and in line with the 5m D-shaped Adit. The central gullet was 6m wide with roof having same profile as that of powerhouse cavity. In order to accommodate the length of rock bolts, the height of central gullet kept as 5m. The central gullet excavated full face by drilling and blasting method. The drilling done by 2-boom Hydraulic jumbo (Boomer) and the mucking of the excavated material was done by a combination of dozer, loader and dumpers. For smooth excavated surface and least disturbance to surrounding rock mass, periphery holes at a closer spacing of 500mm provided.

Figure 4: Heading & Benching of Cavern

Benching excavation is generally carried out with vertical drillholes as in a quarrying operation, or with horizontal drillholes. Vertical drill holes adopted for benching operation as the drilling of holes is then independent of other sections of the excavation cycles except during blasting and ventilation. The holes drilled with crawler mounted drilling rigs. The cavern excavation divided into two parts and in benches of suitable height.

Figure 5: Excavation sequences of Cavern Benching

The height of benches finalised based on following factors:

1. Access, mucking can be possible through tunnels located at suitable levels.
2. Blast hole deviation, the longer holes will be chances of greater deviation.
3. Stability of walls, cavern wall can be unstable if too high, and successive bench excavation and support can be required.
4. Cost effective construction, optimization of the excavation cycles.
5. Limitation on the quantity of explosive discharged in a round given by blast vibration acceptance criteria

Blast Design

All blasting operation causes vibrations which are transmitted to the environs. If sufficiently strong, these vibrations can cause damage to structures and equipment, the vibration that can be accepted may limit the size of the blast or necessitate vibration mitigation blast design. The rock which forms the final surface of the excavation can be damaged if design or execution is unsuitable. As PSP Cavern is close to already operational Tehri HEP powerhouse cavern, so it was very crucial to design economical and safe blasting without disturbing operational powerhouse.

Vibrations are measured in operational powerhouse cavern using vibrographs for designed peak particle velocity. And accordingly, size of each blast designed based on accepted peak particle velocity criteria.

Figure 6: Blast design for Top heading- Right -side & Left-side slashing

The objective of blast design is to break the rock, leaving the smoothest possible rock walls, with minimal overbreak and damage to the surrounding rocks. The parallel hole cut method adopted for excavation of Pilot tunnel of top heading of cavern, the cut holes blasted first with heavy charged and create the free face for rest of the blast.

Blast holes break the bulk of the rock. Contour holes, on the edge of the blast, define the excavation perimeter and limit the overbreak and damage to the surrounding rock.

Blast Vibration Monitoring

Dedicated blasting monitoring team CIMFR deputed at site for independently monitoring various blasting operation and recorded blast induced ground vibration at different locations using advanced seismograph supplied by CIMFR Roorkee. Three sets of engineering seismographs are deployed for monitoring of the blast induced ground vibration. CIMFR deployed one Mini Mate Plus seismographs which is an advanced engineering seismograph with low trigger level (0.125-254mm/s) and higher range of the recording capability. All the equipment used of monitoring consist of a micro-processor-based logger, tri-axial geophone and a mic with 1.5 m three-piece stand. These instruments are manufactured by Instantel Canada shown in Figure 7.

Figure 7: Blast vibration Monitoring Equipment

CIMFR team have continuously supervised and recorded blasting operation at THDC PSP site during this reporting period. More than 100 blast events have been supervised and monitored for blast induce ground vibration at various critical locations. Major rock excavation during this reporting period have been carried out in powerhouse Cavern.

Figure 8: Blast vibration Monitoring at Powerhouse cavern

Fast furrier Transform (FFT) analysis of the observed data reveals that the principal frequency of blast vibration is greater than 50Hz in majority of the cases. As per project Permissible Peak Particle Velocity (PPV) in mm/s for structures containing electro-mechanical equipment is 20 mm/s irrespective of the frequency. As per DGMS criterion, RCC building may sustain vibration upto 50mm/s having dominant frequency more than 50Hz. In light of the above discussion, all the blasting operation has been optimized to ensure peak particle velocity level to be less than 20 mm/s.

Figure 9: Blast event monitoring

Analysis of the observed vibration data reveals minimum and maximum value of Peak Particle Velocity observed during this period is 0.648 and 19.98 mm/s. Majority of the observed vibration data remained less than 5.0 mm/s. All the blasts were carried out using controlled blasting, additional line of line drilling and multi-drifting. This has successfully helped in blast induced ground vibration and all the values are found to be well within the recommended safe limit.

Blast design for excavation of various critical structures has been carried out by conducting trial blast. The blast design parameters are optimized based on the results of the trial blast with due consideration to proximity of the critical structures.

Conclusion

Excavation of large underground cavern of Tehri Pumped Storage project (PSP) was most crucial considering safety and stability of nearby operational cavern of Tehri HEP, and application of proper execution and design of controlled blasting led to successful completion of cavern excavation. The following measures were adopted at site for successful excavation of the cavern:

• Due to the complex geology of the powerhouse cavern, multiple openings at different levels, and location of cavern close to operation powerhouse, the ramp was divided in two parts of cavern width. The turbine pits were also excavated in multiple stages in a controlled way.
• For stability of large underground openings, controlled blasting alone was not an effective solution. Timely installation of designed support system around the excavation face was equally important to prevent rock mass from deteriorating.
• Besides vibration monitoring, the impact of blasts on the roofs and walls of large underground openings were evaluated by instrumentation. As per the instrumentation monitoring data, it was found that the measured ground vibration was within safe limit and no adverse effects were noticed in the powerhouse walls and roof. Multipoint bore hole extensimeter data indicated insignificant movement in the walls of the cavern.
• The line drilling and smooth blasting adopted to maintain designed excavation line of cavern walls, and the overbreak in some sections was due to unfavourable geology and small planar and wedge failures.
• The charge concentrations in the drill hole close to wall of cavern was adjusted so that the damage zone from each hole coincides with the expected excavation limit.
• Analysis of the observed vibration data reveals that minimum and maximum value of peak particle velocity observed during this period is 0.648 and 19.98 mm/s, respectively. Majority of the observed vibration data remained less than 5.0 mm/s. All the blasts were carried out using controlled blasting with additional line of line drilling and multi-drifting. This has successfully helped in blast induced ground vibration and all the values are found to be well within the recommended safe limit.
• After analysis of 133 blast records, site specific attenuation characteristics of blast induced vibration have been suggested. The prediction model can also be used for calculation and to determine the safe maximum charge per delay (MCD) in blast design so that the Peak Particle Velocity (PPV) remained less than the permissible limits.

References

1. NIRM Report-NM-13/09C/04/ Dec 2014, 3-D Stress analysis report of Powerhouse complex and other tunnels of Tehri PSP.
2. Govind Raj Adhikari, R. Balachander and A.I. Theresraj- NIRM Karnataka, India- Execution of safe blasting under adverse condition of a powerhouse complex: A revisit to Sardar Sarovar Project, India.
3. Xiao Liu, Peng Yan, Ming Chen, Sheng Luo- Wuhan University, China- Optimization analysis of excavation procedure design of underground powerhouse under high in-situ stress in China.
4. Geotechnical Engineering Office, Civil Engineering Department, The Hong Kong,- Geoguide 4, Guide to Cavern Engineering
5. Rajeev Prasad & Nishith Sharma- Hydro Nepal/ Issue No. 24/ January 2019- HCC India, Engineering Geological and Geotechnical approaches for the construction of powerhouse cavern of Tehri Pumped storage Plant (100MW)- A case study.
6. F. Vaysse, J. Dufour, EDT GG 12-0178, EDF report – Geological, geotechnical and hydrogeological input data for the conduct of calculations on the stability of the Machine Hall Cavity- Tehri PSP.