1、Standard Method of Test for Measurements of Pore Pressures in Soils AASHTO Designation: T 252-09 (2013) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-1b T 252-1 AASHTO Standard Method of Test for Measurements of
2、Pore Pressures in Soils AASHTO Designation: T 252-09 (2013) INTRODUCTION The principal reasons for measuring pore pressures in highway construction are: 1. To monitor strength increases during construction; 2. To estimate remaining settlements after construction; and 3. To determine existing pore pr
3、essures in landslides. This method consists of installing the pore water pressure measuring device in a soil layer at a point determined by detailed knowledge of the field conditions and theoretical evaluation of the need required. Pore pressure measurement may be made by determining the total pore
4、pressure at the point or determining the differential pore pressure at that point relative to the surrounding normal groundwater table. The data are usually reduced to “excess” pore pressure, which is the difference between the measured pore pressure and the “normal” pore pressure under the general
5、groundwater table for the area at the time of reading. Interpretation of pore pressure measurements in the field are complicated by the variability both vertically and horizontally in the soils. Therefore, interpretation of any pore pressure measurement can be misleading if the complete soil profile
6、, loading history, and detailed soil parameters are not adequately known. 1. SCOPE 1.1. This method covers procedures for determining field pore pressure measurements in natural soils. 1.1.1. This does not include methods of pore pressure measurement in the laboratory or field installations in dams.
7、 1.2. The values stated in SI units are to be regarded as the standard. 2. APPARATUS 2.1. Open SystemThe following equipment may be used to obtain pore pressure in soils of relatively high permeability (Note 1): Note 1This equipment is also satisfactory for low-permeability soils. 2.1.1. A simple op
8、en observation well consisting of an open hole supported by plastic or metal casing, if necessary. 2.1.2. A standard well point installation that consists of a screened well point on the end of standard pipe. Various screen lengths and openings are available for use in different situations. 2.1.3. T
9、he open well Casagrande-type (Hydraulic) piezometer that consists of a porous tip connected to a small diameter riser tube surrounded by filter (Ottawa) sand (100 percent passing 0.850-mm 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication
10、is a violation of applicable law.TS-1b T 252-2 AASHTO (No. 20) sieve, 100 percent retained on 0.425-mm (No. 40) sieve) and sealed in the bottom of a drilled hole. The tips are available in various sizes and porosities and may be made of ceramic filter stone, porous plastic, or porous metals. The ris
11、er tube may be plastic and, for convenience in reading, should have an inside diameter no smaller than 6.35 mm (1/4in.). 2.2. Closed SystemsThe following equipment may be used to obtain pore pressure in soils of low permeability (Note 2): Note 2This equipment is also satisfactory for high-permeabili
12、ty soils. 2.2.1. Closed Casagrande-Type PiezometerThis consists of a porous tip (as in Section 2.1.3) sealed into a soil layer and connected to the ground surface by a fluid filled tube (or tubes) connected to a gauge. The porous tip is installed at the bottom of a drilled hole and surrounded by fil
13、ter (Ottawa) sand (100 percent passing 0.850-mm (No. 20) sieve, 100 percent retained on 0.425-mm (No. 40) sieve). The tubing may be 6.35 to 12.7 mm (1/4to 1/2in.) or larger-diameter plastic or flexible metal. Clean water should be used to fill the tube. Where freezing is a problem, consider using a
14、pneumatic or electrically operated piezometer. A Bourdon tube-type compound gauge (of sufficient capacity to accommodate the expected pore pressure change) should be used. The gauge should be calibrated for the conditions under which it will be used (position, temperature, etc.). This type system me
15、asures the differential water pressure between the surrounding groundwater table and the pressure within the soils system. 2.2.2. Pneumatic PiezometerThis consists of a pneumatic pressure transducer in a plastic or stainless steel case with a porous (ceramic, plastic, or metal) opening to allow the
16、pore water to reach the transducer diaphragm. Two small-diameter plastic air leads are connected to the transducer. A supply of compressed air, bottle compressed, or liquified gas (clean and without moisture) is required to operate the transducer. An air pressure gauge of sufficient capacity to reco
17、rd the existing pressure at the depth of the piezometer tip plus the expected pore pressure is needed. 2.2.3. Electrically Operated PiezometerThis consists of a calibrated diaphragm (with a strain gauge attached) sealed in a plastic or stainless steel case, with one side open to the pore water throu
18、gh a porous filter. The strain gauge attached to the diaphragm may be resistance type or vibrating wire type. Pore pressure is obtained by use of an electrical readout system compatible with the type of strain gauge used. 3. INSTALLATION PROCEDURES 3.1. Driven or Drilled in-Type Installations: 3.1.1
19、. Open-Well Piezometers: 3.1.1.1. Open HoleDrive standard pipe (with disposable point) of suitable size and strength to reach the soil layer in which pore pressure is to be obtained. Pull back the pipe a sufficient distance to obtain pore pressure response and cap the pipe with a vented cap. 3.1.1.2
20、. WellpointDrive standard pipe (with suitable wellpoint attached) of suitable size and strength to reach the soil layer in which the pore pressure is obtained. The wellpoint may be of any size, length, and screen opening suitable for the soil in which it is installed. However, the largest diameter o
21、f the wellpoint shall not be greater than the diameter of the pipe coupling above it. Insert a suitable flexible hose through the pipe and into the wellpoint. Pump clean water through the hose to clean the wellpoint (and screen). Flushing should continue for about 5 min after the water flows clean o
22、ut of the top of the pipe. Remove the hose and install a vented cap on the pipe. 3.1.1.3. Low-Volume Change WellpointA small-diameter 9.5- to 12.7-mm (3/8- to 1/2in.) plastic tube is firmly sealed (by mechanical coupling or suitable glue) to a suitable wellpoint. The seal must be 2015 by the America
23、n Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1b T 252-3 AASHTO strong enough to withstand the total pore pressure without allowing leakage into the drive pipe. The wellpoint is attached to a standard pipe of sufficie
24、nt size and strength to reach the soil layer in which pore pressure is to be measured. The pipe threads for the first 6.1 m (20 ft) of pipe shall be sealed with a suitable pipe sealing compound to prevent leakage through the pipe joints. The plastic tube is threaded through the pipe sections during
25、driving. After driving, the tube is suspended (with sufficient tension to keep the tube straight) from the top of the pipe, and the pipe capped with a vented cap. 3.1.2. Closed-System Piezometers: 3.1.2.1. Closed-system piezometers may be driven into the proper soil layer in a manner similar to that
26、 explained in Section 3.1.1.3 above. Special filters and points may be used for special situations. Connections to standard pipe or standard drill rods are acceptable provided the seals are watertight for the pore pressures used and the diameter of the point does not exceed the diameter of the drill
27、 rod or the drive pipe. For Casagrande-type piezometers, the point and enough tubing to reach the ground surface shall be completely filled with clean water before the start of driving. The upper end of the tube shall be sealed to prevent entrance or loss of water from the system during driving. No
28、tubing connectors should be used in the vertical (Riser) tube. 3.1.2.2. Surface ConnectionsClosed-system piezometers that are to be extended vertically through the embankment fill may be extended in any manner suitable for the type of piezometer being installed, provided that (1) adequate protection
29、 is provided, and (2) there is a method of recalibrating the pore pressure reading with each extension. Closed-system piezometers with remote readout shall be installed in the following manner: 3.1.2.2.1. Excavate a trench 610 mm (2 ft) deep by 305 mm (1 ft) wide from the piezometer location to the
30、readout protection box. The bottom of the trench shall be a minimum of 610 mm (2 ft) below future construction (ditches, pipes, etc.). 3.1.2.2.2. The top of the drive pipe shall be removed to a point at least 1.52 m (5 ft) below ground surface. The tube (or leads) shall be centered over the pipe for
31、 at least 305 mm (1 ft) as sand is placed in the hole. The tube (or leads) shall then be coiled in a spiral approximately 610 mm (2 ft) in diameter, upward to the bottom of a trench approximately 610 mm (2 ft) below ground surface, as sand is placed loosely around it. 3.1.2.2.3. The horizontal leads
32、 shall be layered in a zigzag pattern in the trench on a bed of 152 mm (6 in.) of clean sand. When more than one tube or lead is used in a trench, the lines shall not cross. 3.1.2.2.4. All horizontal tubes or wires shall be checked for continuity, leakage, etc., before final connections are made to
33、the vertical piezometer leads. If possible, the leads or tubes should be in one continual length with no connections, from the piezometer tip to the readout protection box. 3.1.2.2.5. All electrical leads shall be immersed in clear water and the resistance checked. If there is any change in resistan
34、ce upon immersion, the leads shall be discarded and replaced. 3.1.2.2.6. The air leads for pneumatic piezometers shall be immersed in clear water and checked for leaks under pressures exceeding the expected pore pressure. Also, the air leads shall be flushed with high pressure air to clear any dust
35、or moisture from the leads before final connections are made. The air used for flushing should come from a tank that holds only filtered air and that has been bled for excess moisture. 3.1.2.2.7. The horizontal tubes for hydraulic piezometers shall be vacuum tested to 70 kPa (10 psi) of vacuum prior
36、 to filling the tubes with colored water. There shall be two horizontal tubes with T connection to the vertical riser from the piezometer tip. The tubes shall be completely filled with colored fluid, with the tubes in a configuration such that any air bubbles that occur during filling 2015 by the Am
37、erican Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1b T 252-4 AASHTO can be seen and removed. The preferable way of filling is to immerse one end in the colored fluid, seal the T at the rise with a plug, and apply a v
38、acuum to the free end of the horizontal tube. Care must be taken to prevent the formation of air bubbles during filling operations. Once the tube is completely filled with fluid, the connections should be made at the readout box. The vertical riser from the piezometer tip shall then be filled with w
39、ater and the plug removed from the horizontal tube T, and the connection between the vertical riser and the horizontal tubes shall be made underwater. The elevation of the T and the readout gauge must be determined prior to backfilling the trench. 3.2. Piezometers Installed in Drill Holes: 3.2.1. Th
40、e following additional equipment is required for installation of drilled-in piezometers: 3.2.1.1. A tamping hammer, made of a 610-mm (2-ft) length of seamless steel tubing, 41.3-mm (15/8-in.) outside diameter (O.D.), and 15.9-mm (5/8-in.) inside diameter (I.D.) or cast bronze of the same dimensions
41、and having a mass of at least 11.3 kg (25 lb). These dimensions are for a 50.8-mm (2-in.) ID casing. Larger hammer dimensions are required for larger casing sizes. At the upper end, a loop of 3.2-mm (1/8-in.) diameter galvanized airplane cable should be firmly attached to the hammer and to a grooved
42、 ring. The inside surface of the hammer should be smooth and all edges that touch the tubing should be rounded. This hammer should be supplied with a 41.3-mm (15/8-in.) diameter disc 12.7-mm (1/2-in.) thick that can be firmly attached to the bottom. This disc shall have a slot in the center (of suit
43、able size to accommodate the size tubing or leads used) with rounded edges. This hammer-cable assembly is used for the following purposes: 3.2.1.1.1. To tamp the bentonite layers and thereby assure a watertight seal between the casing and the risers. 3.2.1.1.2. To center the risers while the bentoni
44、te seal is being tamped into place. 3.2.1.1.3. To measure depths at various stages of the installation. 3.2.1.2. Galvanized preformed airplane cable 6.35-mm (1/4-in.) diameter, of sufficient length to permit installation on the deepest piezometers. This should be securely fastened to one end of a sn
45、ap-type swivel hook. Mark the cable at 1.52-m (5-ft) intervals, starting at the bottom face of the hammer. 3.2.1.3. A tripod and sheave for operating the tamping hammer. 3.2.1.4. Drive sample drilling equipment. 3.2.1.5. Ottawa sand or thoroughly washed sand passing the 0.850-mm (No. 20) sieve and r
46、etained on the 0.425-mm (No. 40) sieve. 3.2.1.6. Bentonite balls about 12.7 mm (1/2in.) in diameter, which are formed at a water content somewhat above the plastic limit but below the sticky limit (i.e., at a putty-like consistency), rolled in talcum powder to prevent sticking, and stored in glass j
47、ars to protect them from drying, or the commercial pellets. 3.2.1.7. Rounded pebbles approximately 12.7 mm (1/2in.) in diameter. 3.2.2. Drive casing, 50.8-mm (2-in.) ID or larger, to the approximate elevation of the bottom of the piezometer cell. The bottom 3.05-m (10-ft)-long section must be in one
48、 piece, without joints or couplings, and it is not to have a drive shoe on the lower end. The casing may be advanced by any means, except for the final 6.1 m (20 ft) of penetration. It shall then be advanced in 1.52-m (5-ft) increments, and the casing must be washed out after each 1.52-m (5-ft) adva
49、nce. The casing shall be kept filled with water at all times and no washing below the casing will be permitted. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1b T 252-5 AASHTO 3.2.2.1. Obtain a spoon sample of the material for 305 mm (12 in.) below the bottom of the casing and deliver the samples in sealed jars to the engineer. Drive the casing 305 mm (12 in.) below the piezometer cell elevation and clean out the remaining soil to the bottom of the casing. Replace the water
