The 普氏压实试验 establishes the maximum unit weight that a particular type of soil can be compacted to using a controlled compactive force at an optimum water content. This is the most common laboratory soil test and the basis for all engineered compacted soil placements for embankments, 人行道上, 和结构填充. In-place measured densities of the compacted fill are compared to the 普洛克特 test results to determine the degree of soil density.
土壤 are the most basic and least expensive construction materials available. 纵观整个人类历史, construction projects from primitive burial mounds to modern interstate highways have removed, 取代, 填满, or shaped the soil surface to accommodate 人行道上 and structures, or to make the earthwork itself a structure. Experience and experimentation have taught us that compacting the soils we place makes the installation more permanent, 还有其他一些优势.
- Bearing capacity and stability are increased
- Permeability (hydraulic conductivity) is decreased
- Heaving from freeze-thaw cycles is reduced
因为土壤是天然材料, it’s nearly impossible to predict their properties accurately enough for engineering applications without specific tests. Manipulating the characteristics of soils by compaction naturally raises some questions. We know that nearly any mechanical compactive effort will make soil denser, but which methods work best for each soil type? 水分有什么作用呢? 最重要的是, how does the maximum soil density that we find in our laboratory tests related to the soil densities we find in the field?
It has long been known that the moisture content of soils, 尤其是粘性土壤, has a direct impact on how efficiently they can be compacted by construction equipment. But because the soil is a natural material and the ideal moisture content and maximum dry weight varies for each soil type, the questions of the best unit weight and ideal moisture content couldn’t be answered.
1933年，拉尔夫. 普洛克特, 加州大学的一名学生, 加州大学伯克利分校, established a laboratory procedure that easily defines the practical maximum density of a soil sample along with the optimum moisture content needed to achieve that density. That concept is known today as the moisture-density relationship test, laboratory compaction characteristics of soils, 或者仅仅是标准的普洛克特测试, 并在ASTM中详细说明 D698 和AASHTO T 99 测试方法. Unit weights and moisture content field tests could now be expressed as a percent of the laboratory values for the same soil, making it easier for the designer to specify what was required and to document the suitability of the work performed in the field. 除了, knowing the optimum moisture content gave clear guidance for whether the soils being compacted should be made drier or wetter to make everyone’s job easier.
In the latter part of World War ll and later into the 1950s, heavier dynamic loads from larger aircraft and heavier, more frequent truck traffic were placing greater demand on 人行道上 and soil subgrades. Sometimes the compacted soils had passing density tests based on standard 普洛克特 values but were still deflecting too much under load to carry the increased forces. 简而言之, 密度太低, and optimum moistures were too high for these increased loadings. 同时, field compaction equipment was becoming larger and more efficient, making it possible to compact soils to higher densities at lower moisture contents. The modified 普洛克特 test was introduced in 1958 as ASTM D1557 和AASHTO T 180 to help with these applications at higher loads.
Test procedures are similar, but the laboratory compactive effort of the modified method is higher. 使用10lb (4.54公斤)锤子18英寸(457.2毫米)自由落体而不是5.5lb (2.49公斤)锤与12英寸(304.9毫米)下降. This results in higher maximum soil densities at lower optimum moisture contents. The modified 普洛克特 is used today concurrently with the standard 普洛克特. The selection of the method is based on project requirements and specifications. This blog post will focus on the significance of soil compaction (particularly the 普洛克特 Test), 测试是如何执行的, 必要的设备, 和有用的技巧.
The standard and modified 普洛克特 test procedures are straightforward. Prepared soil specimens with progressively higher moisture contents are compacted into molds with known volumes, 重, then their unit weights are calculated in pounds per cubic foot or kilograms per cubic meter. Unit weights will increase with higher moisture contents leading up to the optimum moisture, 然后下降超过这个点. The dry unit weight for each compaction cycle or “point” is calculated by correcting for the moisture content. The results are plotted with moisture on the horizontal axis and dry unit weight on the vertical to form a curve that defines maximum dry density and optimum moisture content.
NOTE: There are variations of test procedures noted in both the standard and modified 测试方法 that are dictated by the maximum particle sizes of the soil samples. The variations affect the preparation of the samples, 测试实践, 以及所需设备的类型. The reader is encouraged to carefully read and understand the ASTM or AASHTO 测试方法.
压实检测设备 required for performing the laboratory proctor test is also straightforward and widely available. 下面的列表就是一个好的开始:
- 压实(普洛克特)模具 with either 4in or 6in diameters to hold compacted samples
- 天天p锤子 are manually operated drop hammers to compact standard or modified 普洛克特 soil specimens into compaction molds
- 土壤机械压实工具 automatically compact 普洛克特 density specimens with the proper number of hammer blows and shut off when a preset number is reached.
- 不锈钢直尺 levels and trims 普洛克特 density specimens to size in molds
- 平衡或规模 compliant with D4753 and 1g readability to weigh the dry unit sample after compaction
- 干燥箱 that maintains uniformity to 230 ± 9°F (110 ± 5°C), for moisture content determinations
- 测试筛子 用于粒度测定
- 样品锅 & 托盘 for air drying, processing, and mixing of soil samples
A representative bulk field sample is obtained for each type of soil material proposed for use in the earthwork operation. Weights required for the bulk sample will range from about 50lb to 100lb (23kg to 45kg) of the moist sample, 取决于指定的测试方法. 回到实验室, processing of the samples begins with gradual air-drying to the desired moisture, usually around 10% or more below the anticipated optimum moisture. 对于粘性土, this can be expedited by breaking down clumps and spreading the sample out on open trays.
Once the soil is friable enough, the breakdown can continue more thoroughly. It is important to read and understand your particular test method carefully as there are several variables that can affect this stage of sample preparation. For most standard and modified 普洛克特 variations, this means reducing the finer materials to pass through either a 4.75mm(#4)或9.5毫米(3/8in)筛. Coarser materials are set aside 用于粒度测定 and in some cases for adding proportionally back into the final test specimens. At this stage, sample breakdown and coarse particle sizing are often performed concurrently.
Four or five specimens are prepared for the compaction points with increasing moisture content that will bracket the estimated optimum water content. This requires some guesswork and experience is always helpful. Approximate optimum moisture of most cohesive soils can be estimated by manually squeezing a portion into a lump that will stick together yet break cleanly into two sections when “bent”. The weight of the specimens should be about 5lb (2.3kg) each for 4in (102mm) molds and 13lb (5.9kg) for 6in (152mm) molds to ensure enough compacted volume to properly fill the molds. Water is added incrementally and mixed thoroughly to increase the individual moisture contents for each specimen by about 2%. The prepared specimens are set aside in closed containers for a prescribed amount of soaking time ranging up to 16 hours for proper moisture conditioning. The containers can be sealed metal cans, but heavy-duty zip-closure bags work well for this step.
NOTE: This article serves as an informational guide to equipment and techniques used to perform the 普洛克特 moisture/density relationship tests as described in ASTM D698 and D1557 or AASHTO T 99 or T 180 测试方法. The information contained herein is in no way intended to supersede the specifications or test protocol of these published 测试方法.
对于每一分的普洛克特, the operator compacts the specimen into the pre-重 soil compaction mold assembly in three to five layers (lifts) according to the method required. The manual soil 压实锤s are lifted to their maximum height and allowed to fall freely onto the soil specimen for the required number of blows. 对于标准的proctor，锤子重5磅.5lb (2.5公斤)，下落12英寸(305毫米). The modified 普洛克特 method uses a 10lb hammer with 18in (457mm) drop height for compaction. Automatic/mechanical compactors are also available to make this process easier. 压实后, 领子被取下, and excess soil is carefully trimmed with a straightedge tool so the compacted soil is flush with the top of the mold. Small voids can be manually 填满 with excess samples. The mold with the sample is then 重 and recorded and the soil is extruded from the mold. A sample of the specimen is obtained to determine the exact moisture content by oven drying, and the process is repeated for subsequent samples.
潮湿的密度, 或单位重量, of each compacted specimen is computed by dividing the mass of the soil (minus the mold assembly) by the volume of the mold. To find the dry density, divide the moist density by the percent moisture divided by 100, plus 1.
对于每个初始点, the mass and the unit weight of the soil will increase as the additional moisture lubricates particles and allows them to be consolidated into a denser state (fewer air voids) using the same compactive effort. After about the third point (if you’re lucky and your estimates were correct) the mass of the sample will decrease as the volume of water reaches a point where it displaces soil particles in a given volume. This indicates that optimum moisture has been exceeded and having another point beyond that will make it a bit easier when constructing the final compaction curve.
The weight of each specimen is used to calculate wet unit weights and the oven-dried moistures are used to determine a dry unit weight for each point.
The results are plotted on a graph as dry unit weight vs. moisture content and will show the curvilinear relationship that allows the maximum dry weight and optimum moisture for each type of soil to be established.
Example of Plotting Wet Density, Dry Density, and Saturation Curves
饱和曲线, 或者零空隙率曲线, is a plot of soil density vs moisture at a theoretical 100% saturation level (zero air voids). Points for plotting the curve are calculated from compacted dry unit weights of the sample material adjusted to moisture contents at 100% saturation, using this formula stated in the ASTM D698 standard test method, 其中:
饱和曲线 serves as a guide when plotting dry unit weights on the soil compaction curve. The wet side of the compaction curve should roughly parallel the saturation curve. 在理论上, 如果两条曲线相交, 在测试中出现了一个错误, 计算, 或者绘制结果图.
当进行磁场密度测试时, the dry unit weight is compared to the maximum dry weight of the 普洛克特 tests to calculate a percent of compaction. When the required densities are hard to reach in the field, the moisture content should be compared to the optimum moisture content from the lab tests. If field moistures differ more than 2% or 3%, it becomes more difficult to compact to the required density. Field moistures can be adjusted by aerating/discing the soil, or by adding water.
当测试场地密度时, the dry unit weight from the density test is compared to the maximum dry weight from the laboratory 普洛克特 to calculate a percent of compaction. If field compaction percentages are too low or seem too high, there are some things to consider: • The compaction energy from heavy equipment dissipates as the depth of the loose soil layers increases. If an 18in (450mm) layer (lift) of soil cannot be compacted to the required percentage, try reducing the lift thickness to 10in or 12in (250mm or 300mm), 或者增加传球次数.
- If the field moisture content of the soil differs from the laboratory optimum moisture by more than 2% or 3%, it becomes more difficult to compact the material to the required density. Field moistures can be adjusted by aerating/discing the soil, or by adding water.
- Check that the compacted soil is the same as the laboratory proctor sample. AASHTO T 272 is a “one-point” field test performed using the same type of mold, 压实锤, and the number of blows as the original lab method. 含水率是用 气压湿度测试仪 or 丙烷抗 dry-back方法. Field density and moisture values are plotted against the original laboratory data, or a family of curves compiled from regional soil data to confirm a match.
- Heavy-duty plastic freezer bags are a convenient option for the storage and moisture conditioning of individual compaction specimens.
- When preparing individual compaction specimens, it doesn’t take much time to prepare a couple of extra, 以防你的估计有一点偏差. Set up one on the drier side and one on the wet side to cover the bases.
- Some methods and materials require adding coarse native material back into the final specimen. Set aside your plus-size material as you break down the bulk sample. It’s also convenient to sieve the coarse material now to reduce handling.
- 使用一个 样本喷射器 expedites the removal of compacted soil from the mold, increasing testing speed and efficiency and making it easier to obtain a representative moisture sample.
- Running five or six points of the proctor test can be physically demanding, especially with a modified 普洛克特 using a 10lb hammer to compact five layers with 56 blows each. If you’ve been staying away from the gym lately, 你也许需要我帮你个忙, 或者更好的是, 考虑一个 土壤机械压实机. This device reduces effort and greatly enhances repeatability and accuracy.
We hope that you have found this blog post helpful in understanding the procedures and equipment used to perform the 普洛克特 compaction tests of soils by the noted ASTM/AASHTO methods. loveBet爱博 supplies a complete line of soil compaction and 普洛克特 equipment and accessories. 请 loveBet爱博体育下载 with questions or to discuss your application.